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Suarez, A. At coitus, human sperm are deposited into the anterior vagina, where, to avoid vaginal acid and immune responses, they quickly contact cervical mucus and enter the cervix. Cervical mucus filters out sperm with poor morphology and motility and as such only a minority of ejaculated sperm actually enter the cervix. In the uterus, muscular contractions may enhance passage of sperm through the uterine cavity. A few thousand sperm swim through the uterotubal junctions to reach the Fallopian tubes uterine tubes, oviducts where sperm are stored in a reservoir, or at least maintained in a fertile state, by interacting with endosalpingeal oviductal epithelium.
As the time of ovulation approaches, sperm become capacitated and hyperactivated, which enables them to proceed towards the tubal ampulla. Sperm may be guided to the oocyte by a combination of thermotaxis and chemotaxis. Motility hyperactivation assists sperm in penetrating mucus in the tubes and the cumulus oophorus and zona pellucida of the oocyte, so that they may finally fuse with the oocyte plasma membrane.
Knowledge of the biology of sperm transport can inspire improvements in artificial insemination, IVF, the diagnosis of infertility and the development of contraceptives. Submitted on July 12, ; revised on September 26, ; accepted on October 4, Passage of sperm through the female reproductive tract is regulated to maximize the chance of fertilization and ensure that sperm with normal morphology and vigorous motility will be the ones to succeed.
Oocytes are usually fertilized within hours of ovulation Austin, ; Harper, On the other hand, in some species, sperm may be inseminated days horses, cattle and pigs or even months some bat species before the arrival of the oocyte. In humans, there is evidence that fertilization occurs when intercourse takes place up to five days before ovulation Wilcox et al.
Because sperm are terminally differentiated cells, deprived of an active transcription and translation apparatus, they must survive in the female without benefit of reparative mechanisms available to many other cells. Sperm are subjected to physical stresses during ejaculation and contractions of the female tract, and they may sustain oxidative damage. Furthermore, because sperm are allogenic to the female, they may encounter the defenses of the female immune system meant for infectious organisms Menge and Edwards, Thus, sperm must somehow use their limited resources to maintain their fertility in the face of numerous impediments.
As it is, of the millions of sperm inseminated at coitus in humans, only a few thousand reach the Fallopian tubes and, ordinarily, only a single sperm fertilizes an oocyte. The site of semen deposition is not easy to establish in many species because it must be determined by examining the female immediately after coitus and by considering the anatomy of the penis, vagina and cervix during coitus. However, it has been accomplished for humans, in which semen has been observed pooled in the anterior vagina near the cervical os shortly after coitus.
Within minutes of vaginal deposition, human sperm begin to leave the seminal pool and swim into the cervical canal Sobrero and MacLeod, In contrast, rodent sperm deposited in the vagina are swept completely through the cervix into the uterus along with seminal plasma within a few minutes Zamboni, ; Bedford and Yanagimachi, ; Carballada and Esponda, Some species, such as pigs, bypass the vagina altogether and deposit semen directly into the uterine cavity, where sperm may quickly gain access to the oviduct Hunter, ; Roberts, Whereas most of the semen of murine rodents is rapidly transported into the uterine cavity, some remains in the vagina where it coagulates to form a copulatory plug.
The plug forms a cervical cap that promotes sperm transport into the uterus Blandau, ; Matthews and Adler, ; Carballada and Esponda, Ligation of the vesicular and coagulating glands of rats prevented the formation of plugs and the transport of sperm into the uterus Blandau, The plugs formed by semen of guinea pigs and mice extend into the cervical canals and thus could form a seal against retrograde sperm loss Blandau, Male mice deficient for the gene encoding the protease inhibitor known as protease nexin-1 PN-1 show a marked impairment in fertility Murer et al.
Vaginal plugs formed in females Women wanting sex 14047 mating with PN-1 null males were small, soft and fibrous and did not lodge tightly in the dual cervical canals. No sperm could be found in the uterus 15 min after mating with PN-1 null males, demonstrating the importance of the plug for promoting transport of mouse sperm into the uterus Murer et al. Human semen coagulates, but it forms a loose gel rather than the compact fibrous plug seen in rodents. The coagulate forms within about a minute of coitus and then is enzymatically degraded in to 1 h Lilja and Lundwall, The predominant structural proteins of the gel are the 50 kDa semenogelin I and the 63 kDa semenogelin II, as well as a glycosylated form of semenogelin II, all of which are secreted primarily by the seminal vesicles Lilja, The gel is degraded by prostate-specific antigen PSAa serine protease secreted by the prostate gland Watt et al.
