[Histopathology of thrombotic thrombocytopenic purpura]

The most striking lesions of thrombotic thrombocytopenic purpura (TTP) are the subendothelial hyaline deposits and multiple occlusive thrombi. They are found in the small arteries, arterioles, and capillaries of many organs, especially the heart, brain, kidneys, and adrenal glands. Immunohistochemical examination demonstrates that the thrombi in TTP are strongly positive for von Willebrand factor (vWF) antigen, but only weakly for fibrinogen/fibrin. Electronmicroscopically, the thrombi in TTP were composed of tightly packed platelets, showing varying degrees of cytoplasmic alteration and degranulation with a small amount of interspersed fibrin and amorphous materials. Many etiological factors causing intravascular platelet aggregation have been postulated, including unusually large vWF multimers, platelet agglutinating proteins and immune abnormalities. All the lesions in TTP are strongly positive for vWF antigen. This result suggests that vWF multimers may play an important role in the pathogenesis of TTP.