Photographs of phase-contrast microscopy and May-Giemsa staining of the colonies developed from day-10.5 mouse yolk sac cells. (A) and (C) show a pure megakaryocytic colony developed in the semisolid medium containing TPO (10 ng/mL) only. (B) and (D) show an erythro-megakaryocytic colony developed in medium containing Epo (2 U/mL) and TPO (10 ng/mL).