Have you ever wondered how our skin protects us? Or how we absorb nutrients from the food we eat, or how we keep things like dust out of our bodies when we breathe? This special tissue that we are going to talk about today is related to all of these. It is called epithelium . You may have heard this word from a doctor. So, let's take a look at what it is, where it is located in the body, and what it does.
What is Epithelium? Simply put...
Epithelium is a special type of tissue in our body . Think about it, it covers almost every surface inside and outside our body, lines the inside of cavities and hollow organs, and is also the main tissue in our glands.
Now you might be wondering, what does this epithelial tissue do to our body? Actually, it has many functions depending on where it is located . The main ones are protection, secretion of various substances, and absorption of necessary things .
You probably know that there are four main types of tissue in our body:
- Epithelial tissue
- Connective tissue
- Muscular tissue
- Nervous tissue
Of these, epithelial tissue is very important. Because if something needs to enter or leave our organs, it must first pass through this epithelial tissue .
You can imagine how many places in our body this epithelial tissue is found. Here are a few examples:
- The outermost layer of your skin (epidermis)
- The lining inside your intestines
- The lining inside your airways
- The lining inside your abdominal cavity
- Your sweat glands
Epithelial tissue is everywhere.
So, what are epithelial cells?
Very simple. Epithelial tissue is made up of epithelial cells . These cells can be of various shapes, and can be arranged in a single layer or in multiple layers. It depends on where the cells are in the body and what kind of function they perform.
In biology, a cell is the smallest unit of life that can survive on its own. Our bodies are made up of trillions of these cells. A cell generally has three main parts:
- Cell membrane: This is the covering around the cell. It controls what enters and leaves the cell.
- Nucleus: This is the most important part of the cell. It contains most of the cell's genetic information, or DNA .
- Cytoplasm: This is the liquid part inside the cell. It contains other cell parts, that is, small structures that perform various functions.
Some cell types, especially epithelial cells, have special structures on their surface that help them perform their functions. Here are some examples:
- Microvilli: These are like little fingers that protrude from the surface of cells. But they don't move. Their main function is to increase the surface area of the cell. This allows the cell to absorb more things. For example, the epithelial cells in your small intestine have thousands of these microvilli . These are what help you absorb nutrients from the food you eat, and they also help protect your body from bacteria in your gut.
- Cilia: These are very fine, hair-like, motile structures. They are found on the surface of cells. They can move an entire cell, or move things along the outer surface of the cell. A cell with cilia usually has hundreds of cilia. Think about it, the epithelial cells in your respiratory tract have these cilia . When you breathe in dust and other particles, they get caught in these cilia, moving them towards the nostrils, protecting them from going into the lungs. Another example is the epithelial cells in the Fallopian tubes of women. These help move an egg from an ovary to the uterus.
- Stereocilia: These are actually a special type of microvilli. But they look similar to cilia. They are also projections from the surface of some epithelial cells. These stereocilia are essential for hearing and balance in the epithelial tissue of your inner ear .
What types of epithelial cells are there?
Because epithelial tissues have different functions, there are several types of epithelial cells, depending on where they are located. Let's take a look at what they are.
Types of epithelial cells by shape
There are three main types of cells based on their shape:
- Squamous epithelium: These cells are flat, like a leaf.
- Cuboidal epithelium: These cells are cuboidal in shape, meaning they are the same height, width, and depth.
- Columnar epithelium: These cells are shaped like a pillar, meaning they are taller than they are wide.
Types based on how cells are arranged
Epithelial tissue also differs in the way the cells are arranged:
- Simple: A simple epithelium is one that has only a single layer of cells .
- Stratified: A stratified epithelium is made up of more than one layer of cells .
- Pseudostratified: This is a bit special. Because the cells are packed together and come in different sizes, it looks like there are many layers. But there is actually only a single layer of cells .
What are the main types of epithelial cells in your body?
Now, combining the shape we discussed above and the way the cell layers are arranged, we can identify several types of epithelial tissue. Here are some of them:
- Simple squamous epithelium: This type usually lines blood vessels and body cavities and controls the passage of substances to the underlying tissues.
- Simple cuboidal epithelium: These are usually found in glandular tissue (which produces secretions) and kidney tubules.
