Fertilization occurs within the ampullar region of the oviduct. Spermatozoa are transported upward toward the ampulla from the vagina (eg., in primates and ruminants). Rodents, pigs, horses, and dogs ejaculate into the cervix or uterus. The ovum migrates down the oviduct toward the ampulla. The approximate fertilizable life of spermatozoa and ova within the reproductive tract of most mammalian females is approximately 48 and 24 hours, respectively (again, use caution concerning rules-of-thumb - eg., live sperm cells can persist for months within hibernating bats).
Sperm. Several barriers impede the ascent of sperm cells through the female reproductive system. Due to secretion of lactic acid by vaginal bacteria, the pH of the vagina is < 5.0. Spermatozoa become immobilized in an acidic milieu. Those spermatozoa reaching the cervical canal can become trapped within a matrix of cross-linked filaments of mucus (defective cells adhere most avidly to strands of mucus). Many sperm cells become lodged within cervical crypts. Granulocytes and antibodies infiltrate the uterine lumen in response to invading sperm cells (consequent subfertility is relatively common in prostitutes - whom become "immunized" to a gamut of sperm antigens). Finally, the intersections of the oviducts and uterus constrict in response to estradiol ("tube-locking"), restricting passage of sperm. Only the fittest of cells (usually fewer than 100) will approach the ovum.
Cellular motility is required for progression of spermatozoa through the cervical mucus. Once sperm cells enter the uterus, contractions of the uterus and oviduct are obligatory for their delivery to the ampulla (dead cells are transported as fast as live cells).
Initial transport of spermatozoa into the oviduct is rapid (~ 5 minutes in women); numbers of cells involved in this phase are small. In most cases fertilization takes place during a second, more prolonged phase of sperm migration; these cells are derived from reservoirs of spermatozoa contained within cervical crypts.
Ova. Fimbria of the oviduct become edematous and are brought into close apposition to the surface of the ovary during the preovulatory period. The ovulated ovum (which is sticky) is thereby captured and moved into the oviduct. A combination of oviductal secretions (fluid), contractions, and beating of cilia is responsible for transport of the ovum. Bidirectional waves of oviductal contractility, allowing sperm and egg transport to occur simultaneously, are propagated toward a focal point within the ampulla; the mechanics of this process are poorly understood.
Hormonal factors. Consistency of cervical mucus, myometrial contractions, and secretory and ciliary activities of oviductal epithelium are mediated by a shift in balance between estradiol and progesterone. Prostaglandins, VIP, and substance P have been implicated in the contractile responses of the uterus and oviducts to estradiol.
Oxytocin is secreted during copulation in both sexes. Stimulation of uterine contractility by oxytocin might therefore aid in transport of sperm (and could have some role in the ejaculatory process).
Artificial insemination. The cervix of appropriately large animals can be negotiated with an insemination rod and sperm deposited directly into the uterus. Livestock producers use AI to improve genetics and control sexually-transmitted diseases. Artificial insemination also is performed if sperm penetration of the cervical mucus is a deterrent to fertility or if mating is impractical.
With livestock, semen is either collected using an artificial vagina or obtained by electrical stimulation of the reproductive tract. The male is allowed to mount and attempt to mate a restrained teaser animal while the penis is manually diverted into the artificial vagina. Rams, boars, and stallions can be trained to mount a dummy. Electroejaculation involves placement of a bipolar electrode over the accessory glands by way of the rectum; this method is an alternative to the artificial vagina if the male is unable or unwilling to mount. Professional AI services generally collect semen from bulls twice a week.
Fertility of an ejaculate is ultimately judged by inseminating females and determining rates of pregnancy (or nonreturn to estrus). Until actual breeding data can be compiled, a predicted judgement of fertility is based on a laboratory examination of semen samples. Criteria used to judge a semen sample include cellular concentration, proportion of live to dead cells, morphology, and motility. The fertilizing capacity of human ejaculates have been enhanced using laboratory techniques (Figure 5-12).
One quality bovine ejaculate can yield enough motile cells to impregnate several hundred females. Diluents have been developed that not only extend the semen sample, but protect it during freeze-thaw (Table 5-4). Methods of cryopreservation of sperm cells have best been worked out for cattle. Breed organizations of certain species (eg., horses) still restrict the use of frozen semen. Insemination units are aliquoted into polyvinyl chloride straws or glass ampules, sealed, and frozen. A second series of microscopic evaluations of an ejaculate are usually performed after thawing.
An inseminating catheter loaded with semen is manipulated through the cervix per rectum in the cow (Figure 5-13) - semen can be deposited at midcervix or into the uterine body just beyond the anterior end of the cervix (a small amount can be discharged into the cervical canal while withdrawing the insemination rod). Ewes can be inseminated with the use of a vaginal speculum, however, passage of an inseminating rod through the cervix is difficult; injection of semen into the uterus with the aid of a laparoscope is gaining in popularity. A spiral-tipped inseminating tube (designed after the corkscrew-like boar penis) can be introduced directly into the cervix of the pig without cervical stabilization. In the mare, a hand is placed into the vagina; the index finger is inserted through the cervical os to help guide the tip of the catheter into the uterine body.
Barrier contraception. Mechanical contraceptives, such as the IUD and diaphragm, impose a physical barrier to sperm transport. Intrauterine devices are made of plastic and(or) metal molded into a variety of shapes and sizes. Some IUDs are medicated (Figure 5-14). The major advantage of the IUD, beyond its high degree of contraceptive effectiveness, is that once put in place there is no conscious effort required for its proper use.
The IUD, being a foreign body, stimulates a chronic inflammatory reaction within the uterus; copper potentiates this response. Products of inflammation are toxic to gametes (and if fertilization occurs, to the embryo itself). The IUD also alters the endometrium preventing implantation. Ovarian cyclicity is not significantly influenced by the IUD in primates (in species in which the uterus exerts a local action in luteolysis, the IUD [ie., prostaglandins] shortens the estrous cycle). Adverse effects of IUDs include pain, uterine perforation, intermenstrual bleeding, prolonged menses, pelvic inflammatory disease, and infertility. Local release of progestin from an IUD improves device tolerance.