Basement membrane
Encyclopedia
The basement membrane is a thin sheet of fiber
s 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.
and the reticular lamina (or lamina reticularis). The lamina reticularis is attached to the basal lamina with anchoring fibrils
(type VII collagen fibers) and microfibril
s (fibrillin). 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 in thickness, and consists of an underlying network of reticular collagen
(type IV) fibrils (fibroblast precursors) which average 30 nanometers in diameter and 0.1–2 micrometer
s in thickness. In addition to collagen, this supportive matrix contains intrinsic macromolecular components.
The Lamina Densa (which is made up of type IV collagen fibers; perlecan (a heparan sulfate proteoglycan) coats these fibers and they are high in heparan sulfate
) and the Lamina Lucida (made up of laminin
, integrin
s, entactin
s, and dystroglycan
s) together make up the basal lamina
.
To represent the above in a visually organised manner, the basement membrane is organized as follows:
--------------------
--------------------
to its loose connective tissue 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 basement membrane is also essential for angiogenesis
(development of new blood vessels). Basement membrane proteins have been found to accelerate differentiation of endothelial cells.
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.
Genetic defects in the collagen fibers of the basement membrane cause Alport syndrome
.
Non-collagenous domain basement membrane collagen type IV is autoantigen (target antigen) 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
.
Fiber
Fiber is a class of materials that are continuous filaments or are in discrete elongated pieces, similar to lengths of thread.They are very important in the biology of both plants and animals, for holding tissues together....
s that underlies the epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
, which lines the cavities and surfaces of organs including skin, or the endothelium
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
, which lines the interior surface of blood vessels.
Composition
The basement membrane is the fusion of two lamina, the basal laminaBasal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
and the reticular lamina (or lamina reticularis). The lamina reticularis is attached to the basal lamina with anchoring fibrils
Anchoring fibrils
Anchoring fibrils extend from the basal lamina of epithelial cells and attach to the lamina reticularis by wrapping around the reticular fiber bundles. The basal lamina and lamina reticularis together make up the basement membrane...
(type VII collagen fibers) and microfibril
Microfibril
The microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, examples are hair and sperm tail. Its most frequently observed structural pattern is 9+2...
s (fibrillin). 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
Lamina lucida
The lamina lucida is a component of the basement membrane which is found between the epithelium and underlying connective tissue...
, while the dense layer closer to the connective tissue is called the lamina densa
Lamina densa
The lamina densa is a component of the basement membrane zone between the epidermis and dermis of the skin, and is an electron-dense zone between the lamina lucida and dermis, synthesized by the basal cells of the epidermis, and composed of type IV collagen, anchoring fibrils made of type VII...
. The electron-dense
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
lamina densa
Lamina densa
The lamina densa is a component of the basement membrane zone between the epidermis and dermis of the skin, and is an electron-dense zone between the lamina lucida and dermis, synthesized by the basal cells of the epidermis, and composed of type IV collagen, anchoring fibrils made of type VII...
membrane is about 30–70 nanometers in thickness, and consists of an underlying network of reticular collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
(type IV) fibrils (fibroblast precursors) which average 30 nanometers in diameter and 0.1–2 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s in thickness. In addition to collagen, this supportive matrix contains intrinsic macromolecular components.
The Lamina Densa (which is made up of type IV collagen fibers; perlecan (a heparan sulfate proteoglycan) coats these fibers and they are high in heparan sulfate
Heparan sulfate
Heparan sulfate is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan in which two or three HS chains are attached in close proximity to cell surface or extracellular matrix proteins...
) and the Lamina Lucida (made up of laminin
Laminin
Laminins are major proteins in the basal lamina , a protein network foundation for most cells and organs...
, integrin
Integrin
Integrins are receptors that mediate attachment between a cell and the tissues surrounding it, which may be other cells or the ECM. They also play a role in cell signaling and thereby regulate cellular shape, motility, and the cell cycle....
s, entactin
Entactin
Entactin also known as nidogen-1 is a protein that in humans is encoded by the NID1 gene. Entactin is a component of the basement membrane alongside other components such as collagen type IV, proteoglycans , laminin and fibronectin.- Function :Entactin is a member of the nidogen family of basement...
s, and dystroglycan
Dystroglycan
Dystroglycan is a protein that in humans is encoded by the DAG1 gene.Dystroglycan is one of the dystrophin-associated glycoproteins, which is encoded by a 5.5 kb transcript in Homo sapiens by chromosome 3. There are two exons that are separated by a large intron...
s) together make up the basal lamina
Basal lamina
The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often confused with the basement membrane, and sometimes used inconsistently in the literature, see below....
.
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
- laminin
- integrins
- entactins
- dystroglycans
- Lamina Densa
- Type IV collagen (coated with perlecan, rich in heparan sulfate)
- Lamina Lucida
- Attaching proteins (between Basal and Reticular Laminae)
- Type VII collagen (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 epitheliumEpithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
to its loose connective tissue underneath. This is achieved by cell-matrix
Extracellular matrix
In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
adhesions through substrate adhesion molecules
Substrate adhesion molecules
Substrate adhesion molecules are proteins that attach cells to specific compounds in the extracellular matrix ....
(SAMs).
The basement membrane acts as a mechanical barrier, preventing malignant
Malignant
Malignancy is the tendency of a medical condition, especially tumors, to become progressively worse and to potentially result in death. Malignancy in cancers is characterized by anaplasia, invasiveness, and metastasis...
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
Carcinoma in situ
Carcinoma in situ is an early form of cancer that is defined by the absence of invasion of tumor cells into the surrounding tissue, usually before penetration through the basement membrane. In other words, the neoplastic cells proliferate in their normal habitat, hence the name "in situ"...
.
The basement membrane is also essential for angiogenesis
Angiogenesis
Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...
(development of new blood vessels). Basement membrane proteins have been found to accelerate differentiation of endothelial cells.
The most notable examples of basement membranes is in the glomerular filtration of the kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...
, by the fusion of the basal lamina from the endothelium
Endothelium
The endothelium is the thin layer of cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. These cells are called endothelial cells. Endothelial cells line the entire circulatory system, from the heart...
of glomerular capillaries and the basal lamina of the epithelium of the Bowman's capsule
Bowman's capsule
The Bowman's capsule is a cup-like sac at the beginning of the tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of blood to form urine. A glomerulus is enclosed in the sac...
, and between lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
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.
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
Alport syndrome
Alport syndrome or hereditary nephritis is a genetic disorder characterized by glomerulonephritis, endstage kidney disease, and hearing loss. Alport syndrome can also affect the eyes . The presence of blood in the urine is almost always found in this condition.It was first identified in a British...
.
Non-collagenous domain basement membrane collagen type IV is autoantigen (target antigen) of autoantibodies in the autoimmune disease Goodpasture's syndrome
Goodpasture's syndrome
Goodpasture’s syndrome is a rare disease characterized by glomerulonephritis and hemorrhaging of the lungs...
.
A group of diseases stemming from improper function of basement membrane zone are united under the name epidermolysis bullosa
Epidermolysis bullosa
Epidermolysis bullosa is an inherited connective tissue disease causing blisters in the skin and mucosal membranes, with an incidence of 1/50,000. Its severity ranges from mild to lethal. It is caused by a mutation in the keratin or collagen gene....
.