The substructure of marginal bundles in human blood platelets.

A fixation protocol was worked out which permitted the visualization of the substructure of microtubules in human blood platelets. This protocol included a brief prefixation with 0.1% glutaraldehyde in a "polymerization mix buffer" including GTP, followed by a fixation mixture containing glutaraldehyde and tannic acid, and postfixation in 0.1% osmium tetroxide. The 13 individual protofilaments of the microtubules were clearly visible and had a triangular cross-sectional area. Filaments could be seen running alongside the microtubules and connected to them by many links. We term these filaments the "intertubular filaments." Some microtubules were incomplete and had a C-shaped profile; by serial sectioning it was seen that such microtubules were close to the terminal ends of the microtubules. A low percentage of the platelets had two marginal bundles oriented perpendicularly to each other.