Megakaryocyte colony and proplatelet formation by ES-derived hematopoietic cells. A hematopoietic colony with unclear borders was formed on day 6 of differentiation (A). On day 12, a megakaryocyte colony with individually identifiable large cells appeared (B). These cells were visualized by immunostaining with anti-CD41 (GPIIb) antibody (C-D) and AChE staining (E). CD41⁺ mature megakaryocytes appeared at the periphery of the colonies. On day 13, almost all cells within the colonies became large (F). These cells were immunostained with an anti-CD41 antibody and were CD41⁺ (G). Cytospin preparation of the culture medium on day 13 was stained by Wright-Giemsa staining (H). On day 14 of culture, cells attached on the layer (I), floated in the medium (J), and in cytospin preparation (K) displayed proplatelet formation from mature megakaryocytes. The cytospin preparation (K) was immunostained with anti-CD41 antibody. Panels T and J were not stained. The proplatelets were CD41⁺ up to the top of the projections. Original magnifications were × 100 for panels A-E, × 200 for panels F-G, × 600 for panel H, and × 400 for panels I-K.