The mesangium is a layer of tissue inside the kidney. It surrounds and supports a knot of extremely tiny blood vessels called glomeruli. There are many glomeruli inside the kidney and they filter blood as part of the process of making urine. The mesangium plays a part in this filtering process as well as helps remove unwanted particles from this area of the kidney. Changes in the mesangium affect kidney anatomy and are involved in the development of some kidney diseases.
Each glomerulus is made up of small blood vessels lined with what are called endothelial cells. On the outside of the blood vessels lies a type of thin sheath or membrane and the cells of the mesangium lie in between this and the endothelial cells. In parts of the glomerulus where two blood vessels lie very close to one another, they may share the same membrane. It is at these points that mesangial cells are frequently found.
When blood is filtered through the glomeruli, it passes out of the blood vessels through holes in their walls, through the membrane and then through gaps in a closely knit layer of cells called podocytes. The rate at which filtering takes place is affected by the surface area available, and the mesangium can alter this. Mesangial cells are able to contract like muscle. This can cause kinking and pinching of blood vessels, and divert blood flow away from some areas of the glomerulus, lowering the area available for filtration. Relaxation of the cells has the opposite effect, increasing the surface area and the rate of filtration.
In some cases, a mesangial cell is able to act like some white blood cells in the immune system, when it consumes foreign particles attached to antibodies. Cells in the mesangium can also secrete substances that are involved in inflammation. This may be an important part of the body's response when part of the kidney is injured.
Sometimes kidney disease may cause the mesangium to expand, with mesangial cells dividing and increasing in number. This can be seen in association with diabetes or in the disease known as glomerulonephritis. The enlarged mesangium and abnormally high numbers of mesangial cells can interfere with kidney function, ultimately leading to kidney failure. Diabetic kidney disease is treated using a combination of drugs and lifestyle changes, while glomerulonephritis treatment depends on the underlying cause. Lowering blood pressure is an important part of treatment in both cases, and dialysis or kidney transplants may be considered as a last resort.