What are epithelial tissues?
Written by Glicy Lou D. Garinggo
Reviewed by Dr. Reuben J C. Los Baños, Ph.D.
No matter how complex your body is, it is only composed of four basic tissue types. These are epithelial, connective, muscular, and nervous tissue.
They make up cells and molecules from the extracellular matrix, but they don’t exist on their own. They work together and in different amounts to form other organs and systems in the body. They are also critical cells because they help the body do its job.
Epithelial tissues are polyhedral layers that are enclosed together. These tissues have very little extracellular material. These tissues stick together very well. They form sheets that cover the body’s surface and line its cavities.
The main functions of epithelial tissues are:
- Covering, lining, and protecting surfaces (e.g., skin)
- Absorbing (e.g., the intestines)
- Secreting (e.g., the epithelial glands)
- Contractility (e.g., myoepithelial glands)
There are some cells in some epithelia that are very good at sensing. These can be like those in the tongue or your nose lining. Everything has to go through an epithelial sheet to get into or leave the body.
How many types of epithelial tissue are there?
We can break Epithelia into two main groups based on their structure and function.
The covering (or lining) epithelial and glandular epithelia.
Covering or Lining Epithelia
It’s called a “covering” because the cells arrange themselves in layers. They cover the outside of the body or line the inside of it. You classify them by how many layers of cells there are and how the cells in the surface layer look. Simple epithelia have only one layer of cells. Stratified epithelia contain more than one layer.
You also base the shape of the cells that make up simple epithelia. They can be squamous (or thin cells), cuboidal(cube-like), or columnar (cells taller than they are wide).
Simple Squamous Epithelium
In simple squamous epithelium, this is in a single layer. The cells of the single-layer is flat and usually very thin. Only the thicker cell nucleus shows up as a bump to show that the cell is there. You can find simple epithelia on the inside of blood vessels.
You can also find them in other areas where substances can get into the rest of the body. They keep substances from getting into the rest of the body. Often, thin cells have transcytosis.
Simple Cuboidal epithelium
Cells in simple cuboidal epithelia can be tall or short. They’re the same height and width, but not always. Their thicker cytoplasm often has a lot of mitochondria in it. Having many powerhouses gives them a lot of energy to move things across the epithelia.
Simple Columnar Epithelium
Cells in simple columnar epithelia are taller than wide and denser. These cells are very good at absorbing. They have microvilli and mix with secretory cells or ciliated cells.
Those epithelial cells always have tight and firm junctions at their top ends. But they are often linked in the more basolateral parts of their bodies. This structure allows for rapid transfer to the space between the cells instead of moving across the cells.
Columnar cells have more cytoplasm. They tend to have more mitochondria and other organelles for absorption and processing. Examples include a renal collecting duct, the oviduct, and the gall bladder. They all have secretory and ciliated cells, like the oviduct and gall bladder.
Stratified Epithelium
We can also divide stratified epithelia into four groups. Again, by basing on the cell shape of the surface layer. These are squamous, cuboidal, columnar, and transitional.
We can call the fragile cell surface cells “nonkeratinized” or “keratinized.” Try to look at your skin and see a stratified squamous keratinized epithelium. Many cells form layers, and the cells near the connective tissue are usually low columnar or cuboidal.
The cells flatten and become irregular in shape as they build up keratin during keratinization. As they move closer to the surface, they become thin. They also got inactive packets of keratin without a nucleus.
Cells on this epithelia’s surface help keep water from leaking across it. Wet cavities have stratified, squamous, or nonkeratinized epithelia like the mouth, esophagus, and vagina.
In these areas where water loss is not a problem, the cells have much less keratin and still have nuclei.
Stratified Squamous Epithelium
Stratified squamous epithelia protect the underlying tissue from microorganisms and water loss. Protection against water loss and drying out is essential for the skin. The epithelia have keratin which means it is hard.
In time, epidermal cells of the skin become filled with keratin and other substances. Still, they also get rid of their nuclei and other parts of their bodies.
Flattened ‘squames’ on the surface form a layer that slows down water loss. They fall off and are replaced. Nonkeratinized epithelial linings are visible on many internal surfaces. Examples of these are the esophagus and the cornea.
