Structure of the definitive placenta of the tenrec, Echinops telfairi.

Until recently, tenrecs were classified with insectivores in the order Lipotyphla, but nucleotide sequence data suggest they have closer affinities with a group of African mammals called Afrotheria. The placenta of Echinops has not been described and no studies involving electron microscopy of the placenta of any species of tenrec have been published. We used light and transmission electron microscopy to examine fixed placentae of embryos ranging from 25-66 mm in length. The placental disk is situated in the antimesometrial portion of the bicornuate uterus. The greater part of the disk consists of a labyrinth underlain by a spongy zone. The interhaemal barrier is unusual in that the trophoblastic component is a single layer of cytotrophoblast. These trophoblast cells have thick areas especially near the nuclei and extensive thin flanges but only occasionally have membrane-closed pore regions. The luminal surface has isolated patches of microvilli, and pinocytotic vesicles are numerous both apically and basally. In the centre of the placental disk is an elaborately folded haemophagous region. The primary folds have allantoic endoderm at one surface and columnar cytotrophoblast at the other. These trophoblast cells have numerous lipid droplets and vesicles, and often contain large yellow pigment crystalloids. The labyrinthine zone ends abruptly at the margins of the placental disk. However, the endoderm and connective tissue of the allantois and a layer of cytotrophoblast extend beyond the placental disk as a paraplacental region. Some of these distinctive features of Echinops placenta are shared with individual afrotherians, but no significant characteristic of definitive placentation is shared by all the Afrotheria.