The basement membrane (membrana basalis) is a thin layer of basal lamina and reticular lamina that anchors and supports the epithelium and endothelium. Epithelium is a type of tissue that forms glands and lines the inner and outer surfaces of organs and structures throughout the body. Endothelium is a type of specialized tissue that coats the inner surface of blood vessels. A portion of this membrane, the basal lamina, is secreted by the epithelial cells that overlie it. The reticular layer lies inside the basal lamina and is composed of fibrous tissue.
Principally, the basement membrane serves to tie the epithelium to the connective tissue beneath it. In the skin, for example, there are three main layers: the outermost epidermis layer, which is composed of epithelial cells and includes the part of the skin that is visible; the middle dermis layer, composed of connective tissue; and a deeper subcutaneous layer. Between the epidermis and dermis lies the basement membrane, which keeps the outer layer adhered closely to the lower layer.
A second function of the basement membrane is that of a protective barrier against foreign objects or malignant cells. Epithelial tissue often lines parts of the body that are in contact with the outer environment, such as the inside of the stomach where food passes or the skin. The tough, semi-permeable nature of this membrane acts as a filter to prevent unwanted objects from entering the inner reaches of the body. In this way, it can also help contain defective, or malign, cells.
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In blood vessels, the basement membrane also aids with angiogenesis, or the manufacturing of new blood vessels from existing ones. During this process, the endothelium, which lines the interior of the blood vessel where blood flows, secretes enzymes into the membrane. The enzymes break down the tissue so that the endothelial cells may migrate outward, multiply, and form a new vessel. Before blood can flow in the new vessel, however, a new basement membrane must be formed.
The glomerular basement membrane is a particularly important example of the tissue’s function as a filter. The glomerulus is a bundle of capillaries found in the nephron of the kidney, where the fluid portions of blood are emptied out to be cleaned and returned to the blood stream. The glomerular basement membrane lining these capillaries is specially designed to select which parts of the blood are filtered out and which components remain in the blood vessel. Negatively charged and particularly thick, these membranes allow small ions, or negatively charged molecules, and fluid to pass while retaining large molecules and positively charged molecules, such as proteins.
Several pathologies may cause weakness or malfunctioning in basement membranes. Anti-glomerular basement membrane disease, also known as Goodpasture’s syndrome, is a rare auto-immune disease in which the body forms antibodies that attack the type IV collagen found in the glomerular and alveolar membranes, resulting in rapidly progressive kidney failure and lung damage. The causes are not decisively known, but likely include virus, genetics, and chemical exposure.
Genetic mutations in the collagen of the basement membranes may cause Alport syndrome, which often leads to kidney failure. Blood in the urine, or hematuria, is the most common symptom of the disorder. Because it is linked to the X chromosome, Alport syndrome is more common in men than in women.