The high viscosity gel phase is composed of a network of filamentous glycoproteins called mucin. Collectively, mucin macromolecules form a complex of interconnected micelles, which comprise a lattice whose interstices are capable of supporting the low viscosity phase, which is predominantly water. Besides hormonal factors, physical processes, such as shearing, stretching, and compression can alter the spaces between molecules and, consequently, orientation of the mucin filaments.
These mechanical forces can be imparted by thrusting and pelvic contraction during coitus, and also by cervical contractions in the pericoital period.
Additionally, rheologic forces associated with the mucus outflow from the cervical crypts tend to align the mucin filaments in a longitudinal fashion within the cervical canal, thus creating aqueous channels between the filaments.
Spermatozoa may retain their fertilizing capacity in human cervical mucus for up to 48 hours and their motility for as long as hours. Another potentially important feature of human cervical mucus is the belief that it is able to restrict migration of human spermatozoa with abnormal morphology.
The percentage of spermatozoa with normal morphology in the cervical mucus and in the uterine fluid is significantly higher than usually seen in semen. Comparison of morphologically normal versus abnormal human sperm in semen has shown that abnormal sperm are less likely to be motile, and those that are motile tend to swim with a lower velocity than normal cells. Little is known about sperm transport within the endometrial cavity.
Sperm motility does not appear to be the only force directing the sperm toward the oviducts, because inert particles deposited within the uterus are transported to the Fallopian tubes. Unfortunately, much difficulty has been met in attempts to recover and quantify uterine sperm.
None of the sperm were motile. A study by Kunz and coworkers used vaginal sonography to demonstrate that uterine peristalsis during the follicular phase of the menstrual cycle exhibits an increasing frequency and intensity of subendometrial and myometrial peristaltic waves as the follicular phase progresses. The ascension of these particles was monitored by serial scintigrams. As soon as 1 minute after placement, the macrospheres reached the intramural and isthmic portion of the oviduct.
Quantitatively, the number of macrospheres progressed dramatically as the follicular phase progressed, with only a few particles entering the uterine cavity during the early follicular phase of the menstrual cycle. By the midfollicular phase, the proportion of macrospheres entering the uterine cavity increased dramatically, and by the late follicular phase, the highest level of macrosphere transported to the oviducts was noted. Perhaps the most striking finding of this particular study was the preferential transport of these inert particles to the oviduct ipsilateral to the side of the dominant follicle.
Other investigators have shown that near the time of ovulation, the number of spermatozoa is higher in the oviduct ipsilateral to the dominant follicle than in the contralateral oviduct on the side of the nondominant follicle.
The results of the above study, however, seem to suggest that lateralizing muscular contractile forces may play a significant role in this preferential movement, in that inert particles are obviously unable to engage in chemotactic migration. The adult human Fallopian tube, about 9 to 11 cm long, consists of five distinct segments: the fimbria, infundibulum, ampulla, isthmus, and intramural segment.
Epithelial cells undergo histologic changes in response to cyclic estrogen and progesterone variations, with the height of the epithelial cells being greatest at the time of the estrogen peak near midcycle.
Sperm movement through the Fallopian tube relies on a combination of forces: intrinsic sperm motility, tubular muscular contraction, and fluid flow. Tubal fluid production is maximal at the time of ovulation, and this fluid sustains the sperm before fertilization.
Although the uterotubal junction does not act as a barrier to inert particles, it may serve as an additional functional barrier to sperm with abnormal morphology or motility. Although tens of millions to hundreds of millions of sperm are deposited in the vagina at the time of ejaculation, anatomic studies have shown that typically only hundreds of sperm are present in the oviduct at various postcoital timepoints.
Parous women undergoing total abdominal hysterectomies for menorrhagia were inseminated with partner or donor semen, and 18 hours later, during surgery, both oviducts were ligated into ampullary, isthmic, and intramural regions.
Using flushing techniques, scanning electron microscopy, and homogenization procedures, patients' oviducts were carefully evaluated for the presence of sperm. A median of only total sperm was recovered from the oviducts of these women, and the ampulla near the ovulating ovary contained a significantly higher percentage of spermatozoa than did the nonovulatory side. The precise role played by tubal fluid in gamete transport and sperm activation is still not entirely understood.
Zhu and colleagues used an in vitro technique to demonstrate that human oviductal fluid maintains sperm motility induced by exposure to follicular fluid longer than does exposure to a simple salt solution. These findings may suggest that tubal fluid potentiates the motility and viability of spermatozoa, thus enhancing the chances of fertilization.
