Placental endocrine function

The placenta produces a number of hormones that are not otherwise synthesized in the organism, suggesting that a distinct set of hormones are required to bring about the physiological changes of pregnancy rather than simply producing more of certain hormones. Therefore, placental hormones may also enter the maternal and fetal circulation, where they are often present at concentrations far in excess than what is found in the nonpregnant adult, to play a pivotal role in the maternal–fetal hormonal dialog. The placenta, decidua, amnion, and chorion actively participate in human reproductive physiology in allowing implantation, the regulation of fetal-placental blood flow, the growth of the conceptus throughout gestation, and in directing the appropriate signals for the timing of parturition. Some of the placental hormones are gonadotropin-releasing hormone (GnRH), growth hormone-releasing hormone (GHRH) and somatostatin (SST). GnRH—a decapeptidic hormone—is produced by the hypothalamus that controls reproduction in vertebrates through the hypothalamic–pituitary–gonadal axis. GHRH and SST—hypothalamic releasing factors—are involved in the regulation of the complex process of growth hormone (GH) synthesis and secretion in the anterior pituitary. While GHRH functions as a specific stimulator of GH secretion, on the contrary, SST acts as the inhibitory counterpart.