Calcium and the control of mammalian cortical granule exocytosis

At fertilization, the release of intracellular calcium is necessary and suf ficient for most, if not virtually all, of the major events of egg activati on that are responsible for the onset of embryonic development. In mammalia n eggs, repetitive calcium oscillations stimulate egg activation events thr ough calcium-dependent effectors, such as calmodulin, protein kinases, and specific proteins involved in exocytosis. One of the earliest calcium-depen dent events is the exocytosis of cortical granules (CGs), a secretory event resulting in the block to polyspermy and the prevention of triploidy. Emer ging studies suggest that CG release in mature eggs is dependent upon calci um-dependent proteins similar to those in somatic cells employed to undergo calcium-regulated exocytosis. In contrast, pre-ovulatory oocytes are incom petent to undergo CG exocytosis due to deficiencies in the ability to relea se and respond to increases in intracellular calcium. The development of co mpetence to release and respond to calcium is relevant to both animal and h uman in vitro fertilization programs that largely utilize ovarian oocytes n ot all of which are fully activation competent.