Epithelial Tissue – Introduction, Characteristics, Types, Importance

Epithelial tissue is what makes up this part of the animal’s anatomy, and it may be found on both the outside and the inside of the body. The body structure of multicellular organisms is more complicated than that of unicellular organisms, in which every important cellular function, such as nutrition, breathing, elimination, and so on, is carried out by a single cell. In these types of creatures, several groupings of cells, each of which performs a unique function, are meticulously arranged into tissues, organs, and organ systems that carry out their jobs in a synchronised way.

Epithelial Tissue

A few of the types of animal tissue known as epithelial tissue, also known as epithelium (from the Greek words epi- upon and thele- nipple), is described as being one of the tissues that cover both the interior and exterior surfaces of an animal’s body. It is made up of a layer of closely packed cells with a minimal amount of intercellular material and is supported by a membrane that is not cellular and acts as a foundation.

Characteristics of Epithelial Cells and Tissues

The following is a list of properties that epithelial tissues have

• There are very few, if any, intercellular gaps since the cells are so clogged together (20–30 nm wide).
• Intercellular junctions are what keep cells that are close to each other connected to one another.
• The epithelial cells are bipolar, which means that they have an apical surface (also known as a free surface), which faces the interior of a cavity or the exterior of a surface, and a basal surface (also known as an attached surface), which faces the surface that lies underneath them.
• This tissue is supported by a thin foundation membrane that is devoid of cells.
• Epithelial tissue does not have its own supply of blood and oxygen.
• The epithelial tissues may be innervated by nerve terminals in certain cases. The Points Where Different Epithelial Tissues Converge

The following is a list of the intercellular connections that are typical of epithelial cells and may be found between them:

• Tight junctions: at the apical area of neighbouring epithelial cells, the plasma membrane gets closely compacted together. These junctions, which are also known as occluding junctions, are responsible for controlling the passage of materials between the cells.
• Adherens junctions are responsible for facilitating the cemented process and maintaining the connection between neighbouring cells, including desmosomes and hemidesmosomes.
• Desmosomes are strong junctions that aid with anchoring activities.
• Gap junctions: These connections are present between the neighbouring cells in the form of thin hydrophilic channels made with the assistance of protein cylinders termed connexin. It helps for the passage of ions, molecules, and other substances between the cells. Because they facilitate the movement of chemicals from one cell to the next within close proximity, they are sometimes referred to as communicative junctions.

Different Types of Epithelial Tissues

The epithelial tissues may be roughly divided into three categories, which are as follows:

• Tissue Consisting Only of Epithelia(or) Simple Epithelial Tissue
• Tissue Composed of Multiple Epithelia(or) Compound Epithelial Tissue
• Tissue Made of Glandular Epithelium(or) Glandular Epithelial Tissue

Simple Epithelial Tissue

• It is composed of a single layer of cells that are packed closely together and that rests on top of a membrane that is not cellularly named the basement membrane.
• It may be found on wet surfaces that experience a moderate amount of friction, which causes some wear and tear.
• In general, it is connected to processes involving the assimilation, excretion, distribution, and transportation of materials. 

It was divided into subsequent types:

Epithelial squamous tissue

It is formed from only one layer of polygon cells that have been flattened and packed closely together in close proximity to one another. There are many different kinds of junctions, but tight junctions are the ones that are most important in holding the cells together. The pavement epithelium is another name for this kind of epithelium, which appears like tiles across a floor.

Locations: Where it may be found include the alveoli of the lungs, the pericardial cavity, the abdominal cavity, and the lining of numerous components of the blood circulatory system. Preservation, evacuation, gaseous exchange, and the secretion of coelomic fluid are the functions of this organ.

Simple Cuboidal Epithelium

On top of the basement membrane, it consists a single layer of cells that are cube-shaped and is made up of these cells. The nucleus or core of the cell may be thought of as a sphere and can be found in the middle of the cell. It’s possible for the free surfaces of the cells to be plain or to have microvilli, which are structures that enhance the surface area. Preservation, intake, elimination, and the generation of gametes are the functions of this organ.

Simple Columnar Epithelium 

It is made up of a single layer of cells that are all stretched and positioned in the same vertical direction as one another. Near the bottom of the cell sits a solitary nucleus that may be described as either oval or elongated. Goblet cells, which are a subset of its cells, are responsible for the production of mucous. These cells may be found in many parts of the respiratory system. 

Position: It may be found in the stomach wall, the larger and smaller intestines, the digestive glands of the stomach, the intestines, and the pancreas, as well as the gallbladder. The lining of the digestive system, the intestines, the respiratory tract, and other organs all contain goblet cells. Functions include protection of the components of the glandular epithelium as well as secretion and absorption of substances.

