Sperm penetration of the ZP and entrance into the perivitelline space are likely achieved by a combination of enzymatic cleavage of the glycoprotein filaments around the spermatozoon and enhanced forward mechanical thrusting by hyperactive beating of the tail or ratcheting movement of the spermatozoon. Possible digestive enzymes include those released during acroso-mal exocytosis (e.g., nonacrosin proteases, serine protease homologues, and glycosidases), a variant of acrosin, or novel enzymes (e.g., hydrolases). For years, acrosin was considered to be the primary proteolytic enzyme to digest the ZP, but results show that acrosin-knockout mice are fertile, and proacrosin/acrosin is associated with inner acrosome membrane-mediated sperm binding to the ZP (e.g., ZP2). In boar spermatozoa, the major role of acrosin is to regulate release of acrosomal matrix proteins during exocytosis, and proacrosin-binding protein is in the acrosome matrix.
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