Abstract
Mammalian embryo development begins when the fertilizing sperm triggers a series of elevations in the oocyte’s intracellular free Ca2+ concentration. The elevations are the result of repeated release and re-uptake of Ca2+ stored in the smooth endoplasmic reticulum. Ca2+ release is primarily mediated by the phosphoinositide signaling system of the oocyte. The system is stimulated when the sperm causes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG); IP3 then binds its receptor on the surface of the endoplasmic reticulum that induces Ca2+ release. The manner in which the sperm generates IP3, the Ca2+ mobilizing second messenger, has been the subject of extensive research for a long time. The sperm factor hypothesis has eventually gained general acceptance, according to which it is a molecule from the sperm that diffuses into the ooplasm and stimulates the phosphoinositide cascade. Much evidence now indicates that the sperm-derived factor is phospholipase C-zeta (PLCζ) that cleaves PIP2 and generates IP3, eventually leading to oocyte activation. A recent addition to the candidate sperm factor list is the post-acrosomal sheath WW domain-binding protein (PAWP), whose role at fertilization is currently under debate. Ca2+ influx across the plasma membrane is also important as, in the absence of extracellular Ca2+, the oscillations run down prematurely. In pig oocytes, the influx that sustains the oscillations seems to be regulated by the filling status of the stores, whereas in the mouse other mechanisms might be involved. This work summarizes the current understanding of Ca2+ signaling in mammalian oocytes.
Highlights
Prior to fertilization, mammalian oocytes are arrested at the metaphase stage of the second meiotic cell division
This review describes the signaling pathway and discusses how it operates in mammalian oocytes to mediate the formation of an embryo, the founder of a new generation
The role of Ca2+ as the trigger of oocyte activation was proved when in medaka oocytes fertilization was shown accompanied by an elevation in the intracellular free Ca2+ concentration (Ridgway et al 1977) and inhibition of this increase in sea urchin eggs blocked changes associated with activation (Zucker and Steinhardt 1978; Whitaker and Steinhardt 1982)
Summary
Mammalian oocytes are arrested at the metaphase stage of the second meiotic cell division. The ovulated oocyte is a highly differentiated cell that, without fertilization, would die within 24–48 h. The fertilizing sperm, provides a stimulus that alleviates the meiotic arrest and activates the oocyte’s developmental program. The content of cortical granules is released into the perivitelline space. This triggers changes in the oocyte’s extracellular matrix, the zona pellucida, to prevent penetration by additional spermatozoa (Jaffe and Gould 1985). The cell cycle resumes, meiosis is completed and after formation of the male and female pronuclei, the activated oocyte ( a 1-cell embryo) enters the first mitotic division. This review describes the signaling pathway and discusses how it operates in mammalian oocytes to mediate the formation of an embryo, the founder of a new generation
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