Placentation in the opossum, Didelphis virginiana

For the first 9 days of gestation, opossum embryos float in uterine secretions, separated from maternal tissues by a shell membrane. Each embryo is part of the wall of its hollow embryonic sphere. By the 10th day of development, the embryo becomes enveloped by both the amnion and yolk-sac. The yolk-sac consists of vascular and non-vascular portions and, together with the surrounding trophectoderm (trophoblast), forms the yolk-sac placenta of the opossum: the allantois does not contribute to formation of the placenta. The vascular portion of the yolk-sac placenta establishes an intimate relationship with the uterine epithelium soon after loss of the shell membrane. The yolk-sac placenta is non-invasive. Cells of the trophoblast exhibit numerous microvilli, an apical endocytic complex and the lateral and basal cell membrane are elaborately folded. These features suggest a cell that is active in the transport of materials. Junctional complexes between cells of the trophoblast and uterine epithelium were not observed. The uterine epithelium changes from ciliated pseudostratified columnar with few infoldings of lateral and basal cell membranes, to non-ciliated simple columnar in which these membranes show elaborate infoldings. The cells show numerous inclusions and mitochondria are polarized to the basal half of the cell. These features suggest a cell that also is active in the transport of materials.