To be present on the free surface epithelia have many special functions. Epithelium creates a selective barrier between the internal and external environment. This permeable barrier facilitates or inhibits the passage of specific substances: any substances that enter the body as the metabolites or discharge from the body as the waste products must pass through the epithelial cell, not between them.
Another functions of epithelial tissues:
- Absorption (as in intestine);
- Excretion (as in tubuli of kidney);
- Secretion (inner surface of stomach, glands)
- Transport (as in the trachea and bronchi - to move particles and mucus trough wave-like activity of motile cilia on its surface);
- Protection (skin).
Epithelium can allow the diffusion of gases, as in the alveoli of the lungs, or it can be almost impervious barrier, as in the urinary bladder. It can serve to receive sensory stimuli, as in the taste buds of the tongue. The function of epithelial tissues varies according to the localization and correlates with morphology.
Characteristics of epithelium
Epithelial cells have some principal characteristics.
1. Covering and lining epithelia form a continuous sheet-like cellular investment that separates the underlying connective tissue from the external environment or environment of internal cavities. They are characterized by close apposition of its constituent cells. Epithelial cells are densely adjoined and strongly attached to one other by means of special junctions. There is little extracellular material between the cells.
2. The cells rest on the basement membrane an extracellular protein-polysaccharide-rich layer that they produce.
3. Epithelial cells exhibit polarity. Each of them has three functionally distinct surfaces: apical or free, lateral and basal. Apical surface is exposed to the lumen or outside world, lateral one contact with neighbouring cell and basal one is attached to the basement membrane. In many epithelial cells their nuclei are somewhat basal. Some organelles, such as the Golgi apparatus, are typically supranuclear (i.e. they are between the nucleus and the cell apex). And secretory granules like apical localisation.
4. Epithelia are avascular. Epithelium does not contain blood vessels. It is nourished by diffusion of substances from underlying capillaries are found in the connective tissue.
5. It is frequently mitotically active which is important since it undergoes various stresses or injuries.
Classifications of Epithelial Tissues
Morphological classification is based on combination of number of cell layers and shape of surface cell.
Description of an epithelium requires two names. The first name can be called simple or stratified on the basis of the number of cell layers, which comprise it. Epithelium is simple, when it is one cell layer thick and all cells sit on the basement membrane. Stratified epithelia have two or more layers of cells.
The second name describes the shape of cells. The individual cell may be:
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- Squamous, very flat cells, when the height of the cell is negligible in comparison of the other dimensions;
- Cuboidal, taller than squamous cells. The width, depth and height are approximately same;
- Columnar, tall cells, where the height appreciably exceeded the its other dimensions.
In simple epithelia all cells reveal same size and shape. In stratified epithelia the shape and height usually vary from layer to layer. But only the shape of the cells forming the surface layer is used in classification.
Exceptions to this method of nomenclature exist and include the following:
Pseudostratified columnar epithelium is a simple epithelium, but appears to be stratified because it contains several kinds of cells whose nuclei occupy varying positions. However, all of the cells touch the basal lamina although not all cells reach the luminal surface.
Stratified transitional is an epithelium, which undergoes a change in appearance depending on whether the organ it lines, is distended or not.
Additional information may be added to epithelia, such as ciliated, with goblet cells, keratinized or non-keratinized and with microvillus (brush) border. These classifications are used in combination to describe a particular type of epithelium. For example, the epithelium lining the trachea is ciliated pseudostratified columnar epithelium, which describes the shape of the cells, the number of cell layers, and the surface specializations.
The morphology of an epithelium often correlates with its function. Simple squamous epithelia are compatible with high rate of transepithelial transport if no intracellular processing of the transported substance occurs (as passive diffusion of gases in the lung due to different concentration and partial pressure in atmospheric air and blood). Epithelia involved in secretion or absorption are typically simple cuboidal or columnar. The height of the cell often reflects the level of secretory or absorptive activity. Stratification of the epithelium usually correlates with transepithelial impermeability and protection. Keratinization additionally increases protective properties of epithelium.
