THE ORIGIN OF THE SUBEPICARDIAL MESENCHYME IN THE AVIAN EMBRYO - AN IMMUNOHISTOCHEMICAL AND QUAIL-CHICK CHIMERA STUDY

It has been proposed that the subepicardial mesenchymal cells (SEMC) o riginate from the primitive epicardium and also from migration of extr acardiac mesenchyme from the liver area. We have studied the possibili ty of an origin of SEMC through transformation of the proepicardial me sothelium, as well as the potential of the early proepicardium to gene rate epicardium and SEMC in quail-chick chimeras. The study was carrie d out in quail and chick embryos between HH16 and HH29 stages. Most pr oepicardial cells, mesothelial as well as mesenchymal, were cytokerati n and vimentin immunoreactive, suggesting a cytoskeletal shift from th e epithelial to the mesenchymal type. Furthermore, we immunolocated, i n the proepicardial mesothelium, three proteins specifically expressed during the endothelial-mesenchymal transition of the endocardial cush ions, namely the JB3/fibrillin-associated antigen, the ES/130 protein and the smooth muscle cell alpha-actin. Grafts of proepicardial tissue from HH16-17 quail embryos into chick embryos of the same age origina ted large areas of donor-derived epicardium, including mesothelial, me senchymal, and vascular cells. The donor-derived primitive epicardium showed segment-specific features, being squamous and adhered to the my ocardium on the atrial wall and showing morphological signs of ingress ion in the atrioventricular groove and outflow tract. These morphologi cal traits together with the distribution of vimentin, the ES/130 prot ein, and the JB3/fibrillin-associated antigen suggested a localized tr ansformation of some epicardial mesothelial cells into mesenchyme. Mos t of the donor-derived cells, mesothelial and mesenchymal, showed the vascular marker QH1, which frequently colocalized with cytokeratin. He terotopic grafts of quail splanehnopleura into the pericardial cavity of chick embryos originated a squamous, epicardial-like, cytokeratin-i mmunoreactive cell layer on the heart surface, as well as a few QH1(+) subepicardial and intramyocardial cells. The results suggest that a s ubstantial part of the subepicardial mesenchyme, including the progeni tors of the cardiac vessels, originates from the transformation of pro epicardial and epicardial mesothelial cells into mesenchyme, and that the epicardial transition could be driven by a segment-specific myocar dial signal, (C) 1998 Academic Press.