Oocyte Maturation in Humans: the Potential Relevance to Reproductive Medicine

Mammalian oogenesis is characterized by alternating periods of active meiotic progression and intermittent, long periods of meiotic arrest. The oocyte undergoes major growth and developmental processes in the period prior to ovulation. At the time of birth, most oocytes are arrested in the dictyate stage of meiosis; they remain quiescent for an indeterminate period until they begin growing in response to as yet undefined local signals [1]. From the beginning of the growth phase until the time of ovulation, oocytes increase in volume by at least two orders of magnitude and during this time exhibit intense metabolic activity. Resumption of meiosis only occurs in a ful ly grown, meiotically competent oocyte after the luteinizing hormone (LH) surge when oocytes undergo germina l ves ic le breakdown (GVBD), complete first meiosis, and mature to metaphase II. In many mammals such as the mouse, pig, cow and man, completion of meiosis is dependent on fertilization that triggers progression to anaphase II, and to the first mitotic interphase with formation of female and male pronuclei. The process of maturation encompasses a complex series of molecular and structural events, culminating in the arrest of the oocyte chromosomes on the metaphase II plate in anticipation of sperm penetration and activation for fertilization. This review will focus on the biology of oocyte maturation and the potential relevance of maturation of human oocytes in vitro to reproductive medicine.