| Basement membrane | |
|---|---|
| The epithelium and endobasement membrane in relation to epithelium and endothelium. Also seen are other extracellular matrix components | |
| Image showing the basement membrane of the lining of the mouth, which separates the lining (epithelium) from a loose layer of connective tissue (the lamina propria) | |
| Details | |
| Identifiers | |
| Latin | membrana basalis |
| MeSH | D001485 |
| TH | H2.00.00.0.00005 |
| FMA | 63872 |
| Anatomical terms of microanatomy | |
The basement membrane, also known as the basal lamina, is a specialized form of extracellular matrix (ECM) common to all multicellular animals. [1] [2] It is a very thin, flexible, and strong sheet-like type of ECM that provides a supporting base for all types of epithelial tissue, separates it from another cell layer such as endothelium, and anchors it to the underlying connective tissue (stroma). [1] [3] A basement membrane also surrounds some individual cells, including muscle cells, fat cells, and Schwann cells, separating them from surrounding connective tissue. [1] [4] Its composition can vary from tissue to tissue, and even in different regions of the same tissue. [1] [4] The other type of ECM is the interstitial matrix. [5]
The basement membrane also acts as a platform for complex cell signaling for polarization, migration, and differentiation. [2] [6] It also regulates the exchange of materials between the epithelium and underlying tissues; binds growth factors from the connective tissue to the epithelium that control the development of epithelium. [3] Epithelial cells are pressed closely together having no blood vessels between them but they mostly rest on a bed of loose connective tissue that is rich in blood vessels providing nutrients and removing waste. [3]
Epithelial cells are polarized. The surface of epithelial cells that face the lumen is the apical surface, and the surface facing the basement membrane is the basal surface. [3] The basement membrane may be described as having two layers or laminae, an external basal lamina, facing the epithelium, and an internal basal lamina that faces the connective tissue. [6] The cells in the internal basal membrane that are closest to the connective tissue show high rates of mitosis, needed to replace skin cell abrasions, and in the GI tract replacement of the cells exposed to digestive enzymes and gastric acid. [3] These two laminae are also known as the basal lamina and the reticular lamina. [7]
The underlying connective tissue attaches to the basal lamina with collagen VII anchoring fibrils and fibrillin microfibrils. [8]
The basal lamina layer can further be subdivided into two layers based on their visual appearance in electron microscopy. The lighter-colored layer closer to the epithelium is called the lamina lucida, while the denser-colored layer closer to the connective tissue is called the lamina densa. The electron-dense lamina densa layer 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 and are coated with the heparan sulfate-rich proteoglycan perlecan. [9] In addition to collagen, this supportive matrix contains intrinsic macromolecular components. The lamina lucida layer is made up of laminin, integrins, entactins, and dystroglycans. Integrins are a key component of hemidesmosomes which serve to anchor the epithelium to the underlying basement membrane.
To represent the above in a visually organised manner, the basement membrane is organized as follows:
The primary function of the basement membrane is to anchor down the epithelium to its loose connective tissue (stroma) This is achieved by cell-matrix adhesions through cell adhesion molecules.
The basement membrane acts as a mechanical barrier, preventing malignant cells from invading the deeper tissues. [10] 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. [11]
The most notable examples of basement membranes are 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, [12] and between the alveoli and capillaries of the lung, 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 occurs (gas exchange).
As of 2017, other roles for basement membrane include blood filtration and muscle homeostasis. [2] Fractones may be a type of basement membrane, serving as a niche for stem cells. [13] [14]
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. [15] Diseases involving basement membranes at multiple locations include:
In histopathology, thickened basement membranes are found in several inflammatory diseases, such as lichen sclerosus, systemic lupus erythematosus or dermatomyositis in the skin, or collagenous colitis in the colon. [18]
These are only found within diploblastic and homoscleromorphic sponge animals. The homoscleromorph were found to be sister to diploblasts in some studies, making the membrane originate once in the history of life. But more recent studies have disregarded diploblast-homoscleromorph group, so other sponges may have lost it (most probable) or the origin in the two groups may be separate.
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