This surface is because the differentiating cells have less keratin and keep their nuclei. Because water loss is less of a problem with these epithelia, they don’t need keratin.
Stratified Cuboidal and Stratified Columnar Epithelia
A layer of cells called stratified cuboidal or columnar epithelia isn’t pervasive. Still, you can find them in the excretory tubes of some glands. The double layer of cells makes the lining more durable than a simple epithelium.
Most epithelia are not layered cuboidal or layered columnar cells. We can find it in the conjunctiva, which lines the eyelids. It is both protective and mucus-secreting. Large excretory ducts of sweat and salivary glands only have stratified cuboidal epithelia. This type of tissue is stronger than the simple epithelium.
There are two types of cells in the transitional or urothelium:
Dome-shaped cells that are neither squamous nor columnar. You can see this type of cell only in the bladder, the ureter, and the upper part of the urethra. These cells, sometimes called umbrella cells, protect the body from urine. It is too acidic and could kill cells.
Transitional epithelium or urothelium
The stratified transitional epithelium lines of the urinary bladder have rounded or dome-shaped cells with two unusual features. There are membranes on the surface of the cells.
They can withstand the hypertonic effects of urine and protect the cells below from this toxic solution. Also, the transitional epithelium can change their structure as the bladder fills and the wall stretches.
This action makes the transitional epithelium of a full bladder seem to have fewer cell layers than an empty bladder. It’s called pseudostratified columnar epithelium because all cells attach to the basal lamina.
Their nuclei are at different epithelial levels, and some cells’ height doesn’t reach the surface. This type is also called the stratified columnar epithelium.
Most people know that pseudostratified columnar epithelium lines the upper respiratory tract. This arrangement also has a lot of columnar cells that are very ciliated.
Pseudostratified Epithelium
As the cells move, they appear to be in layers. But, the basal ends of these layers are all in contact with the basement membrane, which can be very thick in these epithelia. The best example of this type is the pseudostratified ciliated columnar epithelium of the upper respiratory tract. It has cell types with nuclei at different levels, making it look like cells piled.
Glandular Epithelia
Glandular epithelia are cells specialized to secrete. The molecules to secrete are in the cells in small membrane-bound vesicles called secretory granules.
Glandular epithelia may synthesize, store, and secrete proteins (e.g., in the pancreas), lipids (e.g., adrenal, sebaceous glands), or complexes of carbohydrates and proteins (e.g., salivary glands).
Mammary glands secrete all three substances. The cells of some glands have low synthetic activity (e.g., sweat glands) and secrete mostly water and electrolytes transferred into the gland from the blood.
The epithelia that form glands can be according to various criteria. Unicellular glands consist of large isolated secretory cells, and multicellular glands have clusters of cells. We use the term “gland” to designate large aggregates of secretory epithelial cells, such as salivary glands and the pancreas.
What epithelium is skin?
It is keratinized stratified squamous epithelium. It is the source of benign and malignant epidermal tumors found on the skin’s surface.
What is squamous epithelium?
The squamous epithelium consists of flat epithelial layers and looks like scales. The layers are more extensive than tall and look like polygons when seen from the top. It gives a smooth, low- friction surface, making it easy for fluids to move over.
Which epithelium is present in the tongue?
Like on the skin, the squamous epithelium layers on top of the connective tissue, or lamina propria, and the tongue’s muscles. This layer is the basal layer.
Epithelial tissues that are subject to friction, like the covering of the skin or tongue, are where they happen the most.
Which epithelium is present in the kidney?
Glands and kidney tubules have simple cuboidal epithelium, the same type found in both.
Which epithelium is present in the urinary bladder?
Lining epithelium: The urinary bladder lining is the urothelium, a type of stratified epithelia. You can find these body parts only in urinary structures like the ureter, urinary bladder, and proximal urethra.
The urothelium has three layers:
Innermost or apical layer: The innermost layer acts as a barrier between the bladder and other tissues below it. A single layer of umbrella-shaped layers (called umbrella cells) breaks down into two. They form an impenetrable barrier. Tight junctions between the cells and a layer of uroplakin, a glycoprotein, form a plaque on the surface that covers the umbrella layers.
