The fate diversity of mesodermal cells within the heart field during chicken early embryogenesis.

In gastrulation stage embryos of birds and mammals, the heart field is established as mesodermal crescents flanking the area rostrolateral to Hensen's node. Subsequent fusion of the bilateral heart primordia gives rise to a single tubular heart consisting of two epithelial layers: an outer myocardium and an inner endocardium. To date, it is uncertain whether these two distinct cell types of the heart arise from common or separate progenitor populations of mesodermal cells within the heart field. By retroviral single cell marking and tracking, we examined the diversity of cell populations present in the heart field of stage 4 chicken embryos. Here we demonstrate that individual mesodermal cells in the heart field gave rise to a clone consisting only of one cell type, either endocardial or myocardial cells; i.e., 95.1% of the mesoderm-derived clones were localized in the myocardium, while 4.9% of them were found in endocardium. No clones containing both of these two cell types were detected. The results suggest that the heart field mesoderm at stage 4 consists of at least two distinct subpopulations, containing more premyocardial cells than preendocardial cells. If there exists a common precursor of both myocardial and endocardial cells, the lineage diversification must occur at or prior to the arrival of mesodermal cells to the heart field.