Idiopathic microscopic polyarteritis nodosa: ultrastructural observations on the renal vascular and glomerular lesions.

Although the glomerulonephritis (GN) and renal vasculitis in polyarteritis nodosa (PAN) are generally considered to be immune-mediated, the pathogenesis of the renal injury and the role of immune complex (IC) deposition are unclear. To better define the nature of the glomerular and vascular injury in PAN, we performed a detailed ultrastructural study of 27 renal biopsies from 20 patients with histologically confirmed PAN of the microscopic or overlap (microscopic/macroscopic) type. A total of 48 arteries and arterioles were studied ultrastructurally, including 20 vessels with recognizable vasculitis in 1 micron-thick survey sections. By immunofluorescence, glomerular and vascular immunoglobulin deposits were generally scanty, primarily located in areas of necrosis or sclerosis. Fibrinogen, C3 and C1 were more commonly detected, often in the absence of demonstrable immunoglobulin. By electron microscopy, discrete electron-dense deposits of probable immune-type were found in the glomeruli of five initial biopsies. No electron-dense deposits were identified in any of the arteries or arterioles studied. In both glomeruli and vessels, endothelial injury and subendothelial fibrin deposition were the earliest detectable ultrastructural changes. The pathogenetic implications of these findings are discussed.