Ovarian function in ewes at the onset of the breeding season.
Transrectal ultrasonography of ovaries was performed each day, during the expected transition from anoestrus to the breeding season (mid-August to early October), in six Western white-faced cross-bred ewes, to record ovarian antral follicles > or = 3 mm in size and luteal structures. Jugular blood samples were collected daily for radioimmunoassay (RIA) of follicle-stimulating hormone (FSH), oestradiol and progesterone. The first 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 distinct increase in circulating concentrations of progesterone, yet no corpora lutea (CL) were detected and luteinized unovulated follicles were detected in only three ewes. Secretion of FSH was not affected by the cessation of anoestrus and peaks of episodic FSH fluctuations were associated with the emergence of ovarian follicular waves (follicles growing from 3 to > or = 5 mm). During the 17 days prior to the first ovulation of the breeding season, there were no apparent changes in the pattern of emergence of follicular waves. Mean daily numbers of small antral follicles (not growing beyond 3 mm in diameter) declined (P < 0.05) after the first ovulation. The ovulation rate, maximal total and mean luteal volumes and maximal serum progesterone concentrations, but not mean diameters of ovulatory follicles, were ostensibly lower during the first oestrous cycle of the breeding season compared with the mid-breeding season of Western white-faced ewes. 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 first ovulation of the breeding season does not result, as previously suggested, from the ovulation of immature ovarian follicles and short-lived CL, but progesterone may be produced by luteinized unovulated follicles and/or interstitial 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, synchronizing it with the preovulatory surge of luteinizing hormone (LH), and preventing premature luteolysis during the ensuing luteal phase. Progesterone may also enhance ovarian follicular responsiveness to circulating gonadotropins through a local mechanism.
Bartlewski
P M
PM
Department of Veterinary Physiological Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Canada.
Beard
A P
AP
Rawlings
N C
NC
D000818:Animals; D001947:Breeding; D003338:Corpus Luteum / Q000000981:diagnostic imaging / Q000502:physiology; D004958:Estradiol / Q000097:blood; D004971:Estrus; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood; D006080:Ovarian Follicle / Q000000981:diagnostic imaging / Q000502:physiology; D010053:Ovary / Q000000981:diagnostic imaging / Q000502:physiology*; D010060:Ovulation; D012621:Seasons; D012756:Sheep; D014463:Ultrasonography
D004958:Estradiol; D005640:Follicle Stimulating Hormone