The Questions of Developmental Biology

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.