Reproductive Hormones

LH, FSH, and, in some species, PRL are considered pituitary gland regulators of gonadal function. However, the entire endocrine system, through maintenance of metabolic balance, impacts reproductive activity. Gonads respond to and produce protein and steroid hormones. Ovarian follicles and Sertoli cells of the testes produce inhibin and activin. Pituitary activin has positive influences, and gonadal inhibin has negative influences on

FSH secretion from the pituitary gland. These hormones act to influence secretion of FSH and may allow specific regulation of both LH and FSH with a single releasing hormone, GnRH. Castration removes gonadal steroids and results in increased circulating concentrations of LH and FSH and, in boars, decreased insulin and IGF-I. Placenta of pregnant animals are sources of many hormones. Most hormonal proteins are produced in some concentration in placenta. Placental lactogen (somatommotropin, PL) is produced by trophoblast cells of many species, but not sows. Circulating PL concentrations rise in midpregnancy and remain elevated until parturition. PL has GH- and PRL-like activity. Pregnant mares serum gonadotropin (PMSG) is a highly glycosylated protein with primarily FSH-like activity and long half-life in circulation. Human trophoblast cells produce human chorionic gonadotropin (hCG) that has LH-like activity.

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