Mammalian fertilization as a biological machine: a working model for adhesion and fusion of sperm and oocyte.

This review argues that the phenomena of sperm adhesion to the mammalian oocyte and their subsequent fusion can be better understood in the light of recent developments in cell biology. Cell-cell adhesion is mediated, in general terms, by adhesion molecules and their counter-receptors on adhering membranes, and membrane fusion by fusion molecules. The application of these ideas to mammalian fertilization is examined in detail. It is argued that the adhesion of sperm and oocyte is a complex problem which involves not only mutual recognition, but possibly early reorganization of the oocyte cytoskeleton. Mechanisms for activating cytoskeletal reorganization in the oocyte are identified. The site of fusion on the sperm head is discussed, together with the evidence that PH-30 is the fusion protein. It is suggested that current evidence supports the view that PH-30 is an adhesion molecule which is potentially capable of contributing to the reorganization of the oocyte cytoskeleton at the site of sperm attachment, but that PH-30 may not be the fusion molecule. It is argued that an appropriate conceptual framework in which to view sperm-oocyte adhesion and fusion is that of a biological machine into which the role of individual molecular components can be fitted as they become identified. It is hoped that this framework, together with verifiable ideas taken from other areas of cell biology, may facilitate an understanding of this key area of fertilization.