[Primates in the study of oocyte maturation]

Abstract

In the mammalian oocyte, meiosis is initiated during fetal life. Meiosis proceeds up to the diplotene stage of the first prophase and is arrested at birth and the oocyte presents a nuclear structure known as germinal vesicle (GV). Meiotic arrest persists until sexual maturity, when one or more oocytes, the number dependent on the species, reinitiate their reduction division at each cycle. The series of event, initiated by the breakdown of the germinal vesicle (GVBD) and completed with formation of the polar body, leads to the production of a mature, fertilizable oocyte, and is defined as oocyte maturation. Maturation of the oocyte is an essential prelude to fertilization. Normally the meiosis is reinitiated by the preovulatory LH peak but when meiotically arrested oocytes are removed from the antral follicles, they resume meiosis spontaneously in vitro. However primate (human and monkeys) oocytes isolated from antral follicles and cultured within their cumuli for two days, spontaneously resumed meiosis at a very low rate (less than 30%) compared to other mammals. Cynomolgus monkey oocyte then appears as a good model for in vitro studies of maturation initiation. Follicular atresia improve significantly the GVBD rate (about 50%). The cumulus cell mass takes an important part in the maintenance of the meiotic arrest since its mechanical removal is followed by an increase of the GVBD (P less than 0.02). A gonadotropin releasing hormone agonist (GnRHa) and a protein kinase C activator added to the culture medium both improve the GVBD (54% and 55% respectively, P less than 0.01). The GnRHa oocyte maturation induction is probably protein kinase C dependent.(ABSTRACT TRUNCATED AT 250 WORDS)