Epithelial Cells: Definition

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Epithelial cells are the cells that cover the surfaces of the body and line its hollow organs, ducts, and glands. They form the outer layer of the skin and the inner lining of the mouth, stomach, intestines, airways, bladder, and many other organs. Epithelial cells are tightly joined, forming a continuous sheet. That sheet of cells is called the epithelium.

Epithelial cells are a normal part of almost every tissue sample, so their presence is not a diagnosis. They matter in pathology because most cancers start from epithelial cells. This article explains what epithelial cells do, the main types a pathologist describes, and why they are mentioned in your pathology report.

What do epithelial cells do?

Epithelial cells do different jobs depending on where they are in the body. Their main jobs are:

Protection. Epithelial cells form a barrier that keeps germs, chemicals, and other harmful substances out of the body. Examples include the skin and the lining of the mouth.
Secretion. Some epithelial cells make and release substances, such as mucus, sweat, saliva, hormones, and digestive juices. Cells that do this often form glands.
Absorption. Epithelial cells in the small intestine take in nutrients from food and pass them into the blood.
Filtering and exchange. Epithelial cells in the kidneys help filter blood, and those in the lungs let oxygen pass into the blood.

What are the main types of epithelial cells?

Pathologists describe epithelial cells by their shape and function. The types most often named in a pathology report are:

Squamous cells. Squamous cells are flat cells stacked in layers. They protect surfaces that face wear and tear, such as the skin, mouth, esophagus, and cervix.
Glandular cells. Glandular cells are taller cells that make and release substances such as mucus. They line organs such as the stomach, colon, breast ducts, and prostate.
Urothelial cells. Urothelial cells line the bladder and the tubes that carry urine. They can stretch as the bladder fills.
Basal cells. Basal cells sit at the bottom of a layered epithelium. They divide to replace cells that are worn away from the surface.

Some epithelial cells have special names that come from the organ they belong to. Examples include keratinocytes in the skin, pneumocytes in the lungs, and hepatocytes in the liver.

How are epithelial cells arranged?

Epithelial cells can form a single layer or many stacked layers. A single layer is found where substances need to pass through easily, such as the lining of the intestines and the air sacs of the lungs. Many layers are found where the surface needs extra protection, such as the skin. A layered lining made of squamous cells is called a squamous epithelium.

Epithelial cells always rest on a thin sheet of supporting material called the basement membrane. Below it is the stroma, the supporting tissue that contains blood vessels and nerves. This boundary is important in cancer diagnosis, as explained below.

How do pathologists identify epithelial cells?

Pathologists can usually recognize epithelial cells by their shape and arrangement on routine slides. When the cell type is unclear, such as in a tumor that has lost its normal appearance, the pathologist may use immunohistochemistry. This test uses antibodies to detect proteins that particular cells make.

Most epithelial cells contain proteins called cytokeratins. A positive cytokeratin stain, such as pancytokeratin or AE1/AE3, supports that the cells are epithelial. EMA is another stain often used for the same purpose. A few other cell types, such as the mesothelial cells that line the chest and abdomen, also contain cytokeratins. For this reason, the pathologist interprets these stains alongside the cells’ appearance.

Why are epithelial cells mentioned in a pathology report?

Epithelial cells appear in many different findings on a pathology report. Common examples include:

Normal or reactive changes. The report may describe normal epithelial cells, or reactive changes caused by inflammation or injury. Reactive changes are not cancer.
Metaplasia. In metaplasia, one type of epithelial cell is replaced by another type, usually in response to long-term irritation. An example is squamous metaplasia.
Dysplasia. In dysplasia, epithelial cells look abnormal and grow in a disorganized way. Dysplasia is not cancer, but some types can develop into cancer over time.
Carcinoma in situ. In carcinoma in situ, the cells look like cancer cells but remain confined to the epithelium above the basement membrane.
Invasive carcinoma. A carcinoma is a cancer that starts from epithelial cells. Once cancer cells cross the basement membrane into the stroma, the cancer is described as invasive. Carcinomas made of squamous cells are called squamous cell carcinoma, and those made of glandular cells are called adenocarcinoma.
Sample adequacy. In some tests, such as a Pap test or fine-needle aspiration, the report may state whether enough epithelial cells were collected for a reliable result.

Your report may also use the word “epithelioid.” This means cells that look like epithelial cells, often because they are round and plump. Epithelioid cells are not always epithelial cells, so the pathologist may use additional tests to identify them.

Questions to ask your doctor

What type of epithelial cells were found in my sample?
Did the epithelial cells look normal, reactive, or abnormal?
Does my report describe metaplasia or dysplasia?
If cancer was found, did it start from epithelial cells?
Has the cancer grown through the basement membrane into the tissue below?
Were stains such as cytokeratin used to identify the cells?
Did my sample contain enough cells for a reliable result?
Do I need any further tests or follow-up?