Placental evolution from a three-dimensional and multiscale structural perspective.

The placenta mediates physiological exchange between the mother and the fetus. In placental mammals, all placentas are descended from a single common ancestor and functions are conserved across species; however, the placenta exhibits radical structural diversity. The selective pressures behind this structural diversity are poorly understood. Traditionally, placental structures have largely been investigated by grouping them into qualitative categories. Assessing the placenta on this basis could be problematic when inferring the relative "efficiency" of a placental configuration to transfer nutrients from mother to fetus. We argue that only by considering placentas as three-dimensional (3D) biological structures, integrated across scales, can the evolutionary questions behind their enormous structural diversity be quantitatively determined. We review the current state of placental evolution from a structural perspective, detail where 3D imaging and computational modeling have been used to gain insight into placental function, and outline an experimental roadmap to answer evolutionary questions from a multiscale 3D structural perspective. Our approach aims to shed light on placental evolution, and can be transferred to evolutionary investigations in any organ system.