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 embryo s of 9-12 days post coitum (dpc) and chick embryos of 3-5 days of incu bation. In the epicardium and subepicardium of these embryos we have i mmunolocated the proteins cytokeratin (CK), vimentin (VIM), fibronecti n (FN), and two antigens related to the transformation of endocardial cells into valvuloseptal mesenchyme, ES/130 and JB3. In the hamster em bryos, CK+ subepicardial mesenchymal cells (SEMC) were apparently migr ating from the primitive epicardium from 9.5 dpc at the atrioventricul ar (AV) groove and proximal outflow tract (OFT). The morphological sig ns of delamination extended by 11 dpc to the epicardium of the interve ntricular groove and the dorsal part of the ventricle. The relative ab undance of the CK+ SEMC decreased in embryos of 12 dpc. VIM colocalize d with CK in most SEMC, and in some epicardial mesothelial cells, main ly 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 simila r to those reported in the areas of endothelial transformation of the endocardial cushions. We suggest that these observations are compatibl e with an epithelial-mesenchymal transformation involving the epicardi al mesothelium and originating at least a part of the SEMC. (C) 1997 W iley-Liss, Inc.