Transitioning from egg to embryo: triggers and mechanisms of egg activation.

The transition from mature oocyte to developing embryo requires a coordinated series of events, collectively known as egg activation. Egg activation includes changes to egg coverings to prevent polyspermy, release of oocyte meiotic arrest, generation of haploid female and male pronuclei, changes in maternal mRNAs and protein populations, and cytoskeletal rearrangements. In many animals, egg activation is triggered by fertilization, which increases intracellular calcium within the oocyte and thereby regulates molecular events of egg activation. In other animals, fertilization-independent external signals, including mechanical stimulation of eggs and/or changes in ionic milieu, trigger activation. Recent studies have clarified the upstream portion of pathways leading to eggshell changes and cell cycle resumption and have identified activation-induced changes in maternal mRNA and protein profiles that can identify molecular players in the downstream events of egg activation. We review signals that trigger activation and how they link to subsequent molecular events of egg activation.