What Is Fertilization?

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What is fertilization?

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Where does fertilization occur?

Read this next, when does fertilization occur , how long does it take for a sperm to fertilize an egg, can you feel when an egg gets fertilized, how long can sperm live inside you to get pregnant.

As you've no doubt learned by now, the road to fertilization is a bumpy one, with plenty of twists and turns. It takes the right conditions and perfect timing for the egg and sperm to meet up and produce that baby you're hoping for. But once they do, you've embarked on your own amazing journey: the journey of motherhood.

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ScienceDaily

Male fertility gene discovery reveals path to success for sperm

The discovery of a pair of genes that work in perfect harmony to protect male fertility, could provide new insights into some unexplained cases of the most severe form of infertility, research suggests.

Genetic analysis of cases of male infertility revealed that rare mutations in a gene, known as SPOCD1, disrupts the formation of healthy sperm during the earliest stages of their development.

The gene was also found to work in partnership with a previously unknown gene, C19orf84, to protect the early-stage precursors to sperm, known as germ cells, from damage.

The discovery of the essential role of these two key genes could provide the answer to some cases of the most severe forms of male infertility and lead to expanded genetic screening for rare mutations, researchers say.

Cryptozoospermia and azoospermia, in which little or no sperm is produced, affects around 1% of men. In 45% of cases no cause can be found, but they are long suspected of having genetic causes.

Sperm cells biggest challenge starts long before the journey to reach the egg as they are particularly vulnerable during the earliest stages of their development, as germ cells in developing embryos.

Germ cells must protect their DNA from damage during the embryo's development so they can become the pool of self-renewing cells that produce healthy sperm throughout adult life.

A previous study by the researchers had shown that SPOCD1 has an essential role in protecting germ cells in male mice, but it was unclear whether the same process happened in humans.

In collaboration with researchers at the University of Münster and other partner universities, scientists at the University of Edinburgh screened international databases containing genetic data from 2913 men involved in studies on infertility.

They identified three men who carried faulty versions of the SPOCD1 gene which resulted in damage to germ cells that prevented healthy sperm development -- this failure to launch led to infertility.

During their development, germ cells undergo a reprogramming process that leaves them vulnerable to rogue genes, known as jumping genes, which can damage their DNA and threaten fertility.

Germ cells are the vital link between generations but they need unique strategies to protect the genetic information they carry, so it can be passed successfully from parents to their offspring.

The previous study in mice found that the SPOCD1 gene helps to recruit protective chemical tags, known as DNA methylations, to disable jumping genes.

This study revealed that the men with faulty versions of the SPOCD1 gene had the most severe forms of infertility, azoospermia and cryptozoospermia.

Analysis of the mutated variants of the SPOCD1 gene also revealed a new gene, known as C19orf84 which partners with SPOCD1 and forms an important line of defence in early sperm cells.

Further study of the role of these genes in early-stage sperm cells in mouse embryos revealed that both produce proteins are essential in recruiting the protective tags that silence jumping genes.

Scientists have long puzzled over how germ cells escape damage during the reprogramming process, as it temporarily wipes their genetic slate clean of existing protective tags.

C19orf84 protein acts as a matchmaker connecting the SPOCD1 protein with the cell's protective chemical tag-making machinery and directing them towards the jumping genes before they can damage the genome.

Increased understanding of this process together with expanded genetic screening will allow scientists to identify if faulty versions of these genes are the cause of some of these rare cases of male infertility, researchers say.

The study, published in Molecular Cell, was funded by Wellcome. It also involved researchers from University of Oxford, University Hospital Münster, The University of Melbourne, Oregon Health and Science University, University of Utah and Technische Universität Berlin.

Professor Dónal O'Carroll, lead author of the study from the University of Edinburgh, said:

"This was a wonderful collaborative project that led to the discovery of new genetic causes of male infertility. We also advanced our understanding of a process that is fundamental to healthy sperm cell development. These mechanistic insights are leading to a better understanding of the elusive process that allows developing sperm to preserve their genetic integrity and escape an early death."

Dr Ansgar Zoch, first and co-corresponding author of the study from the University of Edinburgh, said:

"A truly collaborative achievement, this study enhances our understanding of male infertility on the molecular and genetic level. I am particularly proud that so many co-authors joined efforts and contributed their expertise. We demonstrate strong evidence for SPOCD1 to be included in genetic screenings of male infertility patients. Providing a genetic diagnosis can help provide closure to affected individuals and potentially prevent unnecessary medical procedures."

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Story Source:

Materials provided by University of Edinburgh . Note: Content may be edited for style and length.

Journal Reference :

  • Ansgar Zoch, Gabriela Konieczny, Tania Auchynnikava, Birgit Stallmeyer, Nadja Rotte, Madeleine Heep, Rebecca V. Berrens, Martina Schito, Yuka Kabayama, Theresa Schöpp, Sabine Kliesch, Brendan Houston, Liina Nagirnaja, Moira K. O’Bryan, Kenneth I. Aston, Donald F. Conrad, Juri Rappsilber, Robin C. Allshire, Atlanta G. Cook, Frank Tüttelmann, Dónal O’Carroll. C19ORF84 connects piRNA and DNA methylation machineries to defend the mammalian germ line . Molecular Cell , 2024; DOI: 10.1016/j.molcel.2024.01.014

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sperm cell journey to the egg

Sperm, Meet Egg: The Process of Fertilisation

sperm cell journey to the egg

Turbo-charged sperm speed through a dimly lit canal, coursing feverishly through vast landscapes and titanic cavities. A lone egg desperately awaits her suitor, pledging her life if left unavowed. Then, the swiftest sperm rises from the depths of darkness, breaking through a fortress wall and presenting himself to his prospective partner. A union is formed. The end. This is the stuff of movies and drama. But it’s also the stuff of fertilisation. When sperm are released into the vaginal canal, an intricate performance ensues. With the ovaries serving as arc lights, the stage womb is set.

Understanding Fertilisation

Every month, one amongst a woman’s two ovaries releases a mature egg into the fallopian tube. This egg remains available for fertilisation for about 24 hours, and if met by a sperm, can lead to conception . Sperm can survive for up to 5 days within the uterine cavity, so it is possible that you will get pregnant if you’ve had intercourse in the 5 days leading up to ovulation .

sperm cell journey to the egg

It’s wise to maintain an ovulation calendar to preempt your fertile periods. Conception involves millions of sperm vying to outpace each other in order to reach the available egg. Only one sperm wins, achieving fertilisation after a long and arduous journey. The resultant embryo, still in the fallopian tube, then descends downward until it reaches the uterine cavity, where it implants itself.

The Steps of Fertilisation

Fertilisation is a complex process, with the female body working in mysterious ways to align the egg and the sperm. Detailed below, are the steps that go into it.

sperm cell journey to the egg

  • Step 1. Ovulation

Ovulation refers to the emergence of a single mature egg from one of the ovarian follicles. The egg only has a 24-hour window to be fertilised . During this time, vaginal discharge becomes wet and slippery, a telling sign that fertility is at its peak . Other ovulation symptoms include bloating and abdominal pain . If unprotected intercourse is had during this time or in the 5 days prior, there is a good chance that it will result in a pregnancy.

sperm cell journey to the egg

  • Step 2. Ejaculation

Semen is a versatile substance, providing both nourishment and protection for sperm . As soon as ejaculation happens, the semen left behind forms a wall across the vagina to save sperm from moving downward. This wall lasts only about half an hour before it starts to trickle out of the vagina. The sperm cells that do make it through after ejaculation begin a long journey up the cervical canal, each holding out hope to make it to the egg first.

sperm cell journey to the egg

Step 3. Journey Through the Cervical Canal

The cervical canal is a warm and conducive environment that allows sperm to thrive and push on in their journey. Generously lined with cervical mucus, the canal is tailored for sperm transportation , especially during the fertile window when mucus is at its maximum. Interestingly, the days before ovulation will herald molecular changes that you may not even be aware of. Microscopic threads of molecules line up along the cervical canal, to allow sperm to latch on as they pace through.

  • Step 4. Biochemical Alterations + Accelerated Movement

Sperm that enter the cervical canal must change their structural form in order to survive. Their new environment triggers biochemical changes that allow them to travel at breakneck speeds through the uterus and fallopian tubes.

sperm cell journey to the egg

  • Step 5. Branching Off

Once sperm reach the uterus, they have a critical decision to make. Do they go right or left? There’s a fallopian tube on either side and it’s anybody’s guess which tube has released an egg this time. Sperm tend to branch out at this point, some gravitating to the left and others to the right. ‍ ‍

Must read - 7 Pregnancy Questions to Ask your Doctor ‍

‍ Sperm that pick the correct tube has a significant chance of reaching the egg in time. Now, with about half the competitors as before, sperm must power to the finish line in time.

  • Step 6. Fertilisation

Only the most resilient sperm reach the egg and even the ones that do must cross another hurdle before they can reach their final destination. Every egg is covered with a tough outer layer and hundreds of sperm engage in a race to see who can penetrate first. When one sperm finally does manage to achieve fertilisation, the egg immediately experiences chemical changes that block other sperm from entering. Then, the chromosomes in the egg and sperm combine, giving rise to a zygote.

‍ Must read - How to Handle the 1st Trimester of Pregnancy?

sperm cell journey to the egg

  • Step 7. Implantation

The zygote divides repeatedly over the next few hours and days. It gradually rolls down through the fallopian tube, reaching the uterine cavity about a week later as a 100-cell ball. The zygote now implants itself into the uterine lining, going on to develop into a baby.

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As frenzied as the fertilisation process is, it marks the beginning of a chapter that will unfold page by page over the next nine months. By being in the know of how fertilisation works, you can stay better prepared in planning your family. So that when the arc lights in your womb are turned on, you’re still the director of the show.

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The journey of the sperm to the egg

The journey of the sperm to the egg

The path of the spermatozoa to reach the egg is not a simple one. This path is divided into a phase in the male reproductive system and another in the female reproductive system.

In the case of the male, the sperm travel from the testicle to the urethra. When vaginal penetration occurs, the sperm continue their journey to the fallopian tubes. This is where the egg is found and the fastest and most capable sperm will fertilize it.

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sperm cell journey to the egg

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The Epic Journey of Sperm Through the Female Reproductive Tract

Millions or billions of sperm are deposited by artificial insemination or natural mating into the cow reproductive tract but only a few arrive at the site of fertilization and only one fertilizes an oocyte. The remarkable journey that successful sperm take to reach an oocyte is long and tortuous, and includes movement through viscous fluid, avoiding dead ends and hostile immune cells. The privileged collection of sperm that complete this journey must pass selection steps in the vagina, cervix, uterus, utero-tubal junction, and oviduct. In many locations in the female reproductive tract, sperm interact with the epithelium and the luminal fluid, which can affect sperm motility and function. Sperm must also be tolerated by the immune system of the female for an adequate time to allow fertilization to occur. This review emphasizes literature about cattle but also includes work in other species that emphasizes critical broad concepts. Although all parts of the female reproductive tract are reviewed, particular attention is given to the sperm destination, the oviduct.

Introduction

Normally only one sperm fertilizes an oocyte despite that billions of sperm are deposited by natural mating into the vagina, or millions are deposited by artificial insemination into the uterus of a cow. The remarkable journey that successful sperm take to reach the oocyte is long and tortuous, filled with viscous fluid, dead ends, and potentially hostile immune cells. Rather than a simple race to get to the oocyte, there is much evidence that complex mechanisms influence sperm transport, immunological tolerance of sperm, sperm selection, sperm storage and release, all before actual fertilization. At steps along the way to the site of fertilization, sperm may interact with the fluid in which they are suspended and the epithelium lining the tract. The very dynamic process of sperm transport helps ensure that there is an appropriate number of fertile sperm at the site of fertilization so that the oocyte can be fertilized by only one sperm. This review considers sperm interaction with fluid in the reproductive tract as well as sperm adhesion to the epithelium. It also reviews how sperm, foreign cells in the female reproductive tract, are tolerated by the immune system. Although it emphasizes literature about cattle, concepts developed in other species are included.

Sperm in the Vagina and Cervix

Sperm are transported through the vagina, cervix, and uterus to the oviduct where they can fertilize oocytes. In cattle and many other mammals, estrus occurs before ovulation so sperm are deposited in the female reproductive tract before ovulation. At normal copulation in cattle, semen is deposited in the cranial vagina. Vaginal fluid is the first luminal medium to which sperm are exposed after semen deposition. The acidic pH of the vagina makes it inhospitable for sperm, although buffers found in semen neutralize the local pH. The cow produces a large volume of vaginal fluid and up to 100 ml can accumulate (reviewed by ( Rutllant et al. , 2005 ). The rheological properties of vaginal fluid appear to influence sperm motility characteristics, although fertilizing sperm may spend only a short time in the vagina ( Rutllant et al. , 2005 ).

