In the human body, the cavities within including passages and tubules along with the external surface of the skin are lined by a specialized tissue structure known as the epithelium. Although these epitheliums can vary from place to place depending on the functional adaptations, they all are arranged on a thin connective tissue structure known as the basement membrane and is separated through this layer from the underlying connective tissue which supplies the epithelium with nutritional needs.
When looking at various structural adaptations of an epithelium, it is evident that, two main categories of cellular arrangement are in existence. These are, single layer arrangement and multi layer arrangement. Thus, the term simple epithelium is used to describe the single layer arrangement of an epithelium which lines certain parts of the body that perform vital tasks to support the bodily functioning.
A simple epithelium can be further classified according to the shape of the cell elements. Thus, the following types of epitheliums are readily identifiable in different parts of the body.
Squamous epithelium
This type of epithelium is readily identified due to the flattened appearance of its cells and it provides a low friction surface that can perform exchange of certain substances as in the case of diffuse passage. One good example of this kind of epithelium is the alveolar surface in which the thin squamous epithelium function as an efficient surface lining that facilitates exchange of oxygen between the inhaled air and the blood vessels. Apart from the alveoli, the same epithelium can be seen lining the blood vessels as well as the inner aspect of heart chambers.
Columnar epithelium
As the name suggest, these cells are tall and appear as a column and can be seen to line the surface of the stomach and the intestines. It can perform secretary and absorptive functions along with specializations of the cellular elements to perform these functions better. Certain columnar epitheliums act as sensory receptors and this is true for the surface lining of nose, ear, tongue and taste buds.
Cuboidal epithelium
This epithelium is given its name because of the cuboidal or square shaped appearance seen in cross sections. The layer performs secretive or absorptive functions as in the case of exocrine pancreas and in kidney tubules respectively. Apart from the above, these cellular structures are the lining epithelium or the germinal epithelium in reproductive organs in both males and females. Thus, it is present in the ovary as well as in the testis.
Apart from these three types, certain other simple epitheliums can be seen to depict a multi layer appearance although in real, they are having only a single layer. The pseudo-stratified epithelium of the intestine is one such example and the reason for this appearance is the different locations of the nuclei within each cell.
