c-kit and Flt3 expression in the hematopoietic hierarchy. The figure indicates expression of c-kit (red, upper symbol on side of each cell) and flt3 (green, lower symbol on side of each cell) on various classes of hematopoietic stem and progenitor cells as well as mature blood cells, as described in the text. Because most hematopoietic cell populations are heterogeneous and hard to purify, it is not possible to exclude c-kit and/or flt3 expression on a minority of cells in the different cell populations. Therefore, the figure illustrates the c-kit and flt3 receptor status on the majority of cells within a specific population, based on studies of receptor expression and/or functional studies. As discussed in the text, the proposed hierarchy of pluripotent stem cells is based solely on different levels of c-kit and flt3 expression and does not take into account other stem cell antigens/characteristics, which are likely to uncover additional heterogeneity. Symbols: (−) most/all cells appear to lack c-kit or flt3 expression; (+) most/all cells appear to express c-kit or flt3; (+/−) the cell type appears to consist of significant receptor-positive as well as receptor-negative populations; (?) sufficient expression or functional data not available; (high and low) cell populations have been separated based on high and low levels of c-kit expression. Abbreviations: BFU, burst-forming units; CFU, colony-forming units; E, erythroid; Mk, mega karyocyte; G, neutrophilic progenitor; M, monocyte/macrophage; DC, dendritic cell; Baso, basophil; RBC, red blood cell; NK, natural killer cell.