The epithelium is a body tissue covering all internal and external body surfaces, lines body cavities and hollow organs, and the primary tissue in glands. Epithelial tissue serves a range of roles depending on its location in the body, including protection, secretion, and absorption.

Based on their shape, they are classified as squamous epithelium, cuboidal epithelium, and columnar epithelium. Squamous epithelium appears flat and sheet-like. It lines organs and blood vessels and also has protective functions. Cuboidal epithelium which is cube-like in appearance has secretory and excretory functions. Columnar epithelium which is taller than they are wide and is specialized for absorption.

Based on their arrangement, they can be simple meaning it is only one layer of cells, stratified which is made up of more than one layer, and pseudostratified which is made up of only one layer of cells, but it looks like they are arranged in layers.

Reference: Cleveland Clinic Medical Professionals. (n.d.-a). Epithelium. Cleveland Clinic. https://my.clevelandclinic.org/health/articles/22062-epithelium