The Molecules of Mammalian Fertilization

in some, ZP3 homologs are implicated as the key adhesion molecules, whereas in others, ZP2 homologs may play a dominant role (Topfer-Petersen et al., 1993). Although O-linked oligosaccharides on ZP3 are Dallas, Texas 75235 † Department of Cell Biology strongly implicated as being responsible for both sperm binding and signaling for the acrosome reaction, the University of Virginia Health Sciences Center composition and structure of the essential carbohydrate moieties remain controversial. Compelling evidence Charlottesville, Virginia 22908 supports roles for both a terminal Gal residue in ␣ or ␤ linkage (Wassarman and Litscher, 1995) as well as for Fertilization proceeds as a well-choreographed cascade of cell–cell interactions between gametes of opposite a terminal N-acetylglucosamine (GlcNAc) in ␤ linkage This review focuses on mammalian fertilization (primarily cient in the glycosyltransferase that amends terminal Gal in ␣ linkage are fully fertile (Thall et al., 1995) points in the mouse) and in particular on proteins involved in the cell–cell adhesion, fusion, and signaling events that to Gal in ␤ linkage or GlcNAc (or both) as critical moieties. Moreover, even though in vitro experiments have failed ensue when sperm meets egg. It is noteworthy that although sperm and eggs are highly specialized, many to show more than a passive role for the ZP3 polypep-tide, intact ZP3 exhibits more effective sperm binding of the mechanisms and some of the players in other cell adhesion and signaling reactions appear to be utilized activity than purified ZP3 glycopeptides, which themselves are more effective than simple O-linked oligosac-in the dance of the gametes. Following their deposition and modification (" capaci-charides. Thus, it remains to be established if the ZP3 polypeptide plays a direct role in adhesion, or if it simply tation ") in the female reproductive tract (Yanagimachi, 1994), mammalian sperm chart a tortuous path, and influences the " presentation " of the oligosaccharides Adhesion/Signaling Molecules gaining access to the egg surface, the remaining steps Having been identified as a key adhesion molecule on of fertilization can ensue. In mammals, sperm–egg inter-mouse eggs, ZP3 has become the lens through which action can be subdivided into six steps (Figure 1): acro-many investigators have sought to identify sperm sur-some-intact sperm first bind to the zona pellucida (ZP) face proteins responsible for ZP adhesion. Several fea-(step 1), a thick extracellular matrix comprising the outer tures of ZP3 have helped to define criteria that sperm layer of the egg; interaction with the ZP …