Special Cytological Features of Epithelia
Basement membrane or basal lamina
Epithelia are separated from the underlying connective tissue by an extracellular supporting layer called the basement membrane. The basement membrane is composed of two sublayers. The basal lamina(about 80 nm thick) consists of fine protein filaments embedded in an amorphous matrix. Membrane proteins of the epithelial cells are anchored in the basal lamina, which is also produced by the epithelial cells. The major components of the basal lamina are two glycoproteins - laminin and (usually type IV) collagen. The reticular lamina consists of reticular fibres embedded in ground substance. The fibres of the reticular lamina connect the basal lamina with the underlying conective tissue. The components of the reticular lamina are synthesised by cells of the connective tissue underlying the epithelium.
In addition to its function as support of the epithelium, the basal lamina acts as a selectively permeable filter between epithelium and connective tissue.
Epithelia are separated from the underlying connective tissue by an extracellular supporting layer called the basement membrane. The basement membrane is composed of two sublayers. The basal lamina(about 80 nm thick) consists of fine protein filaments embedded in an amorphous matrix. Membrane proteins of the epithelial cells are anchored in the basal lamina, which is also produced by the epithelial cells. The major components of the basal lamina are two glycoproteins - laminin and (usually type IV) collagen. The reticular lamina consists of reticular fibres embedded in ground substance. The fibres of the reticular lamina connect the basal lamina with the underlying conective tissue. The components of the reticular lamina are synthesised by cells of the connective tissue underlying the epithelium.
In addition to its function as support of the epithelium, the basal lamina acts as a selectively permeable filter between epithelium and connective tissue.
Unless special stains are used, the basement membrane is rarely visible using light microscopy. You can read more about reticular fibres and ground substance on the Connective Tissues page.
Specialisations of the apical surface
Microvilli and stereociliaare finger- or thread-shaped extensions of the epithelial cells. Their main function is to increase the surface area of epithelial cells. They are typically found in epithelia active in absorption. Microvilli contain actin filaments, which are in contact with the terminal web of the cell. The only difference between microvilli and stereocilia is their length. Microvilli are much shorter than stereocilia. Stereocilia are, despite their name ("cilia"), not actively moving structures.
Microvilli and stereociliaare finger- or thread-shaped extensions of the epithelial cells. Their main function is to increase the surface area of epithelial cells. They are typically found in epithelia active in absorption. Microvilli contain actin filaments, which are in contact with the terminal web of the cell. The only difference between microvilli and stereocilia is their length. Microvilli are much shorter than stereocilia. Stereocilia are, despite their name ("cilia"), not actively moving structures.
Using light microscopy, stereocilia are difficult to discern from cilia.
Specialisations of the lateral and basal surfaces
Connective tissue is responsible for the structural integrity of most organs. As mentioned above, it is absent from epithelia. Instead, tissue integrity as well as the barrier function of epithelia is taken care of by extensive cell-to-cell contacts between epithelial cells. These functions are mediated by several specialisations in the lateral and basal parts of the cell membranes of the epithelial cells.
Connective tissue is responsible for the structural integrity of most organs. As mentioned above, it is absent from epithelia. Instead, tissue integrity as well as the barrier function of epithelia is taken care of by extensive cell-to-cell contacts between epithelial cells. These functions are mediated by several specialisations in the lateral and basal parts of the cell membranes of the epithelial cells.
Desmosomes
are specialisations of the lateral cell membranes which mediate cell adhesion. Proteins inserted into the cell membrane of the adjacent cells form a protein-'zipper' linking the cells. Fibers of the cytoskeleton attach to the cytoplasmic side of the desmosome to stabilise the area of contact. Hemi-desmosomes mediate the attachment of the epithelial cells to the basement membrane.
A group of glycoproteins (cadherins) inserted into the opposing plasma membranes mediate cell-to-cell adhesion at desmosomes and also at the adhesion zones or patches mentioned below. Integrins, another group of proteins, allow the cell to attach to the matrix proteins of the basal lamina.
A group of glycoproteins (cadherins) inserted into the opposing plasma membranes mediate cell-to-cell adhesion at desmosomes and also at the adhesion zones or patches mentioned below. Integrins, another group of proteins, allow the cell to attach to the matrix proteins of the basal lamina.
Intermediate junctions (zonula adherens)
are structurally not as well-characterised as desmosomes. An intermediate junction typically appears as a close and consistent apposition (15-20 nanometers)of the cell membranes near the apical cell surface. Intermediate junctions surround the entire cell. Again, fibres of the cytoskeleton insert into the cytoplasmic side of this membrane specialisation. Patches of adhesion resemble intermediate junctions structurally, but form more localized, patch- or strip-like contacts between neighbouring cells. They are found scattered over the lateral surfaces of the epithelial cell.
The above mentioned membrane specialisations mediate cell-adhesion but are less well suited to support one of the essential functions of epithelia - the isolation of the interior of the body from the outside world. A tight junction (zonula occludens) between epithelial cells mediates this aspect of epithelial function.
Proteins inserted into the cell membranes of adjacent cells 'stitch' the membranes of the cells together and provide an effective barrier to the diffusion of substances from the outside of the epithelium (called luminal side if the epithelium covers the surface of a tubular structure). Several "rows of stitches" may be found. Their number depends on the demand to reduce diffusion across the epithelium. Each of these rows reduces diffusion by about a factor 10 of what it was 'before'.
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