It is likely that bovine sperm, like human sperm ( Suarez and Pacey, 2006 ), that are candidates to fertilize oocytes enter the cervical canal quickly avoiding damage due to the low vaginal pH. The cervix contains many folds and grooves that are filled with mucus. The mucus within the canal is a major barrier to sperm, particularly those that have abnormal motility ( Katz et al. , 1997 ). The composition and structure of cervical mucus changes near estrus, allowing sperm with normal motility to advance, typically through what have been called “privileged paths” that are found in the grooves produced by folds that extend through the cervical canal ( Mullins and Saacke, 1989 ). A microfluidic model has confirmed that sperm migration through these privileged paths is controlled by microgrooves and a gentle flow of fluid ( Tung et al. , 2015b ).

Sperm are foreign cells and can induce an immune response in the cervix. In rabbits, neutrophil infiltration was observed within 30 min of mating ( Tyler, 1977 ). Immunoglobulins IgG and IgA ( Kutteh et al. , 1996 ) and complement proteins have been detected in human cervical mucus ( Mathur et al. , 1988 ). Therefore, sperm retained in the cervix might be attacked by the immune system before moving into the uterus.

Sperm in the Uterus

After natural mating, sperm move from the cervical canal into the uterus. In cattle, artificial insemination (AI) is used frequently. When performing AI, the technician deposits semen directly into the uterine body, so sperm do not enter the vagina and cervix. Depositing sperm directly in the uterus reduces the number of sperm needed for routine AI to 10–20 million ( Moore and Hasler, 2017 ). As few as 2 million sperm are often inseminated when using sperm separated based on their sex chromosome, a process used to bias the sex of offspring ( DeJarnette et al. , 2009 ). Experiments in which the uterotubal junction (UTJ) in heifers was ligated at various times after mating revealed that it took 6–8 hr for sperm to move through the cervix and uterus to infiltrate the oviduct in numbers sufficient for oocyte fertilization ( Wilmut and Hunter, 1984 ). Sperm are transported through the uterus with the aid of uterine smooth muscle contractions in the direction of the oviduct ( Hawk, 1987 ). To measure fluid movement and uterine contractions, technetium-labeled albumin-macrospheres were deposited in the uterus of women. These macrospheres (5–40 μm diameter) could be detected by high-resolution ultrasound. They were transported from the uterus to the oviduct more rapidly in the late follicular phase ( Kunz et al. , 1996 ) which, along with other experiments, indicates that uterine contractions that transport sperm are under endocrine control. Further, this result demonstrates that materials in addition to sperm can move through the UTJ.

Sperm in the uterus of cattle and other species are retained in uterine glands in low numbers per gland ( Hunter, 1995 , Rijsselaere et al. , 2004 ). Retention, at least in swine, is accomplished by sperm binding to uterine epithelial cells ( Rath et al. , 2016 ). Sperm attachment to uterine cells stimulates the production of both pro- and anti-inflammatory cytokines ( Lovell and Getty, 1968 ). There is evidence that porcine sperm bind to sialic acid-containing glycans on the surface of uterine epithelial cells ( Rath et al. , 2016 ). For example, a sialic acid lectin that recognizes sialic acid binds to uterine epithelial cells and blocks sperm binding, in vitro . Although it is not clear whether many sperm in uterine glands move into the oviduct, the fate of the majority of sperm in the uterus is elimination.

Rapid removal of sperm may help reduce the acquired immune response against sperm ( Hansen, 2011 ). Little is known about the immune response elicited by semen deposition in cattle but it has been studied more in rodents and horses ( Katila, 2012 , Bromfield, 2014 , Christoffersen and Troedsson, 2017 ). The primary function of the inflammatory response is to clear excess sperm, seminal debris and bacteria from the uterus. Following semen deposition, there is an infiltration of polymorphonuclear leukocytes. In addition to activation of innate immunity, adaptive immunity is also involved. Several classes of antibodies have been isolated from uterine fluid. In addition to cytokines released from the uterine endometrium, seminal plasma itself contains immune system modulators that affect uterine and oviduct immune cells ( Robertson, 2007 , Schjenken and Robertson, 2014 and 2015 ). There is evidence that a seminal vesicle protein may allow the uterus to tolerate sperm ( Kawano et al. , 2014 ). Interestingly, the seminal fluid fraction of semen also improves preimplantation development and has interesting long-term effects on offspring ( Bromfield et al. , 2014 ). This non-traditional role of seminal plasma has been studied most in rodents; the amount of seminal plasma in cattle that mate normally is low and even lower when artificial insemination is used.

Sperm Entry into the Oviduct through the Utero-tubal Junction

In the bovine UTJ, sperm move through a slit-like lumen with a mucosal pad and into the lower portion of the oviduct, the isthmus, which contains 4–8 primary grooves in tubal segment ( Wrobel et al. , 1993 ). Compared to the major part of the upper oviduct, the ampulla, the isthmus has a narrower lumen with fewer folds but a thicker layer of smooth muscle. Although macrospheres seem to have the ability to pass through the UTJ (discussed above), there is evidence that sperm, at least in mice, require a specific protein to be recognized and to pass through the UTJ into the isthmus. Mouse sperm deficient in ADAM3, due to mutation of the ADAM3 gene or genes whose products affect ADAM3 are not detected beyond the UTJ ( Nakanishi et al. , 2004 , Yamaguchi et al. , 2006 , Yamaguchi et al. , 2009 , Okabe, 2013 ). Even if sperm from a chimeric male derived from a normal and a mutant embryo were deposited, only the normal sperm moved into the oviduct ( Nakanishi et al. , 2004 ). Thus, the presence of normal sperm does not aid in opening the UTJ to allow ADAM3 mutant sperm to pass into the oviduct.

In addition to ADAM3, there also appears to be a rheological barrier in the porcine UTJ, perhaps the viscous mucus present in the grooves of this structure ( Hunter, 2002 , Tienthai, 2015 ). The rabbit and mouse UTJ and oviduct fluid contain proteoglycans with sulfated glycosaminoglycan chains and hyaluronan ( Jansen, 1978 , Suarez et al. , 1997 ). In addition to changing the viscosity and affecting sperm motility, the abundance of hyaluronan in fluid and its receptor, CD44 on the epithelial cells of the UTJ, suggest that CD44 signal transduction might affect the function of the UTJ and lower oviduct ( Bergqvist et al. , 2005a , Bergqvist et al. , 2005b ).

In cattle and other species, there appears to be a valve at the UTJ that can constrict the lumen, restricting sperm entry. This valve is formed by a vascular plexus and surrounded by a thick muscle layer that, in total, can contract the lumen ( Wrobel et al. , 1993 ). The physical constriction, mucus barrier and protein signature requirements emphasize how stringently entrance to the oviduct is regulated.

Sperm in the Oviduct

Once sperm enter the lower oviduct, the isthmus, they can bind to the epithelial cell surface or remain in oviduct fluid. Many studies of the intact oviduct have been performed in mice because the uterus and oviduct can be transilluminated so that sperm can be observed ( Demott and Suarez, 1992 ). Sperm from transgenic mice that have enhanced green fluorescent protein in their acrosomes and red fluorescent protein in their midpiece mitochondria have been followed in the female tract after natural mating ( La Spina et al. , 2016 ). The location of live sperm and their acrosomal status can be followed using fluorescence microscopy.

When sperm in the lumen of the isthmus were observed, groups of sperm were carried by fluid that was moved alternately toward the uterus and then toward the ampulla (back and forth) by contractions of oviduct smooth muscle ( Ishikawa et al. , 2016 ). These contractions were not observed in the ampulla. Most of the sperm in the isthmus were acrosome-intact ( La Spina et al. , 2016 ). Relatively few sperm were found in the ampulla and most were acrosome-reacted ( La Spina et al. , 2016 , Muro et al. , 2016 ), consistent with the recent evidence that the acrosome reaction of fertilizing mouse sperm occurs prior to contact with the cumulus-oocyte complex ( Jin et al. , 2011 , La Spina et al. , 2016 ).

Oviduct Fluid Affects Sperm Function

The fluid in the oviduct is highly viscous, unlike the culture medium in which studies of mammalian fertilization are usually performed. Fluid viscosity is often overlooked in studies of sperm function within the oviduct. More viscous fluid has more internal friction so the wake from a sperm swimming in viscous medium is relatively small compared to less viscous medium ( Kirkman-Brown and Smith, 2011 ). Studies of human sperm demonstrate that resistance of the fluid to be moved results in a sperm tail with multiple bends while beating ( Kirkman-Brown and Smith, 2011 , Hyakutake et al. , 2015 ). In contrast, in less viscous medium, the tail has fewer bends and, instead, remains mostly straight while simply swinging or flapping back and forth ( Kirkman-Brown and Smith, 2011 , Hyakutake et al. , 2015 ). Consequently, in viscous fluid, a motile sperm will have less side-to-side movement (yaw) than in a standard viscosity medium ( Kirkman-Brown and Smith, 2011 ). Sperm also tend to swim near and against solid surfaces, for example epithelial walls or the corners of microchannels ( Denissenko et al. , 2012 ). Sperm that are close to the channel wall swim faster than those moving in the center of the channel ( El-Sherry et al. , 2014 ). Viscoelastic medium induces bovine sperm to swim in coordinated groups that may facilitate sperm migration ( Tung et al. , 2017 ). The majority of sperm orient their swimming so that they swim against the flow of medium when the flow rate is intermediate (33–134 μm/sec) ( Miki and Clapham, 2013 , El-Sherry et al. , 2014 , Tung et al. , 2015a ). This appears to guide sperm upstream in oviduct fluid ( Miki and Clapham, 2013 ). There is controversy about whether a signaling process in sperm aids in orienting sperm in the upstream direction or if sperm rheotaxis is a passive process ( Miki and Clapham, 2013 , Hyakutake et al. , 2015 ).

Interestingly, the viscosity of oviduct fluid varies during the estrous cycle; tenacious mucus is found in the rabbit oviduct lumen at estrus and disappears after ovulation ( Jansen, 1978 ). Most studies of sperm-oviduct interaction or fertilization have used standard culture medium and ignored its low viscosity, compared to oviduct fluid. A few have tried to recapitulate the viscosity of oviduct fluid by adding components like methylcellulose or polyvinylpyrrolidone to medium ( Suarez and Dai, 1992 , Alasmari et al. , 2013 , Gonzalez-Abreu et al. , 2017 ). In addition to effects on normal motility, discussed above, physiological viscosity converts the wild thrashing motion and high yaw of hyperactivated sperm to motility with less yaw and a more forward movement ( Suarez and Dai, 1992 ).

In addition to the rheological properties of oviduct fluid, specific components of oviduct fluid such as secreted proteins, proteoglycans, and lipids may influence fertilization by affecting sperm function ( Coy et al. , 2010 , Killian, 2011 ). This complex fluid can affect sperm prior to encountering the oocyte and during fertilization ( Rodriguez-Martinez, 2007 , Killian, 2011 ). For example, bovine sperm take up phospholipids that are abundant in oviduct fluid ( Killian et al. , 1989 )( Evans and Setchell, 1978 ). Oviduct fluid glutathione peroxidase, superoxide dismutase and catalase can protect bovine sperm from damage by reactive oxygen species that may otherwise reduce sperm viability and motility ( Lapointe and Bilodeau, 2003 ). Proteoglycans found in oviduct fluid promote capacitation of bovine sperm through their glycosaminoglycan side chains ( Parrish et al. , 1989 , Bergqvist et al. , 2006 ).

Oviduct fluid components, for example glycosaminoglycans, can also cause proteolysis or loss of sperm membrane proteins, including those that are implicated in sperm binding to the oviduct epithelium. The best studied of these proteins originate from accessory gland secretions and bind to sperm at ejaculation. Some bovine Binder of Sperm (BSPs) and porcine sperm adhesins are lost as sperm are capacitated ( Topfer-Petersen et al. , 2008 , Hung and Suarez, 2010 ). Although the significance of protein loss or proteolysis is uncertain, in sperm bound to the oviduct epithelium, it might contribute to their release prior to fertilization ( Topfer-Petersen et al. , 2008 , Hung and Suarez, 2010 ).

In addition to losing proteins, sperm also gain proteins while they reside in the oviduct. The first of two examples is Oviduct Specific Glycoprotein (OGP) or oviductin, also known as OVGP1, found in oviducts of many mammals. Although it has homology to the chitinase family of proteins, OGP does not have enzymatic activity ( Jaffe et al. , 1996 , Araki et al. , 2003 ). Bovine sperm incubated in OGP have improved motility and viability ( Abe et al. , 1995 ). Hamster sperm treated with recombinant OGP have increased phosphorylation of tyrosine residues on proteins, an indication that capacitation was enhanced ( Yang et al. , 2015 ). There is also evidence in mice and swine that OGP binds to the zona pellucida to increase fertilization success by rendering the zona matrix more permissive to penetration by sperm ( Lyng and Shur, 2009 , Algarra et al. , 2016 ).

