Calcium oscillations, sperm factors and egg activation at fertilisation

When an egg is fertilised by sperm, the first intracellular signalling event observed is a large transient increase in cytoplasmic free Ca2+ ions. Elevated Ca2+ is known to play a vital role as an intracellular messenger in all cells and the Ca2+ signal occurring in the egg at fertilisation triggers the subsequent events that mediate early embryo development. In mammalian eggs, the Ca2+ response is first observed as a Ca2+ wave that initiates near the point of sperm-egg fusion, spreads across the entire egg, and then continues as a series of intracellular Ca2+ oscillations. The way in which the fertilising sperm generates the Ca2+ response in the egg has been the subject of much debate over recent years. One proposal for which there is growing evidence suggests the mechanism of egg activation at fertilisation involves the introduction of a soluble sperm protein into the egg shortly after sperm-egg fusion.