Gonadotropic control of ovarian follicle maturation: the two-stage conceptand its mechanisms

Most research on the control of oocyte maturation by luteinizing hormone (L H) in teleosts and amphibians has focused on the production and action of m aturation-inducing hormone (MIH), the follicular hormone that directly trig gers the resumption of oocyte meiosis. However, current information indicat es that LH regulates maturation in two stages, and that 'oocyte maturation' can be appropriately described within the broader context of 'ovarian foll icle maturation'. During the first stage of maturation the follicle (somati c) cells acquire the ability to produce MIH and the oocyte to respond to MI H (i.e. oocyte maturational competence, OMC), whereas in the second stage t he follicle cells produce MIH and, consequently, the oocyte is released fro m meiotic arrest. A number of factors such as insulin-like growth factor-I, serotonin, and others may mediate or modulate the OMC-inducing action of L H. Like the acquisition of MIM-producing ability, the acquisition of OMC re quires activation of the protein kinase A pathway. Two major cellular event s associated with OMC acquisition are increases in homologous and heterolog ous gap junction contacts and in oocyte MIH receptor activity. The increase d oocyte MIH receptor activity is presumably associated with OMC acquisitio n, but the significance of changes in gap junction contacts is at present u ncertain. To eliminate inconsistency and ambiguity associated with current terminology we propose that the term, ovarian follicle (or oocyte) maturati on be used for teleosts without qualifiers such as 'final' to define the fi rst and second stages of follicular maturation. Elsevier Science Inc. All r ights reserved.