Contact and Recognition between Egg and Sperm

Contact and Recognition between Egg and Sperm
Most marine invertebrates and many marine fishes simply release their gametes into the ocean. Although an egg is a large target for a sperm, the enormous dispersing effect of the ocean and limited swimming range of a spermatozoon conspire against an egg and a sperm coming together by chance encounter. To improve likelihood of contact, eggs of numerous marine species release a chemotactic factor that attracts sperm to the egg. The chemotactic molecule is speciesspecific, attracting to the egg only sperm of the same species.

In sea urchin eggs, sperm first penetrate a jelly layer surrounding the egg, then contact the egg’s vitelline envelope, a thin membrane lying just above the egg plasma membrane (Figure 8-3). At this point, egg-recognition proteins on the acrosomal process of the sperm (Figure 8-4) bind to speciesspecific sperm receptors on the vitelline envelope. This mechanism ensures that the egg will recognize only sperm of the same species; all others are screened out. This is important in the marine environment where many closely related species may be spawning at the same time. Similar recognition proteins have been found on the sperm of vertebrate species (including mammals) and presumably are a universal property of animals.

Figure 8-4 Sequence of events during sperm contact and penetration of a sea urchin egg.