It is clear that steroid hormones of placental and fetal adrenal origin have critically important roles in regulating key physiological events essential to the maintenance of pregnancy and development of the fetus for extrauterine life. Thus, progesterone has suppressive actions on lymphocyte proliferation and activity and on the immune system to prevent rejection of the developing fetus and placenta (see Fig. 9). Progesterone also suppresses the calcium-calmodulin-MLCK system and thus activity of uterine smooth muscle, thereby promoting myometrial quiescence to ensure the maintenance of pregnancy. Estrogen enhances uteroplacental blood flow and possibly placental neovascularization to provide optimal gas exchange and the nutrients required for the rapidly developing fetus and placenta. In turn, estrogen has specific stimulatory effects on the receptor-mediated uptake of LDL by, and P-450scc activity within, syncytiotrophoblasts, thus promoting the biosynthesis of progesterone. Moreover, there is an estrogen-dependent developmental regulation of expression of the LDL receptor and NAD-dependent 11 beta-HSD in the placenta, processes reflecting functional/biochemical differentiation of the trophoblast cells with advancing gestation. The increase in 11 beta-HSD causes a change in transplacental corticosteroid metabolism, which results in activation of the HPAA in the fetus. As a result of this cascade of events, there is an increase in expression of pituitary POMC/ACTH and key enzymes, e.g. 3 beta-HSD and P-450 17 alpha-hydroxylase, important for de novo cortisol formation by, and consequently maturation of, the fetal adrenal gland. In turn, cortisol has well defined actions on surfactant biosynthesis and consequently fetal lung maturation, as well as effects on placental CRH/POMC release, which may be important to the initiation of labor. At midgestation, estrogen also selectively feeds back on the fetal adrenal to suppress DHA and maintain physiologically normal levels of estrogen. Preparation of the breast for lactation and nourishment of the newborn appears to involve a multifactorial system of regulation that includes estrogen. It is apparent, therefore, that autocrine/paracrine, as well as endocrine, systems of regulation are operative within the fetoplacental unit during primate pregnancy. A major goal of this review has been to illustrate the critically close functional communication existing between the developing placenta and fetus in the biosynthesis and the actions of steroid hormones during primate pregnancy. The functional interaction of the human fetal adrenal and placenta with respect to the biosynthesis of estrogen was demonstrated many years ago. However, the recent studies presented in this review show that the endocrine interaction between the fetus and placenta is more extensive, involving complex physiological regulatory mechanisms. Thus, as illustrated in Fig. 9, estrogen, acting via its receptor within the placenta and other reproductive tissues, orchestrates the dynamic interchange between t...

Substantial advances in our understanding of placental function have resulted from recent establishment of in vitro approaches, such as cell culture, and application of molecular methods to study placental steroidogenesis. Insight into the processes of placental cell differentiation and hormonal function has been gained from culture of relatively pure preparations of cytotrophoblast. Various factors, e.g. cAMP and peptide growth factors, have been shown to have striking effects on progesterone and estrogen formation by placental tissue under in vitro conditions. Using advanced molecular approaches, the genes governing specific enzymes critical to placental steroidogenesis have been identified. Regulation of the mRNAs encoding specific enzyme peptides and thus expression of the genes by factors, such as cAMP, have been elucidated by Northern analysis and other techniques. It is critical that these contemporary approaches continue to be implemented aggressively to further elucidate placental function. However, it is clear from a survey of the literature, particularly of the past decade, that the vast majority of investigation in the area has been conducted in vitro. It is essential to determine whether the factors that have been observed to regulate placental endocrine function in vitro are operable in vivo. It is only with in vivo study that the dynamics of steroidogenesis and the complex functional relationships between placenta, fetus, and mother will be uncovered and understood. It is increasingly evident that the regulation of placental steroidogenesis involves autocrine and/or paracrine mechanisms, similar to those integral to hormone biosynthesis within other reproductive organs, e.g. ovary and testis. For example, as discussed above, estrogen regulates LDL uptake and P-450scc, and thus apparently is involved in generating substrate for progesterone production within the placenta. Conversely, progesterone has effects on 17 beta-hydroxysteroid oxidoreductase and thus the metabolism of estradiol, while androgens exert marked inhibitory effects on placental progesterone formation, at least in vitro. Not surprisingly, the regulation of placental progesterone and estrogen formation also is multifactorial. Thus, aromatase activity is stimulated synergistically by cAMP and phorbol esters, an effect that is suppressed by peptide growth factors. Therefore, the autocrine/paracrine and multifactorial regulation of hormone biosynthesis that has been relatively well documented in other tissues should be recognized as important in the primate placenta. Finally, the basic mechanisms underlying regulation of steroidogenesis within the fetoplacental unit during primate pregnancy appear similar, in important ways, to those of widely used laboratory animals, such as the rat and rabbit.(ABSTRACT TRUNCATED AT 400 WORDS)

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