Ultrastructure of the canine placenta and amnion

The zonary placenta of the dog forms an endotheliochorial labyrinth. It shares the major ultrastructural features of the typical carnivore placenta. Fetal components include a trophoblastic syncytium, cellular trophoblast (which is markedly reduced at at term), transitional trophoblast, capillaries, and connective tissue. The columnar trophoblast of the green border is phagocytic. Maternal components include thick-walled capillaries and a hypertrophied basal lamina. Endometrial giant cells are lacking. Ultrastructurally the amnion resembles that of most other mammals. The dog's placenta differs from that of the cat in its reduced terminal web and increased pericellular deposit. The prominent noncellular barrier of the dog's placenta may be related to the degree of trophoblastic invasiveness and the interaction of antigenically dissimilar tissues.