WASp is involved in inhibition of proplatelet formation induced by α2 integrin ligation. (A) The Lin^- bone marrow cells harvested from wild-type 129Sv mice or WASp-deficient mice, grown for 72 hours in the presence of TPO were plated onto PLL-, CI-, CVX-, or GFOGER-coated coverslips for 2 hours. Adherent cells were fixed, permeabilized, and stained with anti-VWF and TRITC-phalloidin, and the percentage of MKs displaying VWF⁺ proplatelets was evaluated by counting at least 300 MKs showing VWF⁺ staining in each experiment. The graph represents the mean ± SD of the percentage of MKs showing VWF⁺ proplatelets in the indicated substrates in 3 independent experiments. (B) Phase-contrast (i) and diffuse F-actin staining (ii) images illustrate proplatelet formation in WASp-deficient MKs adherent to GFOGER peptide, the high-affinity substrate of α2 integrin. Bar, 10 μm.