Intermediate Layer: It comprises two to three layers of polygonal layers.
Basal Layer: It has two or three layers of small cuboidal layers. At rest, the urothelium is five to seven layers thick. When the bladder is full of urine, its wall expands to fit the extra space. It doesn’t hurt the bladder when the urothelium reorganizes into two or three layers in a distended bladder. Because the urothelium can move from one place to another, it is also called the transitional epithelia.
Which type of epithelium is in the respiratory tract?
The respiratory epithelium is a ciliated pseudostratified columnar epithelium covering most of the respiratory tract. You can’t find it in the larynx or pharynx, but it covers most of the respiratory tract.
Some epithelial layers are more likely to grow abnormally. This abnormal growth leads to cancer which we call neoplasia. Neoplastic growth has cured and does not always lead to cancer.
Metaplasia is another reversible process in which one type of epithelial tissue changes into another.
The ciliated pseudostratified epithelia that line the bronchi can become the stratified squamous epithelia in many people who smoke. In people who have a long-term lack of vitamin A, epithelial tissues like those found in the bronchi and urinary bladder become stratified squamous epithelia.
Metaplasia is not only found in epithelial tissue. It can also happen in connective tissue.
References:
Mescher, A. L., & Junqueira, L. C. U. (2016). Junqueira’s basic histology: Text and atlas (Fourteenth edition.). New York: McGraw-Hill Education.
Mescher, A. L., & Junqueira, L. C. U. (2016). Junqueira’s basic histology: Text and atlas (Twelfth edition.). New York: McGraw-Hill Education.
Microbiologynote (n.d.) Epithelial Tissue Definition, Characteristics, Types, and Functions. Date retrieved March 31, 2022 from https://microbiologynote.com/epithelial-tissue/
Biology online (2001). Squamous Epithelium. Date retrieved April 1, 2022 from https://www.biologyonline.com/dictionary/squamous-epithelium
National Library of Medicine (2018). Tongue and Taste Organ Biology and Function: Homeostasis
Maintained by Hedgehog Signaling. Date retrieved April 1, 2022 from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5966821/#:~:text=Similar%20to%20the
%20skin%2C%20the,or%20lamina%20propria%2C%20and%20muscle.
NCBI (2022). History, Bladder. Date retrieved April 1, 2022 from
https://www.ncbi.nlm.nih.gov/books/NBK540963/#:~:text=Lining%20epithelium%3A% 20The%20urinary%20bladder,urinary%20bladder%2C%20and%20proximal%20urethra.
University of Thi-Qar (n.d.) Glandular Epithelia. Date retrieved April 2, 2022 from https://med.utq.edu.iq/wp-content/uploads/sites/7/2020/09/histology-lecture- 3.pdf
Crystal R. Cassidy
MT 30 – AA
April 3, 2024
From the article, I learned about the structure, function, and importance of epithelial tissues in our bodies. Epithelial tissues are important for covering, lining, protecting surfaces, absorbing nutrients, secreting substances, and for other necessary functions. I learned that they come in different types, such as simple squamous, cuboidal, and columnar, as well as stratified squamous and transitional epithelium. I learned that each type has its own unique characteristics and locations in the body. I also learned that epithelial tissues also have special features like tight junctions, microvilli, and cilia which can help these tissues perform their roles effectively.
The tissues in the body indeed have such amazing coordination—the structure complements the function. Each type of epithelial tissue has its own distinct composition that aligns with how it operates to maintain homeostasis. For instance, simple squamous epithelium, made up of a single layer of flattened cells, can be found in the blood vessel to facilitate processes that require permeability. During inflammation, the blood vessel dilates and becomes more permeable to allow the exit of phagocytic cells to go into the site of injury. If the blood vessel is not made of simple squamous epithelium, but instead of cuboidal or columnar cells which are much thicker and taller, then it would be difficult to achieve chemotaxis. Imagine what more could happen if that’s the case.
