They are not connected to the filtration system; they are smooth muscle.
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Mesangial cells are found in a part of the kidney called the glomerulus — a ball of tiny blood vessels, or capillaries, involved in the filtration of blood and production of urine. Water, waste, and excess nutrients are removed from the blood by filtration through the capillary walls into the surrounding Bowman's capsule. The resulting urine then drains into a duct, known as a renal tubule, from where it eventually passes to the bladder. Mesangial cells are found between the capillaries and help regulate the filtration process while providing support for the glomerular structure. They are also involved in the kidney's response to injury and disease.
Generally, intraglomerular mesangial cells have an irregular shape and are thought to be related to smooth muscle cells. They contain similar proteins, such as myosin and actin, and have the ability to contract. Situated in the gaps between glomerular capillaries, these cells connect to the basement membrane that helps make up the capillary walls and forms part of the filter through which blood passes before entering Bowman's capsule.
Connected firmly to the membrane by cell processes and tiny tubules known as microfibrils, the bridging arrangement of each mesangial cell helps to ensure glomerular structure is stable. Otherwise, the glomerular capillaries could unravel or distend, with a subsequent effect on filtration. The hemodynamics of glomerular filtration is such that there is pressure on the basement membrane to expand and mesangial cells may help by making small, compensatory contractions.
Through the opening where vessels supplying blood to and from the glomerular capillaries enter Bowman's capsule, the mesangial cells and their matrix extend to form the extraglomerular mesangium. Typically, extraglomerular mesangial cells are long and flat, with a bunch of processes at each end. They are arranged in layers within the matrix and, like their intraglomerular counterparts, resemble smooth muscle cells. Their cell processes connect to the basement membrane of Bowman's capsule and the blood vessels passing through, helping to strengthen and close the glomerular entrance.
In diseases such as glomerulonephritis, where the glomeruli become inflamed, the mesangial cells and matrix increase and expand. This inhibits filtration, possibly leading to high blood pressure and ultimately renal failure. Signs of glomerulonephritis include blood or foam in the urine, and swelling of the face, abdomen, or lower limbs. Treatment consists of a combination of medication to lower blood pressure and relieve symptoms, and dietary regulation of salt, water, and protein. If the kidney begins to fail, dialysis or a transplant will usually be necessary.
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