||After work published
in Gilkey, J.C., Jaffe, L.F. Ridgway, E.G., & Reynolds, G.T. (1978) A free
calcium wave traverses the activating egg of the medaka, Oryzias latipes.
J. Cell Biol. 76: 448-466
A transient rise in the intracellular calcium ion concentration: (1) helps prevent more than one sperm fertilizing an egg; and (2) plays a role in "activating" a fertilized egg. In fish, frogs, sea urchins, and mammals, when sperm and egg make contact, there is a fast and a slow "block" preventing more than one sperm fertilizing an egg (polyspermy). The rapid, transient, influx of sodium ions depolarizes the egg's membrane potential establishes the fast block to polyspermy. The slower, permanent, block to polyspermy is established by the fusion of the cortical vesicles with the plasma membrane. This vesicle fusion is driven by the increase in the cytosolic calcium ions due to the release of calcium ions from the egg's endoplasmic reticulum (ER). The release of calcium ions also activates the fertilized egg to increase its cellular respiration and to begin dividing (cleavage). The wave of release of calcium ions sweeps across the egg (tsunami).
What you see in the sequences below is the calcium ion wave in transparent fish eggs (Oryzias latipes). To see the wave of release of calcium ions in eggs, they are injected with aequorin, a protein that emits light when it binds to calcium ions. In the first two sequences, the position of the micropyle (where sperm enter to fertilize an egg) is noted. In the last sequence, an unfertilized egg with aequorin is exposed to an ionophore that makes the egg permeable to calcium ions in the medium. All sequences are approximately twice as fast as they actually occur.