I also like how this article included the information about the locations of specific types of epithelial tissues and the related complications associated with unhealthy habits of man. I just learned that smoking can cause the ciliated pseudostratified epithelia that line the bronchi to become stratified squamous epithelia in people who practice such vice. Although a more elaborated explanation was not provided, the presented information enabled me to find other sources and read about it. In my reading, I discovered that the toxicants from cigarette smoke can actually paralyze the cilia, which are tiny hair-like projections that protect our airways, and eventually damage them, making the tissue lose its significant portion (the cilia) and appear as a bare squamous epithelia. With this, the lungs would be more susceptible to foreign materials.
To those who want to know more about how smoking affects the respiratory system tissues, you can visit the link below.
https://www.cdc.gov/tobacco/data_statistics/sgr/2010/highlight_sheets/pdfs/scientific_respiratory.pdf
Marinella F. Boltron
BSMT-II
MT30 (LAB) – B
Eojjann Tuñacao
BSMT – II | MT 30 (LEC) – AA
April 03, 2024
If I were to leave a one-sentence remark about this article, it would be how I am impressed in finding out how we can learn so much about what comprises our body even if we are only tackling a specific portion – the tissues.
Epithelial tissues are tissues that I am familiar with. Whenever I hear epithelial tissues being uttered, what immediately comes to mind is how they represent protection by acting as a covering/lining, used for absorption, and even on secretion. In this specific article, I came across knowing epithelial tissues as they are – their totality, how they function, the many types/kinds they exhibit, and how each of them plays significant roles in our body that no other element can replicate. Through reading, the first thing I noticed and learned was how each specific type of epithelial is designed to fulfill distinct functions based on the needed requirements of the organ or tissue it covers. For example, the urinary bladder composed of transitional epithelium functions in a way that allows itself to expand, and in return can accept different urine volumes. Furthermore, the skin is made up of another type of epithelial tissue, the stratified epithelium, which is commonly known as the one that protects against differing external components. At this point, it provides a realization of how our body is as diverse as it is from element to specificity in function, promoting at all costs, efficiency, and suitability in maintaining homeostasis no matter the complexity.
As it is, learning epithelial tissues is not only subjected to surface level. Another thing that came across in reading the article is how every aspect of this basic tissue is carefully constructed. This is seen not only from the way they are located in different parts of the body but also in noticing that the way their cell shape is made corresponds to the role they have to play. For example, cuboidal epithelial cells are cube-shaped, facilitating a larger surface area for absorption and secretion. Another is how the squamous epithelial cells are flat and thin, allowing efficient diffusion and filtration. Given this, I conclude that studying epithelial tissues should be given importance. Epithelial tissue is not only fascinating in the ways we have viewed it under the microscope, but it also provides us valuable insights into how specific organs in the body function.
Kylemaxinne Panzo
MT 30 – AA
April 3, 2024
When I hear the word “histology” or “tissues”, I actually visualize epithelial cells first. Perhaps it is because it is almost always the first to be discussed in lectures like Anatomy and Physiology, but regardless, it is through learning about epithelial tissues where I first realized just how related the functions and location of tissues are. For example, the tall structure columnar epithelial tissues are more able to perform absorption and that’s why they are located in areas like the renal collecting duct, the oviduct, and the gallbladder — places where absorption is essential. Another example is also with the structure of simple epithelial tissues, a flat single layer, and how they’re located in places such as the alveoli of the lungs to allow the exchange of gases. Lastly, this article neatly explains the phenomenon of neoplasia and metaplasia. Placing it at the last portion of this article is perfect as it reminds me that we really need to take care of our body and maintain the proper structure of tissues, as they are perfectly placed by God in their locations to fulfill their specific functions in our very complex body.
Princess Niña M. Millor
BMT – II | MT 30 (LEC) – AA
April 3, 2024
Each person may differ in attitudes, physical appearance, skin color, etc., but one thing common in our body is that we are composed of four basic tissue types. In this article entitled “What are epithelial tissues?”, I learned how epithelial cells make our body works or how its complexity affects our body. Epithelial tissues are very important because it protects the body from the environment, absorb substances, secrete substances, and help the body sense its surroundings. I also learned that there are different types of epithelial tissues that make up our body. There are two main types, Covering Epithelial and Glandular Epithelial. Covering epithelial lines our organs and surfaces, providing varying degrees of protection and permeability. Glandular epithelial specialize in producing and secreting substances essential for various bodily processes. Under covering epithelial tissues, it is divided into Simple and Stratified. On the other hand, glandular epithelial tissues can be unicellular or multicellular.
