Epithelium tissues are the cell structures that line body cavities, blood vessels and other passages as well as the surfaces which are susceptible to friction. As such, these tissues are present in many parts of the body and will perform different functions according to its location.
An epithelium tissue will be laying on a basement membrane almost all instances and can consist of either a single layer or multiple layers of cells. These cells can vary in its appearance as well as its cellular integrity. Based on these elements, the epitheliums are categorized according to the following classification.
Basically, all epithelial tissues are classified according to its complexity. An epithelium with only a single layer will be known as a ‘simple epithelium’ whereas a multi layered epithelium will be defined as a ‘stratified epithelium’. First, let us see how a single layer epithelium or a simple epithelium can be further categorized according to the cell type.
Squamouse epithelium : These cells appear flat and often lines structures which needs to contain a surface that allows diffusion of substances or air. Therefore, it lines the blood vessels as well as alveolar walls in the lungs.
Cuboidal epithelium : The cells will be square in its appearance and often will be lining the secretary glands such as the pancreas. It will also line the germinal epitheliums in the ovary and the testis in which germ cells are produced.
Columnar epithelium : The cells in these tissues will appear tall and column like and will be part of the surfaces which facilitates absorption such as in the intestine and in the stomach. Certain columnar cells are specialized in providing sensory reception as in the nose and the taste buds in the tongue.
Psudostratified epithelium : The nuclei of a columnar cell is usually located in the base of the cell and in the pseudostratified epithelium the nuclei are present at various levels of a simple columnar epithelium and thus gives false impression of a multi layered epithelium. The surface of the cell can have specialized structures known as ‘cilia’ and these are located in the air passages of the lungs as well as in fallopian tubes. The ‘cilia’ will facilitate the transport of mucus secretions or an ovum respectively and play an important role in the functioning of these organs.
When talking about the stratified epitheliums, it should be remembered that, the basal layer of the tissue can be either squamouse, cuboidal or columnar and towards the surface, the cells will become more flattened than the base. But, the distinct feature of these kinds of tissues will be the presence of keratin in the surface cells. Thus, accordingly, they are classified as either ‘keratinized’ or ‘non-keratinized’.
Keratinized stratified epithelium is a structure that can withstand stressors and therefore will be present in areas such as the skin. The non-keratinized versions can be present on moist surfaces which are exposed to direct external insults such as in the upper layers of the esophagus.
Apart from these, another stratified epithelium known as the ‘transitional epithelium’ will be preset in structures which are susceptible for distension. Therefore, bladder, urethra and ureters are structures that are lined by the transitional epithelium.