Written by Sean Nathaniel Banayag
Edited and Reviewed by Reuben J C. Los Baños, Ph.D.
Epithelial tissues, also called Epithelium, are a group of specialized cells found throughout your body, specifically forming your skin, the linings of your body cavities and hollow organs, and major glands.
Epithelial tissue is one of the four primary types of tissues that comprise the human body, along with Nervous Tissue, Muscle Tissue, and Connective Tissue. All four types of tissue work together to make up every single part and organ of the human body.
Epithelial cells act as your body’s first line of defense from physical, chemical, and biological damage. Most substances must pass through the Epithelium, as it acts as the gatekeeper of your organs, selectively controlling its permeability to allow certain substances in while keeping others out, much like the plasma membrane of your cells, but on a larger scale.
Epithelial Tissues perform a variety of essential functions, depending on the type and location, such as filtration, protection, absorption, secretion, sensation, and diffusion.
What organs contain epithelial tissue?
Epithelial Tissues are found in almost every organ of your body. They compose the lining of all hollow organs such as your digestive tract, respiratory tract and the blood vessels of your circulatory pathway.
These tissues also make up both internal and external body surface such as your skin as well as composes secreting organs such as various glands found throughout your body such as your sweat glands.
What is the structure of Epithelial tissue?
Epithelial tissue is composed of continuous layers of cells, the most external of which is the apical surface, facing either the external environment or the lumen of an organ.
The Apical Surface, also sometimes called apical poles, often contains other structures called apical specializations, which not only change the surface’s shape but also aid in its function.
Apical Specializations are fingerlike projections that differ in length, function, and motility depending on their type. There are currently three kinds of Apical Specialization in the human body, which are the following:
1. Microvilli
- Microscopic non-motile protrusions, which function to increase the cell’s surface area for better absorption and fluid transport.
- Microvilli are found in Epithelium that lines the linings of your intestinal walls, as these microvilli help in absorbing nutrients from the foods you eat and protect your intestines from harmful microbes.
2. Cilia
- Tiny hair-like motile protrusions that either enable the cell to move around or move substances along the cell’s surface.
- Cells that contain Cilia are often referred to as “Ciliated,” and there can be hundreds of cilia on top of a cell’s surface.
- Ciliated Epithelium can be primarily located in your respiratory tract, as these cells utilize their cilia to trap debris and other microbes and move them away from your lungs.
- Ciliated cells can also be found in a female’s fallopian tube, as the cilia can aid in transporting an egg cell from the fallopian tubes to the uterus.
3. Stereocilia
- Specialized microvilli that are longer than microvilli and similarly resemble cilia. It’s primarily located in epithelial tissue in your inner ear, and its function is to aid in hearing and balance.
- Between cells lies the Lateral surface, which connects adjacent cells to form a tightly packed group that further enhances the defensive capabilities of the Epithelial tissue.
The opposite surface to the apical surface is the basement membrane, as the cells are anchored to the underlying connective tissue through a mixture of fibers and proteins.
Epithelial cells release proteins and collagen to create a thin layer called the basal lamina. This layer attaches to a second layer made by the underlying connective tissue. These two layers together form the basement membrane.
There are two types of epithelial membranes: the Serous membrane and the Mucous membrane.
1. Serous membrane
- Serous membrane forms the linings of the closed cavities of your organs that do not directly open to the outside environment, such as the linings of your pericardial and pleural cavities.
- They always come in a double layer: the parietal layer, which lines the cavity wall, and the visceral layer, which covers the organ.
- Between these two layers is a thin serous fluid secreted by the epithelial tissue to act as a lubricant to reduce friction and abrasion when the organs move against each other
2. Mucous Membrane
- Sometimes called mucosa or mucosae, they are epithelial membranes that compose the lining of your organs that directly open up to the outside world, such as the entire digestive tract and respiratory tract.
- Due to being exposed to the external environment, mucous membranes contain goblet cells that produce mucous to prevent the tissue from drying out, as well as trap any debris or microbes.
Epithelial tissue is made up of tightly packed, continuous layers of cells with a little intracellular matrix. These cells are conjoined by specialized junctions located between individual cells.
There are three basic types of cell-to-cell junctions: tight junctions, anchoring junctions, and gap junctions.
a. Tight Junctions
A tight junction seals the space between cells to prevent the leakage of liquid and restricts the passage of electrolytes and other small molecules.Tight junctions can be selectively permeable as they can limit the diffusion of water-soluble molecules.
