Decreased proplatelets and altered ultrastructure of Pak-null megakaryocytes. (A) Bone marrow imaged for megakaryocytes from wild-type and Mx1cre⁺Pak2^−/− mice analyzed by transmission electron microscopy, 14 DPI. Wild-type megakaryocyte (upper, left, ×5000), periphery magnified (upper, right, ×12 000). Pak2^−/− megakaryocyte (lower, left, ×5000), periphery magnified (lower, right, ×20 000). Scale bar = 2 μm. (B) Proplatelet formation imaged from cultured megakaryocytes from wild-type and Cag-Cre-ERT2⁺Pak2^−/− bone marrow, induced with 500 nM 4-hydroxytamoxifen and adhered to fibrinogen. White arrows indicate proplatelet. (C) Percentage proplatelet-forming megakaryocytes expressed as a percentage of total megakaryocytes per visual field ± SEM, n = 5/genotype. Scale bar = 200 μm. *P < .05. (D) Representative image of proplatelet formation in vehicle and Frax1036-treated bone marrow-derived megakaryocytes. Scale bar = 200 μm. White arrows indicate proplatelet.