Significance of the need for sperm capacitation before fertilization in eutherian mammals.

In order to fertilize, the spermatozoa of eutherian mammals must undergo capacitation in the female tract. The subcellular changes involved in capacitation finally seem to permit the influx of Ca2+ required for onset of the acrosome reaction, and they result also in a hyperactivated form of motility. However, why capacitation has appeared as an essential prerequisite for eutherian fertilization is unknown. Both these facets of capacitation may reflect new cellular control mechanisms for regulating the sperm's activities, necessitated by evolutionary change in the oocyte. The first may reflect a loss of the oocyte's stimulation of the acrosome reaction, and the second a concomitant appearance of unusually formidable egg vestments the spermatozoon must penetrate. Coordination of the rate of capacitation appropriate to fertilization in vivo may depend not only on the minimal time ordained for the species, but also on a heterogeneity among subpopulations of spermatozoa within any one sample and the timing of sperm transport to the oviduct.