ABSTRACT
Resumption of meiosis in the oocyte is an essential prelude for successful fertilization. The meiotic division of the mammalian oocyte is initiated during fetal life. It proceeds up to the diplotene stage of the first prophase and arrests at birth. The chromatin in the meiotically arrested oocytes is decondensed and a nuclear structure known as germinal vesicle (GV) is present (Fig 8.1a). Meiotic arrest persists throughout infancy until the onset of puberty. In a sexually mature female, at each cycle one or more oocytes, according to the species, reinitiate the meiotic division. Upon reinitiation of meiosis the GV disappears (Fig 8.1b), the chromosomes recondense and align on the newly formed meiotic spindle, and the pairs of homologous chromosomes segregate between the oocyte and the first polar body (Fig 8.1c). Emission of the first polar body, which represents the completion of the first round of meiosis, is immediately followed by the formation of the second meiotic spindle with the remaining set of homologous chromosomes aligned on its equatorial plate. The whole series of events, initiated by GV breakdown (GVB) and completed at the metaphase of the second round of meiosis (MII), leads to the production of a mature fertilizable oocyte, also known as an egg. The egg is arrested at MII and will complete the meiotic division only after penetration of the spermatozoon.1
