The placenta plays an essential role in fetal growth and the maintenance of pregnancy. Successful development and maturation of the embryo is totally dependent on placental function. The main endocrine participation of the placenta is attributed to placental lactogens (PLs). Progesterone is essential for pregnancy in all mammals and is secreted by the ovary and placenta, depending on the animal species. In the rat, the main source of progesterone throughout pregnancy is the ovary, and 20a- hydroxysteroid dehydrogenase (20a-HSD) is a key enzyme for ovarian progesterone secretion. The primary action of prolactin (PRL) in the maintenance of ovarian progesterone secretion is suppression of the activity of ovarian 20a-HSD. In this review, the sequence homologies between cDNAs for PLs and PRL and the intimate functional relationship between the ovary and placenta are discussed. We postulate the possibility of co-evolution of PLs and ovarian 20a-HSD.
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