Sperm-egg adhesion and fusion in mammals

Fertilisation is an orchestrated, stepwise process during which the participating male and female gametes undergo irreversible changes, losing some of their structural components while contributing others to the resultant zygote. Following sperm penetration through the egg coat, the sperm plasma membrane fuses with its oocyte counterpart, the oolemma. At least two plasma membrane proteins essential for sperm-oolemma fusion--IZUMO and CD9 on the male and female gametes, respectively--have been identified recently by classical cell biology approaches and confirmed by gene deletion. Oolemma-associated tetraspanin CD81, closely related to CD9, also appears to have an essential role in fusion. Additional proteins that may have nonessential yet still facilitating roles in sperm-oolemma adhesion and fusion include oolemma-anchored integrins and oocyte-expressed retroviral envelope proteins, sperm disintegrins, and sperm-borne proteins of epididymal origin such as CRISP1 and CRISP2. This review discusses these components of the gamete fusion mechanism within the framework of gamete structure, membrane biology, cell signalling and cytoskeletal dynamics, and revisits the topic of antipolyspermy defence at the oolemma level. Harnessing the mechanisms of sperm-egg fusion is of importance to animal biotechnology and to human assisted fertilisation, wherein male patients with reduced sperm fusibility have been identified.