Role of fetal membranes in signaling of fetal maturation and parturition.

The fetal membranes fulfill several functions during pregnancy. In addition to containing the products of conception and amniotic fluid, they presumably have barrier functions and fulfill paracrine signaling functions between the maternal (decidual) and fetal compartments. As the membranes are in an ideal place to receive both maternal and fetal signals and transmit signals to uterine myometrium, there has been a specific focus on the role of membranes in the initiation and maintenance of parturition. In this review, we summarize the data obtained in our laboratories as well as the data reported in the literature particularly with regard to the synthesis of steroids and prostaglandins in the fetal membranes, in signaling fetal maturation and in parturition. The fetal membranes are a major site both of prostaglandin synthesis and of prostaglandin metabolism. In addition, the abundant expression of 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD1), which converts biologically inactive cortisone into active cortisol, in the fetal membranes may provide an extra-adrenal source of glucocorticoids for the fetal compartment during gestation. Accumulating evidence indicates that a positive feedback loop involving glucocorticoids, proinflammatory cytokines, prostaglandins, surfactant protein-A (SP-A) and 11beta-HSD1 is formed locally in human fetal membranes towards term or in preterm labor. This positive feedback loop would produce abundant biologically active glucocorticoids and prostaglandins in the fetal membranes or amniotic fluid, which would ultimately promote fetal organ maturation and initiate parturition.