Epithelium: What to Know

The epithelium is a type of tissue that covers many different surfaces on the inside and the outside of your body. Epithelial cells are packed tightly together and serve as a barrier between the interior and the exterior of your body. You have countless epithelial cells throughout your body that make up what is known as the epithelium.

What Is the Epithelium?

The epithelium is a layer of cells that are bound together tightly to form sheets. These continuous sheets cover different surfaces of your body that can come into contact with foreign substances.

What are epithelial cells? These cells make up the main tissues in your body. They can be found everywhere throughout your body, so the epithelial cell location will determine its function. 

Almost all the organs that are covered in epithelium have unbroken sheets of epithelial cells. These cells work together to form a protective boundary. Heat, sensations, and gases all have to cross this boundary.

Epithelial cell types. Epithelial cells come in several shapes to form different types of epithelial membranes. These types include:

• Columnar
• Cubical
• Squamous (flat)
• Irregular 
• Ciliated (hair-like)

Epithelial tissues can be just one cell thick or organized so that they have several layers.

What Does the Epithelium Do?

Since there are several different epithelial cell types, the epithelium performs several different essential functions. 

Wherever it is, though, the epithelium's main job is to provide protection from the outside world. This could include defense against physical, chemical, or biological damage. You can think of the epithelial cells as bodyguards. They decide what gets to enter by allowing materials to permeate the surface of the epithelium. Any substance that goes into your body has to cross the epithelium at some point.

Epithelial cell function can include a combination of these things:

• Protection
• Secretion
• Absorption
• Excretion
• Filtration
• Diffusion
• Sensory reception

Some epithelial cells can adapt to act as sensory receptors. Some examples are taste buds and the lining of the nose, eyes, and ears.

Where Are Epithelial Cells Located?

Epithelial tissue is composed of embryonic layers. They form from ectoderm, mesoderm, and endoderm. Since they are made up of these materials, epithelial tissues line areas of the body like body cavities, skin, and organ surfaces.

Specific epithelial cell location depends on the type. For example:

• Simple squamous: These cells form a thin, single layer that creates a membrane. They are found in the walls of your capillaries, along the linings of the alveoli in your lungs, and in the linings of the pericardium.
• Simple columnar: These tall cells are packed together to form a row. You’ll find them in areas with high secretion volumes, like the stomach wall, or sites of absorption, like the small intestine. They have cellular extensions that are also found in other places, like the cilia along the female reproductive tract.
• Simple cuboidal: These are wide, single-layer tissues that are also needed for secretion and absorption. These cells can be found in the ducts of the kidneys, salivary glands, and pancreas.
• Pseudostratified: These are columnar epithelial cells that have different heights. They have hair-like extensions called cilia. These cells are found in airways, like the nose and bronchi, as well as the uterus and fallopian tubes.
• Keratinized: These contain keratin, a tough, waterproof protein that helps to protect the body. It’s found in the skin and the lining of the esophagus.
• Transitional: Found in stretchy tissues, this is sometimes called the urothelium since it’s present in the bladder, urethra, and ureters.