Author Topic: Epithilial Tissue  (Read 21329 times)

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Offline Dr.Adnan

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Epithilial Tissue
« on: February 26, 2008, 10:06:32 AM »
Epithelia are tissues consisting of closely apposed cells without intervening intercellular substances. Epithelia are avascular, but all epithelia "grow" on an underlying layer of vascular connective tissue. The connective tissue and the epithelium are separated by a basement membrane. Epithelium covers all free surfaces of the body. Epithelium also lines the large internal body cavities, where it is termed mesothelium. Furthermore, the internal surfaces of blood and lymph vessels are lined by epithelium, here called endothelium.

Epithelia are classified on the basis of the number of cell layers and the shape of the cells in the surface layer.

If there is only one layer of cells in the epithelium, it is designated simple.
If there are two or more layers of cells, it is termed stratified.
Cells in the surface layer are, as a rule, described according to their height as squamous (scale- or plate-like), cuboidal or columnar.

Simple Epithelia

Simple squamous epithelium
This type is composed of a single layer of flattened, scale- or plate-like cells. It is quite common in the body. The large body cavities and heart, blood vessels and lymph vessels are typically lined by a simple squamous epithelium. The nuclei of the epithelial cells are often flattened or ovoid, i.e. egg-shaped, and they are located close to the centre of the cells.

Simple cuboidal epithelium
Cells appear cuboidal in sections perpendicular to the surface of the epithelium. Viewed from the surface of the epithelium they look rather like small polygons. Simple cuboidal epithelium occurs in small excretory ducts of many glands, the follicles of the thyroid gland, the tubules of the kidney and on the surface of the ovaries.
Can there be "low cuboidal" epithelia?

Simple columnar epithelium
The cells forming a simple columnar epithelium are taller than they are wide. The nuclei of cells within the epithelium are usually located at the same height within the cells - often close to the base of the cells. An example is the simple columnar epithelium which lines the internal surface of the gastrointestinal tract (GIT) from the cardia of the stomach to the rectum.


Identifying Epithelia?

The outlines of individual epithelial cells are not always visible, and it may be difficult to identify the shape of the cells.

It is often helpful to look at the shape, location and spacing of the nuclei in the epithelium, which together will allow a very good guess at the shape of the cells forming the epithelium.

How many cell layers seem to be visible in a section depends very much on the angle between the plane of the section plane the surface of the epithelium.

Oblique sections of epithelium will be visible in almost all slides of organs in which epithelium lines a surface with a very irregular profile. A single surface is usually not lined by several types of epithelia.

The number of epithelial cell layers will usually be the smallest number of layers visible anywhere along the surface lined by the epithelium.

Sublingual Gland, Human, H&E
Blood vessels are probably present in all sections you will ever see. With very few exceptions, they are lined by a simple squamous epithelium. The individual epithelial cells are extremely flattened and form a much larger part of the surface than individual cells in cuboidal or columnar epithelia. The nuclei of the squamous epithelial cells are also flattened and often stain darkly. Not every epithelial cell nucleus will be included in the plane of the section, and if the vessel is very small (e.g. a capillary), there may not be any visible nuclei in the epithelial lining.
Capillaries and other small vessels are easily deformed during tissue processing, and the epithelium of larger vessels may be damaged or look corrugated. It may therefore take a little more patience than you expect to find a "good" simple squamous epithelium.
Draw a small vessel with its epithelial lining, label the features visible in your drawing and include a suitable scale.

sublingual gland

Duodenum, Rat, H&E and Ileum, Human - H&E
The small intestines are lined by a simple columnar epithelium. Most of the epithelial cells (enterocytes) are involved in the absorption of components of the digested food in the lumen of the intestines. Complex folds of the intestinal lining increase the surface area available for absorption. The plane of the section will therefore often pass at an oblique angle through the epithelium. The epithelium may look stratified where this happens. Scan along the epithelium until you find a spot where it is cut perpendicular to its surface, i.e. where it looks like a simple columnar epithelium. Mucus producing goblet cells are a second cell type of this epithelium. Mucus stains only weakly or not at all in H&E stained sections. Round, light "hollows" in the epithelium represent the apical cytoplasm of the goblet cells, which is filled with mucin-containing secretory vesicles.
Microvilli extend from the apical surface of epithelial cells into the intestinal lumen. They increase surface area by a factor of ~20 and thereby facilitate absorption. Together, the microvilli are visible as a light red band along the apical limit of the epithelium, i.e. the side of the epithelium facing the lumen of the intestine. This band is call the brush border.

