Developmental Control of Oocyte Maturation and Egg Activation in Metazoan Models

Meiotic arrests during oocyte development. Oocytes from most species undergo a first arrest at prophase I (PI) that is maintained for a few days (Drosophila) or for decades (humans). On hormonal or developmental stimulation, oocytes undergo meiotic maturation, release the primary arrest, and enter a second arrest at metaphase I (MI), metaphase II (MII), or postmeiotic G₁ depending on the species. Fertilization triggers release from the second arrest and the completion of meiosis in vertebrates (Xenopus and mammals). Drosophila releases the secondary arrest in a sperm-independent manner. GV, germinal vesicle; PI, prophase I; GVBD, germinal vesicle breakdown; MI, metaphase I; PB1, polar body 1; MII, metaphase II; PB2, polar body 2. (Adapted from Sagata 1996; reprinted with express permission from Noriyuki Sagata.)