THE MAMMALIAN ACROSOME REACTION - GATEWAY TO SPERM FUSION WITH THE OOCYTE

Mammalian sperm undergo discharge of a single, anterior secretory gran ule following their attachment to the zona pellucida surrounding the o ocyte. This secretory discharge is known for historical reasons as the acrosome reaction. It fulfils a number of purposes and without it, sp erm are unable to penetrate the zona pellucida and fuse with the oocyt e. In this review, we focus on the role of the acrosome reaction in th e development of fusion competence in sperm. Any naturally occurring m embrane fusion has two major sequential steps: a docking or adhesion s tep, in which two membranes adhere, and a fusion step, in which their lipid bilayers are destabilized and merged and a cellular compartment is either created or destroyed. Recent evidence suggests that there is an important role for oocyte integrins and sperm-bound disintegrins i n mammalian sperm/oocyte adhesion and fusion. The fusion mechanism emp loyed by sperm remains poorly understood, however, and circumstantial evidence suggests it is more complex than the interaction between a si ngle protein species and its target. Sperm/oocyte fusion is probably t he most accessible eukaryotic model for intercellular fusion currently available, partly because it is temporally separated from gene expres sion. Elucidation of the mechanism of sperm/oocyte fusion may throw li ght on the mechanism of other intercellular fusions such as myoblast f usion, and the evolutionary relationship of intercellular membrane fus ion to intracellular membrane fusion.