The molecular players of sperm–egg fusion in mammals

Fertilization includes a series of cellular interactions culminating with the fusion of gamete membranes, creating a zygote. Two ADAM proteins present on sperm, fertilin β and cyritestin, drew much attention. However, gene deletion in mice showed that fusion can happen in their absence. The presence of the integrin α6β1 on egg, a putative fertilin β receptor, is also dispensable. In contrast, sperm lacking Izumo, a molecule with a single Ig domain, are unable to fuse. On the egg side, a role for GPI-anchored molecules has been shown, and in mice lacking both tetraspanins CD9 and CD81 fertilization is completely blocked.