Mast cell formation in liquid culture of yolk sac cells. Yolk sac cells collected at E9.5 were subjected to long-term in vitro liquid culture with KL and IL-3 for mast cell formation. Mast cell formation from wild-type cells (A) and GATA-2^−/− cells (B) was examined by Toluidine blue staining after 3 weeks in culture. mc, macrophage; m, mast cell. This finding was reproduced in 3 independent experiments.