The sea urchin egg is 10,000 times bigger than the sperm (Fig. 7.4). It is covered with the jelly coatand the vitelline "membrane", a protein "shell" just outside the plasma membrane. The vitelline membranecontains proteins that bind to sperm of the same species.

Because the egg must supply all of the needs of the embryo until it has developed a mouth and digestive system, it is jammed full of yolkgranules, mitochondria, metabolic precursors, endoplasmic reticulum and protein synthetic machinery. In addition to the nucleus, the sea urchin egg also contains 25-50,000 maternal mRNAspecies that control early development.

Finally, the egg contains a set of secretory vesicles, the cortical granules, lined up just inside the plasma membrane (Fig. 7.6).

When a sperm cell encounters an egg of the same species, components of the jelly coat bind to specific "egg receptors" in the plasma membrane ( Fig. 7.8). This triggers a signal transduction cascaderesulting in a series of events that facilitate fertilization.

First, an influx of calciuminduces the fusion of the plasma membrane with the membrane surrounding the acrosome, releasing a set of hydrolytic enzymes which digest a channel in the jelly coat. This also results in the replacement of the plasma membrane over the front of the sperm by what had previously been the inside of the acrosome vesicle membrane. This exposes a new "egg receptor" to the environment (Fig. 7.10, 7.15).

Actin filaments form at the tip of the sperm head extending the acrosomal process, a long proboscis that penetrates through the jelly to contact the vitelline membrane of the egg. The tip of the acrosomal process binds to the egg plasma membrane (Figure 7.19).

The egg also has specific "sperm receptors" which extend through both the plasma membrane of the egg and the vitelline membrane. They bind to newly exposed surface proteins that had previously been inside the acrosome. This selects for sperm that have undergone the acrosome reaction. Therefore, the two binding events have to occur sequentiallyand help to ensure species-specificity.

The binding of sperm to the receptor triggers a second signal transduction cascade resulting in 1) the opening of Na+ channelscausing a transient depolarization; 2) a wave of "free" Ca++and 3) a rise in intracellular pH.