The structure of the tupaia placenta. I. Histology and vascularisation.

The placenta of the tupaia is bidiscoidal, labyrinthine and endotheliochorial in structure. The two placental discs show a characteristic partition into three broadly fusing main lobules on the mesometrial, and some very small accessory lobules on the antimesometrial side. The accessory lobules are separated incompletely from each other. Their inner lobular structure is rather homogeneous. All vessels seem to exhibit a rather random distribution. The maternal vessels of the three main lobules are regularly arranged in such a way that each lobule can be subdivided into three zones. The lobular centre is the maternal inflow area. The intermediate zone surrounding the centre is composed of maternal and fetal capillaries; the peripheral capsule collects the maternal blood into coarse networks of veins. Each lobule corresponds to one maternal circulatory unit in which up to ten fetal circulatory units are included. The fetal vertical lobular arteries are evenly distributed. Fine networks of fetal capillaries spread out centrifugally towards small collecting veins. Opposite the maternal veins the fetal ones do not form meshworks. Within the main lobules fetal and maternal capillaries are arranged parallel, antiparallel or in varying angles to each other, resulting in a complex crosscurrent flow system.