Osmond J. D'Cruz, Ph.D

Section of Reproductive Endocrinology and Infertility, Department of Obstetrics and Gynecology, University of Oklahoma Health Sciences Center, Oklahoma City, OK 73190

2. INTRODUCTION

Physiologic fertilization in humans results from a highly ordered sequence of events. The major events include gamete maturation, regulated sperm transport, sperm-oocyte recognition, sperm-zona penetration, and sperm-oocyte fusion. Sperm maturation involves capacitation and acquisition of a hyperactive state, and expression of cell-cell adhesive ligands. Binding of capacitated sperm to zona pellucida (ZP) glycoproteins, which is a species-specific event, results in sperm activation and induction of acrosome reaction - an exocytotic event that releases hydrolytic enzymes from the acrosome. Proteolytic and hydrolytic enzymes released during the course of the acrosome reaction then facilitate the penetration of the fertilizing sperm through the ZP and into the perivitelline space. Finally, acrosome-reacted sperm bind to and fuse with the oocyte plasma membrane (oolemma) during fertilization. Following fusion of the sperm head and oolemma at discrete sites, sperm and egg pronuclei are formed, and fusion of both pronuclei results in the completion of fertilization, activation of the oocyte, and the initiation of embryonic development (1-2).

Despite four decades of research on the biology of fertilization carried out in a variety of laboratory animal models and more recently in human in vitro fertilization (IVF) systems, much of the biochemical and molecular aspects of this recognition process remains unknown. It is now thought that several complementary mechanisms may be involved in sperm-oocyte recognition and fusion and that the recognition signals on the oocyte reside on the ZP as well as on the oolemma (3-4).