What is the epithelium?
The epithelium is a type of body tissue that forms the covering on all internal and external surfaces of your body, lines body cavities and hollow organs and is the major tissue in glands. Epithelial tissue has a variety of functions depending on where it’s located in your body, including protection, secretion and absorption.
The organs in your body are composed of four basic types of tissue, including:
All substances that enter or leave an organ must cross the epithelial tissue first.
You have many different kinds of epithelial tissue throughout your body. Some examples of epithelial tissue include:
- The outer layer of your skin (epidermis).
- The lining of your intestines.
- The lining of your respiratory tract.
- The lining of your abdominal cavity.
- Your sweat glands.
What are epithelial cells?
Epithelial tissue is made up of epithelial cells. The cells can be different shapes and can be arranged in a single layer or multiple layers depending on where they are in your body and what kind of functions they have.
In biology, a cell is the smallest unit that can live on its own. Cells make up all living organisms and the tissues of your body. More than 30 trillion cells make up your body.
A cell has three main parts:
- The cell membrane: The cell membrane surrounds the cell and controls the substances that go into and out of the cell.
- The nucleus: The nucleus is a structure inside the cell that contains most of the cell’s DNA (genetic material).
- The cytoplasm: The cytoplasm is the fluid inside the cell. It contains other cell parts that have certain functions.
Some types of cells, including some epithelial cells, have characteristics on the surface of the cell that help them perform certain functions, including:
- Microvilli: Microvilli are non-motile (they don’t move) finger-like structures on the surface of epithelial cells that function to increase the cell’s surface area so that it can better absorb substances. The epithelial cells that line your small intestine have thousands of microvilli that absorb nutrients from the food you eat and protect your body from intestinal bacteria.
- Cilia: Cilia are tiny, hair-like, motile (they can move) structures on the surface of the cell that help move entire cells or can move substances along the outer surface of the cell. Ciliated cells usually have hundreds of cilia on their surfaces. Epithelial cells lining your respiratory tract have cilia that trap dust and other substances you breathe in and move them toward your nostrils so that they don’t go into your lungs. Another example of cells with cilia are the epithelial cells that line the fallopian tubes that help move an egg from an ovary to the uterus.
- Stereocilia: Stereocilia are specialized microvilli that resemble cilia and project from the surface of certain epithelial cells. Stereocilia are needed on the epithelial tissue in your inner ear for hearing and balance.
What are the different types of epithelial cells?
There are several different types of epithelial cells because epithelial tissues have many different functions depending on where they are in your body.
Types of epithelial cells based on their shape
Different types of epithelial cells based on shape include:
- Squamous epithelium: Squamous epithelial cells are flat and sheet-like in appearance.
- Cuboidal epithelium: Cuboidal epithelial cells are cube-like in appearance, meaning they have equal width, height and depth.
- Columnar epithelium: Columnar epithelial cells are column-like in appearance, meaning they are taller than they are wide.
Types of epithelial cells based on their arrangement
Epithelial tissue can also vary based on how the cells are arranged. The descriptors, or adjectives, for the way the cells are arranged, include:
- Simple: A simple epithelium means that there’s only one layer of cells.
- Stratified: A stratified epithelium is made up of more than one layer of cells.
- Pseudostratified: A pseudostratified epithelium is made up of closely packed cells that appear to be arranged in layers because they’re different sizes, but there’s actually just one layer of cells.
Types of epithelial cells in your body
Given the different shapes and types of layers of epithelial cells, there can be several types of epithelial tissue, including:
- Simple squamous epithelium: This type of epithelium typically lines blood vessels and body cavities and regulates the passage of substances into the underlying tissue.
- Simple cuboidal epithelium: This type of epithelium is typically found in glandular (secreting) tissue and kidney tubules.
- Simple columnar epithelium: This type of epithelium is often specialized for absorption and usually has apical cilia or microvilli. These cells line your stomach and intestines.
- Stratified squamous epithelium: This type of epithelium usually has protective functions, including protection against microorganisms from invading underlying tissue and/or protection against water loss. The outer layer of your skin (the epidermis) is made of stratified squamous epithelial cells.
- Stratified cuboidal epithelium: This type of epithelium is not as common and is found in the excretory ducts of your salivary and sweat glands.
- Stratified columnar epithelium: This type of epithelium is not as common and is seen in the mucous membrane (conjunctiva) lining your eyelids, where it’s both protective and mucus-secreting.
- Pseudostratified columnar epithelium: This type of epithelium lines your upper respiratory tract and usually has a lot of cilia.
Epithelial cells based on specialized functions
Epithelial cells can also be categorized by the special functions they have, including:
- Transitional epithelium: A transitional epithelium (also known as urothelium) is made up of several layers of cells that become flattened when stretched. It lines most of your urinary tract and allows your bladder to expand.
- Glandular epithelium: This type of epithelium is specialized to produce and secrete (release) substances. It’s found in your glands, which are specialized organs that can make, store and/or release substances such as hormones, proteins and water.
- Olfactory epithelium: The olfactory epithelium, located within your nasal cavity, contains olfactory receptor cells, which have specialized cilia extensions. The cilia trap odor molecules you breathe in as they pass across the epithelial surface. Information about the molecules is then transmitted from the receptors to the olfactory bulb in your brain, where your brain then interprets the smell.
