Runx1^−/− primitive erythrocytes show defect in morphology. (A,B) May-Grunwald Giemsa stained cytospin preparation of peripheral blood cells at E12.5. Although normal erythrocytes () can be seen in Runx1^−/− embryo, 27% of them show apparent deformed shape (). (C) Quantification of abnormal primitive erythrocytes in Runx1^+/− (n = 3) and Runx1^−/− (n = 4) embryos at E12.5. *Significance (P = .002, Student t test). (D-I) Scanning electron microscopy of primitive erythrocytes at E12.5. Shown are representative pictures of primitive erythrocytes from Runx1^+/− (n = 3) and Runx1^−/− (n = 3) embryos. Characteristic large holes (white arrowheads) were observed in Runx1^−/− erythrocytes (9 of 20), not in Runx1^+/− erythrocytes (0 of 16). This difference is statistically significant (P = .002 by Fisher exact probability test). Original magnifications: A,B, ×400; D-G, ×8000; H, ×8500; I, ×9000. A Leica (Wetzlar, Germany) DC500 CCD camera with IM50 Imaging Manager through Leica DMRXA microscope was used to capture the images using a 20×/0.50 NA objective (A,B).