ABSTRACT
INTRODUCTION Resumption of meiosis in the oocyte is an essential prelude to successful fertilization. e meiotic division of the mammalian oocyte is initiated during fetal life, proceeds up to the diplotene stage of the rst prophase, and arrests at birth. Meiotic arrest persists throughout childhood 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. e chromatin in the meiotically arrested oocytes is encapsulated by a nuclear structure known as the germinal vesicle (GV; Figure 8.1a). e GV in oocytes resuming meiosis disappears (Figure 8.1b), the condensed chromosomes align on the newly formed meiotic spindle, and the pairs of homologous chromosomes segregate between the oocyte and the rst polar body (Figure 8.1c). Emission of the rst polar body, which represents the completion of the rst round of meiotic division, is immediately followed by the formation of the second meiotic spindle with the remaining set of homologous chromosomes aligned on its equatorial plate. e 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. e egg is arrested at MII and will complete the meiotic division only aer the penetration of the spermatozoon (1). e physiological stimulus for re-initiation of meiosis is provided by the preovulatory surge of luteinizing hormone (LH) (2). Once oocyte maturation is completed, LH further induces ovulation, during which the follicle releases the mature oocyte that is picked up by the infundibular mbria of the oviduct.
