Processes controlling sperm-egg fusion.

Sperm interaction with the egg envelopes triggers the acrosome reaction. Indeed, sperm-egg fusion is accomplished by the fusion of the acrosomal process (or of the exposed inner acrosomal membrane in mammals) with the egg plasma membrane. Fusion must be preceded by the establishment of molecular contact between the two membranes. It is suggested that, as in the case of artificial phospholipid membranes, the two major obstacles to the establishment of molecular contact are electrostatic repulsion and the hydration barrier. It is argued that morphology of the acrosome is such as to favour the overcoming of such barriers. By analogy with the conditions governing fusion of artificial phospholipid membranes and cell fusion, it is proposed that the following processes play a role in sperm-egg fusion. The large calcium uptake accompanying the acrosome reaction may help fusion either through the known effect of calcium on fusion of phospholipid membranes or by shielding the surface charges of the acrosomal process. Fusogenic proteins at the surface of the acrosomal process are likely to play a role in the fusion of the acrosomal process with the egg plasma membrane. The activation of phospholipases in conjunction with the acrosome reaction may also be instrumental in sperm-egg fusion through the transient production of lysophosphatides. Clearance or translocation of intramembraneous proteins in the egg plasma membrane at the site of contact with the acrosomal process may also be required for fusion. Lastly it is suggested that a translocation or a conformational change of some proteins of the egg plasma membrane, which is required for fusion, may be induced by the depolarization of the egg plasma membrane that follows molecular contact with the acrosomal process.