Regulation of the follicular hierarchy and ovulation

Studies are discussed which investigate the regulation of follicular maturation and the ovulation sequence of the domestic hen. The number of FSH receptors of ovarian granulosa cells decreases as the follicle matures, and this decrease in receptor number is paralleled by a gradual loss of FSH-stimulable adenylyl cyclase (AC) activity. By contrast, LH-stimulable AC activity increases as the follicle progresses through the hierarchy. In addition, FSH stimulates progesterone secretion by granulosa cells of the smaller preovulatory follicles, whereas these cells are only minimally responsive to LH. These data suggest that the maturation of less mature (smaller) follicles is primarily controlled by FSH, while LH may serve primarily as the ovulation-inducing hormone. The ability of LH to stimulate progesterone release and induce premature ovulation is dependent upon the stage of the sequence. Injection of ovine LH 12 hr prior to ovulation of the first (C1) egg of the sequence induces fully potentiated preovulatory plasma progesterone surges and 100% premature ovulation, whereas injection prior to the second (C2) ovulation of the sequence fails to stimulate prolonged progesterone release and induces premature ovulation in less than 50% of injected hens. These results are consistent with data obtained in vitro which suggest that granulosa cells obtained 12 hr prior to a C1 ovulation secrete more progesterone in response to chicken LH compared to those obtained 12 hr prior to the C2 ovulation. These data are discussed in terms of the ovary's ability to act as a regulator of the ovulatory cycle.