A second example of an oviduct protein that affects sperm is osteopontin. Although it is already bound to bovine sperm before semen is deposited in females ( Erikson et al. , 2007 ), addition of osteopontin during in vitro fertilization reduces polyspermy ( Goncalves et al. , 2008 ). Neither osteopontin nor OGP is necessary for fertility in mice because animals deficient in each are fertile ( Rittling et al. , 1998 , Araki et al. , 2003 ).

In addition to oviduct fluid proteins being added as peripheral membrane proteins, integral membrane proteins could be added by fusion with sperm of oviductosomes secreted by the oviduct. For example, a portion of the major Ca 2+ efflux pump is added to mouse sperm by oviduct exosomes ( Al-Dossary et al. , 2015 ). The proteins secreted by bovine oviduct cells and found in oviduct fluid have recently been profiled and include growth factors, metabolic regulators, immune modulators, enzymes and extracellular matrix components ( Lamy et al. , 2016 , Pillai et al. , 2017 ). They function in immune homeostasis, gamete maturation, fertilization and early development ( Pillai et al. , 2017 ). The abundance of some depend on the stage of the estrous cycle and whether they were found in oviducts ipsilateral or contralateral to the ovary that ovulated ( Lamy et al. , 2016 ).

The Oviduct as a Functional Sperm Reservoir

The oviduct, along with the UTJ in some species, appears to be the major location in which sperm are stored before fertilization. In contrast, although sperm are retained in the cervix or uterus, it is not clear that they are eventually released to move to the oviduct. So the UTJ and oviduct appear to be the major sperm storage sites in many mammals. To be a true “functional sperm reservoir”, as coined by Hunter, ( Hunter et al. , 1980 ), in addition to retaining sperm, the oviduct must affect sperm function and lengthen sperm lifespan beyond the inherent longevity of sperm ( Orr and Zuk, 2014 ). More than simple adhesion occurs because binding to the oviduct epithelium prolongs the lifespan of sperm and suppresses capacitation and motility ( Pollard et al. , 1991 , Rodriguez-Martinez, 2007 , Hung and Suarez, 2010 )( Rodriguez-Martinez et al. , 2005 ). Thus, the oviduct isthmus meets these requirements. But the ability of sperm reservoirs described in a variety of species to prolong the lifespan of a highly differentiated and transcriptionally inactive cell is enigmatic.

The reservoir also releases a finite number of stored sperm, acting as a buffer for sperm number to prevent polyspermy but still provide an appropriate number of fertile sperm to the upper oviduct ( Hunter and Leglise, 1971b ). The isthmic epithelium binds and retains preferentially sperm that have intact acrosomes and normal morphology ( Teijeiro and Marini, 2012 )( Teijeiro et al. , 2011 ). All together, the isthmus functions to increase the probability that a suitable number of fertile sperm are present at the site of fertilization.

The Oviduct Epithelium Retains Sperm and Modulates Sperm Function

In mammals, the oviduct epithelium binds and retains sperm so they accumulate to form the reservoir. Adhesion is very specific. The sperm head binds to oviduct epithelial cells but not all cells ( Pacey et al. , 1995 , Kervancioglu et al. , 2000 ). And the ability of sperm binding to maintain viability is not a common property of all cells ( Boilard et al. , 2002 ). The ability to maintain viability requires direct contact between sperm and oviduct epithelial cells ( Dobrinski et al. , 1997 , Murray and Smith, 1997 , Smith and Nothnick, 1997 ). Adhesion to the oviduct regulates sperm capacitation ( Dobrinski et al. , 1997 , Boilard et al. , 2002 , Fazeli et al. , 2003 ) and suppresses the normal increase in sperm intracellular free calcium that occurs during capacitation ( Dobrinski et al. , 1996 , Dobrinski et al. , 1997 ).

Studies performed in several mammals have concluded that glycans are the components in oviduct epithelial cells that bind sperm ( Lefebvre et al. , 1997 , Green et al. , 2001 , Suarez, 2001 , Cortes et al. , 2004 , Topfer-Petersen et al. , 2008 ). The evidence in most studies underpinning a role for oviduct glycans is a competition assay in which different glycans are added to sperm before challenging these sperm by allowing them to bind oviduct epithelial cells in vitro. If few sperm bind to oviduct cells, this result is interpreted as an indication that the specific glycan is related to the authentic oviduct glycan that binds sperm. A frequent problem with these studies is that most test high concentrations of a small number of monosaccharides or small oligosaccharides.

Identification of Glycans that Bind Porcine Sperm Using a Glycan Array

The development of glycans immobilized on an array provided an opportunity to test hundreds of glycans for their ability to bind sperm. Using such an array, nearly 400 glycans were tested for their ability to bind porcine sperm ( Kadirvel et al. , 2012 ). All the glycans that bound sperm contained one of two glycan motifs, either a Lewis X trisaccharide (Le X ) or a structure with with core mannose and two antennae terminating in the sialylated lactosamine trisaccharide bi-SiaLN or in simply lactosamine ( Figure 1 ). There were several examples demonstrating that sperm bound these two motifs with high specificity. In all sialic acid-containing structures that bound sperm, sialic acid was linked to the 6 position of galactose; structures that were identical except that sialic acid was attached to galactose at the 3 position did not bind sperm. Furthermore, the branched structure on a mannose core was required because single sialylated lactosamine trisaccharides (Neu5Acα2–6Galβ1–4GlcNAc) did not bind sperm ( Kadirvel et al. , 2012 ).

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Structures of glycans that bind bovine (Le A ) and porcine sperm (bi-SiaLN, bi-LN, and Le X ), and the related glycan that does not (LN). Le A is found on the bovine oviduct epithelium. bi-SiaLN is abundant on the epithelium of the porcine ampulla and isthmus including ciliated and non-ciliated cells. Le X is found in the porcine isthmus but not the ampulla.

The Le X trisaccharide was found as a monomer, dimer or trimer in the remaining glycans that bound sperm ( Kadirvel et al. , 2012 ). This trisaccharide is composed of Gal and Fuc linked to GlcNAc ( Figure 1 ). The Le X trisaccharide also bound sperm with high specificity; the closely related Lewis A trisaccharide (Le A , a positional isomer; the carbons in GlcNAc to which Gal and Fuc are linked are exchanged) did not bind porcine sperm. Contrarily, bovine sperm bind Le A but not Le X ( Suarez et al. , 1998 ). Binding specificity was further supported because porcine sperm did not bind to Galβ1–4GlcNAc; fucose substitution on Le X was necessary ( Kadirvel et al. , 2012 ).

To confirm that the glycans on the array that bound sperm were present in the oviduct isthmus and to determine the complete structures of the oviduct glycans that bound sperm, oviduct glycans and glycolipid structures were identified by tandem mass spectrometry ( Kadirvel et al. , 2012 ). The Le X and branched sialylated motifs (bi-SiaLN) that bound sperm were found on larger structures that were the most abundant of the complex-type glycans on epithelial cells ( Kadirvel et al. , 2012 ). Most of the complex-type oligosaccharides linked to proteins through asparagine residues were branched with two antennae and several had a sialyl residue on at least one terminus. Some biantennary glycans had both motifs, a sialyl residue on one terminus and a Lewis structure on the second. This kind of hybrid glycan was not present on the array but it is possible that, because it includes both motifs, it might bind sperm with highest affinity.

Because tandem mass spectrometry did not distinguish between Le A and Le X and between glycans with sialyl residues attached to the 6-carbon and the 3-carbon of Gal, an additional strategy was used. An antibody and specific lectin, Sambucus nigra agglutinin (SNA) were used that recognize sialic acid attached to galactose in an α-2,6 linkage preferentially and not sialic acid attached to galactose in an α-2,3 linkage ( Naito et al. , 2007 , Song et al. , 2011 ). Both reagents detected 6-sialylated structures that were abundant on the epithelium throughout the oviduct including on ciliated and non-ciliated cells ( Kadirvel et al. , 2012 ).

Similarly, an antibody to Le X was also used to confirm the identity of the oviduct Lewis trisaccharide structures identified by MS ( Kadirvel et al. , 2012 ). Interestingly, Le X was found in a punctate pattern at the luminal surface of porcine isthmic epithelial cells ( Machado et al. , 2014 ) but was not found in the ampulla.

bi-SiaLN and Le X Glycan Motifs Bind to the Porcine Sperm Head

The head is the portion of sperm that binds to the oviduct epithelium and is where ( Suarez et al. , 1991 ) authentic receptors for glycans with bi-SiaLN and/or Le X motifs should be localized. Fluorescein-labeled Le X and bi-SiaLN bound preferentially to the apical edge of the head in 60–70% sperm prior to capacitation ( Kadirvel et al. , 2012 , Machado et al. , 2014 ). Binding of fluoresceinated glycans could be displaced by an excess of the same glycan that did not have a fluorescent tag. The binding specificity was confirmed by testing sperm binding to oviduct glycans attached to Sepharose beads ( Figure 2 ). Tethering a motile cell to a solid phase glycan rather than a soluble glycan more closely mimics sperm binding to the oviduct and requires a higher affinity.

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Sperm bind to oviduct cell aggregates isolated from the isthmus (A, porcine sperm) and beads to which Le A trisaccharide has been attached (B, bovine sperm).

Porcine Sperm Binding to Oviduct Cells Requires Glycans with bi-SiaLN and Le X

Experiments using immobilized glycans (i.e. the glycan array and glycans linked to Sepharose) showed that bi-SiaLN and Le X were each sufficient to tether a motile sperm. Necessity experiments were performed in which either the glycans or putative receptors were blocked. The result of blocking was assessed by sperm binding to aggregates of epithelial cells stripped from the isthmus ( Figure 2 ). Results of these experiments indicated that each glycan or glycan receptor was necessary for sperm to bind oviduct cells.

Receptors on Sperm for Oviduct Glycans

The identity of the sperm molecules that mediate binding to the oviduct is controversial. It appears that different species may use different adhesion molecules. Using bovine tissues, one group found that two oviduct proteins, the chaperones GRP78 and HSP60, bound to sperm ( Boilard et al. , 2004 ). In contrast, a second group, also using bovine sperm, proposed that oviduct plasma membrane annexins containing fucose bind to accessory gland proteins deposited on sperm at ejaculation ( Ignotz et al. , 2007 ). This result was a bit surprising because annexins are usually considered as cytosolic proteins and they lack signal peptides that would direct them through the secretory pathway to become fucosylated. A proteomic study found that annexin A1 is the most abundant protein in oviduct fluid ( Lamy et al. , 2016 ). Perhaps it is released into fluid without passing through the secretory pathway. But in the fluid, it would be expected to compete with annexin A1 located on oviduct epithelial cells and decrease sperm binding to the oviduct.

Studies of porcine sperm also implicated accessory gland secretions added to sperm ( Ekhlasi-Hundrieser et al. , 2005 , Topfer-Petersen et al. , 2008 ). The spermadhesin AQN1 originating from accessory gland secretions is a glycan-binding protein ( Ekhlasi-Hundrieser et al. , 2005 , Topfer-Petersen et al. , 2008 ). Spermadhesins represent 90% of the total boar seminal plasma protein and they become peripherally associated with the sperm plasma membrane after ejaculation ( Sanz et al. , 1993 ). Sperm AQN1 is reported to bind mannose and galactose residues on oviduct cells, but not Le X or bi-SiaLN structures ( Ekhlasi-Hundrieser et al. , 2005 ).

The observation that the accessory gland proteins do not bind Le X and bi-SiaLN motifs ( Ekhlasi-Hundrieser et al. , 2005 ) and sperm obtained from the cauda epididymis are still able to bind oviduct cells, although in reduced number ( Petrunkina et al. , 2001 ), suggested that other glycan receptors were important. Indeed, in cattle there is no evidence that the fertility of epididymal sperm, not exposed to accessory gland proteins, is lower that normal ejaculated semen that includes accessory gland secretions ( Amann and Griel, 1974 ). The fertility of cauda epididymal sperm motivated the investigation of glycan receptors on porcine sperm from the epididymis, which also avoided interference from the very abundant accessory gland proteins ( Silva et al. , 2014 ).

Membrane lysates from porcine cauda epididymal sperm were separated chromatographically and each fraction was subjected to SDS-PAGE, transferred to nitrocellulose and incubated with biotinylated Le X and bi-SiaLN. This “glycan blot” was used to identify proteins with appropriate glycan affinity. Several proteins were identified including the peripheral membrane protein MFG-E8, also known as lactadherin, P47 or SED1 ( Silva et al. , 2017 ). Competition experiments showed that lactadherin bound to oviduct cells and that inhibition reduced sperm binding ( Silva et al. , 2017 ).

