Ultrastructure and function of thrombocytes in a family with congenital bleeding tendency

Freshly isolated blood from controls and from three members of a family with congenital bleeding tendency were examined with respect to ultrastructure of platelets. The fixation protocol, which included tannic acid, permitted the visualization of substructures in the marginal bundle. Whereas nearly all platelets from the controls had a discoid shape, those from one of the patients (mother of the other two cases) were mostly flat or else roundish with two or three marginal bundles in different orientations. One portion of the thrombocyte fraction of each person was briefly exposed to adenosine diphosphate in order to activate the platelets and thereby to induce shape changes. Most platelets from the controls reacted as expected by rounding up and by projecting some pseudopods. A certain percentage (between 10 and 20%) of the microtubules in these activated platelets had 14 or even 15 protofilaments. The platelets from patients (and in particular the mother) either reacted by an abnormal bulging of the cytoplasm or did not react visibly at all. The microtubules retained their 13-subunit composition or in the abnormally reacted ones often had an open C-shaped cross-sectional profile. The bleeding tendency in this family might be due to a defect in the cytoskeleton of the platelet making it unable to react properly to stimuli.