Fig. 1. Frequency of endothelial or hematopoietic progenitors in E7.5 embryos. (A) Stroma-dependent hematopoietic colonies (a,b) and vascular endothelial colonies (c,d) are representatively shown. These colonies were formed by cells obtained from E7.5 embryos in co-culture with OP-9 stromal cells. Endothelial colonies were immunostained with anti-CD31 antibody. Red, CD31; blue, DAPI. Scale bar: 100 µm. (B) Limiting-dilution analysis of endothelial progenitors and stroma-dependent hematopoietic progenitors in E7.5 embryos. Cells were plated in 96-well plates and were co-cultured with OP-9 stromal cells for 7 days in the presence of IL3, TPO, SCF, EPO and VEGF. The frequency of endothelial progenitors was estimated to be one in 0.05 embryo equivalents. The 95% confidence interval (CI) was 1/38-1/56. The frequency of stroma-dependent hematopoietic progenitors was estimated to be one in 0.14 embryo equivalent, and the 95% CI was 1/115-1/181.