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.
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 stereocilia
are 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.
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.
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'.
![thumbnail]( "Special Cytological Features of Epithelia")
Title:
Special Cytological Features of Epithelia
by:
om
at
2013-02-15T02:46:00+07:00
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Special Cytological Features of Epithelia