Morphometry of platelet internal contraction.

Blood platelets have a characteristic discoid shape supported by a circumferential band of microtubules. Following stimulation by aggregating agents or foreign surfaces, platelets lose their discoid form, extend pseudopods, and undergo a process of internal reorganization. Randomly dispersed cytoplasmic organelles become concentrated in cell centers within rings of microtubules and masses of microfilaments. Questions have been raised about this process and its contractile nature by studies demonstrating that platelet microtubules dissolve within seconds after activation and reassemble several minutes later in new locations. Earlier investigations showed that Taxol, a microtubule-stabilizing agent, did not inhibit platelet shape change, internal transformation, secretion, aggregation, or clot retraction. In the present study the diameters of microtubule coils in discoid platelets treated or not treated with Taxol and in platelets activated by thrombin, ADP, and a foreign surface were measured. The results of the study reveal no significant differences in diameters of microtuble rings in control or Taxol-treated cells. However, after activation by ADP, thrombin, or the grid surface, the diameter of coiled microtubules decreased by 30% or more. The results support the concept that internal transformation is a contractile event.