Morphogenesis of the placenta and fetal membranes of the tree shrews (family tupaiidae)

The paucity of information on the early developmental stages and the necessity for re‐evaluation of the phylogenetic implications of the tupaiid fetal membranes have stimulated the present study. Neonatal tupaiids possess prominent bilateral endometrial pads that run the length of each uterine horn. These gland‐free pads are the sites of implantation and placentation in adults. Implantation is bilateral and superficial. Multinucleated giant cells differentiate from the cytotrophoblast of the attachment sites; they facilitate the marginal expansion of the placental disc by detaching the paraplacental epithelium from its basement membrane. Prominent bilateral chorio‐vitelline placentae develop and remain functional through early limb bud stages. Decidual cells fuse to form “decidual knots” during this period. The bilateral chorio‐allantoic placentae are labyrinthine endotheliochorial. The trophoblast of the labyrinth is entirely syntrophoblastic, similar to the condition in the armadillo placenta. Epithelioid mesenchymal cells in the labyrinth are strikingly similar to those of the shrew placenta. The large allantoic vesicle and free vascular yolk sac persist. Amniogenesis occurs by folding. Uterine glands remain secretory throughout pregnancy; iron is probably supplied to the fetus in this manner.