Does the subepicardial mesenchyme contribute myocardioblasts to the myocardium of the chick embryo heart? A quail‐chick chimera study tracing the fate of the epicardial primordium

Abstract

Morris (J. Anat., 1976;121:47-64) proposed that the subepicardial mesenchyme might represent a continuing source of myocardioblasts during embryonic and fetal development. Recent studies have shown that the epicardium and subepicardial mesenchyme, and the coronary vasculature are all derived from a region of the pericardial wall, called the proepicardial serosa. In avian embryos, the cells from the proepicardial serosa colonize the heart via a secondary tissue bridge formed by attachment of proepicardial villi to the heart. In the present study, Morris's hypothesis was tested by tracing the fate of the proepicardial serosa. This was achieved by constructing quail-chick chimeras. The proepicardial serosa was transplanted from HH16/17 quail embryos to HH16/17 chick embryos (ED3). A new transplantation technique facilitated an orthotopic attachment of the quail proepicardial villi to the chicken heart, and prevented the attachment of the chicken proepicardial villi to the heart. The fate of the grafted quail cells was traced in chimeras from ED4 to ED18 with immunohistochemistry, using quail-specific antibodies (QCPN, QH-1). From ED4 onward, the transplant was connected to the dorsal heart wall via its proepicardial villi. Starting from the point of attachment of the quail proepicardial villi to the heart, the originally naked myocardium became almost completely covered by quail-derived epicardium, and quail mesenchymal cells populated the subepicardial, myocardial, and subendocardial layers including the av-endocardial cushions. Quail cells formed the endothelial and smooth muscles cells of the coronary vessels, and the perivascular and intramyocardial fibroblasts. Quail myocardial cells were never found in the subepicardial, myocardial, and subendocardial layers. This suggests that the subepicardial mesenchyme normally does not contribute a substantial number of myocardioblasts to the developing avian heart. The new transplantation technique presented facilitates the production of chimeric hearts in which the derivatives of the proepicardial serosa are almost completely of donor origin. This technique might be useful for future studies analyzing the role of certain genes in cardiac development by the creation of somatic transgenics.

Extracardiac septum transversum/proepicardial endothelial cells pattern embryonic coronary arterio–venous connections

“…G2-Gata4-driven reporter expression first is confined to the epicardium and its mesenchymal derivatives (1,(20)(21)(22) and then expands to the CoE.…”

Preotic neural crest cells contribute to coronary artery smooth muscle involving endothelin signalling

“…Quail-chick proepicardium (PEO) chimeras have been described previously 57 . Stage 16-17 quail hearts were inspected to confirm that the PEO was not attached to the dorsal wall of the heart.…”

Signaling During Epicardium and Coronary Vessel Development

“…15,17 This study was followed by others evaluating the ability of the primitive epicardial epithelium to undergo EMT 7,48 and the developmental fate of EPDC. 9,18,19 Many of these studies cultured isolated epicardial progenitor cells (PE) in vitro or simply transplanted them into a host embryo in ovo. As a result of both procedures, the PE, after making contact with the myocardial surface, recapitulates its normal transformation into a monolayered epicardial epithelium.…”

“…The contribution of PE cells to the coronary endothelium (CoE) is less clear. Avian PE gives rise to endothelial cells both in vivo [17][18][19] and in vitro. 16,20,21 In these in vitro experiments, endothelial differentiation was dependent on the addition to the culture medium of the provascular growth factors vascular endothelial growth factor (VEGF) and fibroblast growth factor-2 (FGF2; Figure 1F); these factors, which are produced by the embryonic myocardium, are required for coronary vessel development in the embryo.…”