Placenta as a source of 'brain' and 'pituitary' hormones

Placental peptides, such as human chorionic gonadotropin (hCG) and human placental lactogen (hPL), which have marked homologies to pituitary peptides, were described in the early part of this century. Recently, the presence in placenta of additional peptides previously demonstrated as occurring in other tissues, such as brain and pituitary, has been reported. Their presence in placenta has been attributed to similar embryological origin of the tissues which share these peptides, although this has by no means been proven. Placental concentrations of these recently described peptides are several orders of magnitude lower than described for their original sites of production. In many instances, definitive characterization of structural identity with their extraplacental counterparts has not been performed, but has been based on indirect evidence obtained by immunoassay or immunocytochemistry. Evidence of placental synthesis has been obtained for hCG, hPL, and pro-opiomelanocortin (the precursor molecule for adrenocorticotropic hormone (ACTH), B-lipotropin (B-LPH), alpha-melanocyte-stimulating hormone (alpha-MSH), beta-endorphin, and other thus far uncharacterized peptides). Possible functions for the recently described peptides might include actions on the maternal or fetal systems or local (paracrine) actins affecting other placental constituents, although to date no definitive physiological roles have been demonstrated.