Although there is compelling evidence that oviduct glycans are at least partially responsible for sperm binding, there is also evidence that sperm binding to oviduct epithelial cells is mediated to some degree by other interactions. Perturbation of glycans or their candidate receptors decreases sperm binding to oviduct cell aggregates by a maximum of 60% ( Kadirvel et al. , 2012 , Machado et al. , 2014 ). Protein-based interactions may account for the residual binding. For example, fibronectin from oviduct cells can bind α5β1 integrin on bovine sperm ( Osycka-Salut et al. , 2017 ) and the adhesion protein E-cadherin is found in both sperm and oviduct epithelial cells ( Caballero et al. , 2014 ). ( Pollard et al. , 1991 , Lefebvre et al. , 1995 )

Oviduct Epithelial Cells Respond to Sperm Binding

In addition to the effect of adhesion on sperm, sperm adhesion to the oviduct modifies the transcriptional profile of oviduct epithelial cells ( Fazeli et al. , 2004 , Georgiou et al. , 2005 , Georgiou et al. , 2007 , Lopez-Ubeda et al. , 2015 ). Genes related to the inflammatory response, molecular transport, protein trafficking, and cell-to-cell signaling are among those most affected by sperm ( Lopez-Ubeda et al. , 2015 ). In the sow, there is evidence that the ovary has a local effect on the transcriptome of the oviduct. Unilateral ovariectomy reduces expression of genes believed to be involved in sperm survival and early embryonic development ( Lopez-Ubeda et al. , 2016 ). The effect of sperm on the sperm reservoir appears conserved between birds and mammals. Infiltration of porcine sperm into the UTJ and rooster sperm into the chicken utero-vaginal junction alters the expression of genes involved in pH regulation and immune-modulation ( Atikuzzaman et al. , 2017 ). Even more surprisingly, the transcriptional response of oviduct cells is different in response to insemination of either X chromosome- or Y chromosome-bearing sperm ( Alminana et al. , 2014 ). Thus, the presence of sperm changes the behavior of oviduct cells in addition to its consequences for sperm. The result of altered production of specific proteins by oviduct cells is not clear.

Sperm Release from Oviduct Epithelial Cells

The fertilizing sperm must be released from the oviduct reservoir to move to the ampulla and meet the oocyte. There are several models to explain sperm release. One paradigm is that there is a controlled release of stored sperm near ovulation in response to a signal, perhaps from the ovulated oocyte or the released follicular fluid. There is an alternative hypothesis that a subset of sperm are released more stochastically at all times so that there is always a small number of sperm in the ampulla prepared to fertilize an oocyte. But even in this paradigm, it seems likely that there is some control over release of the heterogeneous population of sperm. Sperm release may be due to a change in the oviduct epithelial cells, in the sperm or in the fluid surrounding the cells.

The observation that capacitated sperm have reduced ability to bind oviduct glycans ( Kadirvel et al. , 2012 , Machado et al. , 2014 ) supports a model in which capacitation, the programmed maturation that sperm undergo, causes a change in oviduct glycan receptors so that sperm are released from the oviduct epithelium. Because capacitation does not occur synchronously, in this model sperm release would be expected to be stochastic. The mechanism behind this reduced binding during capacitation is not clear but it may be related to modification in the function of oviduct glycan receptors on sperm, perhaps by proteolysis. There is some preliminary evidence for proteolysis because one candidate glycan receptor, MFG-E8, co-precipitates in sperm lysates with a proteasomal subunit ( Miles et al. , 2013 ).

Another alternative is that the development of hyperactivated motility may be sufficient to detach a sperm from the oviduct epithelium ( Curtis et al. , 2012 ). In support of this, mouse sperm deficient in CatSper calcium channels that cannot hyperactivate do not detach from the oviduct ( Ho et al. , 2009 ).

There is evidence that the cumulus cells of the ovulated cumulus-oocyte complex can release chemical signals, such as progesterone ( Schoenfelder et al. , 2003 , Tosca et al. , 2007 ), which might activate localized sperm release by promoting Ca 2+ influx through CatSper channels ( Lishko et al. , 2012 ). Release may also be controlled by components from the oviduct itself, such as disulfide reducants ( Talevi et al. , 2007 , Brussow et al. , 2008 ), glycosidases that cleave oviduct glycans from the epithelium ( Carrasco et al. , 2008a , Carrasco et al. , 2008b ), and oviduct smooth muscle contractions ( Chang and Suarez, 2012 ). There is evidence that locally produced anandamide activates cannabinoid receptors and TRPV1 to induce a Ca 2+ influx and sperm release ( Gervasi et al. , 2016 ). Anandamide may also activate nitric oxide production by sperm to promote their release ( Osycka-Salut et al. , 2012 ). Finally, the production of unknown sulfated glyconjugates may release sperm by competing for binding sites on the oviduct epithelium ( Talevi and Gualtieri, 2010 ). The dynamic nature of sperm interaction with the oviduct suggests that a variety of factors may regulate sperm release that may aid in providing a constant supply of competent fertilizing sperm.

Immunological Tolerance of Sperm in the Oviduct

The oviduct lumen must maintain an aseptic state for successful fertilization and early embryonic development while regulating maternal responses to allogenic sperm and semi-allogenic embryos ( Marey et al. , 2016 ). Under pathologic conditions, the mucosal immune system produces a proinflammatory response. But sperm binding to oviduct epithelial cells induces an upregulation of IL-10 , TGFβ and increased production of prostaglandin E 2 , inducing an anti-inflammatory response ( Marey et al. , 2016 , Yousef et al. , 2016 ). This produces an environment that suppresses sperm phagocytosis by PMNs and allows sperm greater opportunity to survive in the oviduct and fertilize oocytes. In essence, sperm induce their own protection from an immune response in the oviduct.

Practical Applications of Unraveling the Complexities of Sperm-Female Tract Interaction

Understanding how the oviduct stores sperm should provide insight into how we could improve semen diluents to store sperm for longer periods of time without cryopreservation ( McGetrick et al. , 2014 ). This would be of great benefit to species whose sperm are stored as liquid semen for a few days because they do not survive cryopreservation. It would also benefit regions of the world that have poor infrastructure for storing cryopreserved semen (i.e. irregular delivery of liquid nitrogen) or use fresh semen routinely due to easy transportation and rapid use of the semen ( Vishwanath and Shannon, 2000 ). As proof of principle, addition of a specific soluble heat shock protein (HSPA8) to bull, boar and ram sperm can improve viability after a 24–48 hr incubation ( Elliott et al. , 2009 , Lloyd et al. , 2009 , Lloyd et al. , 2012 , Holt et al. , 2015 ).

A second application of this line of research is that it may lead to methods to lengthen sperm lifespan in the oviduct. This ability would improve fertility of females if semen deposition was not properly timed with,ovulation, a significant problem with AI in cattle and many other species. It may be possible to reduce the need for estrus detection in females because an accurate estimate of ovulation time might be less critical. Fertility despite the uncoupling of mating with ovulation has been accomplished by some mammals, notably some species of bats that store sperm for months, as well as snakes, reptiles and insects ( Holt and Fazeli, 2016 ). Although the opportunity to reduce estrus detection by lengthening sperm lifespan may be overly optimistic, the examples in nature of species that store sperm for a long duration suggest that it may be possible.

Implications

Sperm interaction with the cow reproductive tract after semen deposition has a profound influence on pregnancy rates and provides perplexing fundamental questions that are unresolved despite considerable study. The fertilizing sperm are selected by the tract from the millions or billions of sperm deposited at mating or artificial insemination. Successful sperm interact with luminal fluid and epithelia, while evading destruction by the immune system. They respond to rheotactic, chemical and adhesive stimuli to undergo functional changes and arrive at the site of fertilization. An understanding of how these processes are coordinated can improve in vitro fertilization success, contraception effectiveness, sperm lifespan in the oviduct, improved semen storage, and fertility.

Acknowledgements

Work in the author’s laboratory was supported by Agriculture and Food Research Initiative Competitive Grant no. 2011–67015–20099 and 2015–67015–23228 from the USDA National Institute of Food and Agriculture and the National Science Foundation. The author acknowledges Rebecca Winters and Lantana Grub for comments that improved the manuscript and for being unable to discuss other important work due to length restrictions.

Conflict of Interest

There are no conflicts of interest.

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Module 12: Development and Inheritance

Fertilization, learning objectives.

By the end of this section, you will be able to:

  • Describe the obstacles that sperm must overcome to reach an oocyte
  • Explain capacitation and its importance in fertilization
  • Summarize the events that occur as a sperm fertilizes an oocyte

Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. This new single cell, called a zygote , contains all of the genetic material needed to form a human—half from the mother and half from the father.

Transit of Sperm

Fertilization is a numbers game. During ejaculation, hundreds of millions of sperm (spermatozoa) are released into the vagina. Almost immediately, millions of these sperm are overcome by the acidity of the vagina (approximately pH 3.8), and millions more may be blocked from entering the uterus by thick cervical mucus. Of those that do enter, thousands are destroyed by phagocytic uterine leukocytes. Thus, the race into the uterine tubes, which is the most typical site for sperm to encounter the oocyte, is reduced to a few thousand contenders. Their journey—thought to be facilitated by uterine contractions—usually takes from 30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3–5 days. Thus, fertilization can still occur if intercourse takes place a few days before ovulation. In comparison, an oocyte can survive independently for only approximately 24 hours following ovulation. Intercourse more than a day after ovulation will therefore usually not result in fertilization.

During the journey, fluids in the female reproductive tract prepare the sperm for fertilization through a process called capacitation , or priming. The fluids improve the motility of the spermatozoa. They also deplete cholesterol molecules embedded in the membrane of the head of the sperm, thinning the membrane in such a way that will help facilitate the release of the lysosomal (digestive) enzymes needed for the sperm to penetrate the oocyte’s exterior once contact is made. Sperm must undergo the process of capacitation in order to have the “capacity” to fertilize an oocyte. If they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte’s thick outer layer of cells.

Contact Between Sperm and Oocyte

Upon ovulation, the oocyte released by the ovary is swept into—and along—the uterine tube. Fertilization must occur in the distal uterine tube because an unfertilized oocyte cannot survive the 72-hour journey to the uterus. As you will recall from your study of the oogenesis, this oocyte (specifically a secondary oocyte) is surrounded by two protective layers. The corona radiata is an outer layer of follicular (granulosa) cells that form around a developing oocyte in the ovary and remain with it upon ovulation. The underlying zona pellucida (pellucid = “transparent”) is a transparent, but thick, glycoprotein membrane that surrounds the cell’s plasma membrane.

As it is swept along the distal uterine tube, the oocyte encounters the surviving capacitated sperm, which stream toward it in response to chemical attractants released by the cells of the corona radiata. To reach the oocyte itself, the sperm must penetrate the two protective layers. The sperm first burrow through the cells of the corona radiata. Then, upon contact with the zona pellucida, the sperm bind to receptors in the zona pellucida. This initiates a process called the acrosomal reaction in which the enzyme-filled “cap” of the sperm, called the acrosome , releases its stored digestive enzymes. These enzymes clear a path through the zona pellucida that allows sperm to reach the oocyte. Finally, a single sperm makes contact with sperm-binding receptors on the oocyte’s plasma membrane. The plasma membrane of that sperm then fuses with the oocyte’s plasma membrane, and the head and mid-piece of the “winning” sperm enter the oocyte interior.

How do sperm penetrate the corona radiata? Some sperm undergo a spontaneous acrosomal reaction, which is an acrosomal reaction not triggered by contact with the zona pellucida. The digestive enzymes released by this reaction digest the extracellular matrix of the corona radiata. As you can see, the first sperm to reach the oocyte is never the one to fertilize it. Rather, hundreds of sperm cells must undergo the acrosomal reaction, each helping to degrade the corona radiata and zona pellucida until a path is created to allow one sperm to contact and fuse with the plasma membrane of the oocyte. If you consider the loss of millions of sperm between entry into the vagina and degradation of the zona pellucida, you can understand why a low sperm count can cause male infertility.

This figure shows the process of sperm fertilizing an egg. There are many sperm trying to attach to the egg.

Figure 1. Before fertilization, hundreds of capacitated sperm must break through the surrounding corona radiata and zona pellucida so that one can contact and fuse with the oocyte plasma membrane.

When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to prevent polyspermy , which is penetration by more than one sperm. This is critical because if more than one sperm were to fertilize the oocyte, the resulting zygote would be a triploid organism with three sets of chromosomes. This is incompatible with life.

