Sperm transport in the female reproductive tract

Spermatozoa are actively transported from the vagina via the cervical canal and the uterine cavity to the ampulla of the oviducts, where fertilization occurs. In the human vagina, the ejaculated semen is deposited near the external cervical opening where the environment is very acidic due to lactic acid and thus hostile to spermatozoa (Harper 1988). The alkaline pH of the ejaculate protects spermatozoa in this acidic environment (Speroff et al. 1994). This protection is, however, temporal, and most spermatozoa only remain motile in the vagina for a few hours (Fordney-Settlage 1981, Speroff et al. 1994). The spermatozoa are transported into the cervical canal by the pressure alterations in the vagina due to the female orgasm assisted by the normal motility of sperm (Fox et al. 1970, Speroff et al. 1994). During their transit in the female reproductive tract, the spermatozoa are stored in cervical crypts (Harper 1988). The passage of spermatozoa through the cervix is thought to maintain the muscle contractions of the reproductive tract wall and some properties of the spermatozoa. The interactions between sperm and mucus and the motility of spermatozoa during the transport are important, and one cause of infertility is presumably the impaired sperm movement through cervical mucus (Speroff et al. 1994). The change in the composition of cervival mucus at mid-cycle also affects the passage (Fordney-Settlage 1981, Harper 1988, Barratt & Cooke 1991). 

The release of spermatozoa from human cervical crypts may continue for several days (Yanagimachi 1994). Motile sperm has been recovered in the uterine cavity up to 24 h after intercourse (Rubinstein et al. 1951, Moyer et al. 1970), occasionally even at 85 h (Harper 1988). The transport of spermatozoa from the cervix to the uterotubal junction is mainly attributable to uterine wall contractions (Yanagimachi 1994). The human endometrium prepares for ovulation by secreting a unique kind of fluid to the uterine lumen. The fluid has a different protein pattern, ionic composition, and volume than at the other stages of the cycle (Casslén & Nilsson 1984, Casslén 1986, Harper 1988). This fluid serves to suspend spermatozoa and to keep them viable during the transport process, and it also contains macrophages that remove dead and nonviable spermatozoa. 

The uterotubal junction and the lower isthmus in some animals act as a site of storage for spermatozoa before the arrival of the ovum to the site of fertilization (Suarez 1998). During their storage, spermatozoa remain weakly motile and attached to the mucosal surface of luminal epithelial cells, especially in specific epithelial crypts (Cooper et al. 1979, Yanagimachi 1994, Suarez 1998). Whether the human oviduct has this function is, however, not fully determined (Harper 1988, Williams et al. 1993, Baillie et al. 1997). 

The ampulla of the oviduct is the site of fertilization, and motile spermatozoa can be found there up to 85 h after intercourse. The transport of spermatozoa through the oviducts is a combination of sperm motility, fluid flow, and contractive movements of the oviduct walls (Harper 1988). 

During its transport in the female reproductive tract, spermatozoa first undergo a maturational change called capacitation. Capacitation includes a variety of changes in the sperm plasma membrane, intracellular ions, metabolism, adenylate cyclase-cAMP system, nucleus, and acrosome (Yanagimachi 1994, Fraser 1995). Human spermatozoa do not need to experience the uterus or the isthmic region of the oviduct to became capacitated, and it has been proposed that capacitation is initiated and possibly already completed in the cervix (Speroff et al. 1994, Yanagimachi 1994). 

 Capacitation is followed by an acrosome reaction, which occurs when spermatozoa come into close contact with the ovum in the ampulla of the oviduct. The acrosome reaction enables spermatozoa to penetrate through the zona pellucida and fuse with the egg plasma membrane. In this reaction, the plasma membrane and the outer acrosomal membrane fuse, enabling release of the acrosomal content. Acrosome contains certain enzymes, e.g. hyaluronidase and acrosin, important for the events preceding fertilization (Yanagimachi 1994).  

Sperm hyperactivation occurs before the acrosome reaction (Yanagimachi 1994). Hyperactivation takes place in the oviduct and helps the spermatozoa to swim in the viscous oviduct fluid and to penetrate the zona pellucida (Stauss et al. 1995, Suarez 1996).

If fertilization occurs, the fertilized ovum enters the uterine lumen at the morula stage 4 days after ovulation. Approximately one day later, the morula develops into a blastocyst. Implantation begins about 7 days after ovulation. The implantation process involves apposition and adhesion of the blastocyst to the uterine surface, followed by invasion of the trophoblast. During apposition, uterine fluid is pinocytosed by microprotrusions of the apical surface of the luminal endometrial epithelium, resulting in a close association of the blastocyst with the uterine epithelium. Apposition is followed by adhesion of the outer layer of the trophoblast cells to the uterine epithelium, which initiates the invasion of trophoblasts. In humans, the invasion process is called interstitial invasion, since the syncytiotrophoblasts invade first between uterine epithelial cells and thereafter through the basal lamina (Klentzeris 1997).


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