Meis1 and Meis2 increase the numbers of ES cell–derived definitive hematopoietic colonies in semisolid media. (A) ES cells with dox-inducible Meis1 (A2lox.Meis1) or Meis2 (A2lox.Meis2) were differentiated as EBs for 6 days before plating in methylcellulose media with cytokines as described in “Methods.” Hematopoietic colonies were quantitated after 6 days of growth on methylcellulose based on morphologies. Data represent the average of 3 experiments. Error bars represent SD. (B) Bright-field microscopy of definitive hematopoietic colonies derived from A2lox.Meis1 or A2lox.Meis2 ES cells with (+) or without (−) treatment with doxycycline (dox). Original magnification ×40. (C) A2lox.Meis1 or A2lox.Meis2 EBs were dissociated on day 6 after differentiation and plated in MegaCult-C media. After another 6 days, megakaryocyte colony formation was visualized by acetylthiocholine iodide and Harris hematoxylin counterstain, and CFU-Mk and mixed Mk colonies were quantitated. Data represent the average of 4 experiments. Error bars represent SD. (D) Bright-field microscopy of CFU-Mk and mixed Mk colonies derived from A2lox.Meis1 ES cells in the presence of dox. Original magnification ×100. CFU-Mk appeared brown because murine megakaryocytes express acetylcholinesterase, producing brown precipitate. Mixed Mk colonies were distinguished by the presence of nonmegakaryocytic cells within brown-staining cell clusters.