An approach to the structure and function of the glomerular mesangium

The mesangium of the glomerulus is a connective tissue tree arising at the vascular pole of the glomerulus and supporting the glomerular capillaries. It is partly covered by a basement membrane that follows the epithelial cells from the peripheral glomerular capillary wall over the supporting tissue. The capillary endothelium does not normally have a separate basement membrane. The endothelium has fenestrations that open directly into the mesangium and allow blood plasma and tracers to flow into the mesangium. The fenestrations partially restrict (or sieve) particles over 405 A in mean length from entry. Tracers move in intercellular channels and are filtered and concentrated by the basement membrane at the sides of the mesangium or by mesangial matrix filaments in the channels between cells. The irregular distributions of flow, matrix, and concentrations of tracers may account for irregular lobular reactions in glomerular disease. Two main pathways of flow seem to be (1) through the basement membrane and between the epithelial foot processes to form part of the glomerular filtrate and (2) into the efferent capillaries through their mesangial fenestrations. Intrinsic mesangial cells can now be regarded as myofibroblasts associated with the production of the connective tissue matrix. These cells hold the basement membrane to maintain the shape of the glomerular capillaries, they swell readily, and they can constrict like smooth muscle cells with appropriate stimulation. These reactions may enable them to control the flow of blood through the capillary network in glomerular disease. Mesangial cells can take up large amounts of foreign material within 24 h. Intrinsic mesangial cells and monocytes can increase in numbers in disease.(ABSTRACT TRUNCATED AT 250 WORDS)