Oocyte activation in invertebrates and humans

Oocytes are metabolically repressed cells that, in most cases, require a signal from the fertilising spermatozoon to trigger activation. Physiological competence of the activated oocyte, a prerequisite for early embryogenesis, depends, therefore, not only on the quality of the oocyte, but also on the quality of the spermatozoon (Dale, 1983; Tosti & Dale, 1992). There are two current opinions as to how the spermatozoon triggers the oocyte into metabolic activity (Dale & DeFelice, 1990). The first hypothesis is that following gamete fusion the spermatozoon releases a soluble activating factor into the oocyte cytoplasm that is then amplified globally by second messengers in the oocyte. The second is that sperm binding to receptors spanning the oocyte plasma membrane is the signal that is transduced to the second messenger system of the oocyte (Swann, 1993; Foltz & Shilling, 1993).