Follicular dynamics and ovarian steroid secretion in sheep during the follicular and early luteal phases of the estrous cycle

In this study, we investigated follicular dynamics and ovarian steroid secretion during the follicular and early luteal phases of the estrous cycle in sheep. Six Finn-Merino ewes with ovarian autotransplanted ovaries were monitored for 10 days during the follicular phase and subsequent early luteal phase after luteal regression was induced with cloprostenol (a potent analogue of prostaglandin F-2 alpha). Over this period, follicular diameter was measured by serial ultrasound scans, and the concentration of gonadotropins and steroids in ovarian venous blood was measured at intervals of 6-12 h. All animals had an LH surge (Day 0) 59 +/- 4.7 h after injection of cloprostenol. The ovulatory follicles were derived mainly from large antral follicles present at the time of injection of cloprostenol (5.1 +/- 0.4 mm; mean +/- SEM, n = 6), although in some animals recruitment of additional small follicles was observed after luteolysis. The concentration of FSH decreased during the follicular phase and peaked synchronously with the LH surge, while estradiol and androstenedione concentrations in ovarian venous plasma increased progressively from luteal regression to a maximum at the LH surge. The rise in concentration of FSH on Day 1 was followed by the growth of a new cohort of follicles. Follicular size and ovarian steroid secretion increased in a linear fashion from Day 1 to Day 3, with ovarian steroid secretion reaching a maximum when the first wave of luteal phase follicles achieved a diameter of 5 mm or more. On Day 4, steroid secretion began to decline without significant changes in follicular diameter, and a second wave of follicles emerged. We conclude that 1) the preovulatory follicles are usually derived from the large follicle population present at the time of luteal regression, but the sheep has the ability to promote smaller follicles if required; and 2) the second peak of FSH stimulates the development of large estrogenic follicles during the early luteal phase, but the period of functional dominance is shorter than the period of morphological dominance.