Pathology of thrombotic microangiopathy (TMA). The pathology of TMA on light microscopy includes glomerular and vascular changes. Glomerular changes: thrombi in glomerular capillaries, mesangiolysis, and, in the chronic phase, thickened capillary walls with double contour formation. Vascular changes: thrombi, fragmented red cells, intimal swelling, and fibrous thickening with onion skinning. Representative findings of TMA in a case of aHUS (a,b), antiphospholipid syndrome (c), scleroderma (d), drug (cocaine) (e), and infection (Shiga toxin hemolytic uremic syndrome; STEC-HUS) (f). (f) STEC-HUS shows severe TMA with cortical necrosis. Thin black arrow points to thrombi in glomerular capillaries and arteries; thick black arrow points to myxoid change and onion-skinning of arterial walls. In general, the glomerular and vascular TMA findings are not specific for a particular etiology. (a,b: silver methenamine stain 40×; c, periodic acid–Schiff stain 40×; d, hematoxylin and eosin 20×; e, toluidine blue 10×; and f, hematoxylin and eosin 10×).