Schematic representation of SP cells in YS. (A) 10.5 dpc YS SP cells identified by cKit staining localize to the mesodermal layer and to clusters of cells within blood vessels (arrowheads). They exhibit endothelial and hematopoietic phenotypic characteristics and demonstrate enriched hematopoietic activity (endodermal layer pseudocolored green). Original magnification, × 1000. (B) Schematic depicting the orientation of endodermal (green) and mesodermal (blue) yolk sac layers in relation to the developing embryo. In the YS close-up, some cKit^HI (dark blue) YS SP cells embedded in the endothelium acquire the ability to generate hematopoietic cells (red cells in vessel lumen). Green cells are endodermal cells; light blue cells, endothelial cells.