ADDITIONAL CELL TYPES

The mesangial cells provide structural support to the glomerular capillaries. These cells are irregularly shaped and send long cytoplasmic processes between endothelial cells. They are similar to modified smooth muscle cells and stain positive for smooth muscle actin and myosin. These cells can contract in response to various signals, narrowing the capillary loops and reducing glomerular flow. Signals that modulate mesangial tone include angiotensin II (see Plate 3-18), antidiuretic hormone (see Plate 3-17), norepinephrine, and thromboxane. In addition, mesangial cells are capable of phagocytosing local macromolecules and immune complexes, as well as generating inflammatory mediators in response. The mesangial cells are embedded in the mesangial matrix, which contains collagen, various proteoglycans, and other molecules. In histologic sections of normal glomeruli, one or two mesangial cells are typically seen per matrix area, with a greater number seen in certain pathologic states.