Simple epithelium consists of a single layer of cells that lines body cavities and blood vessels. The shape of the cells in the epithelium reflects the function of the tissue. Cells in simple squamous epithelium appear as thin scales with flat, elliptical nuclei that mirror the form of the cell.

Because of the thinness of the cells, simple squamous epithelium is present where the rapid passage of chemical compounds is observed. For example, the endothelium that lines the capillaries and vessels of the blood and lymphatic systems is a simple epithelial tissue which allows the exchange of nutrients. Similarly, the alveoli of the lungs, and segments of kidney tubules, are also composed of simple squamous epithelial tissue.

Additionally, simple epithelia can also provide a smooth and protective surface. The mesothelium is a simple squamous epithelium that forms the surface layer of the serous membrane that lines body cavities and internal organs. Mesothelial cells secrete a fluid that lubricates the mesothelium.

In simple cuboidal epithelium, the nucleus of the box-like cells appears round and is generally located near the center of the cell. These epithelia are active in the secretion and absorption of molecules. Simple cuboidal epithelia are observed in the lining of the kidney tubules and the ducts of glands.

Simple columnar epithelium contains tall column-like cells with elongated nuclei located near the cells' basal end. Like cuboidal epithelia, the primary function of this epithelium is the absorption and secretion of molecules. Simple columnar epithelium forms the lining of the digestive tract and parts of the female reproductive tract. Ciliated columnar epithelium is composed of simple columnar epithelial cells with cilia on their apical surfaces. These epithelial cells are found in the lining of the fallopian tubes and parts of the respiratory system. The beating of the cilia in the fallopian tube helps the ovum move toward the uterus, and those in the respiratory tract help remove particulate matter.

Some specialized simple epithelia perform essential bodily functions. One such type is the pseudostratified epithelium, a specialization of simple columnar epithelia that contains cells of varying height and nuclei levels, giving an illusion of having multiple layers. The pseudostratified columnar epithelium is found in the respiratory tract, where these cells have cilia to propel dust and other foreign bodies toward the outside of the body.

Another type of epithelium is the myoepithelia or basket cells, which are contractile cells composed of actin and myosin filaments. They form a layer around secretory cells of various glands, such as salivary glands, mammary glands, and sweat glands. Upon receiving nerve signals, myoepithelia undergo contractions and help release glandular products from the secretory cells.

This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial tissues.