Immunohistochemical identification of epithelial and mesenchymal cell types in the chorioallantoic and yolk sac placentae of the guinea-pig.

To define the epithelial and mesenchymal cell types of the guinea-pig placenta, immunostaining patterns were determined for the intermediate filament proteins cytokeratin and vimentin. Chorionic and yolk sac placentae were studied at 15, 20, 25, 29-30, 44-45, 55 and 65 days of gestation. Immunohistochemistry was performed on 5-microm thick sections of paraffin embedded tissue using specific antibodies against cytokeratin, a marker for epithelial cells, including trophoblast, and vimentin, a marker for mesenchymal cells and stromal decidua. Immunostaining was identified by the avidin-biotin-peroxidase technique with diaminobenzidine as the chromogen. Most of the surface of the placenta is covered by the columnar epithelium of the parietal yolk sac, beneath which is found a layer of chorionic giant cells. In the guinea-pig, a sheet of mesenchymal cells interposed between these cell layers immunostained for vimentin, a protein that is expressed only intracellularly, and had nuclei orientated parallel to the surface of the placenta. This cell layer is quite different from Reichert's membrane in the rat or mouse, which is acellular. Within the main placenta, cytokeratin immunostaining demonstrated that the trophoblasts lining the large maternal blood sinuses are different in character from the surrounding syncytiotrophoblast, confirming earlier ultrastructural observations. In the subplacenta, some trophoblast did not immunostain for cytokeratin and there was non-specific staining of cellular debris, so that immunostaining for vimentin provided the clearest indication of the maternal-fetal interface. In later stages of gestation (30-55 days), trophoblasts invading the walls of maternal arteries immunostained for cytokeratin and were vimentin negative. In early gestation, however, trophoblast invasion of the maternal vessels was indicated by cells that were immunoreactive for both cytokeratin and vimentin.