Covering epithelium are sheets of tissue that cover the external surfaces (skin, lungs, gut) and line the internal cavities (blood and lymphatic vessels, pleura) of the body. Pleura is a type of covering epithelium known as mesothelium. It coats the serous cavities of the body and internal organs. Endothelium is a type of covering epithelium that lines the interior surfaces of blood and lymphatic vessels. Epithelium shows almost no extracellular matrix and its cells are tightly joined by molecular anchoring complexes. Some epithelia show a high rate of cell turnover where cell death and cell proliferation are frequent. For example, there are adult stem cells in the inner layers of epidermis that continuously produce new keratinocytes, whereas the cell death occurs in the outer layers. Some epithelial cells can have apical specializations that allow them to function as sensory receptors, and some animals show complex structures in their epithelia, such as hair, feathers or scales.
Epithelium is usually classified according to two features: the number of cell layers and the shape of the cells of the more superficial layer (Figures 1 and 2). Simple epithelium is a single cell layer where all the cells are in contact with the underlying basal lamina and have an apical free surface. The shape of the cells can be flat (wider than high), cuboidal (as wide as high), or columnar (higher than wide). That is why simple epithelium may be named as simple squamous, simple cuboidal, and simple columnar, respectively. Pseudostratified epithelium contains cells contacting with the basal lamina, but not all of them show a free apical surface. Then, not all of the cells show the same height and the epithelium looks like it was stratified, but it is not. Stratified epithelium contains two or more layers of cells. Only cells of the deeper layer are in contact with the basal lamina and only cells of the upper layer show free surfaces. Stratified epithelium can be classified as squamous, cuboidal and columnar, depending on the shape of the cells of the upper layer. Transitional epithelium is another type of stratified epithelium that can be stretched, changing the shape of its cells.
Cell arrangement Simple Stratified Pseudostratified Transitional
Squamous epithelium
The outer layer is formed by flat cells.
Cuboidal epithelium
The outer layer is formed by cube-like cells.
the excretory duct of a gland.
Columnar epithelium
The outer layer is formed by columnar shaped cells
Transitional epithelium
It can be streched. The cell shape changes during these movements.
Pseudostratified epithelium
It contains cells at different levels, but all of them contact with the basal lamina. That is why this epithelium looks like stratified.
Epithelium 