Oocyte maturation: a story of arrest and release.

The release of a mature healthy egg for fertilization is the center of the entire reproductive process. From the time of embryonic development till fertilization, the oocyte undergoes several stop-and-go periods. In most animals, oocytes are held in meiotic arrest in prophase I prior to ovulation. The ovulatory luteinizing hormone (LH) surge promotes the resumption of meiosis of the arrested oocytes and their progression through the second meiotic cycle, only to be arrested again at metaphase II until fertilization. This review addresses the underlying mechanisms involved in maintaining the oocyte in meiotic arrest as well as the signaling pathways responsible for releasing it from the arrested phase just prior to ovulation until the completion of meiosis at the time of fertilization.