Stratified Epithelia

Stratified squamous epithelium
Stratified squamous epithelia vary in thickness depending on the number of cell layers present. The deepest cells, which are in contact with the basement membrane, are cuboidal or columnar in shape. This layer is usually named the basal cell layer, and the cells are called basal cells. Basal cells are mitotically active and replace the cells of the epithelium which are lost by "wear and tear". The basal cell layer is followed by layers of cells with polyhedral outlines. Close to the surface of the epithelium, cells become more flattened. At the surface of the epithelium, cells appear like flat scales - similar to the epithelial cells of simple squamous epithelia.
Remember that it is the shape of the cell which form the surface of the epithelium which gives the name to the epithelium.

Stratified cuboidal and columnar epithelia
are not common. A two-layered cuboidal epithelium is, for example, seen in the ducts of the sweat glands. Stratified columnar epithelia are found in the excretory ducts of the mammary gland and the main excretory duct of the large salivary glands.

Oesophagus, human - H&E
The oesophagus is lined by a stratified squamous epithelium consisting of many cell layers. Basal cells often form a well defined layer at the border of the epithelium to the underlying connective tissue. The underlying connective tissue forms finger-like extensions towards the lumen of the oesophagus, which are called papillae. The border between epithelium and connective tissue may appear quite irregular because of the papillae. This irregular border aids in anchoring of the epithelium to the connective tissue. If these extensions are not cut exactly along their long axis, they may look like isolated small islands of connective tissue and blood vessels within the epithelium.

Parotid Gland, Human - H&E
Stratified columnar epithelia are found in the largest excretory ducts of some glands. The parotid gland, a large salivary gland, is one of them. Several epithelial types are found in the duct system of the parotid. The smallest ducts, which are embedded in the secretory tissue (intralobular ducts), are lined by cuboidal or columnar epithelia. Small ducts, which are embedded in connective tissue located between areas of secretory tissue (interlobular ducts), are lined by columnar or pseudostratified epithelia. These ducts finally coalesce to form the main excretory duct of the parotid which is lined by a stratified columnar epithelium.

Pseudostratified and Transitional Epithelia
These two types of epithelia are difficult to classify using the shape of the cells in the surface layer and the number of the cell layers as criteria

Transitional epithelium

Transitional epithelium is found exclusively in the excretory urinary passages (the renal calyces and pelvis, the ureter, the urinary bladder, and part of the urethra).

The shape of the cells in the surface layer of a transitional epithelium varies with the degree of distension of the organs whose lumen is lined by this type of epithelium. In the 'relaxed' state of the epithelium, it seems to be formed by many cell layers. The most basal cells have a cuboidal or columnar shape. There are several layers of polyhedral cells, and, finally, a layer of superficial cells, which have a convex, dome-shaped luminal surface. In the distended state of the epithelium only one or two layers of cuboidal cells are followed by a superficial layer of large, low cuboidal or squamous cells. In the distended state the epithelium will resemble a stratified squamous epithelium.

Pseudostratified columnar epithelium

All cells of this type of epithelium are in contact with the basement membrane, but not all of them reach the surface of the epithelium. Nuclei of the epithelial cells are typically located in the widest part of the cell. Consequently, the nuclei of cells which do or do not reach the surface of the epithelium are often located at different heights within the epithelium and give the epithelium a stratified appearance. The epithelium will look stratified but it is not - hence its name "pseudostratified". Pseudostratified columnar epithelia are found in the excretory ducts of many glands.

Bladder, Monkey - H&E

At a first glance a transitional epithelium looks like a stratified cuboidal epithelium. Several rows of nuclei appear to be topped by a layer of dome-shaped cells which bulge into the lumen of the ureter. The shape of the surface cells and the number of rows change if the bladder is distended. The number of rows decreases. This decrease should tell us that many of the nuclei located in different layers of the epithelium belong to cells which are all in contact with the basement membrane. With distension, the shape of the cells in the surface layer will become squamous.

Trachea, Human - H&E

At least two, sometimes three rows of nuclei are seen in the pseudostratified columnar epithelium lining the trachea. The nuclei belong to cells which are all in contact with the basement membrane. The epithelial lining of the trachea is also one of the few examples of a basement membrane clearly visible in H&E stained sections. Epithelial cells can be ciliated or they can be goblet cells (unicellular exocrine glands). Basal cell regenerate other cell types of the epithelium. Capillaries and small vessels are visible in the connective tissue beneath the epithelium.
A ciliated pseudostratified columnar epithelium with goblet cells is a characteristic feature of parts of the respiratory system, where it is call respiratory epithelium. It contains several cell types in addition to ciliated, goblet and basal cells.

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.

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