The basement membrane anchors and supports epithelium and endothelium, serving as a barrier against foreign bodies and helping with angiogenesis. Diseases can weaken or malfunction the membrane, such as Goodpasture syndrome and Alport syndrome.
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 internal and external surfaces of organs and structures throughout the body. The endothelium is a type of specialized tissue that lines the inner surface of blood vessels. A portion of this membrane, the basal lamina, is secreted by the epithelial cells that cover it. The reticular layer is located within the basal lamina and is composed of fibrous tissue.
Primarily, the basement membrane serves to bind the epithelium to the underlying connective tissue. In the skin, for example, there are three main layers: the outermost epidermal layer, which is composed of epithelial cells and includes the visible part of the skin; the middle layer of the dermis, composed of connective tissue; and a deeper subcutaneous layer. Between the epidermis and the dermis is the basement membrane, which keeps the outer layer snugly attached to the lower layer.
A second function of the basement membrane is that of a protective barrier against foreign bodies or malignant cells. Epithelial tissue often lines the parts of the body that are in contact with the external 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 internal areas of the body. In this way, it can also help contain defective or malignant cells.
In blood vessels, the basement membrane also helps with angiogenesis, or the production of new blood vessels from existing ones. During this process, the endothelium, which lines the inside of the blood vessel where blood flows, secretes enzymes into the membrane. The enzymes break down the tissue so that the endothelial cells can migrate outward, multiply and form a new vessel. Before blood can flow into the new vessel, however, a new basement membrane must be formed.
The glomerular basement membrane is a particularly important example of tissue filtering function. The glomerulus is a bundle of capillaries found in the nephron of the kidney where the fluid portions of blood are emptied to be cleaned and returned to the bloodstream. The glomerular basement membrane lining these capillaries is specially designed to select which parts of the blood are filtered and which components remain in the blood vessel. Negatively charged and particularly thick, these membranes allow the passage of small ions, or negatively charged molecules, and fluids while retaining large molecules and positively charged molecules, such as proteins.
Several diseases can cause weakness or malfunction of the basement membranes. Anti-glomerular basement membrane disease, also known as Goodpasture syndrome, is a rare autoimmune disease in which the body forms antibodies that attack type IV collagen found in the glomerular and alveolar membranes, resulting in rapidly progressive kidney failure and lung damage. The causes aren’t known for sure, but likely include viruses, genetics, and chemical exposure.
Genetic mutations in basement membrane collagen can 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.
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