Epithelial Cells

Varieties of Epithelia

Epithelial cells are specialized to stick tightly to one another. These cells line the interior of hollow organs or cover over surfaces like the skin. There are dozens of types of epithelial cells in the body. Each cell can have one of three shapes:

  • flat (squamous)
  • cube-shaped (cuboidal)
  • tall (columnar)

Also, an epithelium can be only one cell layer thick (a simple epithelium) or many layers thick (a stratified epithelium).

Simple Squamous Epithelia

A single layer of thin, flattened epithelial cells lines the interior of blood vessels. They provide a slick surface that promotes a smooth flow of blood cells through the vessel. These lining cells can be surrounded by smooth muscle cells, which form a thick, contractile wall for a blood vessel. If these flat cells become detached from the lining of a blood vessel, harmful chemicals (cholesterol) can enter the wall of the vessel and cause it to become thickened. This condition is called atherosclerosis, or “hardening of the arteries.” Atherosclerosis can reduce the diameter of blood vessels and slow the passage of blood through the vessel. In the brain, this can cause damage to brain cells (a stroke). In the heart, reduced blood flow can damage heart muscle and cause a heart attack. So these cells play important roles in fighting diseases.

Simple Columnar Epithelia

Simple columnar epithelial cells are found lining the interior of the small intestine. An important job that they fulfill is to transport nutrients from food into the connective tissue beneath the epithelium, so that they can be absorbed into the bloodstream.

Control of the Shape of Epithelial Cells

Why do some epithelial cells have a flat shape, and others have a columnar shape? Part of the answer to this question is a layer of a protein that forms an interconnected meshwork on the interior of the cell membrane. This protein, called spectrin, regulates the overall shape of a cell.

Stratified Squamous Epithelia

Stratified squamous epithelial cells are found covering the lining of the lips and tongue or providing the covering for the skin. There are many layers of cells in this epithelium. Cells at the bottom of the epithelium are rounded in shape. These basal stem cells constantly divide to produce 1) exact copies of themselves and 2) flatter cells that migrate towards the top of the epithelium. When the cells reach the top of the epithelium, they can be worn away and thus often need to be replaced by newer cells.

What causes the appearance of so many layers of cells in a stratified epithelium? Recent study has shown that a protein called P63 is responsible. This protein stimulates the basal stem cells to divide. If P63 is inactivated, the body cannot produce a stratified epithelium. In experiments with P63 in developing mice, it has been shown that such mice will develop a skin that is only one cell thick! Such a skin cannot perform its protective job, and the mice would not survive. So, a stratified epithelium has a vital function.

Pseudostratified Columnar Epithelia

Two types of cells are present in a pseudostratified columnar epithelium: 1) round-looking basal stem cells, which continually divide, and 2) taller, columnar cells that resemble tiny cylinders. This type of epithelium is found within the lungs and trachea. The top of each columnar cell is decorated with tall filaments called cilia. These cilia beat in synchrony with each other and propel mucus out of the lungs and towards the throat. The function of this mucus is to trap particles and other inhaled material so that they don’t enter the lungs. Cilia move the mucus towards the mouse and esophagus, so it can be swallowed and destroyed in the stomach.

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Written by John Young

John K. Young is a retired professor of Cell Biology. He worked in the Department of Anatomy at Howard University College of Medicine in Washington, DC for 35 years, teaching his students about cells. During his career, Dr. Young published scientific articles about a part of the brain called the hypothalamus and also wrote a number of books about cells and about the brain.

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