Fig. 7. Heart cells and their lineages, and a model for pnt and pnr function in the heart. (Top) The heart and heart cell lineages. In the whole heart schematic, Tin-positive pericardial cells are not shown in the posterior domain because in this region they reside beneath the cardioblasts. Segment names are provided above the schematic. The lineages of all anterior domain heart cells (left) and all heart cells found within one segment of the posterior heart domain (right) are shown below the whole heart schematic. Owing to sample size, Tin expression in pericardial cells 1-9 in the anterior domain should be considered preliminary. (Bottom) Cardioblasts and pericardial cells develop from the dorsalmost region of the mesoderm: the cardiac mesoderm (CM). Initially, all dorsal mesodermal cells express Tin (top). Tin expression then resolves into two stripes of cells: a dorsal stripe that defines the cardiac mesoderm and a ventral undulating stripe that labels the primordium of the visceral mesoderm (VM). As development proceeds, cardioblasts and pericardial cells develop from the cardiac mesoderm with pericardial cells residing interior and slightly ventral to cardioblasts. In the absence of pnr function, most of the cardiac mesoderm does not form; however, when cardiac mesoderm forms, the subsequent development of cardioblasts and pericardial cells is normal. In the absence of pnt, cardiac mesoderm forms normally but the subsequent development of pericardial cells is perturbed such that cardioblasts appear to develop in their place.