8.2: Epithelial Tissues and Their Functions

Epithelial tissues are large sheets of cells covering all of the surfaces of the body. These surfaces can be internal or external, for example, skin, airways, the digestive tract, the urinary system, and the reproductive system. Hollow organs and body cavities that do not connect to the body's exterior, including blood vessels and serous membranes, are lined by epithelial tissue known as the endothelium.

Epithelial tissues provide the body's first line of protection from physical, chemical, and biological wear and tear. They act as the gatekeepers of the body and allow the selective transfer of materials across a physical barrier. This means that all substances that enter the body must cross an epithelium. Many epithelial cells secrete mucus or other chemical compounds with specific functions onto their apical surfaces. For example, the intestinal epithelium releases digestive enzymes to help digest food, while the respiratory tract lining secretes mucus to trap incoming microorganisms and particles.

Additionally, the epithelium also forms much of the glandular tissue of the body. Glandular tissue consists of single or multiple secretory cells that yield a variety of secretions. These secretions are discharged on the epithelial surface or released into the bloodstream to reach target cells.

Epithelial tissues also have a large supply of sensory nerves that help to generate an immediate response to stimulation.

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

Epithelial tissues are sheets of closely-packed cells covering the body and compartmentalizing specific environments to protect the body from pathogenic invasion, fluid loss, or mechanical injuries.

They occur in two forms, covering or lining epithelia, and glandular epithelia.

Covering or lining epithelia function as gatekeepers. They cover exposed body surfaces and line internal tracts of organs, allowing selective substances to enter and exit the body.

For example, the epithelial lining of the kidney tubules allows the reabsorption of nutrients from the urine and the release of waste products from the nearby capillaries.

Glandular epithelia are arranged into specialized structures or glands, which can be a single cell or a group of cells. They produce and release different secretory substances, such as mucus, sweat, and hormones.

Epithelial tissues are richly supplied with sensory nerves that quickly relay information between the external and internal environments, producing an immediate response.

For instance, a mosquito bite instantly stimulates nerves under the skin epithelium and triggers the body to swat at the location.