Structure of the proplatelet membrane skeleton. (A-B) Representative electron micrographs of the detergent-insoluble proplatelet cytoskeleton. Proplatelets were permeabilized with 0.75% Triton X-100, 0.1% glutaraldehyde, and 5μM phallacidin in PHEM buffer. Examination of the proplatelet membrane skeleton through electron microscopy reveals an intact membrane skeleton that laminates the underside and extends along the entire length of proplatelets. Inset: DIC image of murine proplatelets. Scale bar indicates 500 nm. (C) High-magnification, 3D electron micrograph of the proplatelet membrane skeleton showing the lattice-like network of elongated filamentous strands, which is similar in nature to the spectrin-based meshwork in erythrocytes and platelets. The membrane skeleton continuously laminates the underside of the proplatelet. A cytoplasmic bridge is shown (left) connecting to a swelling (right). Scale bar indicates 200 nm.