Ovarian function in ewes at the onset of the breeding season

Transrectal ultrasonography of ovaries was pei formed each day, during the expected transition from anoestrus to the breeding season (mid-August to ea rly October), in six Western white-faced cross-bred ewes, to record ovarian antral follicles greater than or equal to 3 mm in size and luteal structur es. Jugular blood samples were collected daily for radioimmunoassay (RTA) o f follicle-stimulating hormone (FSH), oestradiol and progesterone. The firs t ovulation of the breeding season was followed by the full-length oestrous cycle in all ewes studied. Prior to the ovulation, all ewes exhibited a di stinct increase in circulating concentrations of progesterone, yet no corpo ra lutea (CL) were detected and luteinized unovulated follicles were detect ed in only three ewes. Secretion of FSH was not affected by the cessation o f anoestrus and peaks of episodic FSH fluctuations were associated with the emergence of ovarian follicular waves (follicles growing from 3 to greater than or equal to 5 mm). During the 17 days prior to the first ovulation of the breeding season, there were no apparent changes in the pattern of emer gence of follicular waves. Mean daily numbers of small antral follicles (no t growing beyond 3 mm in diameter) declined (P < 0.05) after the first ovul ation. The ovulation rate, maximal total and mean luteal volumes and maxima l serum progesterone concentrations, but not mean diameters of ovulatory fo llicles, were ostensibly lower during the first oestrous cycle of the breed ing season compared with the mid-breeding season of Western white-faced ewe s. Oestradiol secretion by ovarian follicles appeared to be fully restored, compared with anoestrous ewes, but it was not synchronized with the growth of the largest antral follicles of waves until after the beginning of the first oestrous cycle. An increase in progesterone secretion preceding the f irst ovulation of the breeding season does not result, as previously sugges ted, from the ovulation of immature ovarian follicles and short-lived CLI b ut progesterone may be produced by luteinized unovulated follicles and/or i nterstitial tissue of unknown origin. This increase in serum concentrations of progesterone does not alter the pattern of follicular wave development, hence it seems to be important mainly for inducing oestrous behaviour, syn chronizing it with the preovulatory surge of luteinizing hormone (LH), and preventing premature luteolysis during the ensuing luteal phase. Progestero ne may also enhance ovarian follicular responsiveness to circulating gonado tropins through a local mechanism. (C) 1999 Elsevier Science B.V. All right s reserved.