Follicular Steroidogenesis and Gonadotropin Binding to Ovine Follicles during the Estrous Cycle

The interrelationships among [¹²⁵I]hCG binding in thecal and granulosa cells, antral fluid steroid concentrations, follicular size, and ovarian steroid secretion were examined at three different stages of the estrous cycle. Group 1 ewes were ovariectomized during the luteal phase of the estrous cycle, and the other groups were ovariectomized before (group 2), or after (group 3) the peak of the preovulatory LH surge. Times of luteolysis and the LH surge were assessed by measurement of peripheral concentrations of progesterone and LH. Three patterns of [¹²⁵I]hCG binding to follicles were noted: 1) binding to both thecal and granulosa cells (activated follicle), 2) binding to the thecal cell layer only, and 3) no observed binding. In general, there was one active follicle per ewe, or one per ovary, and the number of active follicles was not different from the number of corpora lutea in each of the three groups. The active follicles were significantly larger than the other two classes of follicles. Antral fluid estradiol concentrations were significantly greater in the active follicles and were higher in group 2 ewes than in the other two groups. In group 2, antral fluid testosterone concentrations were significantly higher in follicles with LH receptors in the thecal cell layer only. Ovarian secretion of testosterone and estradiol increased during the early follicular phase (group 2), with the major secretion coming from the ovary containing the active follicle. Ovarian progesterone secretion was high in ovaries containing active corpora lutea which prevented the assessment of ovarian follicular secretion of progesterone. The follicle with LH receptors in thecal and granulosa cells was responsible for the increased estradiol secretion observed during the preovulatory period and is presumed to be the ovulatory follicle.