RECENT PROGRESS IN THE STUDY OF EARLY EVENTS IN MAMMALIAN FERTILIZATION*

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Abstract
During the past 25 yr, great advances have been made in understanding the physiology, morphology and biochemistry of fertilization in invertebrate animal species. In contrast, there is a paucity of knowledge pertaining to mammalian fertilization. Major areas in which information is lacking are the nature of changes undergone by spermatozoa in preparation for fertilization, and the mechanisms involved in sperm penetration of the egg investments. The present state of knowledge of these events is outlined, and the weaknesses of some current concepts are evaluated. Fertilization of mammalian eggs in vitro seems an attractive method for studying gamete interaction, but experience has shown that numerous problems are associated with this technique. As a result, the information on mammalian fertilization derived from studies conducted in vitro has fallen short of expectations; some factors contributing to this discrepancy are described. Recent findings concerning the regulation of sperm motility and fertilizing ability seem to have considerable significance for mammalian fertilization in vivo and in vitro. These findings were utilized to refine existing procedures; fertilization of hamster eggs in vitro was accomplished in the presence of numbers of spermatozoa comparable to those believed to be present at the site and time of fertilization in vivo. This improved technique, by more closely approximating the physiological situation, may substantially assist the derivation of useful information from in vitro fertilization studies.