1. MDIS
Management Development Institute of Singapore
Diploma in Biomedical Sciences
Anatomy Essay, Structure of Epithelium
S9326006F
Tan Wen Juan Karmaine

2. 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

3. 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)

4. 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