9 - Gamete Fusion in Mammals

Abstract

This chapter gives a preview of gamete fusion in mammals. Sperm–egg fusion is the culmination of gametogenesis. The known topology of sperm–egg fusion suggests that there is considerable specificity as to which regions of the sperm and egg plasma membrane can fuse. The fertilizing sperm makes initial contact, via its inner acrosomal membrane (IAM), with the microvillar surface of the egg. The region of the egg plasma membrane that has a low concentration of microvilli, the cortical granule-free region, generally does not participate in sperm binding or fusion. The inner acrosomal membrane of mammalian sperm does not fuse with the egg plasma membrane, but is taken up into the egg cytoplasm, along with a piece of the egg plasma membrane. The engulfment of the inner acrosomal membrane has been described as a type of phagocytotic process. A major difference from classical phagocytosis in terms of the morphology is that the “phagosome” is surrounded by a hybrid membrane that includes sperm IAM and egg plasma membrane. In mammals various somatic cell types also exhibit cell-cell fusion. These include fusion of cytotrophoblasts to produce a protective nutritive layer in the placenta; fusion of myoblasts to form myotubes; fusion of monocytes to form osteoclasts; and fusion of macrophages to produce “multinucleated giant cells”. In general, less is known about these somatic cell-cell fusion systems than about gamete fusion. This chapter concludes that new strategies, particularly gene knockout and structural analysis of sperm protein–egg protein complexes, may prove to be effective in providing deeper insight into the molecular basis of gamete fusion.