The evolving placenta: convergent evolution of variations in the endotheliochorial relationship.

Endotheliochorial placentas occur in orders from all four major clades of eutherian mammal. Species with this type of placenta include one of the smallest (pygmy shrew) and largest (African elephant) land mammals. The endotheliochorial placenta as a definitive form has an interhemal area consisting of maternal endothelium, interstitial lamina, trophoblast, individual or conjoint basal laminas, and fetal endothelium. We commonly think of such placentas as having hypertrophied maternal endothelium with abundant rough endoplasmic reticulum (rER), and as having hemophagous regions. Considering them as a whole, the trophoblast may be syncytial or cellular, fenestrated or nonfenestrated, and there may or may not be hemophagous regions. Variations also appear in the extent of hypertrophy of the maternal endothelium and in the abundance of rER in these cells. This combination of traits and a few other features produces many morphological variants. In addition to endotheliochorial as a definitive condition, a transitory endotheliochorial condition may appear in the course of forming a hemochorial placenta. In some emballonurid bats the early endotheliochorial placenta has two layers of trophoblast, but the definitive placenta lacks an outer syncytial trophoblast layer. In mollosid bats a well developed endotheliochorial placenta is present for a short time even after a definitive hemochorial placenta has developed in a different region. It is concluded that the endotheliochorial placenta is more widespread and diversified than originally thought, with the variant with cellular trophoblast in particular appearing in several species studied recently.