The epithelium is a tissue composed of numerous epithelial cells packed tightly together. These cells, which often come into contact with substances foreign to the body, may vary greatly in shape and function.

Columnar epithelium can be found along the intestinal tract, spanning from the end of the esophagus to the rectum. This type of tissue also occurs in the ducts of various glands. Epithelial cells are taller than they are wide and contain nuclei along their bases. The membranes that surround them are relatively thin, but can be easily viewed with the aid of a microscope. One of the best-known examples of columnar epithelial cells appears as a covering of the projections called villi found in the small intestine. The functions of these and other cells of the epithelium are various, but essentially involve absorption, secretion, and protection.