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.

Multiple modes of proepicardial cell migration require heartbeat

“…[16,24,33-35]. However, whether epicardial cells contribute to formation of cardiac muscle has been controversial [25,36-40].…”

“…In chick, the PE forms asymmetrically on the right sinus horn and migrates to the dorsal surface of the ventricular myocardium via an extracellular matrix bridge, which connects the PE and myocardium [13-16]. Epicardial coverage proceeds over the myocardium in a sheet-like manner [17].…”

Cardiac fibroblast in development and wound healing

“…Cells from the pro-epicardium detach and attach on the beating ventricular surface to form the epicardium[14]. Subsequently epithelial cells of the epicardium undergo epithelial-mesenchymal-transition (EMT) to form mesenchymal cells that invade the developing myocardium[15]. A subset of these mesenchymal cells after EMT acquire migratory properties and invade the developing myo-fascial planes to occupy an interstitial position in between cardiac myocytes to become resident cardiac fibroblasts[16, 17].…”

The Epicardium and the Development of the Atrioventricular Junction in the Murine Heart

“…The AV-EPDCs derive from the AV-epicardium, which, just like the rest of the epicardium, with the exception of the epicardium found on the outflow tract [19,20], originates from the proepicardium [21–23]. Until a few years ago, our knowledge of the importance of the epicardium in relation to the formation of the coronary vasculature, cardiac fibroblasts, and AV junction was largely based on experimental studies using avian model systems [24–27]. More recently, however, our insight into the role of the epicardium in heart development has been significantly advanced by studies in the murine heart.…”

“…The initial studies on the fate of EPDCs in the developing heart using avian models strongly suggested that EPDCs were able to migrate into the AV cushions [24,25]. These studies did, however, not provide much insight into the spatiotemporal contribution of the EPDCs to the respective leaflets.…”