|| The nature of the epithelium on
different surfaces reflects the primary functions performed by the
epithelium. These functions that epithelium performs include protection,
absorption, secretion and filtration.
|| On surfaces that require protection
and resistance against wear and tear the epithelium will have many layers
of cells. The
skin is an example of a location where the epithelium has
many layers. The epithelium is stratified and can be very thick. The
uppermost surface layers of the epithelium are dead and are densely packed
with a protein called keratin. Keratin
increases the toughness of the skin and gives it a water-proof quality
that prevents water loss.
|| In other surfaces of the body the
epithelium is designed to permit the passage of a variety of molecules. In
some locations, the lining has to be as thin as possible to allow for the
rapid passage of molecules by diffusion. For example, the surfaces of the
lung where gas exchange takes place is exceedingly thin. It is only one
cell layer thick and the cells are exceedingly flat. This increases the
speed by which gases such as oxygen and carbon dioxide can pass through
the epithelium by diffusion.
|| In other locations, molecules are
transported across the surface by active transport and the cells that form
the epithelium the cells need to expend energy to move molecules across
the surface. For example, the inner lining of the
small intestines is
where nutrients and water is rapidly absorbed. The epithelium here is
again only one cell layer thick but the cells have more bulk. These cells actively
transport nutrients and water against their concentration gradients.
This requires energy and cellular machinery. Hence, the bulk of the cells
is due to the presence of organelles to perform this energy demanding