Aspects of follicular development and intrafollicular oocyte maturation in gilts

When ovarian follicles begin their development from primordial stages, respective oocytes start their growth and granulosa cells proliferate to form multilaminar structures. Within 84 days follicles reach the antral stage and gain competence to ovulate within the subsequent estrous cycle. Follicles develop to their final size during the early follicular phase of the cycle between days 14 and 17. Simultaneously the number of growing follicles decreases from day 16 onwards. Changes in gonadotropin secretion pattern are responsible for preovulatory production of steroids and the growth of follicles. The role of gonadotropins in follicular development has been demonstrated by gonadotropin deprivation after GnRH agonist application. Other than in normally cycling gilts the total number of follicles decreases and growth of follicles larger than 3 min in diameter is inhibited. Replacement therapy with infusion of pFSH via osmotic pumps shows that FSH stimulus of more than 2 mug/h is necessary for the recruitment of follicles >2 min in diameter. The influence of the preovulatory LH surge on cumulus oocyte complex (COC) and nuclear maturation has been examined in cyclic gilts. Results show that during a period of about 35 h after the LH surge cumulus morphology and oocyte maturity shift from a wide heterogeneity to a pronounced synchronity of maturation and cumulus cell maturation precedes nuclear maturation. The importance of gonadotropins on intrafollicular oocyte development as well as for the follicular development has been demonstrated when the meiotic configuration of oocytes derived from antral follicles was analyzed during the estrous cycle and a long-term treatment with a GnRH agonist. The hormonal content of progesterone, androstendion and testosterone in the follicular fluid was higher in PMSG treated prepuberal and estrous synchronized gilts as compared to normocyclic gilts and sows. But no correlation was found between hormonal levels in the follicular fluid and in vitro fertilization rates of such pretreated animals.