Fig. 7. (A-D) Blast colonies develop into haematopoietic and endothelial cells. Blast colonies (A), typically composed of 50-200 round cells emanating from a dense core, proliferated in liquid culture (B) supplemented by VEGF, SCF, IL3 and EPO to give haematopoietic cells of myeloid and erythroid lineages (C, May-Grunwald-Giemsa stained cytocentrifuge preparation) and adherent endothelial cells (D) visualized by antibodies to CD31. (E,F) Frequency of blast colony-forming cells (Blast-CFCs) in GFP⁺FLK1⁺, GFP⁺FLK1^-, GFP^-FLK1⁺, GFP^-FLK1^- populations of cells isolated from differentiating Mixl1^GFP/w ES cells. (E) In day 3 EBs, Blast-CFCs were distributed throughout the four populations, reflecting the dynamic nature of the differentiation process at this time (see Discussion). (F) By day 4, the GFP⁺FLK1⁺ fraction contained the highest frequency and the majority of Blast-CFCs. UN, unsorted. Error bars represent 1 s.d. (n=3 for day 3; n=4 for day 4).