Comparative Placental Anatomy: Divergent Structures Serving a Common Purpose.

The placenta, one of the most important transient organs, forms by the apposition of fetal membranes and maternal tissues. Its role is to mediate physiological exchanges between mother and fetus. The word "apposition" covers a wide range of structural variations. It includes approximation, adhesion, interdigitation, or actual fusion between fetal and maternal tissues. Formation of the placenta establishes hemotropic nutrition for the fetus: essential metabolites must be provided to maintain the growing fetus, and these must come to it via the maternal circulatory system. Equally important, the placenta also provides oxygen and removes metabolic waste products from fetal blood. Nutritive and excretory roles of the placenta are not its only functions: it also has immune and endocrine activities. Nutrient and gas transport, waste removal, immunological protection of the fetus, and hormonal secretion influencing the maternal metabolism are all complex functions. They may also to some extent be conflicting purposes; hence, the placenta is a complex fetal organ. It is structurally adapted to perform its roles somewhat differently in different species, but the set of functions remain the same. Understandably, the placenta has been the subject of extensive research, and it will continue be an important topic thanks to its complexity. The intent of this chapter is to provide a simple description of placental anatomy using classic categories and to describe anatomical species variations in humans, important domestic animals, and the major laboratory species.