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Translocation and Capacitation

The reproductive tract of female mammals plays a very active role in the mammalian fertilization process. While sperm motility is required for mouse sperm to encounter the egg once it is in the oviduct, sperm motility is probably a minor factor in getting the sperm into the oviduct in the first place. Sperm are found in the oviducts of mice, hamsters, guinea pigs, cows, and humans within 30 minutes of sperm deposition in the vagina, a time “too short to have been attained by even the most Olympian sperm relying on their own flagellar power” (Storey 1995). Rather, the sperm appear to be transported to the oviduct by the muscular activity of the uterus.

By whatever means, mammalian sperm pass through the uterus and oviduct, interacting with the cells and secretions of the female reproductive tract as they do so. These interactions are critical for their ability to interact with the egg. Newly ejaculated mammalian sperm are unable to undergo the acrosomal reaction without residing for some time in the female reproductive tract (Chang 1951; Austin 1952). The set of physiological changes that allow the sperm to be competent to fertilize the egg is called capacitation. The requirement for capacitation varies from species to species (Gwatkin 1976). Capacitation can be mimicked in vitro by incubating sperm in tissue culture media (containing calcium ions, bicarbonate, and serum albumin) or in fluid from the oviducts. Sperm that are not capacitated are “held up” in the cumulus and so do not reach the egg (Austin 1960; Corselli and Talbot 1987).

As mentioned above (and contrary to the opening scenes of the Look Who’s Talking movies), “the race is not always to the swiftest.” Although some human sperm reach the ampullary region of the oviduct within a half hour after intercourse, those sperm may have little chance of fertilizing the egg. Wilcox and colleagues (1995) found that nearly all human pregnancies result from sexual intercourse during a 6-day period ending on the day of ovulation. This means that the fertilizing sperm could have taken as long as 6 days to make the journey. Eisenbach (1995) has proposed a hypothesis wherein capacitation is a transient event, and sperm are given a relatively brief window of competence in which they can successfully fertilize the egg. As the sperm reach the ampulla, they acquire competence, but if they stay around too long, they lose it. Sperm may also have different survival rates depending on their location within the reproductive tract, and this may allow some sperm to arrive late but with better chance of success than those that have arrived days earlier.

The molecular changes that account for capacitation are still unknown, but there are four sets of molecular changes that may be important. First, the fluidity of the sperm plasma membrane is altered by the removal of cholesterol by albumin proteins found in the female reproductive tract. If serum albumin is experimentally preloaded with cholesterol, it will not permit capacitation to occur in vitro. Second, particular proteins or carbohydrates on the sperm surface are lost during capacitation (Lopez et al. 1985; Wilson and Oliphant 1987). It is possible that these compounds block the recognition sites for the proteins that bind to the zona pellucida. Third, the membrane potential of the sperm becomes more negative as potassium ions leave the sperm. This change in membrane potential may allow calcium channels to be opened and permit calcium to enter the sperm. Calcium and bicarbonate ions may be critical in activating cAMP production and in facilitating the membrane fusion events of the acrosomal reaction (Visconti et al. 1995; Arnoult et al. 1999). Fourth, protein phosphorylation occurs (Galantino-Homer et al. 1997). However, it is still uncertain whether these events are independent of one another and to what extent each of them causes sperm capacitation (Figure 7.12).

There may be an important connection between sperm translocation and capacitation. Timothy Smith (1998) and Susan Suarez (1998) have documented that before entering the ampulla of the oviduct (where mammalian fertilization occurs), the uncapacitated sperm bind actively to the membranes of the oviduct cells in the narrow passage (isthmus) preceding it (Figure 7.13). This binding is temporary and appears to be broken when the sperm become capacitated. Moreover, the life span of the sperm is significantly lengthened by this binding, and its capacitation is slowed down. This restriction of sperm entry into the ampulla, the slowing down of capacitation, and the expansion of sperm life span may have very important consequences. First, this binding may function as a block to polyspermy by preventing many sperm from reaching the egg at the same time. If the isthmus is excised in cows, a much higher rate of polyspermy results. Second, slowing the rate of sperm capacitation and extending the active life of sperm may maximize the probability of there being some sperm in the ampulla to meet the egg if ejaculation does not occur at the same time as ovulation.

Hyperactivation and Chemotaxis

Different regions of the female reproductive tract may secrete different, regionally specific molecules. These factors may influence sperm motility as well as capacitation. For instance, when sperm of certain mammals (especially hamsters, guinea pigs, and some strains of mice) pass from the uterus into the oviducts, they become hyperactivated, swimming at higher velocities and generating greater force than before. Suarez and co-workers (1991) have shown that while this behavior is not conducive to traveling in low-viscosity fluids, it appears to be extremely well suited for linear sperm movement in the viscous fluid that sperm might encounter in the oviduct.

In addition to increasing the activity of sperm, soluble factors in the oviduct may also provide the directional component of sperm movement. There has been speculation that the ovum (or, more likely, the ovarian follicle in which it developed) may secrete chemotactic substances that attract the sperm toward the egg during the last stages of sperm migration (see Hunter 1989). Ralt and colleagues (1991) tested this hypothesis using follicular fluid from human follicles whose eggs were being used for in vitro fertilization. Performing an experiment similar to the one described earlier with sea urchins, they microinjected a drop of follicular fluid into a larger drop of sperm suspension. When they did this, some of the sperm changed their direction to migrate toward the source of follicular fluid. Microinjection of other solutions did not have this effect. These studies did not rule out the possibility that the effect was due to a general stimulation of sperm movement or metabolism. However, these investigations uncovered a fascinating correlation: the fluid from only about half the follicles tested showed a chemotactic effect, and in nearly every case, the egg was fertilizable if, and only if, the fluid showed chemotactic ability (P < 0.0001). It is possible, then, that like certain invertebrate eggs, the human egg secretes a chemotactic factor only when it is capable of being fertilized.

The female reproductive tract, then, is not a passive conduit through which the sperm race, but a highly specialized set of tissues that regulate the timing of sperm capacitation and access to the egg.