What does the epithelium do?
Epithelial tissue has several important functions that are essential to life. Since epithelial cells are found throughout your body, their function and purpose change based on their location.
Epithelial tissue can have one or a combination of the following several functions:
- Protection: Epithelial tissue protects several aspects of your body. For example, your skin is made up of epithelial tissue and protects the tissues deeper in your body, such as blood vessels, muscle and internal organs. The cilia on the epithelial cells that line your intestines protect the rest of your body from intestinal bacteria.
- Secretion: Epithelial tissue in your glands (glandular epithelium) can secrete (release) enzymes, hormones and fluids.
- Absorption: The epithelial lining of your internal organs, such as your liver and lungs, can allow the absorption of certain substances. For example, the internal epithelial lining of your intestines absorbs nutrients from the food you eat.
- Excretion: Excretion is the removal of waste from your body. The epithelial tissue in your kidneys excrete waste, and the epithelial tissue in your sweat glands excrete sweat.
- Filtration: The epithelium of your respiratory tract filters out dirt and particles and cleans the air that you breathe in. Epithelial tissue in your kidneys filters your blood.
- Diffusion: In biology, diffusion is the passive movement of molecules or particles from regions of higher concentrations to regions of lower concentration. Simple squamous epithelial cells form a membrane that allows selective diffusion of materials to pass through. Diffusion helps with filtration, absorption and secretion functions.
- Sensory reception: Sensory nerve endings that are embedded in epithelial tissue allow your body to receive outside sensory stimuli. As an example, the stereocilia on the surface of the epithelial tissue in your ear are essential for hearing and balance. In addition, your taste buds are embedded in the stratified squamous epithelium of your tongue.
What conditions affect epithelial tissue?
One of the biggest concerns with epithelial tissue is the potential for malignancy development as adenocarcinoma or papillary thyroid carcinoma, which are two types of cancer.
Adenocarcinoma cancers affect the glandular epithelial tissue that lines the organs and is the most common type of cancer affecting organs. Adenocarcinoma is responsible for:
- Almost all prostate cancers.
- Most breast cancers.
- About 96% of colorectal cancers.
- Approximately 95% of pancreatic cancers.
- Around 40% of non-small cell lung cancers.
Papillary thyroid carcinoma represents up to 80% of all thyroid cancers. This cancer type grows slowly. Although papillary thyroid cancer often spreads to lymph nodes in the neck, the disease responds very well to treatment. Papillary thyroid cancer is highly curable and rarely fatal.
In addition to cancer, several other epithelial tissue issues can occur in various organs. Some more common conditions associated with epithelial tissue include:
- Asthma: In asthma, a condition in which your airways narrow and swell and may produce extra mucus, the bronchial epithelium is modified and appears fragile.
- Celiac disease: If a person with celiac disease ingests gluten, a protein found in certain types of grain, their immune system attacks and damages the microvilli of epithelial cells lining their intestines. This makes it more difficult for the intestines to properly absorb nutrients.
- Human papillomavirus (HPV) strains 1-4: HPV can cause warts on the squamous epithelial cells of the outer layer of your skin (epidermis).
- Vertigo: Vertigo is a condition that’s often described as a sensation of motion, most commonly rotational motion. There are several different causes of vertigo, one of which is when an abnormal collection of stratified squamous epithelial cells (a cholesteatoma) develops deep inside your ear. If left untreated, it can cause vertigo (balance problems) as well as hearing issues.
What are the different kinds of epithelial cell tests?
Since epithelial cells exist in several important parts of your body, several types of tests examine epithelial cells to check for certain medical conditions. In medicine, pathology is the laboratory examination of cells in samples of body tissue or fluids for diagnostic purposes. A scientist called a pathologist examines the cells.
Some examples of tests that involve epithelial tissue include:
- Epithelial cells in urine: Part of urinalysis, a test that measures different substances in your urine, is checking epithelial cells in your urine. A lab technician looks at your urine sample under a microscope to see if the number of epithelial cells is within the normal range. It's normal to have a small number of epithelial cells in your urine. A large amount may indicate an infection, kidney disease or other serious medical condition.
- Pap smear: A Pap smear (also called a Pap test) is a test that checks for abnormal epithelial cells in the cervix that are cancerous or have the potential to become cancerous. During a Pap smear, a healthcare provider swabs cells from the cervix to examine under a microscope.
- Certain biopsy tests: A biopsy is an examination of tissue — usually under a microscope — that has been removed from a part of your body to look for signs of disease. Several different biopsies analyze epithelial cells. For example, a prostate biopsy involves removing epithelial cells from the prostate with a needle to look for signs of cancer. A celiac disease biopsy involves an endoscopy to remove epithelial cells in the lining of your small intestine to check for damage to the microvilli of the cells.
- Certain cytology tests: Cytology (also known as cytopathology) involves examining cells from bodily tissues or fluids to determine a diagnosis. Many types of cytology tests involve epithelial cells, such as gynecologic cytology, thyroid cytology, respiratory cytology, and eye cytology.