Role of zona pellucida glycoproteins in gamete recognition

Recent advances in many areas of reproductive technology, such as intracytoplasmic sperm injection (ICSI) and other forms of assisted fertilization, have been rapid. In many respects, these advances have outstripped our knowledge of the fundamental processes of human and animal sperm–egg interactions at fertilization. One of the major unresolved issues is the identity of the complementary molecules on the mammalian sperm surface involved in the early interactions with the extracellular matrix of the egg (the zona pellucida) and this review focuses on this controversial area. Other recent reviews on this topic include: Snell and White (1996), Brewis and Moore (1997), Benoff (1997) and McLeskey et al., (1998). For general reviews on the mammalian sperm acrosome reaction and fertilization, see: Yanagimachi (1994) and Allen and Green (1997). When the fertilizing spermatozoon reaches the egg (ovum), it must first penetrate the surrounding cumulus mass or oophorus, consisting of follicular cells dispersed in a polymerized hyaluronic acid matrix (Yanagimachi, 1994). In most mammals that have been studied, an acrosome-intact spermatozoon usually makes contact with the egg to undergo a specific gamete recognition process. Gamete recognition is mediated by complementary molecules associated with the extracellular matrix of the oocyte (the zona pellucida) and the plasma membrane over the sperm head. Subsequent to initial (primary) sperm–zona binding, the fertilizing spermatozoon undergoes the acrosome reaction. The acrosome reaction is a major exocytotic event over the entire apical region of the sperm head, characterized by multiple-point fusions between the outer acrosomal membrane and the overlying plasma membrane, which exposes the acrosomal contents of the spermatozoon and its resistant inner acrosomal membrane (Yanagimachi, 1994; Allen and Green, 1997). Acrosomal exocytosis is essential for fertilization and, once acrosome-reacted, the fertilizing spermatozoon undergoes secondary sperm–zona binding before penetration of the zona pellucida (Fig. 1). After passing through the zona, the fertilizing spermatozoon fuses with the egg membrane (oolemma), then enters the egg to enable pronuclear formation and syngamy to occur.