Pathology of the human mesangium in situ

Mesangial cells play an important role in the development and progression of human glomerular disease. This article summarizes some important aspects of mesangial properties and behaviour in situ. Intrinsic mesangial cells express alpha-smooth muscle actin and are best characterized as myofibroblasts or glomerular pericytes. The main integrin receptor in the mesangium is the alpha 1 beta 1 integrin. The beta 2 and beta 3 integrins have not been detected. Mesangial cells in situ fail to react with many monoclonal antibodies which stain human mesangial cells in culture, including leukocyte activation antigens. Prominent reactions in glomerular disease are mesangial expansion and progressive glomerular sclerosis, which are preceded by or associated with mesangial cell hypertrophy and/or proliferation. Mesangial enlargement is accompanied by an altered integrin expression and an abnormal composition of extracellular mesangial matrix. From the numerous autocrine and paracrine mediators identified in vitro which stimulate or inhibit mesangial cell growth and extracellular matrix synthesis, up to now only a few factors have been shown to be present in selected human glomerulopathies. These include platelet derived growth factors and platelet derived growth factor receptor beta, transforming growth factors beta, interleukin 1 beta, tumor necrosis factor alpha, and interleukin 6. Further identification of such mediators in situ will improve our understanding of pathological glomerular processes, particularly with respect to the multifunctional properties of the mesangial cell.