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

Abstract

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. Dev.

The epicardium and epicardially derived cells (EPDCs) as cardiac stem cells

“…The space that is sandwiched between the myocardium and epicardium is known as the subepicardium or subepicardial space. Within the subepicardial space, subepicardial mesenchymal cells can be found that are derived from the epicardium as a result of epicardial EMT (Pérez-Pomares et al, 1997, 1998aDettman et al, 1998;reviewed in Mä nner et al, 2001). These cells are therefore generally referred to as EPDCs (Gittenberger-de Groot et al, 1998).…”

“…Whereas in the avian embryo the initially acellular proepicardial ECM is rather extensive, in other species there is relatively little ECM and the entire proepicardium is rather compact. As a result of proepicardial EMT, the ECM becomes populated with mesenchymal cells (Pérez-Pomares et al, 1997, 1998a,1998b, 2002a, 2002b) (see Fig. 1C).…”

“…In the mammalian embryo, the primary mechanism through which the proepicardial cells connect to the cardiac surface is via the release of groups, or vesicles, of cells from the proepicardial protrusions into the pericardial cavity (Van den Eijnde et al, 1995;Pérez-Pomares et al, 1997;Mä nner et al, 2001). These cells attach to the myocardial surface of the heart in more or less the same areas in which the proepicardial protrusions attach in the avian heart.…”

“…At this point of development a new layer of epithelial cells starts to populate the myocardial surface of the heart (Ho and Shimada, 1978;Virá gh and Challice, 1981;Kuhn and Liebherr, 1988;Hiruma and Hirakow, 1989;Fransen and Lemanski, 1990;Hirakow, 1992;Mä nner, 1992Mä nner, , 1993Muñ oz-Chá puli et al, 1997). This cell layer is known as the epicardium.…”

“…During and after the formation of the epicardial sheet, a subset of epicardial cells detach from their epithelial context ( Fig. 1D) and generate in the subepicardium a population of epicardially derived cells (EPDCs) through an epicardial EMT (Muñ oz-Chá puli et al, 1996;Pérez-Pomares et al, 1997, 1998aDettman et al, 1998;Gittenberger-de Groot et al, 1998). The epicardial EMT is observed at the AV junction, in the ventricular epicardium, and in the epicardium at the junction between the ventricles and the OFT, but not in the atrial epicardium.…”