The ruminant estrous cycle depends on a complex interplay of hormones from the hypothalamus, pituitary, ovaries, and uterus, as well as changes in ovarian structures and in behavior. During the luteal phase of the cycle, the corpus luteum is the dominant ovarian structure and the secretion of progesterone is critical to fertility. Progesterone regulates frequency of pulses of LH, through negative feedback on the hypothalamus, and also prevents an LH surge. In addition, progesterone programs uterine secretion of PGF2a, which times initiation of luteolysis. Follicles are stimulated by FSH; some are selected to continue growth and become dependent upon LH. Luteolysis removes negative feedback by progesterone on LH, allowing ovulation. High concentrations of progesterone lead to timely atresia of older follicles. However, if concentrations of progesterone are low, an increase in frequency of pulses of LH allows development of a persistent dominant follicle, increased secretion of estradiol-17ß, and, if that follicle ovulates, fertility may be decreased.
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