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Follicle stimulating hormone (FSH) is secreted into the blood circulatory system by the pituitary and as the name implies stimulates growth and development of follicles within the female ovary. Thus, the administration of exogenous FSH or FSH-like substances (e.g., pregnant mare's serum gonadotropin, PMSG) to cattle has been used for the induction of multiple ovulations and, subsequently, twin or multiple births. These same substances have been used extensively for the induction of superovulation in embryo donor females. Consequently, it was speculated that cows producing twin births naturally from the spontaneous ovulation of multiple ovarian follicles would have higher circulating concentrations of FSH in their blood.
Circulating concentrations of FSH are relatively constant during the estrous cycle except at estrus (Day 0), when a preovulatory surge of FSH is released from the pituitary; a secondary release of FSH occurs about 24 hr later. Current information suggests that the preovulatory release of FSH is mediated by the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus of the brain and its subsequent transport to the pituitary via the portal blood vessels. Similarly, a surge of FSH release can be induced in a dose-related response by the administration of exogenous GnRH.
Recent research has suggested that a growth factor, insulin-like growth factor-I (IGF-I), secreted predominantly by the liver can enhance the stimulatory effect of FSH on follicular growth, development, and steroidogenesis within the ovary. The significance of this finding is that increasing the amount of IGF-I to the ovary would have the same result as increasing the amount of FSH to the ovary.