Passage V

When a sperm fertilizes a sea urchin egg, the egg's cortical granules (spherical structure, each bound by a membrane) fuse with the egg's plasma membrane. The contents of the granules, which include Ca²⁺ (unbound calcium ions), are released outward into the area between the plasma membrane and the vitelline layer (the egg's outer layer) in a process called the cortical reaction. The vitelline layer then expands to form the fertilization membrane. The fertilization membrane prevents other sperm from fusing with the egg (see figure 1).

Two experiments were done to study the hypothesis that the sperm causes the egg to release Ca²⁺ into the cytoplasm, which then triggers the cortical reaction.

Experiment 1

Three suspensions of unfertilized sea urchin eggs in seawater containing no Ca²⁺ were prepared. Aequorin, a substance that luminesces (shines) in the presence of Ca²⁺, was injected into the eggs' cytoplasm. Sea urchin sperm or an ionophore (a substance that can enter an egg and transport Ca²⁺ across membranes) was then added to 2 of the suspensions. Eggs were examined for luminescence in the cytoplasm and for the presence of a fertilization membrane (see Table 1).

Experiment 2

Experiment 1 was repeated except that both aequorin and a substance called EDTA (which binds with Ca²⁺, keeping it from reacting with other substances) were injected (see Table 1).