This type of junction plays a crucial role in tissues that often contain liquids, such as the bladder or the intestine.
b. Anchoring Junctions
Anchoring Junctions either tether cells together (desmosomes) or to the underlying basement membrane (hemidesmosomes).
The main purpose of this kind of junction is to provide mechanical strength against abrasion. These junctions influence your epithelial tissue’s folding and shape.
These junctions are essential in protecting organs that often experience constant stretching or heavy friction, such as your heart or skin.
c. Gap Junctions
Contains Transmembrane proteins called connexons. Two connexons from adjacent cells align to form a channel, enabling certain molecules, ions, and electrical signals to move between the neighboring cells.
Epithelium can be classified into three categories according to shape, which are the following:
Squamous (Flat-shaped), Cuboidal (cube-shaped), and Columnar (column-shaped).
It can also be classified based on the number of layers, as there are two types: Simple (single layer) and Stratified (two or more layers).
When an epithelial tissue is composed of multiple layers (stratified), it is important to take note that, in classifying epithelial tissue based on shape, it is the most superficial layer (apical surface) that determines its classification.
Several types of Epithelial Tissue vary due to shape and the number of layers. We will focus on the seven types of Epithelium commonly found in the human body, which are the following:
1. Simple Squamous Epithelium
- Epithelium that consists of a single layer of flat-shaped cells is commonly found in the lining of blood vessels and the lining of the air sacs within your lungs.
- Due to its thinness, it is able to facilitate the rapid diffusion and exchange of chemical compounds such as oxygen, water, and carbon dioxide between cells.
2. Stratified Squamous Epithelium
- Epithelium that consists of multiple layers of flat-shaped cells, and it is the most common type of Epithelium found in the human body.
- Unlike its single-layer counterpart, the stratified squamous epithelium’s primary function is to protect against environmental hazards and other microorganisms.Keratanized Stratified Squamous Epithelium contains the fibrous protein Keratin, and most of its apical surface is dead. This kind of Stratified Squamous Epithelium is most evident in your skin.
- Unkeratinized Stratified Squamous Epithelium lacks Keratin and makes up the lining of your oral cavity.
3. Simple Cuboidal Epithelium
- Epithelium that consists of a single layer of box-like cells, which can generally be found forming the lining of your kidney tubules and secreting glandular ducts.
- The main function of this kind of Epithelium is for the secretion and absorption of molecules during transport.
4. Stratified Cuboidal Epithelium
- Epithelium that consists of multiple layers of box-like cells. One of the rarest types of Epithelium found in humans is primarily located in large excretory glands, such as the glands in the pancreas, sweat glands, and salivary glands.
- Its primary purpose is to be another line of protection, as well as further reinforcing the structural walls of your glands. It also makes up certain parts of the male urethra.
5. Simple Columnar Epithelium
- Epithelium that consists of a single layer of tall and slender cells and is often ciliated. Its main purpose is tofacilitate the secretion and absorption of molecules, particularly nutrients.
- Simple Columnar Epithelium mainly forms the lining of your digestive track, as well as certain parts of the female reproductive system.
6. Stratified Columnar Epithelium
- A type of Epithelium that consists of multiple layers of tall and slender cells. Its primary purpose is protection and secretion of mucousa
- It is primarily located in the conjunctiva, which lines the eyelid. It can also be found in certain parts of the male urethra and lobar ducts of your salivary gland.
- It is one of the rarest kinds of Epithelium found in humans, similar to Stratified Cuboidal Epithelium.
7. Pseudostratified Columnar Epithelium
- It is a type of Simple Columnar Epithelium that appears to be stratified, but it is actually just one layer. Furthermore, all the cells do not have uniform height, unlike other Columnar epithelia.
- It is often ciliated and is primarily located in the upper respiratory tract, where it aids in the secretion of mucus and utilizes its cilia to either trap or move debris around the lungs.
Epithelium can also be further categorized based on specialized functions, and there are currently three kinds of Epithelium that can be categorized this way:
a. Transitional Epithelium
- Also called urothelium, it is a type of stratified epithelial tissue that lines the urinary tract. It has the special ability to stretch to accommodate the current volume of liquid within the bladder.
- When the bladder is full, it stretches and flattens itself to become squamous in appearance to accommodate the liquid, but when it is empty, it reverts back to a cuboidal-like structure.
b. Glandular Epithelium
- A specialized type of Epithelium designed for the production and secretion of various substances such as sweat, digestive enzymes, saliva, hormones, and breast milk.
- As the name suggests, Glandular Tissue comprises the various kinds of glands in your body. Glands can be classified into either Exocrine or Endocrine Glands.
