Early changes in the substructure of the marginal bundle in human blood platelets responding to adenosine diphosphate

Human blood platelets were exposed to 10 μM adenosine diphosphate in order to induce shape changes and their ultrastructure was examined in preparations fixed with a glutaraldehyde-tannic acid mixture. Within 5 sec after the addition of ADP the marginal bundle along the perimeter of the platelet had undergone profound changes. The intertubular filaments of the marginal bundle were lost and, probably as a consequence, the microtubules were dispersed. The microtubules also had fragmented and appeared as straight rods rather than smoothly curved coils. Profiles with 14 or 15 protofilaments rather than the common 13-unit pattern were seen in 16% of the cross-sectioned microtubules. The proportion of incomplete (C-shaped) microtubules or of microtubules with an electron-dense interior also had increased and were interpreted as sections near the terminal ends of a microtubule. We interpret the C-shaped and the “filled” microtubules as signs of a disassembled microtubule and the 14- and 15-protofilament profiles as probably representing new (ends of) microtubules reassembled with a defective control of the proper structure.