Sox6^–/– erythroid cells have decreased F-actin levels that affect maturation. (A) FACS profiles of F-actin in bone marrow cells from P7 control and Sox6^–/– littermates. Cells were stained with TER119 and phalloidin. (Left panels) FSC/phalloidin profiles, with TER119^– cells shown in blue and TER119⁺cells in orange. Based on the FSC and TER119 staining, 4 subpopulations were identified: I, nonerythroid/early erythroid cells; II, proerythroblasts; III, early erythroblasts; and IV, late erythroblasts. (Middle and right panels) Distribution of F-actin in each subpopulation. The mean and standard deviation are indicated for 4 control and 3 mutant littermates. * indicates statistically significant differences between controls and mutants (Student t test, P < .05). (B) Confocal microscopy of representative P7 bone marrow cells stained with phalloidin (green) and the nucleic acid dye Hoechst 33258 (blue). The arrows point to a control cell with a thick F-actin cytoskeleton and a mutant cell with a thinner F-actin cytoskeleton. (C) FSC/TO profiles of control and Sox6^–/– TER119⁺ erythroid cells from cultures on fibronectin for 48 hours with 0.2 U/mL Epo and with or without cytochalasin D (cytoD) and jasplakinolide (Jasp). (D) Graph combining cell counts and FACS profile data for the experiment described in panel C.