Some Aspects of the Physiology of the Placenta

Abstract

One cannot properly consider the physiology of the fetus without that of the placenta, a fetomaternal organ characteristic of mammalian pregnancy, upon which the fetus is dependent not only for the transfer of oxygen and a variety of nutrients, but the elaboration of a number of hormones and growth factors in the ``maternal-placental-fetal'' unit. Thus, it is perhaps appropriate to review some of the concepts regarding development of that organ and its function, the intervillous space, villous structure, and placental classification. The placenta differs from other organs in being formed by the interaction of both fetal and maternal tissues, shows extreme diversity in its structure among the species, and is of limited lifespan. Development of the placenta must be correlated closely with that of the fetus. Near or at term in humans the placenta weights {\textasciitilde}0.5 kg, and throughout pregnancy, by mechanisms poorly understood placental mass, maternal and fetal blood flows, villous surface area for nutrient exchange, and the elaboration of hormones must be matched closely to fetal growth and development. A history of placental biology might be considered from a number of perspectives. These include: embryology, variation by species, morphologic form, microscopic cellular organization, maternal and fetal circulations, the interface between maternal and fetal exchange surfaces, respiratory gas and nutrient exchange, metabolic, endocrine, and immunologic functions and others. Quite obviously, considerations of these subjects in detail would require a multivolume treatise far beyond a simple historical discussion (Figs. 8.1 and 8.2).