Placental Steroid Hormone Biosynthesis in Primate Pregnancy*

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Abstract
I. Introduction A. Physiological significance PROGESTERONE and the estrogens are the principal steroid hormones produced by the placenta in primate pregnancy. These hormones are essential to several important events in the establishment and maintenance of mammalian pregnancy. For example, in preparative stages estrogen and progesterone play a role in ovum transport (1), endometrial cell proliferation (2) and differentiation (3), decidualization (4), and the process of implantation (5). Progesterone is essential to the maintenance of pregnancy thereafter, and a loss of progesterone is associated with termination of pregnancy (6). Because progesterone has remarkable antiinflammatory and immunosuppressive qualities, Siiteri et al. (7) have suggested that it may be important locally in protecting the products of conception from immunological rejection by the mother. These investigators have proposed that withdrawal of progesterone near term may initiate molecular and cellular events associated with immune rejection and tissue inflammation; events which may generate prostaglandins and result in a subsequent onset of labor. In deed, changes in the ratio of estrogen and progesterone are involved in prostaglandin release and thus the onset of labor in several laboratory species, such as sheep (8), although the relevance to human parturition remains uncertain. Progesterone and estrogen also appear to be important, along with PRL and several other hormones, in lobuloalveolar proliferation and growth of the mammary glands during the course of pregnancy (9). Finally, estrogen generates and maintains the receptors for progesterone in the uterus, as well as other target tissues (10).

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