The Role of Nonsteroidal Regulators in Control of Oocyte and Follicular Maturation

Abstract

This chapter discusses the role of nonsteroidal regulators in control of oocyte and follicular maturation. The ovarian follicle is bathed in a fluid rich in steroid hormones as well as nonsteroidal regulators which interact to serve to control its maturation, responsiveness to gonadotropins, as well as to lead to control of the maturation of its oocyte. The orderly maturation of an ovarian follicle and its oocyte is controlled by pituitary LH and FSH in concert with local intrafollicular regulators. The principal nonsteroidal follicular regulators are an oocyte maturation inhibitor, a luteinization inhibitor, a luteinization stimulator, FSH receptor binding inhibitor, and inhibin-F. OMI is a polypeptide <2000 daltons and is elevated in small and medium sized follicles and declines in the follicle under the influence of the preovulatory LH/FSH surge. The decline of OMI immediately prior to ovulation might be responsible in part for oocyte maturation. The action of FSH upon the granulosa cell to stimulate accumulation of LH receptors and stimulate progesterone secretion is inhibited by a low- and high-molecular-weight luteinization inhibitor which is present in greater amounts in small compared to large follicular fluid. Conversely, as the follicle enlarges a luteinization stimulator appears which enhances these actions of FSH upon the granulosa cell. The follicle also secretes inhibin-F which reaches the ovarian venous blood and acts upon the pituitary to selectively inhibit FSH secretion. Changes in follicular inhibin-F might play a role in modulating the change in FSH secretion at puberty, at menopause, and at selected times during the menstrual cycle.