Epithelium is a thin layer of specialized cells on the surface of a tissue. This could be an outside surface like the skin, or an inside surface like the cells that line the inner surface of the colon. The cells in the epithelium are called epithelial cells. The epithelial cells stick together to form a barrier.
Below the epithelium is specialized layer of connective tissue called lamina propria. The epithelium and lamina propria are separated by a very thin layer of tissue called the basement membrane.
There are many different types of epithelium throughout the body and each serve a specific purpose. The most common types of epithelium are squamous, glandular, and urothelial.
Many different types of cancer can start from the cells in the epithelium. All cancers in this group are called carcinomas. These cancers can start in any area of the body where there is an epithelium.
Types of cancer that start from the cells in the epithelium include:
Sometimes the abnormal cells are seen only inside the epithelium. Pathologists call this condition dysplasia. Carcinoma in situ is a diagnosis pathologists use to describe cells that look the same as cancer cells but are still seen only in the epithelium. Carcinoma in situ is called a non-invasive type of cancer because the abnormal cells have not spread to the tissue below the epithelium. The movement of abnormal cells out of the epithelium and into the tissue below is called invasion.