Calcium and fertilization: The beginning of life

The explosive increase in Ca ²⁺ that occurs in the cytosol at fertilization is brought about by the activation of Ca ²⁺-release channels in the intracellular stores. Inositol 1,4,5-trisphosphate (InsP ₃) is traditionally considered to be the messenger that initiates the increase and spreading of the activating Ca ²⁺ wave. In line with this hypothesis, recent evidence suggests that the penetrating sperm delivers into mammalian eggs a novel isoform of phospholipase C (PLC), which promotes the formation of InsP ₃. By contrast, data from echinoderms studies indicate that the newly discovered second messenger nicotinic adenine dinucleotide phosphate (NAADP) promotes an initial, localized increase in Ca ²⁺, which is then followed by the InsP ₃-mediated globalization of the Ca ²⁺ wave. The mechanism by which the interacting sperm triggers the production of NAADP and subsequently that of InsP ₃ remains obscure.