It has been proposed that this coagulum serves to hold the sperm at the cervical os Harper, and that it protects sperm against the harsh environment of the vagina Lundwall et al. Seminal gels are not fully successful at holding sperm at the cervical os. In cattle, several studies have demonstrated loss of sperm from the vagina after mating or insemination reviewed by Hawk, The fate of spermatozoa that are ejaculated or inseminated into the vagina, but that do not enter the cervix, has not been studied extensively in humans.
Like humans, some primates produce semen that forms a soft gel. However, in chimpanzees, a species in which females mate with more than one male in a brief time, the semen coagulates into a compact plug resembling that of rodents Jensen-Seaman and Li2 ; Kingan et al. The plug may serve to prevent other males from mating with the female.
Some carnivores e. The vagina is open to the exterior and thus to infection, especially at the time of coitus; therefore, it is well equipped with antimicrobial defenses. These defenses include acidic pH and immunological responses and can damage sperm as well as infectious organisms.
To enable fertilization to take place, both the female and the male have adopted mechanisms for protecting sperm. In humans, semen is deposited at the external os of the cervix so that sperm can quickly move out of the vagina Sobrero and MacLeod, Human sperm must contend, however briefly, with the acidic pH of vaginal fluid. The vaginal pH of women is normally five or lower, which is microbicidal for many sexually transmitted disease pathogens.
Evidence indicates that the acidity is maintained through lactic acid production by anaerobic lactobacilli that feed on glycogen present in shed vaginal epithelial cells Boskey et al. Lowering pH with lactic acid has been demonstrated to immobilize bull sperm Acott and Carr, ; Carr et al. The pH Women wanting sex 14047 seminal plasma ranges from 6. Vaginal pH was measured by radio-telemetry in a fertile human couple during coitus. The pH rose from 4. Vaginal washings of women with high levels of detectable seminal antigens had a median pH of 6.
Women wanting sex 14047 gel deed to maintain a low vaginal pH after coitus has been shown to immobilize human sperm Women wanting sex 14047 vitro and in vivo Amaral et al. In additions to pH buffers, seminal plasma contains inhibitors of immune responses, including protective components that coat sperm Suarez and Oliphant, ; Dostal et al.
These are most effective when sperm are bathing in seminal plasma and may be gradually shed when sperm leave the seminal plasma behind. Males may also overcome female defenses by inseminating many sperm. This strategy is particularly effective for overcoming cellular immune responses. In the rabbit, deposition of semen in an invasion of neutrophils into the vagina. This invasion takes time, however, to build to an effective level.
Numerous leukocytes, many containing ingested sperm, were recovered from vaginas of rabbits 3—24 h post coitus Phillips and Mahler, a,b. By that time, however, thousands of sperm had already reached the Fallopian tubes Overstreet et al. In some species, the cervical canal widens under the influence of estrogen. Fluoroscopy and scintigraphy have been used in domestic dogs and cats to examine cervical patency.
Opening of the cervix in these species has been correlated with estrus Silva et al. Radioopaque fluid and also human serum albumin radiolabelled with technetium 99 could be seen rapidly passing through the cervix and filling the uterine lumen after deposition in the cranial vagina at estrus. Sperm of humans and cattle enter the cervical canal rapidly where they encounter cervical mucus Figure 1 A.
The extent of hydration is correlated with penetrability to sperm Morales et al. Coitus on the day of maximal mucus hydration in women is more closely correlated with incidence of pregnancy than coitus timed with Women wanting sex 14047 to ovulation detected using basal body temperature Bigelow et al. Human female reproductive tract illustrating stages of gamete transport. A Sperm entering cervical mucus at external os of cervix. The mucus fills the upper half of the inset. B Sperm interacting with endosalpingeal epithelium in Fallopian tube. C Hyperactivated motility of sperm in Fallopian tube.
