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

Abstract

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.

Follicular and Hormonal Dynamics during the Estrous Cycle in Goats

“…While some authors have postulated the existence of follicular dominance especially during first and final follicular waves [29,30], others are still dispute this [31,32]. In ewes, follicular waves have been frequently found to emerge in the presence of growing ovulatory-sized follicles from a previous wave [33,34] and large (presumably dominant) follicles do not inhibit equine chorionic gonadotropin (eCG)-induced growth of other follicles [31].…”

The Effect of the Manipulation of Follicle-Stimulating Hormone (FSH)-Peak Characteristics on Follicular Wave Dynamics in Sheep: Does an Ovarian-Independent Endogenous Rhythm in FSH Secretion Exist?1

“…The oFSH for injection was prepared in saline with 0.05% BSA (w/v; Sigma Chemical Co., St. Louis, MO) and 50% polyvinylpyrrolidone (w/v; Sigma) [12].…”

“…Treatment of ewes with a superovulatory dose of FSH results in increased numbers of follicles growing to an ovulatory diameter [16][17][18][19]; however, the effects of changes in the amplitude of FSH peaks within a physiological range have not been tested. In a recent study from our laboratory [12], we demonstrated that the presence of the largest follicle of a wave induced by a treatment with ovine FSH (oFSH) did not postpone the occurrence of the next endogenous FSH peak. However, it is not known whether an increase in the frequency of FSH peaks, using repeated injections of FSH, would disrupt the rhythmic occurrence of endogenous FSH peaks and the regular emergence of follicular waves.…”

“…The emergence of follicular waves is preceded by a transient peak in circulating FSH concentrations [2,3,6,[8][9][10][11][12][13][14]. However, the relationship between the characteristics of FSH peaks and the emergence of follicular waves in ewes has not been studied [15].…”

“…However, it is not known whether an increase in the frequency of FSH peaks, using repeated injections of FSH, would disrupt the rhythmic occurrence of endogenous FSH peaks and the regular emergence of follicular waves. As injection of oFSH induced a follicular wave that did not disrupt the normal rhythm of FSH peaks and follicular waves [12], a question arose as to the regulation of the peaks in FSH secretion that precede follicular waves. Could there be an ovarian-independent endogenous rhythm of FSH secretion (i.e., periodic peaks in serum FSH concentrations) in the ewe ? We designed a series of three experiments to examine the relationship between FSH peaks and follicular wave dynamics in nonprolific Western White Face ewes.…”