Sperm factor: what is it and what does it do?

There are two current hypotheses as to how the spermatozoon triggers the oocyte into activity; a transmembrane receptor mechanism involving G-proteins and a soluble sperm-factor mechanism. In this short review we show that the present data favours the idea of a soluble factor diffusing from the spermatozoon into the oocyte following plasma membrane fusion of the two gametes that triggers calcium release in the oocyte. Two categories of calcium release mechanisms, inositol-1,4,5-triphosphate-induced calcium release (IICR) and calcium induced calcium release (CICR) are found in oocytes from a variety of species and both appear to be activated at fertilization. Since these calcium release pathways are distinct it is possible that sperm cytosol contains more that one activating factor. Finally, the fact that sperm extracts 'activate' oocytes from different phyla and trigger calcium oscillations in somatic cells infers calcium releasing agents common to other cell types.