Mammalian Oocyte Maturation: Model Systems and Their Physiological Relevance

Abstract

The study of oocyte maturation in vitro illustrates a dilemma often facing the physiologist: the simplest system, consisting of cultured isolated oocytes, does not appear to match the physiological events in vivo in that the oocytes mature spontaneously in hormone-free media (1,2,3). On the other hand, the oocyte ex-planted within the follicle while maintaining its normal association with the cumulus oophorus matures in culture only following a hormonal stimulus (4,5), closely resembling the requirements prevailing in vivo. However, being a multicompartmental model it does not allow to draw unequivocal conclusions about the primary site(s) Of gonadotropin action in inducing oocyte maturation and the location of the primary response to this stimulus. A third approach to the study of oocyte maturation in vitro, namely the culture of oocyte together with follicular constituents (6,7) was adopted in order to test the role of follicle cells in the control of oocyte maturation. In this system, the dependence of maturation on hormonal stimulation is restored, and this approach affords an opportunity to analyze events in the oocyte and follicular compartment separately and to define the primary site of hormone action. Some of the results obtained by each of these approaches will be reviewed with special reference to their physiological relevance.