The basement membrane (membrana basalis) is a thin layer of basal lamina and reticular lamina which 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 endothelium specialized to coat the inner surface of blood vessels. A portion of the basement membrane, the basal lamina, is secreted by the epithelial cells that overlie it. The reticular layer of the basement membrane lies interior to the basal lamina and is composed of fibrous tissue.

The basement membrane has several functions. Principally, it serves to tie the epithelium to the connective tissue beneath it. Consider the skin. The skin has three main layers: the outermost epidermis layer, which is composed of epithelial cells and includes the part of the skin we can see; 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.

The 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 which contact the outer environment, such as the inside of the stomach where food passes, or the skin. The tough, semi-permeable nature of the basement membrane acts as a filter to prevent unwanted objects from entering the inner reaches of the body. In this way, the basement membrane can also help contain defective, or malign, cells.

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 basement membrane. The enzymes break down the basement membrane 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 basement membrane’s function as a filter. The glomerulus is a bundle of capillaries occurring 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 what parts of the blood are filtered out and which components remain in the blood vessel. Negatively charged and particularly thick, these basement 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 basement membrane and alveolar membrane, 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 membrane may cause Alport syndrome, which often leads to kidney failure. Blood in 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.