Contribution of the primitive epicardium to the subepicardial mesenchyme in hamster and chick embryos.

A study about the hypothetical contribution of the epicardial cells to the subepicardial mesenchyme was carried out in Syrian hamster embryos of 9-12 days post coitum (dpc) and chick embryos of 3-5 days of incubation. In the epicardium and subepicardium of these embryos we have immunolocated the proteins cytokeratin (CK), vimentin (VIM), fibronectin (FN), and two antigens related to the transformation of endocardial cells into valvuloseptal mesenchyme, ES/130 and JB3. In the hamster embryos, CK+ subepicardial mesenchymal cells (SEMC) were apparently migrating from the primitive epicardium from 9.5 dpc at the atrioventricular (AV) groove and proximal outflow tract (OFT). The morphological signs of delamination extended by 11 dpc to the epicardium of the interventricular groove and the dorsal part of the ventricle. The relative abundance of the CK+ SEMC decreased in embryos of 12 dpc. VIM colocalized with CK in most SEMC, and in some epicardial mesothelial cells, mainly at the areas of delamination. CK immunoreactivity was also found in some early subepicardial capillaries. Similar observations were made in the chick embryos studied. The immunoreactive patterns obtained at the subepicardium with anti-FN, ES/130, and JB3 antibodies were similar to those reported in the areas of endothelial transformation of the endocardial cushions. We suggest that these observations are compatible with an epithelial-mesenchymal transformation involving the epicardial mesothelium and originating at least a part of the SEMC.