Light microscopy of thrombotic microangiopathies. This group of disorders includes hemolytic-uremic syndrome and thrombotic

thrombocytopenic purpura, malignant hypertension, and renal disease in progressive systemic sclerosis (scleroderma renal crises). A, These lesions are characterized primarily by fibrin deposition in the walls of the glomeruli (fibrin). B, This fibrin deposition is associated with endothelial cell swelling (arrow) and thickened capillary walls, sometimes with a double contour. Variable capillary wall wrinkling and luminal narrowing occur. Mesangiolysis (dissolution of the mesangial matrix and cells) is not uncommon and may be associated with microaneurysm formation. With further endothelial cell damage, capillary thrombi ensue. C, Arteriolar thrombi also may be present. In arterioles, fibrin deposits in the walls and lumina are known as thrombonecrotic lesions, with extension of this process into the glomeruli on occasion (arrow). The arterial walls are thickened, with loose concentric intimal proliferation. D, On electron microscopy, the subendothelial zones of the glomerular capillary wall are widened (arrows). Flocculent material accumulates, corresponding to mural fibrin, with associated endothelial cell swelling. E, With widespread arterial thrombosis, cortical necrosis is a common complicating feature. The necrotic cortex consists of pale confluent multifocal zones throughout the cortex.

FIGURE 5-2 (see Color Plate)

Microangiopathic hemolytic anemia. Bizarrely shaped and fragmented erythrocytes are commonly seen in Wright's stained peripheral blood smears from patients with active lesions of thrombotic microangiopathy. These abnormally shaped erythrocytes presumably arise when the fibrin strands within small blood vessels shear the cell membrane, with imperfect resolution of the biconcave disk shape. The resultant intravascular hemoly-sis causes anemia, reticulocytosis, and reduced plasma haptoglo-bin level.

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