Gamete Transport

Fertilization depends upon the two gametes bumping into one another. In species with internal fertilization, which includes all mammals and birds, both sperm and egg must be transported into the oviduct, which serves as the site of fertilization.

Sperm Transport

Semen is ejaculated and deposited initially into one of two sites: the vagina (e.g. humans, cattle, rabbits) or the uterus (e.g. horses, pigs, rodents). In species such as dogs, semen is probably deposited largely into the vagina, but also forced into the uterus. Despite these differences in deposition site and significant differences in the number of sperm ejacuated, there is remarkably little variation among species in the total number of sperm that reach the oviducts. Typicially, a few hundred to a few thousand sperm reach the oviducts following a single mating, which usually represents far less than one percent of the sperm in the ejaculate.

The vagina represents a hostile environment for sperm, and their continued survival depends on getting into more hospitable regions of the female tract. In their journey from vagina to oviduct, sperm must overcome a series of barriers, each of which eliminates a substantial proportion of the original population of sperm:

The cervix connects the vagina to the uterus. The cervical canal follows an irregular, tortuous route, and the epithelium contains many deep crypts.

The cervical epithelium is richly endowed with mucus-secreting cells, and, as a consequence, the lumen is filled with mucus. Interestingly, the consistency and viscosity of cervical mucus is under endocrine control. When estrogen levels are high and progesterone levels low, as occurs prior to ovulation, cervical mucus becomes watery and its mucin strands assume a parallel orientation. This state apparently greatly facilitates passage of sperm through the cervical canal. Conversely, when progesterone concentrations are high, as in the luteal phase of the cycle, cervical mucus becomes exceptionally viscous and disorganized, which largely precludes entry of sperm into the uterus.

The uterus does not present an active barrier, but sperm must somehow be transported directionally along its length.

Studies in several species have shown that sperm are able to get from the distal uterus to the oviducts in times as short as a few minutes, which is much too fast to be explained by sperm motility. Moreover, dead sperm and inanimate sperm-sized particles are rather efficiently transported upward through the uterine lumen. The conclusion from these types of studies is that sperm transport in the uterus is largely a result of uterine contractions, and that sperm motility plays a minor if any role in the process.

In most, but not all species, the uterus is also a site hostile to sperm. In many animals, sperm within the uterus are rapidly phagocytosed. In other cases, sperm can remain viable in the uterus for several days, but their fertility rapidly declines. There are some dramatic exceptions to these general observations.

The uterotubal junction is the region joining the tip of the uterine horn to the oviduct. The morphology of this region varies considerably among species, and and this structure appears to be a significant barrier to sperm especially in animals like rodents and pigs where huge numbers of sperm are deposited directly in the lumen of the uterus.

In summary, the vast majority of ejaculated sperm are lost are various points between the cervix and oviduct. A few exhausted semifinalists make it to the site of fertilization. Of those, of course, there can be only one "winner" for each egg. Without meaning to, John Wayne provided a good synopsis of the life a sperm.

Egg Transport

Mammalian eggs are ovulated from ovarian follicles as cumulus-oocyte complexes, which consist of the oocyte embedded in a cluster of follicle cells. The image to the right shows such a structure from a cow - the oocyte is encased in its zona pellucida, which is somewhat obscured by a cloud of follicle cells.

In order to reach the site of fertilization, the ovulated egg must be picked up and transported into oviduct through an opening called the ostium. In most mammals the ovarian end of the oviduct flares into a funnel-shaped structure called the fimbria, which is positioned to partially cover the ovary. The fimbria is densely covered with ciliated epithelial cells, which beat toward the ostium and propel the cumulus-oocyte complex into the oviduct.

In species such as the rodents and dogs, the ovary is enclosed completely or nearly completely in a thin membrane called the bursa. Because the ostium of the oviduct is inside the bursa, the eggs are essentially trapped after ovulation with no where to go except into the oviduct.

Once an oocyte enters the oviduct, it is propelled by ciliary motion down into the ampulla, where fertilization takes place. The oviduct provides the appropriate environment not only for fertilization, but for early embryonic development, and it is important that the embryo remain there for a period of about three days.

The Fertilizable Lifespan of Gametes

In most species, both sperm and egg have a short fertilizable lifespan, and once they are delivered into the female tract, the clock starts ticking. What this means, of course, if that mating or insemination must coincide closely with ovulation. If sperm are deposited many days before the egg reaches the oviduct, there is little chance that they will survive to fertilize. Conversely, if sperm reach the oviduct several days after ovulation, they will certainly find an egg that has long since degenerated.

One of many demonstrations of this concept is depicted with data from humans in the graph to the right, which presents data on the probability of conception in healthy women that had sexual intercourse a single time within several days of ovulation (adapted from Wilcox et al. New Eng J Med 333:1517, 1995). A group of 221 women were in the trial and data were collected from a total of 625 menstrual cycles. Over the course of the study, 192 pregnancies were established, of which 129 resulted in delivery of a baby.

There were no term pregnancies established when a single intercourse took place greater than six days before ovulation or even a day after ovulation. The highest probability of becoming pregnant was seen when sex occurred during the two days preceeding ovulation. These and similar data from other studies indicate that in humans, the ovulated egg has a very short fertilizable lifespan. Cautionary note: In this study, no pregnancies were attained when sex took place after the day of ovulation - this does not mean that fertility is zero at that time, but rather, that it is low and was not observed in this sample of women.