Oocyte Metaphase Arrest and Release: Triggers and Pathways

Release of a mature egg is an important pre-requisite of mammalian reproduction.  Starting from the time when oogenesis begins during fetal development, to the time of puberty and until fertilization, the oocyte encounters several stop-and-go periods. Altered pathways may affect fertilization outcome. Understanding of the stop-and-go periods is based on exploring the underlying mechanisms. This review aimed at addressing the triggers and signalling pathways leading to oocyte meiotic II resumption. A detailed literature search was done for studies on various databases such as Google, PubMed/MEDLINE, Cochrane reviews etc.with related keywords such as oocyte meiotic release, oocyte metaphase arrest, oocyte activation, sperm oocyte activating factors etc. Selected studies were reviewed by two observers. All obtained information was analysed and was shaped into manuscript appropriately. It was explained that till ovulation begins oocytes are held in meiotic arrest in diplotene stage of prophase I. Just before ovulation luteinizing hormone (LH) surge directs resumption of meiosis I. The arrested primary oocytes complete meiosis I and progresses through second meiotic division and gets arrested again at metaphase II (MII) until fertilization. Results concluded that the MII arrest persists till sperm enters the oocyte and releases its activating factors into the ooplasm. These activating factors are the triggers unlocking the subsequent pathways releasing oocyte from meiotic arrest.