MDIS

Management Development Institute of Singapore

Diploma in Biomedical Sciences

Anatomy Essay, Structure of Epithelium

S9326006F

Tan Wen Juan Karmaine

4. Describe the statement – The structure of epithelium is well-adapted for its

functions.

Epithelial tissue, or the epithelium, is made a layer of epithelial cells. Epithelium is the ‘covering’ tissue of the body. Cells all of the outer and inner surfaces of the body; they line cavities and cover flat surfaces. The outer layer of the skin, the inner lining of the stomach and respiratory tubes, and the lining of all blood vessels all contain epithelia. Generally, they can be said to occur at the interfaces between two different environments. They both protect the underlying tissues and contain nerve endings for sensory reception. Hence, epithelia serve as interface tissues and boundary layers. Epithelia are the gatekeeper cells – nearly all substances that are received or given off by the body must pass across an epithelium.

Epithelial cells have several special characteristics. They have cellularity, meaning that they are almost completely composed of cells, with a minimal amount of extracellular material. The extracellular material is also mainly comprised of projections of integral membrane proteins belonging to the epithelial cells themselves. Epithelial cells have specialized contacts – adjacent cells are directly joined at many points by special cell junctions. They exhibit polarity, in that the cell regions near the apical surface differ from those at the basal surface. All epithelial sheets are supported by an underlying layer of connective tissue. Whereas most tissues in the body contain blood vessels, epithelium does not. It is avascular but innervated; containing nerve endings but lacking blood vessels, it receives its nutrients from the underlying connective tissue. Epithelial tissue has a high regenerative capacity, as they are exposed to friction or destroyed by hostile substances in the external environment. Epithelial cells regenerate quickly as long as they receive adequate nutrition.

The functions of epithelial cells include diffusion, filtration, ion transport, protection, secretion and absorption. As we will see in the following paragraphs, the structure of epithelia has specialized features at all surface levels – apical, lateral and basal.

Epithelia have special apical surface features of microvilli and cilia. The apical surface of most epithelial cells contain microvilli, which increase surface area and may also serve to anchor sheets of mucus. The longest and most abundant microvilli are found on epithelia that absorb nutrients, like in the small intestine, or transport, such as in the kidney. The microvilli serve to maximize the surface area across which small molecules enter or leave cells. They may also act in helping to resist abrasion. Cilia are whip-like projections of the apical surface membranes of certain epithelial cells that beat to move fluid. Microtubules in the cores of cilia generate ciliary movement. The cilia on an epithelium bend and move in coordinated waves, like waves across a field of grass on a windy day.

In the lateral cell surfaces, special cell junctions called tight junctions close off the extracellular spaces. Adhering junctions and desmosomes bind cells together. Gap junctions serve as spaces through which small molecules can pass from cell to cell.

In the basal epithelial surface, epithelial cells lie on a protein sheet called the basal lamina. The basal lamina acts as a filter and support on which regenerating epithelial

cells form. The basal lamina, together with underlying reticular fibers form the thicker basement membrane.

There are six types of epithelial cells, and each is specially structured in accordance with their function. They are classified by cell shape – squamous, cuboidal and columnar, and by their number of cell layers at the free surface – simple, with just one layer, or stratified, with more than one layer.

Simple squamous epithelium consists of a single layer of thin, flat cells. These cells are tightly adherent to each other and have a smooth surface. Its structure allows molecules to pass through it rapidly, decreasing diffusion distance. Hence simple squamous epithelium is present in many areas requiring efficient gaseous exchange such as in the alveoli of the lungs, the Bowman’s capsule of the kidney and the interior lining of blood vessels.

Simple cuboidal epithelium and simple columnar epithelium play a key role in secretion, absorption and ion transport. Simple cuboidal epithelium is found in the kidney tubules and the small ducts of glands, while simple columnar epithelium lines the stomach and the small and large intestines.

Pseudo-stratified columnar epithelium contains both short and tall cells. Pseudo-stratified epithelia appear multi-layered, but are actually just one layer, hence the name ‘pseudo’, meaning ‘false’. This type of epithelia is best suited to propel substances across it’s surfaces - ciliated pseudo-stratified columnar epithelium lines most of the respiratory passages.

Stratified squamous epithelium is found in the epidermis and the lining of the mouth, esophagus and vagina. It is the type of epithelia most common in areas subject to friction. It resists abrasion as it is multilayered and thick, with flat apical cells.

Transitional epithelium is a stratified epithelium that thins when it stretches. A special structural feature of transitional epithelium is that the cells are rounded instead of flat when they are in the relaxed position. This thus enables distension, as the cells are able to slide over one another, and allows the organ to be stretched or unstretched. Transitional epithelium lines the hollow urinary organs - when the urinary bladder is full, the transitional epithelium stretches, thinning from about six cell layers to three, and its apical cells unfold and flatten.

To conclude, we have seen that different types of epithelia are found in various regions of the body, and the design of each structure serves to best fulfill the role of the processes required in the organs they are found in.

(918 words)

REFERENCES

Marieb, E., Mallatt, J. and Wilhelm, P. (2005). Human anatomy. 1st ed. San Francisco: Pearson/Benjamin Cummings.

Alberts, B., Johnson, A., Lewis, J. et al (2002), Molecular Biology of the Cell, 4th edition, New York, Garland Science

Lodish, H., Berk, A., Zipursky, S.L. et al (2000) Molecular Cell Biology, 4th edition, New York, W. H. Freeman