The epithelial tissue is one of the most important tissues found within the human body. The epithelial tissue is responsible for protecting the body from external conditions and elements. Epithelial tissue is a flat tissue composed of closely bound cells. The uppermost layer of this tissue is usually exposed to the external environment, while the lowest layer of this tissue is in contact with the internal environment of the body.
The epithelial tissue makes up the entire covering of the body, or skin, as well as forming the linings and membranes of the organs and dividers between these organs. Many glands within the body are also composed of epithelial cells. The cells of the epithelial tissue appear so tightly packed together that there is no room for blood vessels or capillaries, however, the epithelial tissue generally sits above a layer of loose connective tissue from which the epithelial tissue is able to obtain nourishment and dispose of waste.
Between the epithelial tissue and the loose connective tissue beneath it, lays the basement membrane; a membrane that contains collagen and adhesive glycoproteins called laminin, fibroconectin, and protein-carbohydrate called heparan sulfate. This basement layer gradually merges into the layer of loose connective tissue and serves as a tether, binding and connecting the epithelial tissue to the loose connective tissue beneath it.
There are two broad classifications used when describing epithelial tissue; simple and stratified.
Simple Epithelial Tissue
Typically, simple epithelial tissue contains a single layer of cells, but some epithelial tissues with more than one layer of cells are still classified as simple, so this requirement is considered somewhat controversial. There are three types of simple epithelial tissue, each named for the shapes of the cells; simple squamous, simple cuboidal, and simple columnar. There is a fourth type, pseudostratified columnar, where not all cells reach the surface, but instead, are obstructed by the taller cells.
Stratified Epithelial Tissue
Stratified epithelial tissue consists of two or more layers of cells, all resting one row upon another. Only the deepest layer is anchored to the basement membrane. Three of the stratified epithelial cells are named for the shape of the cells; stratified squamous, stratified cuboidal and stratified columnar. A fourth type is known as transitional epithilium, and was named so because at the time, scientists believed it was a transitionary stage between stratified squamous and stratified columnar.
Saladin, Kenneth S.. Anatomy & physiology: the unity of form and function. 5th ed. Dubuque: McGraw-Hill, 2010. Print.
Simple Epithelial Tissue