Asymmetrical Ovarian Function during Recruitment and Selection of the Dominant Follicle in the Menstrual Cycle of the Rhesus Monkey

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Abstract
Despite similar exposure to pituitary gonadotropins by perfusion of both ovaries with the same peripheral blood, only 1 of the 2 ovaries sponsors the single dominant follicle in the typical menstrual cycle. In the present study we examined the initiation of asymmetrical ovarian function during recruitment and selection of the dominant follicle in the primate ovarian cycle by comparison of steroid hormones in the ovarian venous effluent. Thirty-four adult female rhesus monkeys were selected because of high estimated fertility based on their reproductive performance. These monkeys underwent laparotomy for ovarian inspection and collection of ovarian venous blood on 1 of days 1, 3, 5, 7, 9, and 11 after the onset of menses. In addition, femoral blood was collected daily. Repeat laparotomies were performed in the midluteal phase to assess the location of the functional corpus luteum. Concentrations of 17²-estradiol, androstenedione, and progesterone were determined in all sera, as well as LH and FSH in peripheral sera, by RIA. In all, 17 of 19 ovulatory monkeys manifested clear asymmetry of 17²-estradiol 5 days before the LH/FSH midcycle surges. Often, asymmetry of androstenedione levels was not apparent until 3 days before the midcycle gonadotropin surge. Uniformly, in ovulatory monkeys, the ovary associated with significantly greater concentrations of 17²-estradiol and androstenedione in ovarian venous serum ultimately bore the functional corpus luteum observed in the midluteal phase and confirmed by elevated progesterone in peripheral serum. We interpret these findings to indicate that asymmetrical ovarian steroid secretion, especially of 17²-estradiol, may be among the earliest indicators that the dominant follicle, or at least the ovary destined to bear it, is already selected by 5 days before the preovulatory FSH/LH surge in the typical menstrual cycle.

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