%0 Journal Article
%T Placental Evolution within the Supraordinal Clades of Eutheria with the Perspective of Alternative Animal Models for Human Placentation
%A Andrea Mess
%J Advances in Biology
%D 2014
%R 10.1155/2014/639274
%X Here a survey of placental evolution is conducted. Placentation is a key factor for the evolution of placental mammals that had evolved an astonishing diversity. As a temporary organ that does not allow easy access, it is still not well understood. The lack of data also is a restriction for better understanding of placental development, structure, and function in the human. Animal models are essential, because experimental access to the human placenta is naturally restricted. However, there is not a single ideal model that is entirely similar to humans. It is particularly important to establish other models than the mouse, which is characterised by a short gestation period and poorly developed neonates that may provide insights only for early human pregnancy. In conclusion, current evolutionary studies have contributed essentially to providing a pool of experimental models for recent and future approaches that may also meet the requirements of a long gestation period and advanced developmental status of the newborn in the human. Suitability and limitations of taxa as alternative animal models are discussed. However, further investigations especially in wildlife taxa should be conducted in order to learn more about the full evolutionary plasticity of the placenta system. 1. Introduction Evolution of the placental system is a key feature of placental mammals or Eutheria. It was already present in their ancient condition (stem species pattern) and had evolved an astonishing diversity following the group¡¯s radiation during the Tertiary [1¨C8]. Indeed, both their chorioallantoic and yolk sac placentas are regarded as most diverse organs [4, 9¨C13]. This diversity affects structural and functional aspects such as trophoblast invasion, the establishment of the fetomaternal interface, immune tolerance, and exchange processes. Significant variations occur also in gene expression patterns. About 3000 genes are specifically expressed in eutherian placentas, associated with gene duplications, specifically expressed genes, and a high percentage of retroviral genes adapted for placental functions [14¨C18]. However, as a temporary organ that does not allow easy access to structure and function, an essential lack in understanding the interaction on various levels remained and placentation was dramatically underrepresented in the traditional surveys on the evolutionary history of mammals. Fortunately, for various reasons the situation has been improved to some degree in the last couple of years. 1.1. Evolutionary Developmental Biology First, due to the rise of the broad
%U http://www.hindawi.com/journals/ab/2014/639274/