What are epithelial tissues?
Written by Glicy Lou D. Garinggo
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.
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.
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 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.
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