Second messengers at fertilization in sea-urchin eggs

The activation of the sea-urchin egg at fertilization is triggered by two ionic changes in the egg cytoplasm. First there is a wave of increase in cytoplasmic free calcium concentration (Ca2+i) that travels across the egg within the first minute of fertilization. The calcium wave is followed by a sustained rise in intracellular pH (pHi). Fertilizing spermatozoa cause these two ionic changes by stimulating the hydrolysis of phosphatidylinositol 1,4,5-bisphosphate (PtdInsP2). PtdInsP2 hydrolysis produces inositol 1,4,5-trisphosphate, which releases calcium from intracellular stores, and diacylglycerol, which stimulates a plasma membrane sodium/hydrogen antiporter causing the rise in pHi. A positive feedback cycle of intracellular calcium release and InsP3 production appears to be part of the mechanism by which the calcium wave is propagated. It has been suggested that the spermatozoon triggers the calcium wave by acting on a cell surface receptor that couples to the egg phosphoinositidase C via a G-protein. We discuss evidence for an alternative hypothesis of egg activation in which a spermatozoon triggers the calcium wave by introducing a soluble activating factor into the egg cytoplasm.