- Simple columnar epithelium: These are often specialized for absorption. These cells often have cilia or microvilli on their surface. These cells line your stomach and intestines.
- Stratified squamous epithelium: This type of tissue usually has protective functions. For example, protecting underlying tissues from microorganisms or water damage. The outermost layer of your skin (epidermis) is made up of these stratified squamous epithelial cells.
- Stratified cuboidal epithelium: This type is not very common. It can be found in the excretory ducts of your salivary glands and sweat glands.
- Stratified columnar epithelium: This is also not very common. It is found in the mucous membrane (conjunctiva) that lines your eyelids. It provides protection and also secretes mucus.
- Pseudostratified columnar epithelium: This type lines your upper respiratory tract. It usually has a lot of cilia.
Epithelial cells with specialized functions
There are some epithelial cells that can be classified according to their specialized functions.
- Transitional epithelium: Also called urothelium . It is made up of several layers of cells. The special thing about this is that when stretched, these cells flatten out. It lines most of your urinary tract . It is also the tissue that allows your bladder to expand.
- Glandular epithelium: This type of epithelium is specialized to produce and secrete (excrete) substances. These are found in your glands. Glands are specialized organs that can make, store, and/or secrete things like hormones, proteins, and water.
- Olfactory epithelium: This is located inside your nasal cavity. It contains olfactory receptor cells . These cells have long, specialized structures called cilia. When you breathe in, odor molecules get caught in the cilia as they pass across this epithelial surface. The information about those molecules then travels from the receptors to the olfactory bulb in your brain. That's where your brain interprets what that odor is.
So, what actually happens to epithelial tissue? (Functions)
Now you can see how important epithelial tissue is to our bodies. They perform several functions that are essential to our lives. Since epithelial cells are found throughout the body, their function and purpose vary depending on where they are located.
Epithelial tissue mainly performs one or more of the following functions:
- Protection: Epithelial tissue protects various parts of our body. For example, your skin is made up of epithelial tissue . It protects the tissues, blood vessels, muscles, and internal organs inside your body. Similarly, the cilia on the epithelial cells lining your intestines protect the rest of your body from bacteria in the intestines.
- Secretion: The epithelial tissue in your glands (glandular epithelium) can secrete enzymes, hormones, and various fluids.
- Absorption: The epithelial tissue that lines your internal organs, like your liver and lungs, can absorb some things. For example, the epithelial lining inside your intestines absorbs nutrients from the food you eat .
- Excretion: Excretion is the process of getting rid of waste products from the body. The epithelial tissue in your kidneys excretes waste products . And the epithelial tissue in your sweat glands secretes sweat.
- Filtration: The epithelium in your respiratory tract cleans the air you breathe by filtering out dust and dirt particles . Similarly, the epithelial tissue in your kidneys filters your blood.
- Diffusion: In biology, diffusion is the movement of molecules or particles from an area of higher concentration to an area of lower concentration. A simple membrane made up of epithelial cells can only allow selected substances to pass through it. This process of diffusion helps in functions such as filtration, absorption, and secretion.
- Sensory reception: Our bodies receive sensory information from outside the body because of sensory nerve endings located within epithelial tissue. For example, the stereocilia on the surface of the epithelial tissue in your ear are essential for hearing and balance . Similarly, the taste buds on your tongue are located in the stratified squamous epithelium of the tongue.
What are the diseases that affect epithelial tissue?
The greatest risk with epithelial tissue is the possibility of developing cancerous conditions, especially types of cancer such as adenocarcinoma or papillary thyroid carcinoma .
Adenocarcinoma is a type of cancer that affects the glandular epithelial tissue that lines organs. It is the most common type of cancer that develops in organs. This is because:
- Most prostate cancers
- Many breast cancers
- About 96% of colorectal cancers
- About 95% of pancreatic cancers
- Non-small cell lung cancers account for about 40% of all lung cancers.
Papillary thyroid carcinoma accounts for about 80% of thyroid cancers. This type of cancer grows slowly. Even if it spreads to the lymph nodes in the neck, the disease responds well to treatment. It is highly curable and rarely fatal.
