The female reproductive system is tightly regulated through negative and positive feedback systems. Negative feedback systems predominate and maintain a steady hormonal milieu. Ovarian steroids and inhibin, also produced by the ovary, negatively feedback to the hypothalamus and the anterior pituitary and control secretion of FSH and LH. However, an exception to this negative feedback system is the positive feedback system that operates during the period immediately preceding ovulation and triggers the LH surge. Under the influence of ever-increasing blood concentrations of estradiol 17-p produced by the rapidly growing ovarian follicles, the pulses of GnRH increase in frequency and amplitude, triggering a similar increase in LH secretion from the anterior pituitary gland. In poultry, rapidly increasing secretion of progesterone from the largest preovulatory follicle increases the secretion of GnRH and the subsequent increase in LH secretion. When this preovu-latory LH surge reaches adequate concentrations in the blood, ovulation occurs. The number of follicles ovulating varies among species. The preovulatory surge of LH is terminated because estradiol-17p or progesterone secretion in the case of poultry subsides due to a lack of response of the ovarian follicles to LH.
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