This article also mentions the types of epithelial tissues found in the different parts of our body like in our skin, tongue, kidney, urinary bladder, and respiratory tract. From the stratified squamous epithelium safeguarding our skin to the pseudostratified columnar epithelium lining our respiratory tract. When I was reading the article, the function of the Transitional epithelium or urothelium is what most amazed me. Imagine your body without its function, “They can withstand the hypertonic effects of urine and protect the cells below from this toxic solution,” it is like living in a body producing something (urine) against your body. I realized that we may not see these complexities happening in our body, but it is just amazing to know that we are made uniquely by Our Creator.
Jessa Belle T. Cabataña
BSMT- II
MT 30 (LEC)- AA
April 3, 2024
Teamwork is not only observed among humans. It is also happening on the tissues that makes the integral components in our body and aids in carrying out its functions. One of the significant tissues in our body is the epithelial tissue. This tissue is really best known for its covering and protection functions. These are the soldiers in our body because everything has to pass through it when getting inside or leaving the body.
Epithelial tissues are classified based on the number of layers in the cell and the shape of cells in its apical surface. Each of these classifications has distinct functions and locations where it is predominantly found. The article provides in depth information about each of these classifications. I am amazed because these epithelial tissues are destined in their locations and functions. Their characteristics are well-coordinated and function well on where they are stationed.
Bea Niezel T. Masayon
MT30 (LEC)-AA
April 3, 2024
Reading the article has deepened my understanding of epithelial tissues and their important role in the proper operation of the human body. I’ve come to understand that despite the intricate complexity of the body, there are just four fundamental types of tissue, with epithelial tissue being one of them. These tissues serve numerous functions, including covering, lining, protecting surfaces, absorbing nutrients, secreting substances, and aiding in contractility.
Moreover, the article emphasized the response of specialized epithelial types to certain physiological demands, such as pseudostratified epithelium in the respiratory tract and transitional epithelium in the urinary bladder. The diversity of glands based on cellular organization and functions as well as their function in secretion has been emphasized by learning about glandular epithelia.
Lastly, analyzing abnormalities and modifications of epithelial tissues, such as neoplasia, and metaplasia, showed how dynamic these tissues are and how they can react to damage or environmental stressors. My understanding of the durability and adaptability of epithelial tissues in preserving homeostasis inside the human body has increased as an outcome of this article.
Francheska D. Badon
BSMT – II | MT 30 (LEC) – AA
April 3, 2024
I do also think that epithelial tissues has a vital significance in comprehending the intricate organization of biological systems at the microscopic scale. The diverse cell types that comprise epithelial tissues are organized precisely to form glands, linings, and coverings across the body. Through their physical and microscopic traits, we learn about the unique characteristics of epithelial cells, such as their polarity, specialized cell-to-cell connections, and closely packed organization. Understanding the fundamentals is crucial to comprehending the physiological roles that epithelial tissues play in sensation, absorption, secretion, and protection. Moreover, the significance of epithelial tissues within the framework of histology demonstrates their remarkable flexibility and ability to react to the various environmental obstacles that various organs face.
Cliantha Marielle S. Asonan
MT 30 – AA
Activity for April 3, 2024
Reading this article made me even more amazed at how intricately and carefully designed the human body is. Through our Human Anatomy and Physiology class, Histology class, and even my Biology class in high school, I have already gained a sense of familiarity with epithelial tissues, their structures, and their functions; with this, I commend the author of this article for having concisely yet more than sufficiently summarized everything that we have to know about epithelial tissues. Through this article, I have learned that epithelial tissues, as one of the four main tissue types in our body, serve a wide variety of functions in our body and take on many forms that are designed to fit their function. There are squamous epithelia, which take on two forms: simple and stratified. Simple squamous epithelia are found in capillaries and alveoli where diffusion takes place, warranting a single layer of thin cells. Stratified squamous epithelia, on the other hand, are found in places that constantly experience abrasive stress, such as the epidermis and the esophagus, which are keratinized and nonkeratinized respectively. Cuboidal cells, owing to their structure, are efficient in secretion and are thus found in glands and ducts; similarly, stratified epithelia are found in places where absorption occurs, such as the inner intestinal lining. Truly, form fits function, as my Senior High School Biology teacher always said.
