Decreased proplatelet formation and abnormal morphology in Cdc42^−/− and Rac1/Cdc42^−/− MKs. (A) TEM of MKs in BM of adult (6-week-old) mice. (i-ii) Normal ultrastructure in MKs derived from wt and Rac1^−/− mice. (iii-iv) Altered ultrastructure in MKs derived from Cdc42^−/− (iii) and Rac1/Cdc42^−/− (iv) mice. (Upper) Overview. Bar sizes are depicted. (Lower) Detailed view. Bar, 0.6 µm. (B-C) Determination of proplatelet formation from cultured MKs derived from wt (black), Rac1^−/− (dark gray), Cdc42^−/− (light gray), and Rac1/Cdc42^−/− (patterned) mice. Results are expressed as percentage of proplatelet-forming MKs per visual field ± SD from at least 5 samples per group. (B) Results from BM-derived MKs (day 6 of culture). (C) Results from fetal liver-derived MKs (embryonic day 14.5, day 4 of culture). ***P < .001; **P < .01.