- Exocrine Glands release their chemical substance to the external environment. An example of an Exocrine Gland is your sweat Gland.
- Endocrine Glands, also known as ductless glands, directly secrete their chemical substance in the human body through blood vessels or to other nearby tissues. An example of an Endocrine Gland is your Pituitary Gland.
c. Olfactory Epithelium
-Located inside your nasal cavity, the Olfactory Epithelium is often ciliated and utilizes cilia to trap odor molecules from the air you breathe. These molecules are then processed and send sensations to the brain to be interpreted
What happens when epithelial cells are damaged?
Epithelial tissue often acts as your body’s first line of defense, as it either covers your entire body in the case of your skin or composes the lining of your organs.
Epithelial tissue is often prone to damage due to friction, environmental factors such as radiation or infection. When epithelial tissue is damaged, your body undergoes a series of four phases in response: hemostasis, inflammation, proliferation, and remodeling (maturation).
Epithelial cells have a high penchant for regeneration. Repairing surface damage to epithelial tissue is achieved through a process called Epithelialization, which utilizes keratinocytes and occurs during the proliferative phase of wound healing.
Epithelialization often occurs a few hours after injury, and it typically begins with keratinocytes at the wound edges slowly migrating towards each other to form a temporary scab that covers the gap.
After some time, the keratinocytes grow over the granulating tissue, slowly changing the color from a deep pink to a lighter purple, signifying a successful healing process.
Once the wound is closed, adult stem cells located at the innermost layer (the basal layer) begin to rapidly divide via mitosis to replace the lost volume of cells.
These cells will then undergo differentiation to become mature cells with identical properties to the cells they replaced, such as the epithelial tissue in your skin (stratified squamous), which becomes keratinized to become more durable and water-resistant.
Illustrated by Elar Athena F. Cataylo
What vitamin helps epithelial cells?
The most important vitamin for epithelial health is Vitamin A, which is the generic term for several fat-soluble vitamins, including retinol, retinyl palmitate, and the provitamin A carotenoids, such as beta-carotene.
Vitamin A, also called the “anti-infective” vitamin, is essential for maintaining the integrity of epithelial barriers. It also plays a crucial role in cell differentiation as Vitamin A aids in the development of young cells into mature cells. For example, Vitamin A can signal your cells to develop into specialized types such as ciliated or goblet (mucous-producing) cells.
Lack of Vitamin A can cause hyperkeratosis or dry, scaly skin that can easily be damaged. Vitamin A deficiency can also cause xerophthalmia, also known as dry eyes.
To prevent Vitamin A deficiency, common sources of Vitamin A include beef liver, eggs, certain types of fish meat such as salmon, as well as yellow and orange fruits and vegetables like carrots and mangoes, and most leafy vegetables.
Insights
Epithelial tissue is an interesting and essential subject to learn, as epithelial tissues are one of the key components that make up who we are.
Without our epithelial tissue, we would die in seconds from infections and the dangers of the external environment. Our organs will be damaged due to rubbing against each other, which can lead to organ failure if sustained long enough.
Our bodies would not be able to survive without serous and mucous membranes, as not only would our organs be defenseless against microbes, but each step would be extremely painful without any of these fluids acting as lubricants.
Epithelial tissues are silent defenders, taking hits for us and regenerating from wounds to keep protecting us. It can regenerate and be constantly replaced, unlike nervous tissue, or be hurt as much if damaged, unlike muscle tissue.
As an individual, I don’t think we take care of our skin and the rest of our epithelial tissue as much compared to other organs, and I think that it is a tragedy.
Taking care of our skin isn’t just for appearance or vanity but for our overall health. We need to not only eat healthy foods rich in vitamins, especially vitamin A, but also apply moisturizer, sunscreen, and lotion to keep our skin healthy and smooth.
Epithelial tissue isn’t just limited to your skin, as I initially thought when I was younger, as it also makes up the linings of your organs. It made me realize how vital epithelium is and how every single cell in our body works together for us to function.
Epithelial Tissue isn’t just tasked with protection but a myriad of other vital functions, from facilitating the exchange of CO2 and O2 to the absorption of nutrients in our intestines.
The more I researched and read for this article, the more intrigued I am by how and why our bodies, including epithelium, were designed that way.
How did we get such efficient and multi-purpose tissues in the first place? How did we develop these tissues, and what were the cells like before during a period where humanity hadn’t existed, and how did they become the cells that compose us today?
This is just some of the many questions that were sparked by this writing, and I wish to learn more about myself biologically, literally, and figuratively.
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