Ultrastructural physiology of platelets with randomly dispersed rather than circumferential band microtubules

Annular bundles of microtubules supporting the discoid shape of blood platelets are temperature-labile. Chilling causes loss of platelet form and complete disappearance of microtubules. In a previous report the author demonstrated that the microtubule-stabilizing agent taxol caused reassembly of microtubules in chilled platelets. However, the microtubules were dispersed singly or in short bundles throughout the cytoplasm. In the present study the author has used this unusual feature of taxol treatment to prepare platelets with randomly dispersed microtubules and compare their physiology with control platelets containing circumferential bands. Platelets with randomly dispersed tubules responded in the same manner as control platelets to identical concentrations of aggregating agents. Secretion measured simultaneously with aggregation was also unaffected by dispersal of the coiled tubule making up the annular band into many single elements or short bundles. Examination in the electron microscope revealed that platelets with randomly dispersed tubules developed shape change and internal transformation comparable to those of control cells following stimulation by aggregating agents. However, centrally clustered organelles in cells with dispersed tubular elements were not encircled by a tight ring of microtubules. Instead, the tubular elements were concentrated in masses or bundles between centrally clustered organelles. The results of this study suggest that it does not matter whether microtubules are organized as a single structure, coiled on itself to produce a circumferential band, or as many single tubules or bundles of tubular elements spread throughout the cell. As long as microtubules are present, internal transformation will develop and platelet function, including shape change, internal contraction, secretion and irreversible aggregation, will not be significantly impaired.