The first mechanism is the fast block, which involves a near instantaneous change in sodium ion permeability upon binding of the first sperm, depolarizing the oocyte plasma membrane and preventing the fusion of additional sperm cells. The fast block sets in almost immediately and lasts for about a minute, during which time an influx of calcium ions following sperm penetration triggers the second mechanism, the slow block. In this process, referred to as the cortical reaction , cortical granules sitting immediately below the oocyte plasma membrane fuse with the membrane and release zonal inhibiting proteins and mucopolysaccharides into the space between the plasma membrane and the zona pellucida. Zonal inhibiting proteins cause the release of any other attached sperm and destroy the oocyte’s sperm receptors, thus preventing any more sperm from binding. The mucopolysaccharides then coat the nascent zygote in an impenetrable barrier that, together with hardened zona pellucida, is called a fertilization membrane .

Recall that at the point of fertilization, the oocyte has not yet completed meiosis; all secondary oocytes remain arrested in metaphase of meiosis II until fertilization. Only upon fertilization does the oocyte complete meiosis. The unneeded complement of genetic material that results is stored in a second polar body that is eventually ejected. At this moment, the oocyte has become an ovum, the female haploid gamete. The two haploid nuclei derived from the sperm and oocyte and contained within the egg are referred to as pronuclei. They decondense, expand, and replicate their DNA in preparation for mitosis. The pronuclei then migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material intermingles. This step completes the process of fertilization and results in a single-celled diploid zygote with all the genetic instructions it needs to develop into a human.

Most of the time, a woman releases a single egg during an ovulation cycle. However, in approximately 1 percent of ovulation cycles, two eggs are released and both are fertilized. Two zygotes form, implant, and develop, resulting in the birth of dizygotic (or fraternal) twins. Because dizygotic twins develop from two eggs fertilized by two sperm, they are no more identical than siblings born at different times.

Much less commonly, a zygote can divide into two separate offspring during early development. This results in the birth of monozygotic (or identical) twins. Although the zygote can split as early as the two-cell stage, splitting occurs most commonly during the early blastocyst stage, with roughly 70–100 cells present. These two scenarios are distinct from each other, in that the twin embryos that separated at the two-cell stage will have individual placentas, whereas twin embryos that form from separation at the blastocyst stage will share a placenta and a chorionic cavity.

Everyday Connections: In Vitro Fertilization

IVF, which stands for in vitro fertilization, is an assisted reproductive technology. In vitro, which in Latin translates to “in glass,” refers to a procedure that takes place outside of the body. There are many different indications for IVF. For example, a woman may produce normal eggs, but the eggs cannot reach the uterus because the uterine tubes are blocked or otherwise compromised. A man may have a low sperm count, low sperm motility, sperm with an unusually high percentage of morphological abnormalities, or sperm that are incapable of penetrating the zona pellucida of an egg.

A typical IVF procedure begins with egg collection. A normal ovulation cycle produces only one oocyte, but the number can be boosted significantly (to 10–20 oocytes) by administering a short course of gonadotropins. The course begins with follicle-stimulating hormone (FSH) analogs, which support the development of multiple follicles, and ends with a luteinizing hormone (LH) analog that triggers ovulation. Right before the ova would be released from the ovary, they are harvested using ultrasound-guided oocyte retrieval. In this procedure, ultrasound allows a physician to visualize mature follicles. The ova are aspirated (sucked out) using a syringe.

In parallel, sperm are obtained from the male partner or from a sperm bank. The sperm are prepared by washing to remove seminal fluid because seminal fluid contains a peptide, FPP (or, fertilization promoting peptide), that—in high concentrations—prevents capacitation of the sperm. The sperm sample is also concentrated, to increase the sperm count per milliliter.

Next, the eggs and sperm are mixed in a petri dish. The ideal ratio is 75,000 sperm to one egg. If there are severe problems with the sperm—for example, the count is exceedingly low, or the sperm are completely nonmotile, or incapable of binding to or penetrating the zona pellucida—a sperm can be injected into an egg. This is called intracytoplasmic sperm injection (ICSI).

The embryos are then incubated until they either reach the eight-cell stage or the blastocyst stage. In the United States, fertilized eggs are typically cultured to the blastocyst stage because this results in a higher pregnancy rate. Finally, the embryos are transferred to a woman’s uterus using a plastic catheter (tube).The diagram below illustrates the steps involved in IVF.

This multi-part figure shows the different steps in in vitro fertilization. The top panel shows how the oocytes and the sperm are collected and prepared. The next panel shows the sperm and oocytes being mixed in a petri dish. The panel below that shows the fertilized zygote being prepared for implantation. The last panel shows the fertilized zygote being implanted into the uterus.

Figure 2. Click for a larger image. In vitro fertilization involves egg collection from the ovaries, fertilization in a petri dish, and the transfer of embryos into the uterus.

IVF is a relatively new and still evolving technology, and until recently it was necessary to transfer multiple embryos to achieve a good chance of a pregnancy. Today, however, transferred embryos are much more likely to implant successfully, so countries that regulate the IVF industry cap the number of embryos that can be transferred per cycle at two. This reduces the risk of multiple-birth pregnancies.

The rate of success for IVF is correlated with a woman’s age. More than 40 percent of women under 35 succeed in giving birth following IVF, but the rate drops to a little over 10 percent in women over 40.

Chapter Review

Hundreds of millions of sperm deposited in the vagina travel toward the oocyte, but only a few hundred actually reach it. The number of sperm that reach the oocyte is greatly reduced because of conditions within the female reproductive tract. Many sperm are overcome by the acidity of the vagina, others are blocked by mucus in the cervix, whereas others are attacked by phagocytic leukocytes in the uterus. Those sperm that do survive undergo a change in response to those conditions. They go through the process of capacitation, which improves their motility and alters the membrane surrounding the acrosome, the cap-like structure in the head of a sperm that contains the digestive enzymes needed for it to attach to and penetrate the oocyte.

The oocyte that is released by ovulation is protected by a thick outer layer of granulosa cells known as the corona radiata and by the zona pellucida, a thick glycoprotein membrane that lies just outside the oocyte’s plasma membrane. When capacitated sperm make contact with the oocyte, they release the digestive enzymes in the acrosome (the acrosomal reaction) and are thus able to attach to the oocyte and burrow through to the oocyte’s zona pellucida. One of the sperm will then break through to the oocyte’s plasma membrane and release its haploid nucleus into the oocyte. The oocyte’s membrane structure changes in response (cortical reaction), preventing any further penetration by another sperm and forming a fertilization membrane. Fertilization is complete upon unification of the haploid nuclei of the two gametes, producing a diploid zygote.

Critical Thinking Questions

  • Darcy and Raul are having difficulty conceiving a child. Darcy ovulates every 28 days, and Raul’s sperm count is normal. If we could observe Raul’s sperm about an hour after ejaculation, however, we’d see that they appear to be moving only sluggishly. When Raul’s sperm eventually encounter Darcy’s oocyte, they appear to be incapable of generating an adequate acrosomal reaction. Which process has probably gone wrong?
  • Sherrise is a sexually active college student. On Saturday night, she has unprotected sex with her boyfriend. On Tuesday morning, she experiences the twinge of mid-cycle pain that she typically feels when she is ovulating. This makes Sherrise extremely anxious that she might soon learn she is pregnant. Is Sherrise’s concern valid? Why or why not?
  • The process of capacitation appears to be incomplete. Capacitation increases sperm motility and makes the sperm membrane more fragile. This enables it to release its digestive enzymes during the acrosomal reaction. When capacitation is inadequate, sperm cannot reach the oocyte membrane.
  • Sherrise’s concern is valid. Sperm may be viable for up to 4 days; therefore, it is entirely possible that capacitated sperm are still residing in her uterine tubes and could fertilize the oocyte she has just ovulated.

acrosome: cap-like vesicle located at the anterior-most region of a sperm that is rich with lysosomal enzymes capable of digesting the protective layers surrounding the oocyte

acrosomal reaction: release of digestive enzymes by sperm that enables them to burrow through the corona radiata and penetrate the zona pellucida of an oocyte prior to fertilization

capacitation: process that occurs in the female reproductive tract in which sperm are prepared for fertilization; leads to increased motility and changes in their outer membrane that improve their ability to release enzymes capable of digesting an oocyte’s outer layers

corona radiata: in an oocyte, a layer of granulosa cells that surrounds the oocyte and that must be penetrated by sperm before fertilization can occur

cortical reaction: following fertilization, the release of cortical granules from the oocyte’s plasma membrane into the zona pellucida creating a fertilization membrane that prevents any further attachment or penetration of sperm; part of the slow block to polyspermy

fertilization: unification of genetic material from male and female haploid gametes

fertilization membrane: impenetrable barrier that coats a nascent zygote; part of the slow block to polyspermy

polyspermy: penetration of an oocyte by more than one sperm

zona pellucida: thick, gel-like glycoprotein membrane that coats the oocyte and must be penetrated by sperm before fertilization can occur

zygote: fertilized egg; a diploid cell resulting from the fertilization of haploid gametes from the male and female lines

  • Anatomy & Physiology. Provided by : OpenStax CNX. Located at : http://cnx.org/contents/[email protected] . License : CC BY: Attribution . License Terms : Download for free at http://cnx.org/contents/[email protected]

All about sperm

Sperm Travel Path: Understanding the Route to Fertilization

sperm cell journey to the egg

Short answer sperm travel path: Sperm travel from the testes through the epididymis, vas deferens, and ejaculatory duct before being released through the urethra during ejaculation. The journey takes approximately 64-72 days to complete.

Exploring the Fascinating Journey: Sperm Travel Path

Understanding how sperm travel path affects conception: step by step guide, all you need to know about sperm travel path: frequently asked questions, the intriguing process of fertilization: an in-depth look at sperm travel path, breaking down the miracle of life: inside the male reproductive tract and its role in sperm travel path, discovering the unseen world of conception: the hidden secrets of sperm travel path.

Table of Contents

When it comes to the miracle of human life, there are plenty of fascinating facts and intricate details that often go unnoticed. One aspect of this journey that is particularly intriguing is the path that sperm travel during fertilization. Despite their tiny size, these cells embark on a complex and challenging journey in order to reach the egg.

To begin with, it’s important to understand the basic anatomy of sperm. Each one features a head, midpiece, and tail. The head contains genetic material (DNA) while the midpiece holds mitochondria needed for energy production. Finally, the tail – which resembles a whip-like structure – propels sperm towards its ultimate destination.

The first step in this journey begins when sperm cells are released from the testes. They enter into a part of the male reproductive system known as the epididymis where they mature over several weeks before being released during ejaculation. From here, they must travel through various ducts, including the vas deferens and urethra before exiting through the penis.

Once outside of the male body, sperm face further challenges as they navigate through cervix into uterine cavity via vaginal canal during intercourse or other artificial methods like IVF. Here, they must contend with acidic pH levels in female reproductive tract as well as immune cells that may see them as foreign invaders attempting to harm host (woman carrying embryo).

As if all this wasn’t difficult enough, sperm still have a long way yet to travel! They must then make their way through fallopian tube where fertilization occurs based on their fortunate meeting with an egg cell.

Sperm can remain viable for up to five days within female reproductive tract which provides extra opportunities for fertilization attempting at ovulation time – roughly midpoint menstrual cycle when ovary releases an egg cell into its respective Fallopian tube readying itself for fertilization.

Overall, exploring the fascinating journey undertaken by sperm cells provides us with insight into the incredible complexity involved in human reproduction. From their early maturation in the epididymis to their ultimate goal of reaching and fertilizing an egg, these tiny cells face a veritable obstacle course and yet still manage to achieve what amounts to nothing short of a miracle – when successful fertilization occurs! So let us honor this journey that we often overlooks as we get caught up with complexities of our lives!

When it comes to conception, understanding the process of sperm travel is essential. After all, it takes a single sperm to fertilize an egg and result in pregnancy. So, how does this little swimmer make its way up towards the egg? In this step-by-step guide, we’ll explore the complex journey that sperm must undertake in order to reach their ultimate destination.

The first step in understanding how sperm travel path affects conception is knowing where they come from. Sperm are produced in the testes of males and mature over approximately 72 days. Whenever a man ejaculates, he releases millions of these little swimmers which then navigate through several obstacles before they can reach their final goal.

The next critical step is for them to make their way into the cervix – the opening that separates a woman’s uterus from her vagina. This passage can prove challenging for many sperm as its narrow opening and acidic environment can be quite hostile. Only a relatively small proportion of these hardy cells will remain viable enough to make it through this initial challenge.

Once through the cervix, those swimmer’s that do survive are launched into even rougher waters, weaving their way upstream through thick mucus membranes located inside the female reproductive tract; One by one-many falling by wayside losing momentum along with movement- until only very few finally come close enough to take on and fertilize the coveted ovum or egg.

Fertilization happens when one lucky little guy makes it past all these hurdles and meets a waiting egg emerging around day 14 after ovulation within your fallopian tubes-enabling mothers await with pregnancy tests eagerly due for conception notice.

Overall factors such as age, lifestyle choices such as diet or smoking habits can play vital roles in altering what size or quality of sperms actually end up reaching these prized eggs much later on hence affecting fertility rates drastically causing recurring patterns of infertility rejections for some couples trying to conceive.

