There are four types of tissues in the body: connective, epithelial, muscular, and neural. This article will address the epithelium, closely spaced cells that form borders in the body. Epithelial cells could be separated into three broad classes: simple, pseudostratified, and stratified.

Simple cells are single-layered and may be squamous, cuboidal, or columnar. Squamous cells are found in body structures that need unlimited free exchange of gases and nutrients. These structures include the lungs, circulatory system, loop of Henle, and serous membranes which line the internal body cavities. Cuboidal and columnar cells allow for controlled exchange of nutrients and gases and are found in the intestines.

Pseudo-stratified cells appear to be double layered; however, there is only a single layer, hence the name. These are found only in the upper respiratory system and contain cilia, hair-like organelles that push mucus up and out of the body. This is known as the mucociliary escalator. Mucus traps foreign bodies and contaminants, so expelling mucus helps keep the lungs sterile.

Stratified cells have multiple layers. Stratified cells may be keratinized, non-keratinized, or transitional. Keratinized cells are dead, cover the entire skin surface, and are filled with keratin, which waterproofs the skin. Non-keratinized cells cover the esophagus and mouth. Transitional cells have the ability to change shapes depending on how much volume there is in the organ. For example, the transitional cells in the bladder can flatten out to increase the amount of space in the bladder, so it can hold more urine.