any structural anomaly of the modified smooth muscle cells that are interposed between endothelial cells and the basement membrane in the central or stalk region of the capillary tuft of the renal glomerulus, which regulate blood flow by their contractile activity, and secrete extracellular matrix, prostaglandins, and cytokines; mesangial cells also have phagocytic activity, removing proteins and other molecules trapped in the glomerular basement membrane or filtration barrier