Yao and colleagues used in vitro oviductal cell cultures incubated with spermatozoa to determine that oviductal cells promote capacitation and stabilize the acrosome. Although done in an in vitro setting, new studies such as the ones already discussed will likely provide clarity to the complex interplay between male gametes and the female reproductive tract.
In Chang, while studying rabbits, and Austin, while working on rats, each independently reported that mammalian sperm must reside in the female reproductive tract for a finite period of time before they gain the ability to fertilize ova. Develop hyperactivated motility, with vigorous nonlinear flagellar motion Bind to the zona pellucida Undergo the acrosome reaction Proceed eventually to fusion with the oolemma and egg fertilization Initial investigative work in the area of sperm capacitation was performed using animal models such as rabbits, rats, and hamsters.
In fact, in , Yanagimachi and Chang broke major scientific ground with their finding that hamster epididymal spermatozoa could be capacitated in vitro. Temporally as well, there are also differences in capacitation between species with some species capable of much more rapid capacitation in vitro than others.
Studies of capacitation have sometimes met with controversy, largely because of lack of morphologic criteria by which to assess its occurrence. Despite this, both in vivo and in vitro capacitation enable the spermatozoa to undergo fusion of the plasma and outer acrosomal membrane during the acrosome reaction and thus proceed to subsequent fertilization. These two steps, sperm capacitation and the acrosome reaction, are both essential precursors of normal fertilization.
Evidence of this is seen in sperm that have not been incubated in the female reproductive tract or otherwise capacitated cannot effectively fertilize an egg. Many substances within the female reproductive tract have been examined as potential capacitating factors, but at this time none has been uniquely identified.
Nonetheless, we do know that at the molecular level, several key changes are noted to occur in the spermatozoa as a result of capacitation.
These changes include: Alteration or removal of sperm coating materials. These coating materials become adsorbed to or integrated within the sperm plasma membrane during epididymal transport and also during exposure to seminal plasma 59 , 60 A decrease in the net negative surface charge 61 Changes in the content and location of surface antigens 62 Conformational changes to intrinsic membrane proteins 63 Changes in the permeability of the membrane to various ions, especially calcium 64 Capacitation in Human Spermatozoa.
Very little is known about human sperm capacitation in the female reproductive tract. We do know that human sperm that are recovered from the cervical mucus and placed into a noncapacitating medium are able to penetrate the zona pellucida of the human oocyte and also fuse with zona-free hamster oocytes.
Because of the inherent difficulty in manipulating and subsequently evaluating the in vivo environment of the female reproductive tract, much of what we now know about human sperm capacitation is the result of in vitro studies. Capacitation is associated with significant alteration of the surface of the sperm, with various molecules being removed or rearranged.
This inhibition, like capacitation, is reversible. Capacitation is also characterized by a loss or reduction of cholesterol from the plasma membrane of spermatozoa. Benoff and colleagues have shown that a loss of membrane cholesterol is a necessary feature of capacitation in human spermatozoa.
Membranes are a very dynamic collection of proteins and lipids that are capable of responding to various environmental signals that modify cellular activities.
Part of this ongoing dynamic process involves alterations of membrane topography, with certain cell surface molecules moving to various locations or domains in response to environmental conditions. Cholesterol has been shown to limit the insertion of proteins into lipid bilayers, to prohibit the movement of receptors in cell membranes and to change membrane protein conformation and thus alter their activity.
This is described as one of the hallmark characteristic changes seen as a result of capacitation. Sperm motility becomes more vigorous with a decreased rate of forward progression. Specifically, the sperm develops:. Wider amplitude of lateral head displacement Marked increase in flagellar beating A curved and tortuous trajectory 79 Although the functional significance of these changes remains unclear, they may facilitate sperm transit through the oviduct and provide the necessary force needed to penetrate the granulosa cell layer and zona pellucida surrounding the ovum.
The sperm plasma membrane is composed of a lipid bilayer interspersed with a number of proteins. Lipid types present include cholesterol, glycolipids, and phospholipids.
The proteins found here can traverse the entire membrane from cytosolic compartment to extracellular space. These proteins have important functions, including activation of receptors and transport of ions. The mature human ovum possesses a number of surrounding layers that must be penetrated by the spermatozoa for normal fertilization to occur. This structure contains a number of digestive enzymes, such as hyaluronidase, corona-penetrating enzyme, and acrosin to facilitate membrane fusion and sperm entry into the ovum.
