Primitive and definitive megakaryopoiesis from ES cells. (A) Megakaryocytes on days 9 (i,iii,v) and 13 (ii,iv,vi) are shown in pairwise fashion. Cells are observed under phase-contrast microscopy (i-ii). Consistent with the first-wave platelet production, small megakaryocytes on day 9 already display numerous proplatelets. These megakaryocytes are stained with anti-CD41 (iii-iv) and AChE (v-vi). Panels Ai and Aii were not stained. Original magnification in all photographs is × 200. (B) The diameter of the megakaryocytes on days 9, 11, 13, and 16 was measured as described in “Materials and methods.” The values shown are the mean ± SD in 50 cells. (C) The DNA content of the megakaryocytes on day 9 (top panels) and day 13 (bottom panels) was analyzed by flow cytometry. Wright-Giemsa staining of the typical analyzed cells is shown on the right (original magnification, × 400).