Mesangial Proliferative Glomerulonephritis

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FIGURE 2-22 (see Color Plate)

Light, immunofluorescence, and electron microscopy in mesangial proliferative glomerulonephritis. This heterogeneous group of disorders is characterized by increased mesangial cellularity in most of the glomeruli associated with granular immune deposits in the

mesangial regions. Little if any increased cellularity is seen, despite the presence of deposits. In this latter instance, the term mesangial injury glomerulonephritis is more properly applied. The disorders are defined on the basis of the immunofluorescence findings, rather than on the presence or absence of mesangial hypercellularity. There are numerous disorders with this appearance; some have specific immunopathologic or clinical features (such as immunoglobulin A nephropathy, Henoch-Schonlein purpura, and systemic lupus erythe-matosus). Patients with primary mesangial proliferative glomeru-lonephritis typically exhibit the disorder in one of four ways: asymptomatic proteinuria, massive proteinuria often in the nephrot-ic range, microscopic hematuria, or proteinuria with hematuria. A, On light microscopy, widening of the mesangial regions is observed, often with diffuse increase in mesangial cellularity commonly of a mild degree. No other alterations are present. B, Depending on the specific entity or lesion, the immunofluorescence is of granular mesangial deposits. In the most common of these disorders, immunoglobulin M is the dominant or sole deposit. Other disorders are characterized primarily or exclusively by complement C3, immunoglobulin G, or C1q deposits. C, On electron microscopy the major finding is of small electron-dense deposits in the mesangial regions (arrow). Foot process effacement is variable, depending on the clinical syndrome (eg, whether massive proteinuria is present).

FIGURE 2-23 (see Color Plate)

Crescentic glomerulonephritis. A crescent is the accumulation of cells and extracellular material in the urinary space of a glomerulus. The cells are parietal and visceral epithelia as well as monocytes and other blood cells. The extracellular material is fibrin, collagen, and basement membrane material. In the early stages of the disease, the crescents consist of cells and fibrin. In the later stages the crescents undergo organization, with disappearance of fibrin and replacement by collagen. Crescents represent morphologic consequences of severe capillary wall damage. A, In most instances, small or large areas of destruction of capillary walls (cells and basement membranes) are observed (arrow), thereby allowing fibrin, other high molecular weight substances, and blood cells to pass readily from capillary lumina into the urinary space. B, Immunofluorescence frequently discloses fibrin in the urinary space. C, The proliferating cells in Bowman's space ultimately give rise to the typical crescent shape. Whereas crescents may complicate many forms of glomeru-lonephritis, they are most commonly associated with either antiglomerular basement membrane (AGBM) antibodies or antineu-

trophil cytoplasmic antibodies (ANCAs). The clinical manifestations are typically of rapidly progressive glomerulonephritis with moderate proteinuria, hematuria, oliguria, and uremia. The immunomor-phologic features depend on the basic disease process. On light microscopy in both AGBM antibody-induced disease and ANCA-associated crescentic glomerulonephritis, the glomeruli without crescents often have a normal appearance. It is the remaining glomeruli that are involved with crescents. D, Anti-GBM disease is characterized by linear deposits of immunoglobulin G and often complement C3 in all capillary basement membranes, and in approximately two thirds of affected patients in tubular basement membranes. The ANCA-associated lesion typically has little or no immune deposits on immunofluorescence; hence the term pauci-immune crescentic glomerulonephritis is used. By electron microscopy, as on light microscopy, defects in capillary wall continuity are easily identified. Both AGBM- and ANCA-associated cres-centic glomerulonephritis can be complicated by pulmonary hemorrhage (see Fig. 2-25).

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