Comparative placentation: reasons for diversity and phylogenetic fit

Abstract

Although there are common features in placental structure, such as short diffusion distances between vascular systems in late pregnancy, there is great morphological variation both in definitive placental structure and in the way placentas develop. In species with a long gestation period and precocial young, placentas with more intervening layers, which tend to reduce fetal to maternal cell exchanges, are an obvious advantage. In other instances, there are no apparent advantages in different placental structures. Several factors favor development of diversity in placental morphology. 1. The extraembryonic membranes of one organism, the conceptus, must develop in the uterus of another organism. 2. Early conditions for the developing conceptus are anaerobic whereas later efficient aerobic conditions are essential for continued growth of the fetus. 3. There are potentially two partially sequential placentas, the yolk sac and the chorioallantoic placenta. Despite these factors favoring various methods of forming the definitive placenta, the placental type is consistent within families, but less consistent at the order and superorder levels. The consistency within families suggests that the basic classification of mammals was reasonably accurate even before genomics, and that divergence in placental structure accompanied the initial divergence of different mammalian groups.