Mechanism of Ca2+ release at fertilization in mammals

At fertilization in mammals the sperm triggers a series of oscillations in intracellular Ca2+ within the egg. These Ca2+ oscillations activate the dev elopment of the egg into arm embryo. It is not known how the sperm triggers these Ca2+ oscillations. There are currently three different theories for Ca2+ signaling in eggs at fertilization. One idea is that the sperm acts as a conduit for Ca2+ entry into the egg after membrane fusion. Another idea is that the sperm acts upon plasma membrane receptors to stimulate a phosph olipase C (PLC) within the egg which generates inositol 1,4,5-trisphosphate (InsP(3)). We present a third idea that the sperm causes Ca2+ release by i ntroducing a soluble protein factor into the egg after gamete membrane fusi on. In mammals this sperm factor is also referred to as an oscillogen becau se, after microinjection, the factor causes sustained Ca2+ oscillations in eggs. Our recent data in sea urchin egg homogenates and intact eggs suggest s that this sperm factor has phospholipase C activity that leads to the gen eration of InsP(3). We then present a new version of the soluble sperm fact or theory of signaling at fertilization. (C) 1999 Wiley-Liss, Inc.