No abstract

Ovarian antral follicles in sheep grow in an orderly succession, producing typically three to four follicular waves per 17-day oestrous cycle. Each wave is preceded by a transient increase in circulating FSH concentrations. The mechanism controlling the number of recurrent FSH peaks and emerging follicular waves remains unknown. During the ewe's oestrous cycle, the time between the first two FSH peaks and days of wave emergence is longer than the intervals separating the ensuing FSH peaks and follicular waves. The prolonged interpeak and interwave interval occurs early in the luteal phase when low levels of progesterone are secreted by developing, or not fully functional, corpora lutea (CL). The purpose of the present study was to determine the effect of varying progesterone (P(4)) levels on circulating concentrations of FSH and antral follicular development in sheep. Exogenous P(4) (15 mg per ewe, i.m.) was administered twice daily to six cycling Rideau Arcott×Dorset ewes from Day 0 (ovulation) to Day 4 (the mean duration of the interwave interval); six animals served as controls. Follicular growth was monitored in all animals by daily transrectal ultrasonography (Days 0-9). Jugular blood samples were drawn twice a day from Day 0 to Day 4 and then daily until Day 9 to measure systemic concentrations of P(4), FSH and 17β-oestradiol (E(2)). The first FSH peak after ovulation was detected on Days 1.5±0.2 and 4.2±0.2 in treated and control ewes, respectively (P<0.05). The next FSH peak(s) occurred on Day 3.9±0.3 in the treated group and on Day 6.4±0.5 in the control group. Consequently, the treated group had, on average, three follicular waves emerging on Days 0, 3 and 6, whereas the control group had two waves emerging on Days 0 and 5. Mean serum E(2) concentrations were greater (P<0.05) in control compared with treated ewes on Days 1.3, 2.3, 3.3, 4.0 and 4.3 after ovulation. In summary, creation of mid-luteal phase levels of P(4) in metoestrus shortened the time to the first post-ovulatory FSH peak in ewes, resulting in the emergence of one more follicular wave compared with control ewes during the same time frame. Therefore, P(4) appears to be a key endocrine signal governing the control of periodic increases in serum FSH concentrations and the number of follicular waves in cycling sheep.

Ovarian antral follicles in the ewe grow in an orderly succession, producing 3 to 4 waves per estrous cycle. In prolific sheep, some large antral follicles from the second-to-last wave of the estrous cycle are added to the ovulatory follicles emerging just before estrus to give a higher ovulation rate; it is feasible that regression of these follicles is prevented by an increase in serum concentrations of FSH or LH pulsatility at proestrus. Prolific sheep tend to have a shorter luteal phase than nonprolific ewes and there is a great deal of evidence that luteal progesterone (P), in addition to regulating LH release, may govern the secretion of FSH heralding the emergence of follicular waves. The specific purpose of this study was to determine whether or not extending the duration of the luteal phase in prolific sheep to that typically seen in nonprolific breeds would alter the follicle wave dynamics and ovulation rate. In 2 separate experiments, exogenous P (7.5 mg per ewe intramuscularly) was administered on day 11 at PM and day 12 at AM (day 0 = first ovulation of the interovulatory interval studied) in moderately prolific Rideau Arcott × Polled Dorset ewes (experiment 1, n = 8) and highly prolific Olkuska ewes (experiment 2, n = 7; TRT), whereas the equinumerous groups of animals served as controls (CTR). Transrectal ovarian ultrasonography was performed daily, and jugular blood samples were drawn twice a day from day 9 until the next ovulation. Progesterone injections resulted in relatively uniform increments in serum P levels, but the mean duration of the interovulatory interval did not differ (P > 0.05) between TRT and CTR groups of ewes in either experiment. The mean ovulation rate post-treatment was 1.6 ± 0.2 vs 3.2 ± 0.4 (experiment 1, P < 0.001) and 3.2 ± 0.8 vs 4.0 ± 1.0 (experiment 2, P > 0.05) in TRT vs CTR, respectively. The number and percentage of ovulating follicles from the penultimate wave of the interovulatory interval studied was 0.25 ± 0.16 vs 1.75 ± 0.45 (P < 0.01) and 25.0 ± 16.4% vs 75.0 ± 16.4% (P < 0.05) in experiment 1, and 0.50 ± 0.30 vs 1.60 ± 0.40 (P < 0.05) and 13.8 ± 9.0% vs 53.4 ± 16.7% (P < 0.05) in experiment 2, for TRT vs CTR, respectively. In summary, administration of P at the end of diestrus decreased the incidence of ovulations from the penultimate wave of the estrous cycle in both the moderately and highly prolific strains of sheep, but it reduced the ovulation rate only in moderately prolific ewes.

No abstract

Ovarian antral follicles in sheep grow in an orderly succession, producing typically 3 to 4 follicular waves per 17-day estrous cycle. Each wave is preceded by a transient increase in circulating FSH concentrations. The mechanism controlling the number of recurrent FSH peaks and emerging follicular waves remains unknown. During the ewe's estrous cycle, the time between the first 2 FSH peaks and days of wave emergence is longer than the intervals separating the ensuing FSH peaks and follicular waves. The prolonged inter-peak/inter-wave interval occurs early in the luteal phase when low levels of progesterone are secreted by developing, or non-fully functional, corpora lutea. The purpose of the present study was to determine the effect of varying progesterone (P4) levels on circulating concentrations of FSH and antral follicular development in sheep. Exogenous P4 (15 mg per ewe i.m.) was administered twice daily to 6 cycling Rideau Arcott × Dorset ewes from Day 0 (ovulation) to Day 4 (the mean duration of the inter-wave interval); 6 animals served as controls. Follicular growth was monitored in all animals by daily transrectal ultrasonography (Days 0 to 9). Jugular blood samples were drawn twice a day from Day 0 to 4 and then daily until Day 9 to measure systemic concentrations of P4 and FSH. The first FSH peak post-ovulation was detected on Day 1.4 ± 0.2 and 4.0 ± 0.2 in treated and control ewes, respectively (P < 0.05). The next FSH peak(s) occurred on Days 3.4 ± 0.3 and 5.2 ± 0.2 in the treatment group and on Day 5.5 ± 0.3 in controls. Consequently, the treatment group had, on average, 3 follicular waves emerging on Days 0, 3, and 6, whereas the controls produced 2 waves emerging on Days 0 and 5 (P < 0.05).We then retrospectively analyzed and compared daily serum concentrations of P4 and FSH obtained in cyclic Western White Face ewes (Columbia × Rambouillet) that had 3 (n = 10) or 4 (n = 19) follicular waves per cycle. Mean P4 concentrations were greater (P < 0.05) in sheep with 4 waves per cycle compared with their counterparts, which had 3 waves of follicular growth. Interestingly, the ewes with 3 waves exceeded (P < 0.05) all animals with 4 follicular waves in mean serum FSH concentrations on Days 0 to 2, 6 to 7, and 9 to 15 post-ovulation. In summary, creation of mid-luteal phase levels of P4 in metestrus shortened the time to the first post-ovulatory FSH peak in ewes, resulting in emergence of one more follicular wave compared with control animals during the same time frame. The ewes exhibiting 4 waves of follicular emergence had greater serum levels of P4 but lower FSH concentrations compared with sheep with 3 waves per cycle. Therefore, progesterone appears to be a key endocrine signal governing the control of periodic increases in serum FSH concentrations and the number of follicular waves in cyclic sheep. This study was funded by OMAFRA and NSERC grants. Appreciation is extended to Norman C. Rawlings, Susan Cook, and Sekallu Srinivas (University of Saskatchewan) and the staff at Ponsonby Sheep Research Station.