Effects of a 6-Day Treatment with Medroxyprogesterone Acetate after Prostaglandin F2α-Induced Luteolysis at Midcycle on Antral Follicular Development and Ovulation Rate in Nonprolific Western White-Faced Ewes1

The mechanisms of ovulatory compensation following unilateral ovariectomy (ULO) are still not understood. In the present study, we investigated the short- and long-term effects of ULO in sheep using transrectal ovarian ultrasonography and hormone estimations made during the estrous cycle in which surgery was done, the estrous cycle 2 mo after surgery, and the 17-day period during the subsequent anestrus. The ULOs were done when a follicle in the first follicular wave of the cycle reached a diameter > or =5 mm, leaving at least one corpus luteum and one ovulatory-sized follicle in the remaining ovary. Ovulation rate per ewe was 50% higher in the ULO ewes compared with the control ewes at the end of the cycle during which surgery was performed, but it did not differ between groups at the end of the cycle, 2 mo later. This compensation of ovulation rate in ULO ewes was due to ovulation of follicles from the penultimate follicular wave in addition to those from the final wave of the cycle. Ovulation from multiple follicular waves appeared to be due to a prolongation of the static phase of the largest follicle of the penultimate wave of the cycle. Interestingly, the length of the static phase of waves was prolonged in ULO ewes compared with control ewes in every instance where the length of the static phase could be determined. Changes in follicular dynamics due to ULO were not associated with alterations in FSH and LH secretion. In conclusion, ovulatory compensation in ULO sheep involves ovulation from multiple follicular waves due to the lengthened static phase of ovulatory-sized follicles. These altered antral follicular dynamics do not appear to be FSH or LH dependent. Further studies are required to examine the potential role of the nervous system in the enhancement of the life span of the ovulatory-sized follicles leading to ovulatory compensation by the unpaired ovary in ULO sheep.

Section: Discussionsupporting

confidence: 75%

Short- and Long-Term Effects of Unilateral Ovariectomy in Sheep: Causative Mechanisms1

Duggavathi

Bartlewski

Barrett

et al. 2008

“…Coculturing small follicles with large follicles in sheep in a closed system did not decrease thymidine incorporation by granulosa cells of the small follicles as compared with those of small follicles cultured alone [21]. Recently, it has been demonstrated that follicles from the final and penultimate waves of the estrous cycle in sheep can ovulate together and form healthy corpora lutea [13,15,22]. Therefore, the largest follicles of waves in the ewe may not exert the same functional dominance as is seen in cattle.…”

Section: Discussionmentioning

confidence: 97%

“…Coculturing large and small ovine antral follicles did not result in atresia of the small follicles [21]. In the ewe, two or more follicles have been shown to grow in a wave, and antral follicles from the final and penultimate waves of the estrous cycle have been shown to ovulate at the same time [13,15,22]. However, when cycling ewes were treated with low levels of progesterone for several days, the life span of large antral follicles was prolonged, and emergence of follicular waves was suppressed [10].…”

Section: Introductionmentioning

confidence: 99%

Patterns of Antral Follicular Wave Dynamics and Accompanying Endocrine Changes in Cyclic and Seasonally Anestrous Ewes Treated with Exogenous Ovine Follicle-Stimulating Hormone During the Inter-Wave Interval1

Duggavathi

Bartlewski

Barrett

et al. 2004

Self Cite

In the ewe, ovarian follicular waves emerge every 4 to 5 days and are preceded by a peak in FSH secretion. It is unclear whether large antral follicle(s) in a wave suppress the growth of other smaller follicles during the inter-wave interval, as is seen in cattle. In this study, anestrous (n = 6; experiment 1) and cyclic (n = 5; experiment 2) Western white face ewes were given ovine FSH (oFSH) (0.5 microg/kg; two s.c. injections, 8 h apart) during the growth phase (based on ultrasonography) of a follicular wave (wave 1). Control ewes (n = 5 and 6, respectively) received vehicle. In oFSH-treated ewes, serum FSH concentrations reached a peak (P < 0.05) by 12 h after oFSH treatment, and this induced FSH peak did not differ (P > 0.05) from the endogenous FSH peaks. In all ewes, emergence of follicular waves 1 and 2 was seen (P > 0.05). However, in oFSH-treated ewes, an additional follicular wave emerged approximately 0.5 days after treatment: during the interwave interval of waves 1 and 2 without delaying the emergence of wave 2. The growth characteristics and serum estradiol concentrations did not differ (P > 0.05) between oFSH-induced waves and waves induced by endogenous FSH peaks. We concluded that, unlike in cattle, the largest follicle of a wave in sheep has limited direct effect on the growth of other follicles induced by exogenous oFSH. In addition, the largest follicle of a wave may possibly not influence the rhythmicity of follicular wave emergence, as it does in cattle.

“…Emergence of a new follicular wave in the presence of a large follicle(s) from a previous wave in sheep has been also shown in studies where follicles from the penultimate wave of TABLE 1. Comparison of the characteristics (mean 6 SEM) of follicular waves emerging following endogenously driven FSH peaks (waves 1 and 2) of an estrous cycle and after treatment with oFSH (waves A and B) in experiment 1. the estrous cycle were seen to ovulate with follicles from the final wave [2,33,34]. In addition, wave overlap has also been noted in ewes treated with exogenous progesterone [35][36][37].…”

Section: Discussionmentioning

confidence: 88%

Ovarian Follicular Dominance and the Induction of Daily Follicular Waves in the Ewe1

Toosi

Davies

Seekallu

et al. 2010

Self Cite

Large antral follicles grow in waves in the ewe, and each wave is triggered by a peak in serum concentrations of FSH. The existence of follicular dominance in the ewe is unclear. The objective of experiment 1 was to determine if an endogenously driven follicular wave could emerge during the growth phase of a wave induced by injection of ovine FSH (oFSH). Cyclic ewes (n = 7) were given oFSH (two injections of 0.5 microg/kg; s.c.; 8 h apart) on 2 separate days equally spaced in the interval between the first two endogenously driven follicular waves of an estrous cycle. Injection of oFSH induced two follicular waves in the interval between the first two endogenously driven waves of the cycle. The second endogenously driven wave of the estrous cycle emerged in the midgrowth phase of a follicular wave induced by injection of oFSH and its day of emergence, and growth pattern did not differ from that of the equivalent wave in control ewes (emerging 5.4 +/- 0.2 and 4.8 +/- 0.5 days after ovulation, respectively; P > 0.05). Experiment 2 was designed to determine if emergence of follicular waves could be induced on a daily basis. Six anestrus ewes were given oFSH (two injections of 0.35 microg/kg; s.c.; 8 h apart) on each of 4 days, starting 24 h after the expected time of an endogenously driven FSH peak. Each injection of oFSH resulted in a discrete peak in serum FSH concentrations and the emergence of a new follicular wave. The present findings provide evidence for the lack of follicular dominance in the ewe.