Contact and Recognition
between Egg and Sperm
|Figure 8-3 Structure of sea urchin egg at the moment of fertilization.
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
|Figure 8-4 Sequence of events during sperm contact and penetration of a sea urchin egg.