In conclusion, the journey of sperm travel is a complex, multi-step process that’s far from easy. However, understanding how it all works can help you boost your chances of conception. With proper planning regarding sexual timing and fertility wellness strategies like stress management or nutritional interventions advised by professionals, it is possible for many couples to overcome these hurdles and bring their family dream into fruition despite any challenges in the path~!

When it comes to reproduction, people often focus on the act of sex itself without giving much thought to what happens after ejaculation. However, understanding the journey that sperm cells take from release into the vagina to reaching their destination can be essential in ensuring successful conception. In this article, we’ll address some frequently asked questions about sperm travel path and give you all you need to know about reproductive biology.

Q: What is sperm? A: Sperm are tiny male reproductive cells that are produced by the testes and contain genetic material needed for fertilization. During sex, they swim through seminiferous tubules and gather in the epididymis before they’re ready for ejaculation.

Q: How does ejaculation work? A: When a man is sexually aroused, his parasympathetic nervous system takes over and signals his body to prepare for ejaculation. Once he reaches orgasm, the muscles surrounding his urethra contract and push semen out of his penis. The average man releases around 2-5 milliliters of semen per ejaculate, containing millions of sperm cells.

Q: Where do sperm go after ejaculation? A: After being released during ejaculation, sperm enter the vagina through semen. While their journey towards their final destination may seem short, it’s actually very intricate due to several variables including acidity levels in cervical mucus as well as vaginal fluctuation in pH.

Q: How long does it take for sperm to reach an egg? A: On average, it takes up anywhere between 30 minutes to three days for a single sperm cell to travel from the cervix up into one of a woman’s fallopian tubes where fertilization can occur with an available egg cell – but depending on conditions like distance between partners’ bodies or time since ovulation conception might never happen at all!

Q: What requirements do sperms have to meet so that they can successfully fertilize an egg? A: Before fertilizing an egg, sperm have to go through a series of tests such as the cervical mucus test and the penetration test. From there, if sperm find their way into the fallopian tubes where an egg is present then they can potentially meet up with waiting eggs for fertilization.

Q: What are some factors that could decrease sperm count or make them less mobile? A: There are several lifestyle habits that could worsen sperm health like smoking or excessive drinking, sedentary behavior and stress which reduces levels of testosterone in males. Poor nutrition, chronic illness, sexually transmitted infections (STIs), genetics, medications you take regularly for other conditions – such as antidepressants- poor sleep hygiene also negatively impacts reproductive function in both men and women.

In conclusion, there’s more to reproductive biology than just intercourse. Understanding the journey that sperms travel from ejaculation to fertilization is essential in ensuring successful conception. While various variables can come into play at times making it challenging for some couples to conceive according to expectations; taking care of yourself and reducing harmful lifestyle choices

Fertilization is a miraculous process that has fascinated researchers since the beginning of time. The combination of genetic material from two different organisms results in the creation of a totally unique being—a blend of traits and characteristics that make each individual distinct and special.

For conception to occur, a male’s sperm must travel through a complex maze before it meets with the female’s egg. This journey starts from the moment an ejaculation occurs and ends when fertilization takes place.

The path sperm take is nothing short of remarkable. It can take anywhere from five minutes to several days for them to reach their destination—a feat, given that sperm cells are minuscule compared to human beings and have to overcome several obstacles along the way.

Once released into the vagina during intercourse, sperm begin their arduous journey upwards into the fallopian tubes, where the female’s egg awaits fertilization. Sperm swim against gravity through multiple barriers such as cervical mucus, which can be difficult to penetrate even for healthy sperm cells.

Sperm’s motility, or ability to move quickly towards their destination, is also crucial in fertilization. They move like tiny propellers driven by a whip-like tail called flagellum, which helps them traverse through different kinds of fluids present throughout their journey.

As they progress further towards fallopian tubes, they encounter various natural filters such as immunity system cells protecting against foreign invaders trying to access eggs leading up to white blood cell towers defending these pathways along their long course “mountains”.

When finally reaching mature ovum awaiting them in its zone at the end of this journey in ampulla regions or points (depending on woman cycle), millions come close but only one makes it inside and fertilize egg marking beginning stages an embryo genesis phase; all others either die or get lost along this path filled with numerous biochemical obstacles preventing successful fertilization at every step (Incredible!).

The process of fertilization carried out by sperm is truly intriguing; it’s a testament to the amazing power and resilience of these tiny, yet mighty cells. It’s a fascinating journey that spans several days and involves crossing numerous barriers and obstacles.

It’s no wonder that researchers and scientists continue to study this complex process, hoping to unravel its mysteries further. Just imagine what other secrets could yet be hidden within the intricate processes of human pregnancy – who knows what new discoveries await us as we delve deeper into the world of reproductive biology!

The miracle of life is a fascinating phenomenon that never ceases to amaze us. We all know that the human reproductive system plays a vital role in bringing new life into this world, but have you ever wondered about the intricacies of the male reproductive tract and its role in sperm travel path? Let’s dive deeper and explore this miraculous journey.

The Male Reproductive Tract

The male reproductive system mainly comprises two organs, the testes and the penis. The testes are responsible for producing sperm cells, which are then transported out of the body by way of the penis. However, it is not as simple as it sounds; several structures and mechanisms ensure that sperm travel along their designated path.

Sperm Production

Sperm production takes place inside a network of tiny tubes called seminiferous tubules located within each testicle. Within these tubes, cells undergo meiosis – a special cell division mechanism – to produce mature, functional sperm cells with half the genetic material required for reproduction.

Transportation

Once produced, mature sperm cells move from seminiferous tubules toward epididymis (a duct situated above each testis). During that journey, they acquire motility from surrounding fluids secreted by accessory glands like prostate gland and seminal vesicle. The fluid also provides nutrients to sustain their energy needs while they swim to reach their ultimate destination — an egg inside female reproductive tract.

Ejaculation

When sexual stimulation or arousal occurs, muscles surrounding the epididymis contract, forcing sperm into vas deferens – muscular ducts that carry them upwards towards prostate gland. Here they mix with seminal fluid when ejaculation occurs—from here; millions of swimming soldiers commence their incredible race to find familiar eggs.

Travel Path

When semen shoots out through penis during ejaculation inevitably makes some contact with outsides surfaces before it enters female genitalia. Inclined vaginal walls help channel movement up toward cervix opening leading deeper into reproductive tract. There are more hurdles to overcome inside a female’s body than one would expect as the path is full of obstacles. This includes acidic environments in the vagina, and barriers produced by mucus on the cervix.

Final Thoughts

The male reproductive tract and its role in sperm travel can be viewed as a remarkable example of evolution at work. The closely coordinated response between organs & proteins secretion, advanced muscular contractions, transportation throughout several ducts – all these mechanisms adapted to enhance chance of fertilization success showcases nature’s brilliance. As humans, we must appreciate how our bodies have evolved over millennia to bring new life into this world. And now that you know just how incredible and fascinating the male reproductive system is let us take a moment to marvel at it.

Conception is one of nature’s most fascinating and intricate processes. It represents the merging of two cells, the sperm and the egg, which leads to a new life. This process has fascinated scientists for years, leading to numerous studies and research in an attempt to uncover all its mysteries. One particular area of discovery that continues to intrigue researchers is the path travelled by sperm during conception.

Sperm travel through a series of complex environments within the female reproductive system on their journey to fertilize an egg. To understand this journey, it’s essential first to know what happens when a male ejaculates.

When males ejaculate, semen – a mixture made up of sperm, enzymes, proteins and other substances – is released from the penis into the female reproductive system via intercourse or direct injection techniques such as artificial insemination. After entering into this environment created by a woman’s body temperature and hormones, some sperm die instantly due to unfriendly conditions like acidity.

The surviving sperms start their journey through different parts of the female reproductive system such as cervix-uterus-fallopian tubes over several hours or days depending on individuals’ anatomy and physiology before finding themselves in contact with an egg cell. This trip requires them first navigating against gravity’s pull entirely; once they reach the uterus at about 45mins/1hour after ejaculation, contractions of cervical mucus carry them further up into fallopian tubes where most successful healthy fertilisations take place under ideal ovulation timing.

While this process seems straightforward enough in theory: get from point A (ejaculation) to point B (an egg cell), there are no guarantees that any given sperm will reach its destination successfully fertilizing an egg cell ultimately. In fact, only about 300 million out of billions released sperm amount actually get close enough for chance encounter with unprotected eggs for natural conceptions: The vast majority wastes existing energy resources lingering around due unfavourable conditions and possible sperm anomalous health issues.

Overall, for conception to occur, the intricate symphony of events leading up to fertilization must work seamlessly together. Even slight disruptions in any of the stages could result in infertility or no viable pregnancies.

Discovering the secrets of sperm travel path is only part of our knowledge pool about reproductive health sciences- as there are still many things we do not know yet; however, it’s an exciting area of study that has vast potential implications for infertility treatments and fertility preservation efforts. It’s an undiscovered universe that presents much promise for improving human reproduction management in spite of its challenging terrain- uncovering more about this unseen world wouldn’t hurt!

sperm cell journey to the egg

Fertilisation

When the nucleus of a sperm and egg fuse together, the egg is fertilised and it develops into a fetus.

The fetus grows in the mother’s uterus. The mother's body provides all the baby needs until birth.

Human reproduction interactive

Play this game to explore human reproduction and the stages of pregnancy.

Gamete structure and fertilisation

Humans typically reproduce by sexual fertilisation close sexual fertilisation A process in which new organisms are created by combining the genetic information from two individuals of different sexes. and produce offspring close offspring An animal's young. that are genetically unique. Fertilisation is the process in which the nucleus of a sperm cell fuses with the nucleus of an egg cell to produce a zygote close zygote A fertilised ovum (egg cell) before it has divided into an embryo. which will eventually grow into offspring.

Gametes are the sex cells:

  • Sperm are male gametes
  • Eggs are female gametes

Gametes have adaptations to increase the chances of fertilisation and successful development of an embryo close embryo A bundle of several hundred cells that has developed from a fertilised ovum. .

Sperm cell adaptations

a diagram of a sperm cell labelled tail, acrosome, midpiece containing mitochondria, haploid nucleus

  • A tail to move them towards an egg cell.
  • Many mitochondria to release energy for movement.
  • Part of the tip of the head of the sperm releases enzymes to digest the egg membrane to allow fertilisation to take place.
  • The haploid close haploid A cell which contains half the number of chromosomes compared to other cells in the organism. Eg gametes. nucleus contains the genetic material for fertilisation.
  • Produced in large numbers to increase the chance of fertilisation.

Egg cell adaptations

diagram of an egg cell labelled cell membrane, cytoplasm, haploid nucleus, mitochondria

  • The egg cell’s cytoplasm contains nutrients for the growth of the early embryo.
  • The haploid nucleus contains genetic material for fertilisation.
  • The cell membrane close cell membrane This surrounds the outside of animal cells and controls what can enter and exit it. changes after fertilisation by a single sperm so that no more sperm can enter.

Pregnancy and fetal development

After fertilisation, the zygote will divide by a process called mitosis close mitosis A type of cell division that results in two daughter cells that are genetically identical to the parent cell. into a ball of genetically identical cells called the embryo. This embryo will attach itself to the lining of the uterus where it will develop into a fetus close fetus An unborn baby. and finally into a baby.

Development of a fetus

The fetus relies upon its mother as it develops. These are some of the things it needs:

  • Protection against knocks, bumps and temperature changes.
  • Oxygen for respiration.
  • Nutrients (food and water).
  • The developing fetus also needs its waste substances removing.

The fetus is protected by the uterus and the amniotic fluid, a liquid contained in a bag called the amnion.

The role of the placenta

The placenta is an organ that grows into the wall of the uterus and is joined to the fetus by the umbilical cord.

Its main function is to allow the exchange of materials such as oxygen and nutrients between the fetus and mother whilst removing waste substances such as carbon dioxide and urea. This relies on a process called diffusion close diffusion The overall movement of particles of gas or liquid from an area of higher to lower concentration. as there is no mixing of maternal and fetal blood in the placenta.

Oxygen and nutrients such as glucose diffuse across the placenta from the mother to the fetus.

Carbon dioxide and other waste substances diffuse across the placenta from the fetus to the mother.

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Male fertility gene discovery reveals path to success for sperm

by University of Edinburgh

Male fertility gene discovery reveals path to success for sperm

Discovery of a pair of genes that work in perfect harmony to protect male fertility, could provide new insights into some unexplained cases of the most severe form of infertility, research suggests.

Genetic analysis of cases of male infertility revealed that rare mutations in a gene known as SPOCD1 disrupt the formation of healthy sperm during the earliest stages of their development.

The gene was also found to work in partnership with a previously unknown gene, C19orf84, to protect the early-stage precursors to sperm, known as germ cells , from damage.

