The basement membrane is a thin, fiberous, non-cellular region of tissue that separates
the epithelium, mesothelium and endothelium from underlying connective tissue.
The name is somewhat misleading in that the basement membrane is not actually a membrane;
rather, it is a matrix.
Composition The basement membrane is the fusion of two
laminae, the basal lamina and the reticular lamina.
The lamina reticularis is attached to the basal lamina with collagen VII anchoring fibrils
and fibrillin microfibrils.
The two layers together are collectively referred to 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 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: EpithelialEndothelial Tissue
Basement Membrane Basal Lamina
Lamina Lucida laminin
integrins entactins
dystroglycans
Lamina Densa collagen IV
Attaching proteins collagen VII
fibrillin
Lamina Reticularis collagen III
Connective Tissue Function and importance
The primary function of the basement membrane is to anchor down the epithelium to its loose
connective tissue underneath.
This is achieved by cell-matrix adhesions through substrate adhesion molecules.
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 basement membrane is also essential for angiogenesis.
Basement membrane proteins have been found to accelerate differentiation of endothelial
cells.
The most notable examples of basement membranes is the glomerular basement membrane of the
kidney, by the fusion of the basal lamina from the endothelium of glomerular capillaries
and the podocyte basal lamina, 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 CO 2 diffusion happens.
Diseases 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.
Non-collagenous domain basement membrane collagen type IV is autoantigen of autoantibodies in
the autoimmune disease Goodpasture's syndrome.
A group of diseases stemming from improper function of basement membrane zone are united
under the name epidermolysis bullosa.
See also Intima
References
Further reading Kefalides, Nicholas A. & Borel, Jacques P.,
ed..
Basement membranes: cell and molecular biology.
Gulf Professional Publishing.
ISBN 978-0-12-153356-4.