[Clinical pathology of the glomerulus--from phenomenon to entity. The mesangial lesion].

: The numerous findings discussed lead to the following conclusions: 1. The mesangial lesions, which may take a wide range of different forms, can be classified into two groups according to whether an underlying immunological pathomechanism is involved. Those that result from such a pathomechanism represent various types of glomerulonephritis. 2. Amongst these immunologically-mediated glomerulonephritides mesangioproliferative glomerulonephritis (and, of this group, IgA nephritis) is the most common. Membranoproliferative glomerulonephritis is the most severe of these diseases. Either may be idiopathic or secondary, or may occur in association with systemic disease. 3. The number of macrophages in the mesangial lesions in glomerulonephritis correlates with the severity of the glomerulonephritis, the localization of the immune complex deposits and the degree of proteinuria. If the immune complex deposits extend out of the mesangium into the subendothelial space, the number of macrophages is higher, the structural changes are more marked, and proteinuria is more severe. 4. Various pathomechanisms and nosologic entities can lead to mesangial lesions of the type seen in mesangioproliferative glomerulonephritis or membranoproliferative glomerulonephritis. On the other hand, the same entity may be associated with mesangial lesions of different severity, and consequently the prognosis varies. Differential diagnosis of the mesangial lesions, which represent heterogeneous nosologic entities, requires the use of light microscopic, immunohistochemical, and electron microscopic techniques. Exact diagnosis is necessary because of the differences in prognosis. 5. The course and prognosis of mesangial lesions are determined by immunological and nonimmunological factors. Long-term studies have demonstrated that prognostically relevant information can already be gained at the time of biopsy by the assessment of certain morphological features (e.g., immunohistological findings, severity of glomerulonephritis, the presence of focal/segmental lesions) and clinical parameters (e.g., proteinuria, hematuria, hypertension, and serum creatinine concentration). The decisive predictor of an unfavorable prognosis is the presence of interstitial fibrosis.