Epithelial Tissue · What Is It, Where It’s Found, and More

Epithelial tissue is a membranous tissue made up of cells that form various surfaces and linings throughout the body. Although most epithelial tissues are a type of tissue on the surface of bodily structures (e.g., the skin or the lining of the intestines), some are also part of glandular tissue that produce and secrete products, such as mucus and proteins. Most epithelial tissues are consistently regenerating new epithelial cells from epithelial stem cells. In this process, when the superficial layer of cells dies, the new cells are formed in the deeper basal layers. Therefore, younger cells are found deeper than the older cells, which are more superficial.

Epithelial cells typically have three regions or domains in their structure: apical, basal, and lateral. The apical domain is located on the side of the lumen, or external environment. This region often contains structures related to the cell’s function, such as finger-like projections called microvilli found on intestinal epithelial cells in the small intestines, that increase surface area for absorption and fluid transport; cilia that are motile to transport substances in the respiratory tract; and stereocilia to help with hearing and balance in the inner ear. The basal region is the bottom edge of the cell and is adjacent to the basal lamina of the extracellular matrix, which separates the epithelial cell from the surrounding connective tissue. The lateral region is on the sides and typically allows for connections with neighboring cells.

Epithelial cell junctions allow connections between neighboring cells, structural support, and more. There are four main types of epithelial cell junctions located on the lateral sides: tight junctions, adherens, desmosomes, and gap junctions. There is one type of connection that exists between epithelial cells and underlying connective tissue, called hemidesmosomes.  

There are two main types of epithelium: surface and glandular.

The surface epithelium is classified according to its number of layers, cellular structure, and function. There are three epithelial cell shapes: cuboidal, columnar, and squamous. Cuboidal epithelial cells are shaped like a cube and have a spherical nucleus in the center. Columnar epithelial cells are tall, rectangular in shape, and have an oval-shaped nucleus in the basal region. Squamous epithelial cells are slightly rounded, often elliptical and flat in shape, with a small nucleus in the center.

Epithelial cells can also be organized into one or multiple layers. There are two types of epithelial tissues with only one layer of cells: simple and pseudostratified. Simple epithelial tissues have one layer of cells in a single line; an example of this tissue type is simple cuboidal epithelium. This is contrasting to pseudostratified epithelial tissues, which contain cells with irregular shapes and sizes; an example of this tissue type is pseudostratified columnar epithelium. Both subtypes often include specialized cell types located between the epithelial cells, such as the mucus-secreting goblet cells found in a mucous membrane. 

Epithelial tissues may also have two or more layers of cells, known as stratified epithelium. Stratified epithelial tissues have two additional classifications: keratinized and transitional. Keratinized describes epithelium that is filled with a fibrous protein called keratin and where the most apical layers are dead, with minimal nucleus and cytoplasm. This is seen in keratinized stratified squamous epithelium. On the other hand, transitional epithelium consists of many layers that may become flat when they are stretched.

In contrast, glandular epithelium can be classified based on its location relating to the surface epithelium, its secretory mechanism, and what it secretes. Intraepithelial cells are those located between surface epithelium, while extraepithelial cells are located under the surface epithelium. There are two types of intraepithelial glands: goblet cells and secretory epithelium. Goblet cells are singular epithelial cells that often appear as a tube with a large vacuole full of mucus. In contrast, secretory epithelium consists of columnar cells that have a cytoplasm filled with mucus, but without a vacuole. Extraepithelial cells have terminals that connect to the excretory ducts and are involved in secretion.

Surface epithelial tissue is found throughout the body covering all surfaces both inside and outside of the body. Each cell type is prominent in specific locations.  

Single-layered epithelial tissue is often found along the linings of organs. Simple cuboidal epithelium is typically found in the lining of kidney tubules and glandular ducts. Simple columnar epithelium is often found throughout the digestive tract and part of the female reproductive tract, like the fallopian tubes. Pseudostratified columnar epithelium is often located in the respiratory tract.

Stratified squamous epithelium is the most common type of stratified epithelium, and is keratinized along the skin and nonkeratinized in areas like the lining of the oral cavity. The stratified cuboidal and columnar epithelia are also located within some glands and ducts, but are very rare.

There are three primary functions of epithelial tissue: serving as a protective barrier, secreting substances, and absorbing substances. The main function of specific tissues varies by the specific location and type of cell.  Protective barriers are typically formed by stratified epithelium and goblet cells. The stratified epithelium protects the body from physical and chemical damage by creating a barrier of keratinized cells. Goblet cells are involved in maintaining a protective layer of surface tissue, like the inner lining of the stomach, through secretions such as mucus and other substances that provide a barrier. Secretion and absorption are often performed by simple epithelia for molecules that require active transport across a cellular membrane, while extraepithelial glands are involved in direct drainage into ducts.

Due to the high turnover rates of epithelial cells, the most common disorder affecting epithelial tissues is cancer. A benign tumor of epithelial origin is called an adenoma or papilloma, typically appearing along the skin or digestive tract. However, when tumors of epithelial origin have spread past the basement membrane, they are then identified as malignant and referred to as carcinomas. The most common carcinomas worldwide are lung adenocarcinomas, colon adenocarcinomas and thyroid papillary carcinomas.

As epithelial cells are the most common type of cell to undergo metaplasia, a process in which cells change their cell type, there is a risk of the cells becoming cancerous. Metaplasia often occurs in response to environmental changes and stress. For instance, when gastroesophageal reflux disease (GERD) is uncontrolled, the continuous acid exposure causes the esophageal epithelial cells to turn from squamous cells to columnar cells that produce mucus. If this continues, there is a higher risk for developing cancers.

Many other disorders can affect epithelial tissues, ranging from ones that develop in the digestive tract, like celiac disease, to ones on the surface of the skin, like human papillomavirus.

Epithelial tissues make up many surfaces and linings throughout the body, and may include surface and glandular tissues. Surface epithelial tissue is further classified by the number of layers, its structure, and function. Additionally, glandular epithelium is classified by its location to the surface epithelium, its secretory mechanism, and secretory substance. Surface epithelium is found throughout linings and tracts of the body, while glandular epithelium may be found between surface cells and inside glands. The function of epithelia varies by cell type and location, but for the most part, epithelial tissue provides a location for protection, secretion, and absorption. Epithelial cells are commonly associated with cancers, like adenocarcinoma of the colon.