Signal transduction pathways in activation of the mammalian egg.

The mature egg arrested at its second metaphase is a rather quiet cell, and possesses a pre-set developmental program. A sequence of biochemical events that leads to the rapid embryonic mitotic divisions initiates at fertilization when the spermatozoon overcomes this second cell cycle arrest. These events induce a transient rise in intracellular Ca2+ concentration ([Ca2+]i) that leads to the cortical granule exocytosis (CGE) and resumption of meiosis. Various treatments can induce parthenogenetic activation as manifested by the extrusion of the second polar body or CGE. Similar to somatic cells, recent studies in mammalian eggs suggest that signal transduction pathways mediate egg activation. The initial increase in ([Ca2+]i) appears to be critical for egg activation. Other messengers such as protein kinase C and protein tyrosine kinases, were suggested as possible inducers of some aspects of egg activation.