Novel tissue interactions support the evolution of placentation

Organ development occurs through the coordinated interaction of distinct tissue types. So, a question at core understanding evolution new organs is, how do tissue-tissue signalling networks arise? The placenta is great model for organs, because placentas have evolved repeatedly, relatively recently in some lineages, and exhibit intermediate forms extant clades. Placentas, like other form from two tissues, one maternal fetal. If each these tissues produces signals that can be received by other, then apposition likely to result dynamics used as scaffold support development. Using published data examples, this review I demonstrate are derived hormonally active considerable potential exists between fetal egg-laying vertebrates, conserved oviparity-viviparity transition, consequences interactions basis aspects placentation including embryo implantation. argue placental not merely consequence formation, but novel regulatory networks, functions, patterning mechanisms.