Ciliated Epithelium that Is Simplified

On its surface layer, it carries cilia, which are projections that transport materials in a certain orientation across the epithelium. It is located above the basement membrane at its location. Once again, there are two different classes:

• Plain Ciliated tabular Epithelium Simple(or)simple Ciliated Columnar Epithelium: This kind of epithelial tissue contains cells that have cilia on the surface of their tabular epithelia across the whole layer. It may be found in the cervical, various regions of the uterus,  the respiratory system, the reproductive organs, and even the various tubules of the testes.
• The Simple Ciliated Cuboidal Epithelium: It is made up of cuboidal or cubical cells that have cilia on the surfaces of their free sides. It may be found in many locations inside the ependymal cells of the neurological system as well as the tubules of the urinary system. Function: It ensures that mucous, fluid, or particles floating in liquid continue to follow the same path at all times.

Pseudostratified Epithelium of the Endothelium

It is just one cell thick; yet, it gives the appearance of having two layers because not all of the cells extend to the free surface. Be a result of the cells’ attachment to the basement membrane, the structure is referred to as pseudostratified. Again, there are two categories:

• Pseudostratified Columnar Epithelium(or) false stratified tabular epithelium: It is characterised by the presence of columnar cells that are devoid of cilia and line the vast ducts of certain glands, such as the parotid salivary glands and the urethra of male humans.
• Pseudostratified Ciliated Columnar Epithelium(or) False stratified ciliated tabular cells: This kind of epithelium may be characterized by the appearance of columnar(or)tabular cells that contain cilia on the material surface of their cells. It is observed in the bronchi and tracheal lining.
  Functions include shielding, the transfer of glandular fluids like urine and sperm via the urethra, and the transport of mucous that contains dust particles and pathogens down the trachea.

Compound epithelial tissue 

• It is composed of more than 1 layer of individual cells.
• You may find them on surfaces where the continual replacement of cells is necessary because of the quick wear and tear that is caused by friction.

There are two distinct categories of it:

The Structurally Complicated Epithelium (or) Stratified Compound Epithelium

It is made up of numerous layers of cells and has been classified into four categories that follow according to the morphology of the cells that are found in the surface layers:

Squamous Epithelium With Stratification

The cells that make up the germinal epithelium are columnar or cuboidal in shape, and their nuclei are oval in shape. There is a possibility that the basal layer of the tissue contains these cells. Mitosis is the process through which the cells continually reproduce themselves to make new cells.  In addition to this, it is split into the following 2  types

• Keratinized stratified squamous epithelium: This epithelium includes a protein known as keratin, which is a tough protein that is waterproof and is also known as horn. It is very resistant to mechanical abrasions. In terrestrial vertebrates, it is responsible for the formation of the epidermis of the skin.
• Non-Keratinized Stratified Squamous Epithelium: Because it does not contain keratin, this epithelium is unable to prevent the loss of water. It offers a satisfactory layer of safety against damage. It borders the buccal cavity, the throat, the oesophagus, the gastrointestinal system, the lower portion of the urethra, the vocal cord, the genitalia, the cervical, the conjunctiva, and the cornea of the eye, and the inner surface of the eyelids. Other areas that it lines include the cervix and the vagina.

Cuboidal stratification of the epithelium

It comprises a basal layer that is composed of columnar cells and an outer layer that is composed of cuboidal cells. The epidermis of many fishes and amphibians with tails, including toads, are made of this substance. In addition, it covers the bigger salivary and pancreatic ducts and the sebaceous glands.

Columnar epithelium that is stratified and ciliated

Columnar cells make up its foundational layer, whereas ciliated columnar cells make up its outermost layer. It also covers the top section of the soft palate and borders the larynx. The stratified columnar epithelium is the fourth kind. Columnar cells may be found in both the surface and the basal levels of this structure. It also lines the ducts of the mammary glands and sections of the urethra. It wraps around the epiglottis.

Epithelium Composed of Transitional Compounds
It is composed of more than 1 layer of cells, but in comparison to the stratified epithelium that covers the majority of the system, it is far less dense and more elastic. It has cells that are cuboidal at its base, 2 or 3 layers of huge polygonal or pear-shaped cells in the centre, and a surface layer of massive, wide, rectangular, or oval cells.
Spot: It may be found on the inside surface of the renal pelvis, the ureter, and the urinary bladder. 