Ultrastructurally, the acrosome reaction involves regional fusion of areas of the outer acrosomal membrane and the overlying sperm plasma membrane. These fused areas then lyse, serving as portals through which soluble contents of the acrosome can be dispersed to act on the vestments of the ovum. The acrosome reaction is initiated as the spermatozoa arrives at the ovum. The outermost covering of the ovum, the cumulus oophorus, is degraded by hyaluronidase located on the plasma membrane of the spermatozoa.
Next, proacrosin, a zymogen within the acrosomal region, is converted to acrosin, and this facilitates breakdown of the zona pellucida glycoproteins. For this biologic process to occur, the spermatozoa plasma membrane and the outer acrosomal membrane must be removed.
This, in essence, is the hallmark of the acrosomal reaction. After the spermatozoa has proceeded through the zona pellucida, the sperm head crosses the perivitelline space and attaches to the cell membrane of the ovum. Subsequently, the sperm and ovum plasma membranes fuse, the sperm enters the ovum, and fertilization follows. The acrosome reaction is a key component of the fertilization process, and its proper timing is essential. Inappropriately early release of the acrosomal enzymes within the female reproductive tract would result in spermatozoa being unable to fertilize.
Initiation of the acrosome reaction seems to hinge specifically on spermatozoal binding to the zona pellucida. Although the human model is not entirely understood, the murine model has been extensively studied. With the murine model, spermatozoal exposure and binding to the structural zona glycoproteins, ZP3 zona pellucida protein 3 have been identified as the molecule that sets the events of the acrosome reaction into motion.
Influx of calcium into the spermatozoa Activation of the adenylate cyclase, adenosine 3',5'cyclic phosphate cAMP , protein kinase pathway Activation of the guanylate cycle, cyclic guanosine monophosphate cGMP , protein kinase pathway Activation of the phospholipase C, diacylglycerate, protein kinase C pathway. Together, these pathways likely share a complex regulation of the events collectively called the acrosomal reaction. Extensive clinical application has been made of the large body of information accumulated to date regarding sperm transport and capacitation.
The most notable utilization has come with the widespread use of in vitro fertilization techniques since the early s for couples with otherwise untreatable infertility. In particular, spermatozoa capacitation techniques in vitro are now performed readily in the laboratory as a routine part of the in vitro fertilization IVF treatment for both male and female infertility. Because of the large number of sperm required for standard IVF as well as the modest initial fertilization and pregnancy rates associated with IVF, several gamete micromanipulation techniques were developed over the next decade in an attempt to improve successful outcomes.
The first advance involved creation of a nick in the zona pellucida, followed by standard IVF. This was called partial zona dissection PZD. Some of these myths go way back to false notions of sperm exceptionalism, but many of them also obscure the fact that conception, like sex, is much more of an active partnership. Believing these myths can also lead to many inaccurate or toxic presumptions.
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Health Conditions Discover Plan Connect. Medically reviewed by Timothy J. Legg, Ph. Share on Pinterest. Sperm swim like Olympic athletes. Thicker sperm is more fertile sperm.
Sperm only live for a short time after release. Sperm only needs to go straight for the egg. Briefs are bad for your sperm count. Every sperm is healthy and viable. More sperm is better when trying to get pregnant. Sperm are a protein powerhouse. A healthy sperm count can help increase your chances for conception. Here are seven things you can do to promote healthy semen. The odds of a woman getting pregnant by having sex while on her period are low, but not zero.
Learn about the different types of sperm motility and whether Y "boy" chromosomes actually swim faster than X "girl" chromosomes. Are condoms with spermicide more effective than condoms without spermicide? As featured on Good Morning America, we examine expert opinions and survey data in a comprehensive overview of the current fertility landscape in We'll take a look at fertility testing for men and what may or may not be contributing to the challenge of having a child.
Infertility is something many men experience. Here are 10 science-backed ways to increase sperm count and enhance overall fertility in men. Health Conditions Discover Plan Connect. Medically reviewed by Justin Choi, M. Sperm near the body Hot tubs and sperm Spermicide Sperm motility Frozen sperm Outlook We include products we think are useful for our readers.
Can you get pregnant if a man ejaculates in a hot tub or bathtub? Getting pregnant in this case is highly unlikely to impossible. Does spermicide kill sperm? What role does sperm motility play in pregnancy?
Parenthood Pregnancy. Read this next. Medically reviewed by Debra Rose Wilson, Ph. Medically reviewed by Steven O'Brien.
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