Development and structure of the extra-embryonic membranes of the ferret. A light microscopic and ultrastructural study.

Light and electron microscopic observations on the development and structure of the extra-embryonic membranes of the ferret are described. By day 16 of pregnancy trophoblastic villi consisting of a cytotrophoblastic core with a peripherally located phagocytic syncytiotrophoblast have penetrated deeply into the endometrium. During its invasion the syncytiotrophoblast removes endometrial glandular cells and some of the adjacent stromal tissue. The maternal blood vessels remain intact and eventually become surrounded by the syncytiotrophoblast. A marked hypertrophy of the maternal capillary endothelium then begine. By day 28 the maternal capillary endothelial cells attain their maximum height. The cytoplasm of these cells contains numerous cisternae of rough endoplasmic reticulum and a well-developed Golgi apparatus, features which are typical of protein-secreting cells. The maternal blood vessels are separated from the syncytiotrophoblast by a thick layer of amorphous material which stains positively with PAS. Alcian blue staining in the presence of critical concentrations of magnesium chloride indicates the presence of both sulphated and carboxylated acid mucopolysaccharides. It may be that the amorphous material contributes significantly to the nutritional requirements of the developing fetus. Invaginations of the syncytiotrophoblast plasma membrane facing this amorphous layer are often seen. Coated vesicles and substances in contact with the outer surface of the membrane are therefore likely to be endocytozed. The acid phosphatase in the syncytiotrophoblast is consistent with the presence of lysosomes and catabolic function. Regions of paraplacental cellular trophoblast which lie in close association with the endometrium, but do not penetrate it, appear to specialize in endocytosis. The haemophagus organ, which is located at the antimesometrial pole, is the site of rupture of maternal blood vessels, and the extravasated blood lies in close association with the chorio-allantoic membrane. The maternal blood cells ingested by the trophoblast in this area are presumably an important source of iron for the embryo. The cellular trophoblast in the paraplacental regions is well equipped with hydrolytic enzymes, as shown by histochemical tests for acid phosphatase. It seems likely that these regions are concerned with embryotrophic endocytotic nutrition supplementing endothelio-chorial nutrition.