Reduced proplatelet formation and absence of swellings in ADF/n-cofilin–null MKs. Fetal liver cells of embryos at day 14.5 were prepared and cultured in medium containing 50 ng/mL TPO. On day 4, MKs were stained (A,C). Control (Ci), ADF/n-cofilin–null (Cii-iv). Scale bars represent (A) = 10 μm, (Ci) = 50 μm, (Cii-iii) = 25 μm, (Civ) = 10 μm. (B) Fetal liver cells were cultured for 4 days and MKs were analyzed for proplatelet formation under a light microscope using a 20× objective. (●) Average of 20 analyzed visual fields per fetal liver-derived MK culture. The horizontal bars represent the arithmetic mean of each group. ***P < .001. (D) Bone marrow sections (0.5-mm-thick) were incubated with Tyrode buffer supplemented with 5% mouse serum at 37°C. Representative pictures of small MKs, spherical MKs, MKs with protrusion, and MKs with proplatelets are shown. Quantification was performed after 3 and 6 hours of incubation. Arrows indicate MKs (small MKs, spherical MKs, MKs with large pseudopodia). Arrows indicate swellings (MKs with proplatelets). (E) Representative images of bone marrow explants from ADF/n-cofilin–null mice after 6 hours of culture. The morphologies of spherical MKs (i) and proplatelet-like extensions (ii) were compared with control (see also control MKs in panel D) and analyzed. (i) Arrow indicates the MKs. (ii) Arrow indicates short extension of the MKs. (iii) Isolated extensions (black arrows) of ADF/n-cofilin–null MK were observed in the chamber.