Maturation of ovarian follicles in the prepubertal gilt.

The processes of follicle development and puberty are closely related, and both are associated with maturation of the hypothalamic-pituitary-ovarian axis. Prenatal development of the ovary is independent of gonadotrophic stimulation. Beyond 60 days of age (postnatally), tertiary follicles develop and gonadotrophins begin to influence ovarian follicular development. Negative feedback regulation of pituitary gonadotrophins by ovarian secretions develops between 60 and 100 days of age. In the prepubertal gilt, no consistent changes in peripheral FSH, oestrogen or progesterone concentrations have been identified which are associated with recruitment of the first set of preovulatory follicles. Whether LH secretion increases before this recruitment remains equivocal. Few details are available on how gonadotrophic hormones stimulate ovarian function in the prepubertal gilt. On the basis of a follicular maturation model that has been described for the rat, the actions of FSH, LH and oestrogens on follicular cell receptors and the regulation of aromatase activity seem paramount. Aromatization of androgens to oestrogens has been proposed as a central regulator for follicular maturation. In the prepubertal gilt, a selective increase in peripheral FSH concentration occurs on Day 1 after unilateral ovariectomy, followed by significant increases in ovarian venous concentrations of oestradiol and inhibin on Days 2 and 4 and compensatory growth as measured by follicular fluid volume on Days 2, 4 and 8. Administration of pig follicular fluid to the prepubertal gilt during and after unilateral ovariectomy suppresses compensatory ovarian hypertrophy by mechanisms yet to be determined. In pigs a number of intraovarian factors have been identified, but there is little information on how these factors regulate follicular recruitment and growth. The factor(s) that prevents ovarian follicles in the prepubertal gilt from progressing to ovulation after acquiring the ability to ovulate in response to exogenous gonadotrophins remains unknown.