Calcium and the control of mammalian cortical granule exocytosis

At fertilization, the release of intracellular calcium is necessary and sufficient for most, if not virtually all, of the major events of egg activation that are responsible for the onset of embryonic development. In mammalian eggs, repetitive calcium oscillations stimulate egg activation events through calcium-dependent effectors, such as calmodulin, protein kinases, and specific proteins involved in exocytosis. One of the earliest calcium-dependent events is the exocytosis of cortical granules (CGs), a secretory event resulting in the block to polyspermy and the prevention of triploidy. Emerging studies suggest that CG release in mature eggs is dependent upon calcium-dependent proteins similar to those in somatic cells employed to undergo calcium-regulated exocytosis. In contrast, pre-ovulatory oocytes are incompetent to undergo CG exocytosis due to deficiencies in the ability to release and respond to increases in intracellular calcium. The development of competence to release and respond to calcium is relevant to both animal and human in vitro fertilization programs that largely utilize ovarian oocytes not all of which are fully activation competent.