Early heart development: dynamics of endocardial cell sorting suggests a common origin with cardiomyocytes

The myocardial and endocardial cell sorting out processes take place primarily between 19 and 29 hr of development in the avian embryo. This occurs in an apparent rostral to caudal wave through the heart forming region. During heart development considerable uncertainty exists regarding the processes that regulate cell commitments, progressive aggregation, and sorting out of the different precardiac cell populations. The question addressed in this report is whether endocardial and myocardial cells have a common origin or do the endocardial cells arise from a distinct population of cells from within the precardiac mesoderm. These cells then migrate to become localized between the developing myocardium above and the endoderm below. The distribution of preendocardial cells and premyocardial cells has been followed immunohistochemically in quail heart-forming region mesoderm explants from embryos approximately 18 hr in development and incubated for a 24-hr period. Differentiating myocardiocytes were immunostained with anti-N-cadherin and endocardiocytes with QH-1, a monoclonal antibody that recognizes an antigenic determinant on quail endothelial cells. Sparsely localized QH-1 labeled endothelial cells are localized in the stage 5 heart-forming region. These cells are often arranged in a columnar fashion in the mesoderm explants 6 hr after explantation. By 15-22 hr large patches of QH-1 expressing cells are interspersed with the N-cadherin expressing myocardiocytes. A subpopulation of cells express both N-cadherin and QH-1 antigen suggesting that endocardial and myocardial cells may arise from a common precursor population and that N-cadherin regulation may be a mechanism underlying specific cell sorting of these two cell populations during heart development.