Mammalian oocytes enclosed in the ovary are arrested at the diplotene stage of the first meiotic prophase, which is also termed the germinal vesicle (GV) stage. Following proper stimulation (gonadotropin in mammals), the fully grown oocytes reinitiate meiosis. The resumption of meiotic maturation is characterized by germinal vesicle breakdown (GVBD), followed by chromatin condensation, microtubule organization, and emission of first polar body. Then, the oocyte meiosis arrests again at metaphase II (MII) (1 ), which is released by fertilization or parthenogenetic stimulation and followed by the completion of second meiotic division.
