Abstract
BackgroundIn the ewe, ovarian antral follicles emerge or grow from a pool of 2–3 mm follicles in a wave like pattern, reaching greater than or equal to 5 mm in diameter before regression or ovulation. There are 3 or 4 such follicular waves during each estrous cycle. Each wave is preceded by a peak in serum FSH concentrations. The role of pulsatile LH in ovarian antral follicular emergence and growth is unclear; therefore, the purpose of the present study was to further define this role.MethodsEwes (n = 7) were given 200 ng of GnRH (IV) every hour for 96 h from Day 7 of the estrous cycle, to increase LH pulse frequency. Controls (n = 6) received saline. In a second study, ewes (n = 6) received subcutaneous progesterone-releasing implants for 10 days starting on Day 4 of the cycle, to decrease LH pulse frequency. Controls (n = 6) underwent sham surgery. Daily transrectal ovarian ultrasonography and blood sampling was performed on all ewes from the day of estrus to the day of ovulation at the end of the cycle of the study. At appropriate times, additional blood samples were taken every 12 minutes for 6 h and 36 min or 6 h in studies 1 and 2 respectively.ResultsThe largest follicle of the follicular wave growing when GnRH treatment started, grew to a larger diameter than the equivalent wave in control ewes (P < 0.05). Mean serum estradiol and progesterone concentrations were higher but mean serum FSH concentrations were lower during GnRH treatment compared to control ewes (P < 0.05). The increased serum concentrations of estradiol and progesterone, in GnRH treated ewes, suppressed a peak in serum concentrations of FSH, causing a follicular wave to be missed. Treatment with progesterone decreased the frequency of LH pulses but did not have any influence on serum FSH concentrations or follicular waves.ConclusionWe concluded that waves of ovarian follicular growth can occur at LH pulse frequencies lower than those seen in the luteal phase of the estrous cycle but frequencies seen in the follicular phase, when applied during the mid-luteal phase, in the presence of progesterone, do enhance follicular growth to resemble an ovulatory follicle, blocking the emergence of the next wave.
Highlights
In the ewe, ovarian antral follicles emerge or grow from a pool of 2–3 mm follicles in a wave like pattern, reaching greater than or equal to 5 mm in diameter before regression or ovulation
Each follicular wave is preceded by a peak in serum concentrations of FSH lasting 3 to 4 days; this peak has been shown to be an essential trigger for the emergence of a follicular wave [1,2,3,6,7]
It was concluded that the growth of ovarian antral follicles beyond 2 to 3 mm in diameter was largely dependent on FSH, with the final growth and maturation becoming LH dependent [8,9,10,11]
Summary
Ovarian antral follicles emerge or grow from a pool of 2–3 mm follicles in a wave like pattern, reaching greater than or equal to 5 mm in diameter before regression or ovulation. Prior to the use of ultrasonography for repetitive, non invasive monitoring of ovarian structures in the ewe, studies on the endocrine regulation of ovarian folliculogenesis were largely limited to single surgical or post mortem observation of the ovary. From such studies, it was concluded that the growth of ovarian antral follicles beyond 2 to 3 mm in diameter was largely dependent on FSH, with the final growth and maturation becoming LH dependent [8,9,10,11]
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