Mesodermal differentiation of ES cells on OP9 stromal cells. (A) E14 ES cells were cultured on collagen type IV plates for 2 days, and then all cells were cultured on OP9 cells for 9 days. The expression of CD34, c-Kit, Sca-1, CD45, Flk1, and Tie2 in ES cell–derived cells was analyzed at the indicated time points by flow cytometry. Stained Tie-/- cells are represented by purple shaded histograms. Unstained controls are represented by green lines. The percentages of cells in each quadrant are indicated. (B) Gene expression of fractionated cells shown in A (R3-R6) was analyzed by RT-PCR. (C) Fractionated cells (20 000; R3-R6) were cultured on OP9 cells. On day 7 of culture, hematopoietic clusters (red arrowheads) were developed only from the R3 fraction. In other fractions (R4-R6) hematopoietic clusters were not developed, but embryoid body–like colonies (blue arrowheads) developed from R5 and R6 fractions. Cells were analyzed at low (×40) magnification.