The clinical significance of endometrial prolactin

Prolactin (PRL) is produced not only by the anterior pituitary gland but also by human endometrium. That decidual stromal cells secrete PRL from day 22 of the menstrual cycle is demonstrated by: 1) nett accumulation of PRL during in vitro culture; 2) PRL accumulation is prevented by inhibitors of protein synthesis; and 3) identification of PRL mRNA within endometrial decidua. Endometrial PRL is biologically and immunologically equipotent with pituitary PRL and its amino acid sequence is very similar. In human pregnancy, decidual PRL binds to receptors on the fetal chorion and amnion and thereby passes into amniotic fluid in high concentration. Three putative functions of uterine PRL are suggested from current studies: a) a PRL receptor defect is present in the chorion laeve of patients with pregnancies complicated by chronic polyhydramnios and this deficiency in chorionic receptors for endometrial PRL may result in the development of excessive amniotic fluid; b) decidual PRL may modulate prostaglandin synthesis not only within the endometrium prior to menstruation, but also within the chorion and amnion to allow labour to proceed in a timely manner; and c) amniotic fluid PRL may pass into the fetal tracheo-bronchial system to promote surfactant production. Unlike pituitary PRL with distant target organs, decidual PRL appears to have paracrine or cybernetic functions in the human uterus, placental membranes and the fetus.