However, despite learning about epithelial tissues in the past semesters, I was surprised to have also learned something new from this article. I learned about neoplasia, which is a type of cancer that is brought by abnormal growth of some epithelial layers yet can be cured. I also learned about metaplasia, which is a condition wherein one epithelial tissue changes into another type and can be brought about by things such as smoking and long-term deficiency of Vitamin A. This article was truly an enlightening read!
Carlen Faith C. Torres
MT 30 – AA
Activity for April 3, 2024
After reading this article, I have refreshed my knowledge about epithelial tissues, their corresponding functions, and even their specific abnormalities, which are sometimes not included in the discussions. I have also realized the importance and relevance of epithelial tissues in our growth and development as individuals. It has diverse functions that are essential for maintaining our body’s homeostasis, and these tissues also contribute to our body’s overall health as they can be one of the factors in diagnosing some diseases, disorders, and abnormalities in an individual. Not only do their diverse functions amaze me, but also the way that these tissues work together to act as the protective barrier, inside and outside the body.
I like how this author has written this article because it is concise and straight to the point while making sure that all relevant facts and information about the topic are included. I also applaud how this author included the specific abnormalities in the epithelial tissues such as neoplasia and metaplasia. Reading this article, I have learned new things about this topic, even though it has been discussed in our previous classes. Accordingly, I have not only learned a lesson about epithelial tissues today, but I also learned an important lesson in life: We are all capable of growth and we all have something new to learn every day. Thus, we must not take the blessing of learning and growth for granted.
Gwyneth Mae A. Kadile
MT 30 (LEC) – AA
Activity for April 3, 2024
The article highlights a fascinating fact: epithelium, a single tissue type, takes on so many critical roles in our body. This is as diverse as the organs it lines, acting as both a protective barrier and a functional powerhouse.
Specifically, it is interesting for me to learn about the different classifications of epithelia based on structure and function. For example, the stratified squamous epithelium that forms our skin acts as a tough barrier, much like a well-fortified wall. The flattened cells, packed together like bricks, prevent water loss, and shield us from harmful microorganisms and physical injury. Similarly, the urothelium lining in the urinary bladder shields underlying tissue from the harsh environment of urine. For the simple cuboidal epithelium found in the kidneys excels at absorption, working tirelessly to filter products from our blood. The article also highlighted the existence of glandular epithelia. These cells secrete substances like sweat, hormones, and enzymes. Sweat helps regulate body temperature, hormones act as chemical messengers, and enzymes breakdown food molecules for absorption.
Overall, the article effectively explained the various functions of epithelial tissue. Understanding these epithelial classifications is important in understanding the intricate organization of our body and the vital role this tissue plays in maintaining our health.
We have repeatedly learned about the different types of epithelial tissues since we were medical technology freshmen yet admittedly, I still need to be reminded of some of the concepts sometimes. This article in particular has been a great help in refreshing my memory about the notable terms under this topic, most of which are already mostly engraved in my mind and as familiar as the back of my hand. However, there are still new words that I haven’t encountered before such as metaplasia. Because of this article I got into scouring the internet regarding metaplasia and all of the other familiar concepts I needed to revisit as well.
Kylle Christine L. Cabanog
MT30 LEC – AA
Epithelial tissues are fundamental building blocks of the body, forming layers that cover surfaces and line cavities. They play crucial roles in protection, absorption, secretion, support, and more. I remember being so intimidated by this lesson as there are various kinds based on the number of cell layers and their shapes. However, learning about their respective functions and how crucial they are in maintaining homeostasis of the body was fascinating. Viewing them under the microscope was equally fascinating as well. You can really see what makes them distinct from each other. Some terms and concepts in this article, like “neoplasia” and “metaplasia,” are new to me, but the majority of them are still familiar to me, like a lingering thought in the back of my mind. This article is useful as it provides a comprehensive view of epithelial tissue that is informative and valuable to students, and even professionals who just want an active recall.