D Oocyte in cumulus within a transverse section of the tubal ampulla. Artwork by C. Rose Gottlieb. Cervical mucus presents a greater barrier to abnormal sperm that cannot swim properly or that present a poor hydrodynamic profile than it does to morphologically normal, vigorously motile sperm and is thus thought as one means of sperm selection Hanson and Overstreet, ; Barros et al. The greatest barrier to sperm penetration of cervical mucus is at its border, because here the mucus microarchitecture is more compact Yudin et al. Components of seminal plasma may assist sperm in penetrating the mucus border.
Like the vagina, the cervix can mount immune responses. In rabbits and humans, vaginal insemination stimulates the migration of leukocytes, particularly neutrophils and macrophages, into the cervix as well as into the vagina Tyler, ; Pandya and Cohen, Neutrophils migrate readily through midcycle human cervical mucus Women wanting sex 14047 and Saltzman, In rabbits, neutrophils were found to heavily infiltrate cervices within a h of mating or artificial insemination Tyler, Interestingly, it was discovered that if female rabbits were mated to a second male during the neutrophilic infiltration induced by an earlier mating, sperm from the second male were still able to fertilize Taylor, Thus, although the cervix is capable of mounting a leukocytic response, and neutrophils may migrate into cervical mucus, the leukocytes may not present a ificant barrier to sperm.
This can happen in vivo if the female somehow becomes immunized against sperm antigens. Altogether, the evidence indicates that leukocytic invasion serves to protect against microbes that accompany sperm and does not normally present a barrier to normal motile sperm, at least not shortly after coitus.
Immunoglobulins, IgG and IgA, have been detected in human cervical mucus. Secretory IgA is produced locally by plasma cells in subepithelial connective tissue. The amount secreted increases in the follicular phase but then decreases at about the time of ovulation Kutteh et al.
The immunoglobulins provide greater protection from microbes at the time when the cervical mucus is highly hydrated and offers the least resistance to penetration. However, when there are antibodies present that recognize antigens on the surface of ejaculated sperm, infertility can result Menge and Edwards, Complement proteins are also present in cervical mucus Matthur et al. Thus, there is a potential for antibody-mediated destruction of sperm in the cervical mucus as well as leukocytic capture of sperm.
Some anti-sperm antibodies are not complement-activating; however, they can still interfere with movement of sperm through cervical mucus by physical obstruction Menge and Edwards, ; Ulcova-Gallova, An elegant three-dimensional reconstruction of serial sections of the bovine cervix produced by Mullins and Saacke led them to conclude that mucosal folds in the cervical canal form channels leading to the uterine cavity.
Furthermore, based on histochemical staining characteristics of the mucus, they concluded that, during the follicular phase, mucus deep in the channels is different in composition and less dense than that in the central portion of the cervical canal. They proposed that bull sperm enter deep channels at the external os and travel in them all the way to the uterine cavity, thereby avoiding the more viscous mucus in the centre of the cervical canal that serves to discharge uterine contents.
This model is supported by of earlier studies on farm animals. The remaining half of the sperm were found deep in the mucosal grooves. These observations also indicate that the cervix supports the passage of normal motile sperm while discouraging passage of microbes and sperm with abnormal form or motility. Normal, fresh, motile sperm can avoid the area most populated by neutrophils and they appear to be resistant to leukocytic phagocytosis anyway, as discussed above.
In descriptions of human cervical anatomy, mention is made of cervical crypts that are thought to entrap and store sperm Fawcett and Raviola, ; Harper, On the other hand, scanning electron microscopy of the human cervix indicates that mucosal grooves forming a preferential pathway for sperm could be present as in the bovine Figure 2. A comprehensive study of the human cervix is needed to determine whether sperm follow mucosal grooves to traverse the cervical canal.
Sperm may also be guided through the cervix by the microarchitecture of the cervical mucus. Mucins, the chief glycoproteins comprising cervical mucus, are long, flexible linear molecules molecular weight of human mucins is approximately 10 7 Daltons.
The viscosity of mucus is due to the large size of mucins, while elasticity from the entanglement of the molecules Carlstedt and Sheehan,1 ; Sheehan and Carlstedt, ; Sheehan et al.Women wanting sex 14047
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