Scanning electron microscopy of the canine placenta

The zonary placenta of the dog is peculiar in providing three morphologically and functionally distinct areas for transplacental exchange: the endotheliochorial labyrinth of the girdle zone, the hemochorial hemophagous zone of the marginal hematoma and the epitheliochorial free polar zone. Besides, the yolk sac persists throughout pregnancy. Scanning electron microscopy (SEM) was used to study these structures comprehensively with special emphasis on natural surfaces because of their relevance for exchange processes. Unequal cell diameter was typical for trophoblast cells. Surface specializations were observed on trophoblast cells lining the marginal hematoma, the free polar zone and the glandular chambers as well as on uterine epithelial cells of the glandular chambers and of the deep glands. In addition, maternal and fetal endothelial cells in the labyrinth showed major differences. Whereas fetal endothelial cells were extremely thin and polygonal in shape, maternal vessels were lined by thick endothelial cells exhibiting a complex interdigitation pattern of cytoplasmic processes. Thus, most of the natural surfaces could readily be distinguished by SEM due to their typical appearance.