A subpopulation of apoptosis-prone cardiac neural crest cells targets to the venous pole: multiple functions in heart development?

A well-described population of cardiac neural crest (NC) cells migrates toward the arterial pole of the embryonic heart and differentiates into various cell types, including smooth muscle cells of the pharyngeal arch arteries (but not the coronary arteries), cardiac ganglionic cells, and mesenchymal cells of the aortopulmonary septum. Using a replication-incompetent retrovirus containing the reporter gene LacZ, administered to the migratory neural crest of chicken embryos, we demonstrated another population of cardiac neural crest cells that employs the venous pole as entrance to the heart. On the basis of our present data we cannot exclude the possibility that precursors of these cells might not only originate from the dorsal part of the posterior rhombencephalon, but also from the ventral part. These NC cells migrate to locations surrounding the prospective conduction system as well as to the atrioventricular (AV) cushions. Concerning the prospective conduction system, the tagged neural crest cells can be found in regions where the atrioventricular node area, the retroaortic root bundle, the bundle of His, the left and right bundle branches, and the right atrioventricular ring bundle are positioned. The last area connects the posteriorly located AV node area with the retroaortic root bundle, which receives its neural crest cells through the arterial pole in concert with the cells giving rise to the aortopulmonary septum. The NC cells most probably do not form the conduction system proper, as they enter an apoptotic pathway as determined by concomitant TUNEL detection. It is possible that the NC cells in the heart become anoikic and, as a consequence, fail to differentiate further and merely die. However, because of the perfect timing of the arrival of crest cells, their apoptosis, and a change in electrophysiological behavior of the heart, we postulate that neural crest cells play a role in the last phase of differentiation of the cardiac conduction system. Alternatively, the separation of the central conduction system from the surrounding working myocardium is mediated by apoptotic neural crest cells. As for the presence of NC cells in both the outflow tract and the AV cushions, followed by apoptosis, a function is assigned in the muscularization of both areas, resulting in proper septation of the outflow tract and of the AV region. Failure of normal neural crest development may not only play a role in cardiac outflow tract anomalies but also in inflow tract abnormalities, such as atrioventricular septal defects.