Ovarian follicular dynamics during anoestrus in ewes

Abstract

The aim of the present study was to document ovarian antral follicle dynamics throughout seasonal anoestrus in sheep. Daily transrectal ultrasonography was performed during four 17 day scanning periods from March to July in Western White-faced crossbred ewes. Blood samples were collected each day with ultrasonographic scanning for measurement of serum concentrations of FSH, oestradiol and progesterone. Blood samples were also taken every 15 min for 6 h, mid-way through each period of ultrasonographic examination, to determine the patterns of secretion of gonadotrophic hormones. Hormonal data were then related to observed changes in follicular populations and the patterns of antral ovarian follicle turnover. Ultrasonography showed that the ovaries of anoestrous ewes remained active and that the largest ovarian antral follicles grew to a periovulatory size (> or = 5 mm in diameter) at all stages of anoestrus. The total number of all ovarian follicles > or = 3 mm in diameter was lower during early anoestrus compared with at mid-anoestrus because of a significantly smaller number of small (3 mm) and medium (4 mm) ovarian follicles. The largest ovarian follicles (attaining > or = 5 mm in diameter before regression) exhibited a wave-like pattern of growth; an average of three waves of follicular development were recorded in sheep during each of the four 17 day scanning periods in anoestrus, with follicular waves emerging approximately every 5 days. This rhythmic pattern of follicular emergence was found to be associated with the occurrence of fluctuations in serum FSH concentrations. The growth rate of the largest follicles of the wave increased significantly from early to late anoestrus in sheep. In addition, ovarian follicles not growing beyond 3 mm in diameter showed organized patterns of growth and regression; their numbers tended to be lower (P = 0.09) at 3 days before and on the day of follicular wave emergence. Some ewes were seen to maintain synthesis of progesterone throughout anoestrus. This submaximal progesterone secretion tended to occur at irregular intervals and was not coupled with changes in concentrations or patterns of gonadotrophin release, ovulations or detectable morphological luteinization of ovarian antral follicles. It was concluded that the growth of ovarian antral follicles to an ovulatory size was maintained throughout anoestrus in ewes, with a transient shift in the number of small and medium-sized follicles during mid-anoestrus, and that the periodic emergence of waves of large follicles (> or = 5 mm in diameter) occurred in synchrony with an endogenous rhythm of FSH secretion.

Comparison of follicular dynamics and hormone profiles in Boer goats examined during the breeding and non-breeding seasons in the tropics of Queensland, Australia

“…4). In Western White-faced ewes, Bartlewski et al (1998) also reported a greater number of small and medium antral follicles as anoestrus advances, but the causes remained unknown. In our study, the presence of larger follicles in the first and fourth waves during the breeding season could have obscured the presence of smaller follicles, thereby reducing the total number of follicles that were visible in the breeding season.…”

Short, medium or long-term hormonal treatments for induction of synchronized estrus and ovulation in Saanen goats during the nonbreeding season

“…The emergence of a follicular wave is attributed to fluctuations in FSH plasma concentrations. The follicles reach pre-ovulatory size but go into atresia (Bartlewski et al, 1998;Karsch et al, 1993). Exposure to high progesterone concentrations accelerates the emergence of a new follicular wave (Menchaca & Rubianes, 2002).…”

Response of anestrous ewes to the ram effect after follicular wave synchronization with a single dose of estradiol-17?

“…The follicular waves emerge approximately every 5 days during the estrous cycle in the breeding season [9]. This wave pattern continues during the non-breeding season in anestrous ewes [10,11]. As in cattle (for review see [12]), follicle waves are preceded by a transient increase in FSH concentrations that stimulates follicle wave emergence during the breeding [13][14][15] and non-breeding seasons [16].…”