The spermatozoa of most invertebrates (e.g., sea urchins) and nonmammalian vertebrates (e.g., fishes and amphibians) have full capacity to fertilize eggs upon leaving the testis. Testicular spermatozoa of mammals, on the other hand, do not possess the ability to do so. Their fertilizing capacity develops as they pass through the epididymis (Young, 1931; Nishikawa and Waide, 1952; Blandau and Rumery, 1964; Bedford, 1966; Orgebin-Crist, 1967). This process, apparently “unique” to mammals, is referred to as the epididymal maturation of spermatozoa. Even after their maturation, however, spermatozoa require an additional phase of maturation or capacitation within the female genital tract before they are able to fertilize eggs (Austin, 1951, 1952; Chang, 1951a). Thus, epididymal maturation and capacitation are two extra steps that mammaliam spermatozoa must take before they can effect fertilization. In this chapter, I will discuss how mammalian spermatozoa prepare themselves for fertilization and how the spermatozoa and eggs interact during fertilization. The process and mechanisms of sperm transport in the female genital tract will not be dealt with extensively here. Readers are referred to Bishop (1961, 1969), Blandau (1969), Bedford (1970b, 1972b), Thibault (1972, 1973a), Zamboni (1972), Blandau and Gaddum-Rosse (1974), Hafez and Thibault (1975), Overstreet and Katz (1977), Overstreet and Cooper (1978, 1979b), Overstreet et al. (1978), Shalgi and Kraicer(1978), Cooper et al. (1979) and Hunter (1975, 1980). The rejection or elimination of extra spermatozoa by the fertilized egg, one of the most fascinating events in fertilization, will not be discussed here. Instead, readers are referred to the chapter by Dr. Wolf in this volume. The physiology of egg activation has been described and discussed to some extent by Gwatkin (1977) and Yanagimachi (1978a). There are numerous reviews dealing with general aspects of mammalian fertilization. The following are recommended to aid in grasping the outline of mammalian fertilization: Austin and Bishop (1957), Austin and Walton (1960), Austin (1961, 1968), Blandau (1961), Piko (1969), Thibault (1969), Bedford (1970a,b, 1972b), Gwatkin (1976, 1977), Yanagimachi (1977, 1978a), Bedford and Cooper (1978) and Hunter (1980).

Mechanisms of Fertilization in Mammals

The hemizona assay (HZA): development of a diagnostic test for the binding of human spermatozoa to the human hemizona pellucida to predict fertilization potential * † ‡

Oocytes were obtained from patients with tubal infertility at fixed times after the onset of the endogenous LH rise or hCG injection, and were inseminated immediately after recovery or after periods of 4-4 1/2, 5-5 1/2 and 6-6 1/2 h in culture in vitro. Delayed insemination resulted in a marked increase in the proportion of oocytes that were fertilized and developed to normal embryos and maximum rates occurred after 5-5 1/2 h in culture (0-1/2 h, 26%; 4-4 1/2 h, 50%; 5-5 1/2 h, 89%; 6-6 1/2 h, 69%). The range and mean (+/- s.d.) intervals from insemination for the pronuclear and early cleavage stages were 27-43 (35.6 +/- 4.4) h for 2-cell stages, 36-65 (45.7 +/- 8.3) h for 4-cell stages, 45-73 (54.3 +/- 12.6) h for 8-cell stages and 68-85 h for the 16-cell stage. In 7/50 patients receiving 1 or 2 embryos at the 2-, 4- and 8-cell stages, fetal development was normal and 2 women had twin pregnancies (36% success compared with 8% for single embryos). All pregnancies were from the groups in which insemination was delayed for 5-6 1/2 h. It is concluded that a short period of culture in vitro may allow the completion of oocyte maturation, and improve the results of in-vitro fertilization.

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Effect of delayed insemination on in-vitro fertilization, culture and transfer of human embryos.

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.

James W. Overstreet

Papers

Penetration of the Zona Pellucida of Nonliving Human Oocytes by Human Spermatozoa in Vitro