Patterns of growth and regression of ovarian follicles during the oestrous cycle and after hemiovariectomy in mares

Abstract

Summary To increase our understanding of the regulation of ovarian follicular development during the oestrous cycle, 13 mares were used in two experiments. In Experiment 1, the pattern of growth and regression of individual follicles, 15 mm or more in size, was monitored daily during 17 oestrous cycles by real‐time ultrasonography. Each examination was recorded on videotape to ensure accurate monitoring of individual follicles. Results indicated that the development of such follicles occurs in waves, with the most common pattern being one follicular wave per oestrous cycle (12 of 17 cycles). An alternative pattern of two follicular waves per cycle was observed in mares with (n=3) and without (n=2) dioestrous ovulations. The ovulatory follicle appeared earlier in the cycle in mares with one wave than in mares with two waves (Day 6.3 ± 0.9 vs Day 11.0 ± 1.2, respectively; p<0.01). Also, the ovulatory follicle was the largest follicle for a longer period of time in mares with one wave than in mares with two waves per cycle (9.5 ± 0.6 vs 5.8 ± 0.3 days, respectively; p<0.01). Mares with one follicular wave had shorter cycles than those with two waves per cycle (18.2 ± 0.6 vs 21.6 ± 0.5 days, p<0.001). In both cases, the interval between the appearance of the ovulatory follicle and its ovulation was, on average, 10 to 12 days. Overall, a negative correlation was found between the relative size of the ovulatory follicle at the beginning of the follicular phase and the length of the follicular phase (r=‐0.91, p<0.001). To study the control of follicular recruitment, patterns of follicular growth were monitored in nine mares following removal of the ovary bearing the presumptive ovulatory follicle (Experiment 2). Hemiovariectomies were performed between Day 14 of dioestrus and the fourth day of oestrus. Follicular dynamics were characterised by the development of one follicular wave that produced one to three ovulatory follicles in seven of nine mares. The ovulatory follicle reached 15 mm or more 6.3 ± 1.5 days following surgery and the interval from hemiovariectomy to ovulation was 13.7 ± 1.3 days. Two of nine mares hemiovariectomised on Day 14 failed to undergo complete luteolysis after the ovary bearing the ovulatory follicle, but not the corpus luteum, was excised. These mares had not ovulated by 30 days after surgery, although follicular waves producing ovulatory size follicles were observed. This study indicated that the development of ovarian follicles 15 mm or larger in the mare occurs in waves, with a more common pattern of one wave and an alternative pattern of two waves per oestrous cycle.

Follicular dynamics in Mangalarga mares

“…The causes of these relatively high levels of dioestrous follicular activity during the second half of the ovulatory season in Mangalarga mares are intriguing. Although differences between breeds with regard to patterns of follicular development have been well established (Sirois et al 1989;Ginther 1990;Ginther 1992;Plata-Madrid et al 1994), it is reasonable to suspect an influence of environment on ovarian activity in the Mangalarga mares used in the present study . Quintero et al (1995) studied seasonal changes in ovarian activity of Thoroughbred mares maintained in a tropical environment and suggested a direct relationship between latitude and seasonality, the closer the mare being to the Equator, the less seasonal her reproductive pattern.…”

“…However, these findings disagreed with previous studies that had reported the presence of large follicles, and even the occurrence of ovulation during dioestrus, in some mares (Vandeplasche et al 1979). The discrepancy continued until Sirois et al (1989) and Ginther (1990) both reported 2 distinct patterns of follicular dynamics during the equine oestrous cycle. By repeated ultrasonographic monitoring of individual follicles these authors were able to identify cycles that contained one follicular wave and other cycles that contained 2 waves.…”

“…Differences in the patterns of ovarian follicular activity between breeds of horses have also been reported. Sirois et al (1989) studied Standardbred and Thoroughbred mares during late May to late July and found that 5 of 17 interovulatory intervals contained 2 major waves (29.41%). Moreover, 3 of the 5 secondary waves resulted in a dioestrous ovulation, which is consistent with the previous reports of a high incidence of dioestrous ovulations in these breeds (Stabenfeldt et al 1972).…”

Human chorionic gonadotropin-dependent up-regulation of epiregulin and amphiregulin in equine and bovine follicles during the ovulatory process

“…Indeed, cultures of preovulatory follicles or isolated germinal vesicle-stage COCs with EREG or AREG induced cumulus expansion and oocyte meiotic resumption in a manner similar to that observed by LH/hCG in ovarian follicles in vivo [2,35,36]. To increase our understanding of EREG and AREG regulation in follicular cells of large monoovulatory animal species, the equine preovulatory follicle was selected as a model because of its large size (40-45 mm in diameter), protracted ovulatory process (39-42 h interval from hCG to ovulation) and ability to monitor preovulatory follicular development by ultrasonography [53]. The current study revealed the presence of a marked gonadotropin-dependent induction of EREG and AREG mRNAs in equine follicle prior to ovulation, with an induction observed in both granulosa and theca cells.…”

“…In cattle, one study using GnRH as an agonist revealed a significant induction of AREG transcripts in both granulosa and theca cells [27], whereas another investigation using differential display reverse transcription polymerase chain reaction analysis and suppressive subtractive hybridization showed the expression of EREG mRNA in granulosa cells of bovine follicles containing a competent oocyte [41]. The development of the equine preovulatory follicle is of particular interest because of its large size (40-45 mm), relatively long ovulatory process (defined as the interval of time from hCG administration to follicular rupture) and pattern of recruitment and selection, which resembles that observed in women [11,16,53]. Thus, the objectives of the present work were to characterize the gonadotropin-dependent regulation of EREG and AREG in equine follicles prior to ovulation and, using primary cultures of bovine granulosa cells, to document the effect of EGFR activation on the expression of ovulation-related genes in granulosa cells of a large monoovulatory animal species.…”