The discovery of the essential role of these two key genes could provide the answer to some cases of the most severe forms of male infertility and lead to expanded genetic screening for rare mutations, researchers say.

Cryptozoospermia and azoospermia, in which little or no sperm is produced, affect around 1% of men. In 45% of cases, no cause can be found, but they are long suspected of having genetic causes.

A sperm cell's biggest challenge starts long before the journey to reach the egg as sperm are particularly vulnerable during the earliest stages of their development, as germ cells in developing embryos.

Germ cells must protect their DNA from damage during the embryo's development so they can become the pool of self-renewing cells that produce healthy sperm throughout adult life.

A previous study by the researchers had shown that SPOCD1 has an essential role in protecting germ cells in male mice, but it was unclear whether the same process happened in humans.

In collaboration with researchers at the University of Münster and other partner universities, scientists at the University of Edinburgh screened international databases containing genetic data from 2913 men involved in studies on infertility.

They identified three men who carried faulty versions of the SPOCD1 gene, which resulted in damage to germ cells that prevented healthy sperm development—this failure to launch led to infertility.

During their development, germ cells undergo a reprogramming process that leaves them vulnerable to rogue genes, known as jumping genes, which can damage their DNA and threaten fertility.

Germ cells are the vital link between generations, but they need unique strategies to protect the genetic information they carry so it can be passed successfully from parents to their offspring.

The previous study in mice found that the SPOCD1 gene helps to recruit protective chemical tags, known as DNA methylations, to disable jumping genes.

This study revealed that the men with faulty versions of the SPOCD1 gene had the most severe forms of infertility, azoospermia, and cryptozoospermia.

Analysis of the mutated variants of the SPOCD1 gene also revealed a new gene, known as C19orf84 which partners with SPOCD1 and forms an important line of defense in early sperm cells.

Further study of the role of these genes in early-stage sperm cells in mouse embryos revealed that both produce proteins that are essential in recruiting the protective tags that silence jumping genes.

Scientists have long puzzled over how germ cells escape damage during the reprogramming process, as it temporarily wipes their genetic slate clean of existing protective tags.

C19orf84 protein acts as a matchmaker, connecting the SPOCD1 protein with the cell's protective chemical tag-making machinery and directing them toward the jumping genes before they can damage the genome.

Increased understanding of this process, together with expanded genetic screening , will allow scientists to identify if faulty versions of these genes are the cause of some of these rare cases of male infertility, researchers say.

Professor Dónal O'Carroll, lead author of the study from the University of Edinburgh, said, "This was a wonderful collaborative project that led to the discovery of new genetic causes of male infertility. We also advanced our understanding of a process that is fundamental to healthy sperm cell development. These mechanistic insights are leading to a better understanding of the elusive process that allows developing sperm to preserve their genetic integrity and escape an early death."

Dr. Ansgar Zoch, first and co-corresponding author of the study from the University of Edinburgh, said, "A truly collaborative achievement, this study enhances our understanding of male infertility on the molecular and genetic level."

"I am particularly proud that so many co-authors joined efforts and contributed their expertise. We demonstrate strong evidence for SPOCD1 to be included in genetic screenings of male infertility patients. Providing a genetic diagnosis can help provide closure to affected individuals and potentially prevent unnecessary medical procedures."

The study was published in Molecular Cell .

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Robert D. Martin Ph.D.

Testosterone

A sperm’s obstacle course to the egg, men and women have quite different tactics for sperm numbers.

Posted September 6, 2017

Original cartoon by Alex Martin

Some men produce too many sperms. That little-mentioned fact was the focus of my previous blog post. (See Why Too Many Sperms Spoil the Egg , posted on August 11, 2017.) With too many sperms, an unusually dense cloud surrounds the egg and more than one sperm may penetrate ( polyspermy ). In most human cases, two sperms fertilize an egg, yielding an embryo with an extra set chromosomes in addition to the normal pair from the father and mother ( triploid condition ). The additional chromosome set inevitably has catastrophic effects, with loss of the fetus or death of the infant within hours of birth. Some kind of chromosomal abnormality is present in about half of all miscarriages , and a quarter of those abnormalities involve extra chromosome sets.

 NinaSes (2015). Wikimedia Commons; file licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

The sperm’s odyssey

Although ejaculates of male mammals typically contain vast numbers of sperms — around 250 million on average in humans — surprisingly few usually get anywhere near an egg. Indeed, the reproductive tract of female mammals is seemingly specially adapted to minimize the number of sperms reaching the upper stretches of the oviduct (the tube along which the egg travels to reach the womb).

A 2006 paper by Susan Suarez and Allan Pacey expertly reviewed the odyssey facing sperms passing up the female reproductive tract. For starters, only part of the ejaculate escapes from the hostile acidity of the vagina into the neck of the womb ( cervix ). Then, as sperms migrate up the cervix, mucus strands filter out any that have abnormal shapes or swim too slowly. When the cervical barrier is bypassed by injecting semen directly into the womb ( intrauterine insemination — IUI ), pregnancy success levels off above 20 million sperms. This suggests that only 10% of sperms in a natural ejaculate reach the womb. Once sperms enter the womb, muscular contractions assist their passage to the oviduct. Only a few thousand sperms enter the relatively congenial environment of the oviduct. Its lower end, the isthmus , serves as a reservoir where sperms bind to the oviduct lining and are then released in a staggered fashion. After release, sperms undergo capacitation and become hyperactive , which enables them to travel to the upper end of the oviduct ( ampulla ), where fertilization occurs. The outcome of all obstacles encountered is that only a hundred or so sperms are typically present in the ampulla at any one time. Progressive reduction in sperm numbers between insemination and fertilization undoubtedly serves to reduce the risk of polyspermy.

 Shazz (2006). Wikimedia Commons; File licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Among many other suggestions, rampant speculation surrounding possible functions of the human orgasm spawned the hypothesis that it represents an adaptation to facilitate transportation of sperms toward the egg. It is known that orgasm is associated with enhanced release of the hormone oxytocin , which could potentially trigger active sperm transport. However, as Roy Levin noted in 2011, this hypothesis completely ignores the fact that avoiding polyspermy actually requires delicately balanced control of sperm transit through the female tract. Following insemination, the primary challenge for the female reproductive tract is to achieve a staged reduction of sperm numbers, not to speed transit of sperms towards the egg.

 MartaFF (2015). Wikimedia Commons; file licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

Lessons from in vitro fertilization

The advent of test-tube babies in 1978 opened up new possibilities for examining fertilization of the human egg, while also introducing the possibility that errors might arise because of inappropriate sperm densities. Yet this was not seen as a problem when in vitro fertilization (IVF) was first developed. In 1981, IVF pioneer Robert Edwards provided one of the first comments on possible polyspermy. He reported from initial work that a fetus that miscarried in the twelfth week of pregnancy had been found to be triploid. Although Patricia Jacobs and colleagues had previously reported in 1978 results from a major survey showing that triploidy is relatively common (1-3%) in human conception, Edwards stated that this chromosomal anomaly “may not be serious quantitatively” because the vast majority of eggs are fertilized by a single sperm. Admittedly, the frequency that Jacobs and colleagues reported was for natural conceptions. In 1981, no comparable information was available for eggs exposed to an unnatural sperm density in vitro .

In fact, in a 1981 paper Ian Craft and colleagues explicitly discussed sperm numbers in relation to IVF. They noted that the number of sperms surrounding an egg during natural conception was unknown and that the ideal number of sperms for in vitro fertilization had not been evaluated. Edwards and colleagues reportedly used between 100,000 and a million sperms, whereas the Craft team achieved fertilization with only 10,000 motile sperms in the culture medium surrounding the egg. They predicted that far lower numbers would eventually prove to be sufficient, “thus reducing the risk of polyspermic fertilization”.

Adapted from a figure in Wolf et al. (1984).

Subsequently, in 1984, Don Wolf and colleagues reported in more detail on relationships between sperm concentration and in vitro fertilization of human eggs. Fertilization success was actually found to decrease with increasing sperm numbers over the range of 25,000 to 500,000, with maximal fertilization of 80.8% at the lowest density. By contrast, the degree of polyspermic fertilization was directly related to sperm concentration, increasing from zero at less than 25,000 sperms/cc to 5.5% at 500,000 sperms/cc. Wolf and colleagues emphasized that concentrations of between half a million and a million sperms per egg were remarkably high compared with the hundred or so estimated to be present at the fertilization site in natural conception. A 1985 paper by Hans van der Ven and colleagues reinforced the findings reported by the Wolf team. Since the 1980s, relatively little has been published regarding optimal sperm numbers for IVF. As a rule, relatively low sperm densities are now used, and in 2013 Ping Xia reported that under these conditions approximately 7% of fertilized eggs are polyspermic. In modern IVF procedures, routine examination eliminates any such cases before transfer to the womb.

 Filip em (own work). Wikimedia Commons; file licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license.

Diagram of an orchidometer for assessment of testis volume. Numbers indicate volume in ccs. Sizes 1–3 ccs (yellow) are typically found before puberty , sizes 4–12 ccs (orange) generally occur during puberty, and sizes 15–25 ccs (red) are usually found in adults.

Testis size, testosterone and sperm counts

Numerous studies have shown that testis size, testosterone levels and sperm production are all connected together in a functional network. Testis volume is often estimated through calculation from maximum length and width measured with calipers. In a 2004 paper, for instance, Leigh Simmons and colleagues reported a strong correlation between testis size and sperm counts calculated from linear measurements in a study of student volunteers. In many studies conducted by medical professionals, however, testis volume is determined by palpation accompanied by comparison with a standard set of ovoid models originally designed by Andrea Prader (1966). This device, known as an orchidodometer, is composed of twelve wooden or plastic ovoids with volumes of 1, 2, 3, 4, 5, 6, 8, 10, 12, 15, 20, and 25 ccs, respectively. Pediatricians regularly use orchidometers (“the urologist’s stethoscope”) to study individual development. On average, testes grow very little from birth up to the eleventh year (1-3 ccs), after which they begin to increase in size to reach about 12 ccs during puberty. Subsequent growth is very rapid, and the transition to the adult condition (typical range: 15-25 ccs) takes just three years.

Adapted from a figure in Simmons et al. (2004).

With high testosterone levels, large testes and over-the-top sperm counts, some men may be “hypermasculine”. Confirming the suspicions of many women, it may truly be said that men may suffer from testosterone poisoning. A major downside of producing unusually large numbers of sperms is that it increases the potential for polyspermy with consequent disruption of fetal development. Presumably, natural selection generally operates to maintain sperm production at an optimal level reflecting a compromise between maximizing the probability of successful fertilization and minimizing the risk of polyspermy. And the female reproductive tract is evidently adapted for radical reduction of sperm numbers in a staged fashion.

sperm cell journey to the egg

Bujan, L., Mieusset, R., Mansat, A., Moatti, J.P., Mondinat, C. & Pontonnier, F. (1989) Testicular size in infertile men: relationship to semen characteristics and hormonal blood levels. British Journal of Urology 64 :632-637.

Craft, I., McLeod, F., Bernard, A., Green, S. & Twigg, H. (1981) Sperm numbers and in-vitro fertilisation. Lancet 318 :1165-1166.

Edwards, R.G. (1981) Test-tube babies, 1981. Nature 293 :253-256.

Jacobs, P.A., Angell, R.R., Buchanan, I.M., Hassold, T.J., Matsuyama, A.M. & Manuel, B. (1978) The origin of human triploids. Annals of Human Genetics 42 :49-57.

Levin, R.J. (2011) The human female orgasm: a critical evaluation of its proposed reproductive functions. Sexual & Relationship Therapy 26 :301-314.

Pasqualotto, E.B., Daitch, J.A., Hendin, B.N., Falcone, T., Thomas, A.J., Nelson, D.R. & Agarwal.A. (1999) Relationship of total motile sperm count and percentage motile sperm to successful pregnancy rates following intrauterine insemination. Journal of Assisted Reproduction & Genetics 16 :476-482.

Prader, A. (1966) Testicular size: Assessment and clinical importance. Triangle 7 :240-243.

Simmons, L.W., Firman, L.C., Rhodes, G. & Peters, M. (2004) Human sperm competition : testis size, sperm production and rates of extrapair copulations. Animal Behaviour 68 :297-302.

Suarez, S.S. & Pacey, A.A. (2006) Sperm transport in the female reproductive tract. Human Reproduction Update 12 :23-37.

van der Ven, H.H., Al-Hasani, S., Diedrich, K., Hamerich, U., Lehmann, F. & Krebs, D. (1985) Polyspermy in in vitro fertilization of human oocytes: frequency and possible causes. Annals of the New York Academy of Sciences 442 :88-95.

Wolf, D.P., Byrd, W., Dandekar, P. & Quigley, M.M. (1984) Sperm concentration and the fertilization of human eggs in vitro. Biology of Reproduction 31 :837-848.

