The basement membrane is a thin sheet of fibers that underlies the epithelium which lines the cavities and surfaces of organs including skin, or the endothelium which lines the interior surface of blood vessels. The name is somewhat misleading in that the basement membrane is not actually a membrane; rather, it is a matrix beneath any epithelium.
The basement membrane is the fusion of two lamina, the basal lamina and the reticular lamina (or lamina reticularis). The lamina reticularis is attached to the basal lamina with collagen VII anchoring fibrils and fibrillin microfibrils. The two layers are collectively known as the basement membrane.
The basal lamina layer can further be divided into two layers. The clear layer closer to the epithelium is called the lamina lucida, while the dense layer closer to the connective tissue is called the lamina densa. The electron-dense lamina densa membrane is about 30–70 nanometers thick, and consists of an underlying network of reticular collagen IV fibrils which average 30 nanometers in diameter and 0.1–2 micrometers in thickness. In addition to collagen, this supportive matrix contains intrinsic macromolecular components.
The lamina densa, whose collagen IV fibers are coated with the heparan sulfate-rich proteoglycan perlecan, and the lamina lucida (made up of laminin, integrins, entactins, and dystroglycans) together make up the basal lamina.
Integrins are not part of the basal lamina, they are part of desmosomes which are in the basement membrane but not the basal lamina. To represent the above in a visually organised manner, the basement membrane is organized as follows:
- Epithelial Tissue (outer)
- Basement Membrane
- Basal Lamina
- Lamina Lucida
- Lamina Densa
- collagen IV (coated with perlecan, rich in heparan sulfate)
- Lamina Lucida
- Attaching proteins (between Basal and Reticular Laminae)
- collagen VII (anchoring fibrils)
- fibrillin (microfibrils)
- Lamina Reticularis
- Basal Lamina
- Connective Tissue (inner)
Function and importance
The primary function of the basement membrane is to anchor down the epithelium to its loose connective tissue (the dermis) underneath. This is achieved by cell-matrix adhesions through substrate adhesion molecules (SAMs).
The basement membrane acts as a mechanical barrier, preventing malignant cells from invading the deeper tissues. Early stages of malignancy that are thus limited to the epithelial layer by the basement membrane are called carcinoma in situ.
The most notable examples of basement membranes is in the glomerular filtration of the kidney, by the fusion of the basal lamina from the endothelium of glomerular capillaries and the basal lamina of the epithelium of the Bowman's capsule, and between lung alveoli and pulmonary capillaries, by the fusion of the basal lamina of the lung alveoli and of the basal lamina of the lung capillaries, which is where oxygen and CO2 diffusion happens.
Some diseases result from a poorly functioning basement membrane. The cause can be genetic defects, injuries by the body's own immune system, or other mechanisms.
Genetic defects in the collagen fibers of the basement membrane cause Alport syndrome.
A group of diseases stemming from improper function of basement membrane zone are united under the name epidermolysis bullosa.
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