Epithelial Tissues That Are Specialized

This epithelial tissue is specialized to execute certain duties, and as a result, its structures are similarly specialised for this purpose. Several examples of these are as follows:

The Epithelium of the Glandula of the Gland

• The glandular epithelium is formed when a subset of the columnar or cuboidal cells becomes specialised for the process of release.
• There are two distinct varieties of it

Single celled Glandular Epithelium

This kind of glandular epithelium is made up of singular glandular epithelial cells, which are referred to as intraepithelial cells. One example of this type of glandular epithelium is the goblet cells found in the alimentary canal.

• Multicellular Glandular Epithelium: This kind of glandular epithelium is made up of a group of epithelial cells that are referred to as additional epithelial cells. These cells come together to form a gland, such as the salivary gland.
• The germinal epithelial layer in the reproductive organs, this epithelium is made up of specialised cuboidal cells that are able to produce gametes (sperm and ova) (i.e., testes and ovaries).

Epithelium Sensitive to Stimuli(or) impulses

• Neurosensory cells, either alone or in groups, are dispersed throughout this epithelium. These cells may be found anywhere inside the epithelial cells.
• This epithelial layer may be detected in the olfactory region, the retinal of the vision, and the epithelial covering of the tongue, which contains taste buds.

Epithelium with Pigmentation(or) Pigmented Epithelium

This epithelium may be found in the bottom-most layer of the retina, the iris, and the layer of the skin that carries pigment.

Epithelial tissue’s several roles in the body

• It helps to protect the underlying tissues from mechanical damage, the introduction of bacteria and hazardous substances and drying out 2. It prevents the tissues from being mechanically injured.
• The epithelium of nephrons is modified specifically for the production of urine.
• It acts as a check on the body’s ability to absorb potentially dangerous or superfluous elements.
• It assists in the reception of a variety of stimuli and the transmission of those to the brain with the assistance of the sensory epithelia of the sense organs.
• The epithelium that lines the alveoli is responsible for the gas exchange that takes place between the blood and the air.
• The pigmented epithelium of the retina is responsible for the darkening of the space inside the eyeball itself.
• In addition, it creates the glands that are responsible for the release of fluids such as mucus, gastric juice, and intestinal juice.
• The generation of ova(females) and sperm(males) is controlled by the germinal epithelium of the ovaries and the seminiferous tubules of the testes, respectively.
  • In addition, it is responsible for the production of exoskeletal structures including feathers, scales, hair, nails, claws, horns, and hooves.
  • Ciliated epithelia assist in the transportation of mucus and other fluids via the ducts in which they are located.

Conceptual Questions

Question 1: How can epithelial tissue be recognized when seen using a microscope? 

Answer:

When seen via a microscope, a squamous epithelial cell has the appearance of a flat surface, while a cuboidal epithelial cell has the appearance of a square and a columnar epithelial cell has the appearance of a rectangle. Some epithelial cells have transitional forms.

Question 2: Can you give me two instances of epithelial tissues?

Answer:

Different forms of epithelial tissue include simple(or) plain squamous epithelial and simple(or)plain cuboidal epithelial.

Question 3: What types of epithelial tissue exist?

Answer:

• Tissue Consisting Only of Epithelia(or) Simple Epithelial Tissue
• Tissue Composed of Multiple Epithelia(or) Compound Epithelial Tissue
• Tissue Made of Glandular Epithelium(or) Glandular Epithelial Tissue

Question 4: What roles does epithelial tissue play in the body?

Answer:

• It protects the underlying tissues from mechanical injury, bacterial and toxic material invasion, and withering out
• It prevents the tissues from being mechanically injured.
• The epithelium of nephrons is modified specifically for the production of urine.
• It acts as a check on the body’s ability to absorb potentially dangerous or superfluous elements.
• It assists in the reception of a variety of stimuli and the transmission of those to the brain with the assistance of the sensory epithelia of the sense organs.
• The epithelium that lines the alveoli is responsible for the gas exchange that takes place between the blood and the air.
• The pigmented epithelium of the retina is responsible for the darkening of the space inside the eyeball itself.
• In addition, it creates the glands that are responsible for the release of fluids such as mucus, gastric juice, and intestinal juice.
• In addition, it is responsible for the production of exoskeletal structures including feathers, scales, hair, nails, claws, horns, and hooves.
• Ciliated epithelia assist in the transportation of mucus and other fluids via the ducts in which they are located.

Question 5: Represent some features that determined epithelial tissue?

Answer:

• There is a very close arrangement of the cells.
• Inter-cellular connections are what keep adjacent cells connected to one another.
• This tissue is supported by a thin foundation membrane that is devoid of cells.
• Blood vessels are lacking.