Xia, P. (2013) Biology of polyspermy in IVF and its clinical indication. Current Obstetrics and Gynecology Reports 2 :226-231.

Robert D. Martin Ph.D.

Robert Martin, Ph.D., is Emeritus Curator of Biological Anthropology at the Field Museum in Chicago, as well as a member of the Committee on Evolutionary Biology at the University of Chicago.

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primefertilitycenter

A Journey of Sperm

sperm cell journey to the egg

Sperm cells are produced from both testis of male. From germ cell and develop to become fully mature. This process takes around 70-80 days to complete.

After ejaculation, millions of sperm cells are moving to female’s vagina. A lot of sperm cells will be diving through vaginal mucus then aiming to cervix and uterine cavity. In this step, plenty of sperms die during approaching to the target.

Meet the egg

Once sperms (that still survive) reach the fallopian tubes, they will meet 1 fully matured egg which has passed the ovulation then moved to wait for sperm at the fallopian tubes. Each sperm will be approaching to this egg consequently.

Fertilization

Although the egg is surrounded by a lot of sperms, there will be only 1 strongest sperm can penetrate into the egg. After that sperm will throw off its tail and release 23 pairs of chromosomes to match with egg’s chromosomes. Combining to be 1 cell. The egg which has been fertilized already will transform itself in order to reject the approaching of any others sperm cells.

Implant and develop to be the fetus

After fertilization, the cell will be dividing very quickly. Then transfer from fallopian tubes to the uterine cavity in order to implant and grow up to be the fetus later on.

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#ICSI #IUI #IVF #eggfreezing #EmbryoFreezing #SpermFreezing #SemenAnalysis #Hysteroscopy #FET #PGT #PGD #NGS #PESA #TESA #primefertilityclinic #primefertiltycenter #fertilityclinic #bangkokfertilityclinic #thailandfertilityclinic

Reference: Prime Fertility Center Co., Ltd. https://www.primefertilitycenter.com/en/a-journey-of-sperm/

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ScienceAlert

Simulations Reveal What Happens When a Sperm Kisses an Egg

T he moment when a slithering sperm propels itself head-first into a gelatinous egg is one of sudden change. Within seconds to minutes , chemical changes in the egg's membrane and outer coat are enacted to block any more sperm from attaching to and entering the oocyte.

A series of reactions also takes place as the sperm and egg recognize each other, chemically speaking, and then begin to merge their membranes together. But despite the significance of these delicate molecular events, their details haven't been fully resolved.

A new study from researchers at ETH Zurich and Ludwig Maximilian University of Munich in Switzerland now reveals the intricacies of a special protein complex known for its crucial role in the fertilization process.

"It was assumed that the combination of the two proteins [JUNO and IZUMO1] into a complex initiates the recognition and adhesion process between the germ cells, thereby enabling their fusion," explains Paulina Pacak, a bioinformatician at ETH Zurich and first author of the study.

This interaction of JUNO – which is located on the outer membrane of the female egg cell – and IZUMO1, found on the male sperm cell surface, is the first known physical link between two newly fusing sex cells.

However, efforts to develop small molecular inhibitors of the JUNO-IZUMO1 union, as a potential contraception, haven't amounted to much so researchers suspect there might be more to their molecular interactions than we know.

Techniques commonly used to figure out the structure of individual proteins and protein complexes, such as cryo-electron microscopy and protein crystallography , also involve snap-freezing proteins or crystallizing them, which means they only produce a static image of those protein structures and can't capture their dynamic interactions.

But inside cells, proteins are constantly being made and folded into shape, floating around in a watery mix of cytoplasm , binding to and detaching from their partners, and getting recycled.

So Pacak and colleagues used a Swiss supercomputer to simulate the interactions between JUNO and IZUMO1 in water, therefore more closely resembling their natural forms in cells.

Each simulation spanned just 200 nanoseconds each, but they showed that the JUNO-IZUMO1 complex is initially stabilized by a host of short-lived and weak non-covalent interactions between the protein molecules.

These contacts lasted less than 50 nanoseconds each, and understanding what happens when they are interrupted, either by other molecules or mutations, could provide insights into contraceptives and infertility, the researchers suggest .

Next, Pacak and colleagues simulated how longer-lasting bonds holding the JUNO-IZUMO1 complex together could be destabilized by zinc ions.

Minutes after a sperm and egg unite, the fertilized egg releases a flood of charged zinc atoms which are thought to prevent other sperm from entering the egg by hardening its outer coat.

The simulations showed that the presence of zinc ions bent IZUMO1 into a boomerang shape, so it could no longer firmly bind to JUNO. This suggests the egg's zinc release could also hinder the binding of approaching sperm.

While these are just computer simulations based on protein sequences and shapes, the findings do provide a new look at the first moments of fertilization.

"We can only find out something like this with the help of simulations," says Viola Vogel, biophysicist at ETH Zurich and senior author.

"The findings that we derive from them would hardly be possible on the basis of the static crystal structures of the proteins."

The study has been published in Scientific Reports.

Computer illustration of egg cell surrounded by sperm cells.

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  1. How sperm meets egg: a journey from production to fertilization

    To reach the egg cell, the sperm has to go through a long and difficult journey that can take from thirty minutes to several hours. For this reason, fertilization needs a large number of motile spermatozoa for at least one of them to be able to overcome all barriers. Firstly for the sperm to get to the egg, and then to fertilize the egg.

  2. Conception: Fertilization, Process & When It Happens

    If a sperm is successful on its quest to fertilize an egg, the now fertilized egg (called a zygote) continues to move down your fallopian tube, dividing into two cells, then four cells, then more cells. About a week after the sperm has fertilized the egg, the zygote has traveled to your uterus.

  3. Egg meets sperm (article)

    Hundreds of millions of sperm vie for a single egg cell. The sperm cells are streamlined in design for this purpose: a long tail to help them move, lots of mitochondria to power that movement, genetic information to pass on, and enzymatic proteins to get into the egg cell.

  4. Conception Pictures: From Egg to Embryo

    It takes about 24 hours for a sperm cell to fertilize an egg. When the sperm penetrates the egg, the surface of the egg changes so that no other sperm can enter. At the moment of...

  5. Here's how long it takes sperm to reach the egg after sex

    Usually, the sperm reaches the egg within 15 to 45 minutes of ejaculation. However, the process could be much longer than that if you haven't ovulated yet by the time you have sex, because sperm can live inside a reproductive tract and wait for an egg for up to five days.

  6. What Is Fertilization and How Does It Happen?

    To reach the target, though, a sperm cell has to go on a lengthy and strenuous journey. First, it must make its way from the vagina to the cervix, and then it has to swim through the uterus to the fallopian tubes. Once there, if the sperm is the lucky one, it will penetrate the egg and fertilize it.

  7. Male fertility gene discovery reveals path to success for sperm

    Sperm cells biggest challenge starts long before the journey to reach the egg as they are particularly vulnerable during the earliest stages of their development, as germ cells in developing ...

  8. Sperm, Meet Egg: The Process of Fertilisation

    What is Fertilization? how do the sperms meet the eggs? - Process explained | Cloudnine Blog Sperm, Meet Egg: The Process of Fertilisation December 3, 2020 Dr Arockia Virgin Fernando About Pregnancy , ‍ Turbo-charged sperm speed through a dimly lit canal, coursing feverishly through vast landscapes and titanic cavities.

  9. Fertilization: a sperm's journey to and interaction with the oocyte

    Near the eggs, probably stimulated by the cumulus cells and the ZP, sperm release their acrosomal contents by exocytosis and penetrate the ZP. Only acrosome-reacted sperm fuse with eggs, but their competency for fusion does not last long. Cumulus cells are packed together by hyaluronic acid at ovulation and become diffuse during fertilization.

  10. Sperm Cell

    A sperm cell or spermatozoon is a gamete (sex cell) produced in the male reproductive tract. It is a motile cell with a single aim - to fertilize a female egg. Each sperm cell contains the entire genome of the male that produces it. In combination with the female genome contained within the egg, a zygote is formed - a single totipotent stem ...

  11. The journey of the sperm to the egg

    The journey of the sperm to the egg 0 The path of the spermatozoa to reach the egg is not a simple one. This path is divided into a phase in the male reproductive system and another in the female reproductive system. In the case of the male, the sperm travel from the testicle to the urethra.

  12. Fertilization

    The result of fertilization is a cell ( zygote) capable of undergoing cell division to form a new individual. The journey of a fertilized egg in a woman. In mammals, eggs are released by the ovaries. If an egg meets a sperm cell, it may become fertilized. The fertilized egg travels to the uterus, where it grows and develops into a new ...

  13. The Epic Journey of Sperm Through the Female Reproductive Tract

    The remarkable journey that successful sperm take to reach an oocyte is long and tortuous, and includes movement through viscous fluid, avoiding dead ends and hostile immune cells. ... It also reviews how sperm, foreign cells in the female reproductive tract, are tolerated by the immune system. ... Mouse oviduct-specific glycoprotein is an egg ...

  14. Fertilization

    Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell. This new single cell, called a zygote, contains all of the genetic material needed to form a ...

  15. Sperm Meets Egg: The Genetics of Mammalian Fertilization

    Once the sperm have passed the UTJ, they congregate by attaching to the mucosal epithelium in the isthmus of the oviduct; they can wait in the epithelium for several days for ovulation to occur while maintaining their ability to fertilize the egg (Figure 1c).During this time, the sperm interact with the female epithelial cells and also with the molecules in the oviductal fluid (reviewed in 45 ...

  16. Human reproduction

    1. What else is produced by the ovaries besides ova (egg cells)? 2. What is semen made from? Male reproductive system The function of the male reproductive system is to produce sperm cells...

  17. The Ultimate Guide to Understanding the Pathway of Sperm: A Fascinating

    The pathway of sperm is the journey that sperm cells take from the testes to the female reproductive tract during fertilization. The journey involves a series of steps, including production, maturation, and transport.

  18. The Journey of Sperm and Egg: The Fertilization Process

    "The Journey of Sperm and Egg: The Fertilization Process" | Pregnancy | Embryo FormationFertilization is the biological process by which a sperm cell from a ...

  19. Sperm Travel Path: Understanding the Route to Fertilization

    Short answer sperm travel path: Sperm travel from the testes through the epididymis, vas deferens, and ejaculatory duct before being released through the urethra during ejaculation. The journey takes approximately 64-72 days to complete. Table of Contents Exploring the Fascinating Journey: Sperm Travel Path

  20. Fertilisation

    Fertilisation is the process in which the nucleus of a sperm cell fuses with the nucleus of an egg cell to produce a zygote which will eventually grow into offspring. Gametes are the sex...

  21. Male fertility gene discovery reveals path to success for sperm

    A sperm cell's biggest challenge starts long before the journey to reach the egg as sperm are particularly vulnerable during the earliest stages of their development, as germ cells in developing ...

  22. A Sperm's Obstacle Course to the Egg

    Only part of the ejaculate deposited in the vagina enters the cervix. 2. In the cervix, strands of mucus filter out aberrant sperms. 3. Sperms enter the lower end of the oviduct (isthmus) and bind ...

  23. A Journey of Sperm

    Each sperm will be approaching to this egg consequently. Fertilization. Although the egg is surrounded by a lot of sperms, there will be only 1 strongest sperm can penetrate into the egg. After that sperm will throw off its tail and release 23 pairs of chromosomes to match with egg's chromosomes. Combining to be 1 cell.

  24. Computer Models Uncover Fertilization Dynamics at the Moment of ...

    Science has long been fascinated by the dance that occurs when a sperm approaches an egg. This moment is pivotal because what triggers within seconds to minutes are rapid chemical transformations ...

  25. SPE-51, a sperm-secreted protein with an immunoglobulin ...

    Mei et al. present a secreted immunoglobulin superfamily protein, SPE-51, that is on the sperm surface and is required for the sperm to fertilize the eggs. In their secretion assay, the mammalian fertilization molecule SOF1 is also secreted. This work highlights the role of secreted molecules in mediating interactions between the sperm and egg.

  26. About 2% of babies born in the US are from IVF. Here's what you ...

    If 10 eggs are exposed to sperm, about seven will fertilize, she said. Of those seven, only 25% to 50% will grow in the laboratory long enough to be considered a more mature embryo called a ...

  27. Simulations Reveal What Happens When a Sperm Kisses an Egg

    The moment when a slithering sperm propels itself head-first into a gelatinous egg is one of sudden change. Within seconds to minutes, chemical changes in the egg's membrane and outer coat are ...

  28. It's Possible a Stingray Got Pregnant on Her Own

    When an egg cell is developing, it replicates, reorganizes and separates. During that process, leftover genetic material called polar bodies can act like sperm and fertilize the egg. The result is offspring that's slightly different genetically to the mother. ... and you can follow her journey on the Aquarium and Shark Lab by Team ECCO's ...