The Endocrine Function of the Placenta: Human Placental Growth Hormone Variant

The placenta of mammals is a very adaptable organ that accomplishes many critical functions necessary for the well-being of the host and developing fetus. For example, the placenta serves as an attachment site to secure the developing fetus to the uterus, and it transports nutrients from the maternal to the fetal compartment. During the evolution of viviparity, the placenta has assumed various morphologies and a diverse set of endocrine functions. As an endocrine organ, it is very versatile. Its versatility arises from its ability to produce a wide array of protein and steroid hormones. The protein hormones that are elaborated by the placenta share structural and functional overlap with those produced by the hypothalamus and the anterior pituitary gland. The structural and biologic features of the various protein hormones produced by the placenta are very species-dependent. For example, in humans the placenta produces human chorionic gonadotrophin (hCG), a hormone that shares structural and functional properties with pituitary luteinizing hormone; decidual prolactin and chorionic somatomammotrophin (hCS) (also referred to as human placental lactogen), which share structural and functional overlap with pituitary growth hormone (hGH) and prolactin; and human placental growth hormone (human growth hormone variant, hGH-V), which shares structural and functional overlap with pituitary growth hormone (reviewed in ref. 1). In this review, I will limit my discussion to the knowledge that is available on the structure and function of hGH-V.