Representative electron-microscopic views of mature MKs in the bone marrow of WASp-deficient mice. Samples obtained from bone marrow of wild-type 129Sv and WASp-deficient mice were processed for electronic microscopy as described in “Materials and methods.” Ultrastructural analysis showed the abnormal presence of WASp-deficient MKs displaying morphologic features of maturing MKs. (A) Mature MK showing delineated platelet territories (arrows) (magnification, × 2200; bar, 1 μm). (B) Bone marrow MKs in the process of shedding platelets (arrows) (magnification, × 2200; bar, 1 μm). (Inset) Proplatelets (PP) extended from a mature MK within the bone marrow (magnification, × 1650; bar, 1 μm). (C) Platelets (P) and proplatelets (PP) released from MKs and scattered within the bone marrow (magnification, × 2950; bar, 1 μm). (D) Naked MK nucleus (N) found in the bone marrow space (magnification, × 3900; bar, 1 μm).