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Epithelium is the cellular covering of internal and external body surfaces. Simple epithelium is made up of one layer of cells, and it serves many roles in various regions of the body. One of its functions is to secrete fluids that lubricate tissues in order to minimize friction. It also lines body tracts as secretory cells, which release stored chemicals; and absorptive cells, which are efficient in absorbing substances.
Each cell in a simple epithelium is in contact with a thin sheet of fibers underlying the epithelium, known as the basement membrane. This membrane acts as a barrier in preventing tumorous cells from invading deeper tissues. A simple epithelium is one cell layer thick. According to the shape and function of its cells, simple epithelium can be subdivided into simple squamous epithelium, simple cuboidal epithelium, simple columnar epithelium and pseudostratified epithelium.
A simple squamous epithelium is a single layer of flat cells whose appearance is similar to thin, flat plates. Simple epithelium of this type is often permeable and is found where microscopic molecules pass through membranes via diffusion or filtration. Simple squamous epithelia are found in capillaries; in alveoli of the lungs, where gas exchange occurs; and in kidney glomerulus, where urine begins to form.
Simple cuboidal epithelium is found in a single layer of cubelike cells. Cuboidal cells have a spherical nucleus in the center. These cells undertake complex functions such as absorption or secretion. Simple cuboidal epithelia are found in the secretory portions of the endocrine and exocrine glands as well as in the collecting ducts or tubules of the kidney.
As the name suggests, simple columnar epithelia are made up of a single layer of columnar cells that are elongated and column-shaped, resembling skin cells. Simple columnar epithelia undertake absorption or secretion. They are found lining most organs of the digestive tract, including the stomach, small intestine and large intestine. Other regions of the body where simple columnar epithelia are found include the upper respiratory tract, the uterus of females and the central part of the spinal cord.
Pseudostratified epithelium is made up of simple columnar epithelial cells, arranged in a single layer. The term "pseudostratified" is used because the cross-section of the cells falsely resembles more than one layer of cells. Psuedostratified epithelia are involved in absorption or secretion. It is not uncommon to find simple epithelium of this type in the upper airways — the nose and bronchi — as well as in the uterus and fallopian tubes of females.
Some of the other functions of simple epithelia include the lining of blood vessels and in goblet cells, which secrete mucin that dissolves in water to form mucus. The mucous layer formed entraps microbes, dust and debris into the tract. Secretion of the mucus can be stimulated by smoke, dust or other particles in the air.