In addition to cancer, other problems involving epithelial tissue can occur in various organs. Some of the most common conditions include:
- Asthma: Asthma is a condition in which your airways become narrowed, swollen, and possibly produce excess mucus. In this, the bronchial epithelium of the airways changes and becomes fragile.
- Celiac disease: When someone with celiac disease eats gluten (a protein found in some grains), their immune system attacks and damages the microvilli in the epithelial cells that line their intestines. This makes it difficult for the intestines to absorb nutrients properly.
- Human Papillomavirus (HPV) types 1-4: HPV can cause warts on the epithelial cells in the outermost layer of your skin (epidermis).
- Vertigo: Vertigo is a condition that causes a spinning sensation. It can have several causes. One cause is an abnormal buildup of stratified squamous epithelial cells deep inside your ear (called a cholesteatoma ). If left untreated, it can cause vertigo (balance problems) as well as hearing problems.
What tests examine epithelial cells?
Because epithelial cells are found in several important places in our bodies, there are several types of tests that examine epithelial cells to see if they are involved in certain diseases. In medicine, pathology is the examination of cells in body tissue or fluid samples in a laboratory to diagnose disease. The scientist who examines these cells is called a pathologist .
Some examples of tests involving epithelial tissue:
- Epithelial cells in urine: A urinalysis is a test that measures various things in your urine. Part of that is a urine epithelial cell count. A lab technician will look 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 few epithelial cells in your urine. But if you have a lot of them, it could be a sign of an infection, kidney disease, or another serious condition .
- Pap smear / Pap test: A Pap test is a test to check for abnormal epithelial cells in the cervix that are cancerous or may become cancerous. In this test, a healthcare provider takes a sample of cells from the cervix and examines them under a microscope.
- Some biopsy tests: A biopsy is the examination (often under a microscope) of tissue removed from a part of the body to look for signs of disease. There are several types of biopsies that analyze epithelial cells. For example, a prostate biopsy involves taking epithelial cells from the prostate with a needle and checking for signs of cancer. A biopsy for celiac disease involves taking epithelial cells from the lining of the small intestine through an endoscopy and checking for damage to the microvilli in the cells.
- Some Cytology tests (Cytology tests / Cytopathology): Cytology is the examination of cells from body tissues or fluids to make a diagnosis. There are many cytology tests involving epithelial cells, such as gynecologic cytology, thyroid cytology, respiratory cytology, and eye cytology.
What is the difference between epithelium, endothelium, and mesothelium?
These three names can be confusing because they sound a bit similar. Epithelium, endothelium , and mesothelium are three types of epithelial cell layers that line our internal organs, body cavities, and form the outer layer of our skin.
- Epithelium usually lines the passages that open to the outside environment, for example your respiratory tract and digestive system.
- Endothelium normally lines the entire body's internal channels, for example your circulatory system (blood vessels) and the cornea of your eye.
- The mesothelium lines your major body cavities. For example, the abdominal cavity (peritoneum), the lung cavity (pleura), and the heart cavity (pericardium). The mesothelium secretes a lubricating fluid called serous fluid .
Epithelial tissue is a type of tissue that performs many functions that are essential to your body's function and health. You've probably heard your doctor use the word "epithelium" when talking about certain tests or types of cancer. There are a lot of complicated, strange words like these in the medical field. If you're ever unsure about a word your doctor uses, don't be afraid to ask him or her what it means.
The most important things for you to remember (Take-Home Message)
So, I hope you have a simple idea about this 'epithelium' we talked about today.
- Epithelium is a specialized tissue that acts as a covering and is found almost everywhere in our bodies. It is found in places like the skin, intestines, and respiratory tract.
- This performs many important functions, such as protection, secretion, and absorption .
- There are different types of epithelial cells depending on their shape and arrangement.
- Some medical conditions, especially cancer , can affect this epithelial tissue, and are also linked to conditions like asthma and celiac disease.
- The health of these cells can be seen through various tests (urine tests, Pap smears, biopsies) .
Being aware of your body will go a long way in helping you stay healthy. So, be interested in learning more about these things.
` epithelium, epithelial cells, tissue, body covering, glands, protection, secretion, absorption, cell types, squamous, cuboidal, columnar, stratified, simple, pseudostratified, cancer, adenocarcinoma, pap smear
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