The cortical reaction and development of activation competence in mammalian oocytes

Blocks to polyspermic fertilization are necessary to prevent the incorporation of two sperm nuclei into a zygote's genome, which would result in abnormal development. Many mammalian eggs utilize both an extracellular zona pellucida block to polyspermy and a plasma membrane block. Although little is known about the plasma membrane block in mammals, fertilization results in zona glycoprotein modifications caused by enzymes released by the egg and its cortical granules (CG). This article reviews other recent investigations demonstrating that the oocyte's ability to cause CG release and the block to polyspermy develops near the time of ovulation. The development of normal "activation competence' is likely to involve preovulatory changes in the oocyte's ability to signal the release of intracellular calcium as well as to respond to this calcium increase, resulting in CG exocytosis. Because normal activation competence appears to have a brief temporal window after oocyte meiotic maturation is resumed and since the oocytes are collected at various stages in assisted reproductive procedures, these studies are relevant to optimizing clinical success.