title: The evolution of the placenta.
abstract: The very apt definition of a placenta is coined by Mossman, namely apposition or fusion of the fetal membranes to the uterine mucosa for physiological exchange. As such, it is a specialized organ whose purpose is to provide continuing support to the developing young. By this definition, placentas have evolved within every vertebrate class other than birds. They have evolved on multiple occasions, often within quite narrow taxonomic groups. As the placenta and the maternal system associate more intimately, such that the conceptus relies extensively on maternal support, the relationship leads to increased conflict that drives adaptive changes on both sides. The story of vertebrate placentation, therefore, is one of convergent evolution at both the macromolecular and molecular levels. In this short review, we first describe the emergence of placental-like structures in nonmammalian vertebrates and then transition to mammals themselves. We close the review by discussing the mechanisms that might have favored diversity and hence evolution of the morphology and physiology of the placentas of eutherian mammals.
authors.affiliation: C.S. Bond Life Sciences CenterUniversity of Missouri, Columbia, Missouri, USA Division of Animal SciencesUniversity of Missouri, Columbia, Missouri, USA robertsrm@missouri.edu.
authors.forename: R Michael
authors.initials: RM
authors.lastname: Roberts
authors.affiliation: Division of Animal SciencesUniversity of Missouri, Columbia, Missouri, USA.
authors.forename: Jonathan A
authors.initials: JA
authors.lastname: Green
authors.affiliation: Department of ObstetricsGynecology and Women's Health, University of Missouri School of Medicine, Columbia, Missouri, USA.
authors.forename: Laura C
authors.initials: LC
authors.lastname: Schulz
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005260:Female; D006801:Humans; D010920:Placenta / Q000166:cytology*; D010929:Placentation; D011247:Pregnancy