[Physiological mechanisms of oocyte activation]

In all species, eggs are activated at fertilization by an increase in intracellular free Ca2+. In frogs, fishes and echinoderms the increase in cytoplasmic free calcium takes the form of a single explosive wave while, in mammals the spermatozoon triggers a series of calcium oscillation that last for several hours. A major unresolved is determination of the mechanism that the spermatozoon uses to trigger this calcium increase in the egg at fertilization. This review will recall the various hypothesis on the signal transduction system and present the recent argument for a soluble sperm cytosolic factor.