Epithelial Tissue: Definition, Characteristics, Types, and Functions

Epithelial Tissue: Do you know what kind of tissue your skin is made up of? It is made up of Epithelial Tissue that covers both the external and internal surfaces of the animal body. In Unicellular Organisms, all vital cellular functions like digestion, respiration, excretion, etc., are performed by a single cell, but Multicellular Organisms have a complex body organization. In these organisms, different groups of cells having different functions are well-organized into tissues, organs, and organ systems that function in a coordinated manner. There are four different kinds of tissues in animals, i.e., epithelial tissue, connective tissue, muscular tissue, and nervous tissue.

Epithelial Tissue: Table of Contents

1. Epithelial Tissue Definition
2. Epithelial Tissue Characteristics
3. Junctions between Epithelial Tissues
4. Epithelial Tissue Types
5. Epithelial Tissue Functions
6. Summary
7. FAQs

Epithelial Tissue Definition

Epithelial tissue or epithelium (epi- upon; thele- nipple) is defined as one of the animal tissues that covers both external and internal surfaces of the animal body. It consists of a sheet of tightly packed cells with a minimum of intercellular material and rests upon a non-cellular basement membrane.

Fig. Epithelial Tissue

Epithelial Tissue Characteristics

The characteristics of epithelial tissues are as follows:
1. The cells are tightly packed, with little or no intercellular spaces (\(20 – 30\,{\rm{nm}}\) wide).
2. The cells adjacent to each other are held together by intercellular junctions.
3. The epithelial cells are polarised, i.e., having an apical surface (surface) that faces the inside of a cavity or outside of a surface and a basal surface (attached surface) that faces the underlying surface.
4. This tissue lies on a thin, non-cellular basement membrane.
5. Epithelial tissue lacks its own blood supply.
6. The materials between epithelial cells and the blood vessels of the connective tissues across the basement membrane are exchanged by the diffusion process.
7. Nerve endings may innervate the epithelial tissues.

Junctions Between Epithelial Tissues

The common intercellular junctions present between the epithelial cells are as follows:

1. Tight junctions: In the apical region of the adjacent epithelial cells, the plasma membrane becomes tightly packed together. These junctions check the flow of materials between the cells and are also called occluding junctions.
2. Adherens junctions: Facilitate the cementing process so as to keep the neighbouring cells together that include desmosomes and hemidesmosomes.
3. Desmosomes: These are thick and strong junctions that help for anchoring functions.
4. Gap junctions: These junctions are present between the adjacent cells in the form of fine hydrophilic channels formed with the help of protein cylinders called connexin. They help in the exchange of chemicals between adjacent cells and thus are called communicating junctions also.

Fig: Different Types of Junctions Between Epithelial Cells

Epithelial Tissue Types

The epithelial tissues are broadly classified into three types:

1. Simple Epithelial Tissue
2. Compound Epithelial Tissue
3. Specialised Epithelial Tissue

1. Simple Epithelial Tissue

1. It is made up of a single layer of cells that are compactly arranged which rest over a non-cellular basement membrane.
2. It is present over moist surfaces where a little wear and tear occurs by friction.
3. It is generally associated with absorption, secretion, diffusion, and movement of materials.
4. It is further divided into the following types:
   i) Simple Squamous Epithelium
   It is formed of a single layer of closely fitted, flattened, polygonal cells. The cells are held together by various types of junctions, mainly tight junctions. This epithelium appears as tiles over a floor, also known as pavement epithelium.
   Location: It is found in the alveoli of the lungs, Bowman’s capsule, Henle’s loop of the nephron, pericardial cavity, abdominal cavity, the lining of various components of the blood vascular system.
   Function: Protection, excretion, gaseous exchange, and secretion of coelomic fluid.
   ii) Simple Cuboidal Epithelium
   It is composed of a single layer of cube-like cells that lies on the basement membrane. The nucleus is round and placed centrally. The surfaces of the cells may be smooth or bear microvilli that increase the surface area.
   Function: Protection, absorption, excretion, and gamete formation.
   iii) Simple Columnar Epithelium
   It consists of a single layer of tall and slender cells. A single oval or elongated nucleus is situated near the base of the cell. Some of its cells produce mucus, called goblet cells.
   Location: It occurs in the lining of the stomach, small and large intestine, digestive glands of the stomach, intestine and pancreas, gallbladder, etc. The goblet cells are found in the lining of the stomach, intestine, respiratory tract, etc.
   Functions: Secretion, absorption, and protection to the components of glandular epithelium.
   iv) Simple Ciliated Epithelium
   It bears cilia on its surface that move particles in a specific direction over epithelium. It lies over the basement membrane. It is again of two types:
   a) Simple Ciliated Columnar Epithelium: It possesses columnar cells that bear cilia over their surface. It occurs in the respiratory tract, fallopian tubes, parts of the uterus and cervix, and the different tubules of testes, etc.
   b) Simple Ciliated Cuboidal Epithelium: It consists of cuboidal or cubical cells that bear cilia on their surface. It is present in many parts of ependymal cells of the nervous system and parts of uriniferous tubules.
   Function: It maintains a flow of mucus, liquid, or suspended particles constantly in one direction.
   v) Pseudostratified Epithelium
   This epithelium is one-cell thick but appears two-layered because all the cells do not reach the surface. The cells are attached to the basement membrane, hence are called pseudostratified. It is again of two types:
   a) Pseudostratified Columnar Epithelium: It consists of columnar cells without cilia that line the large ducts of certain glands, like parotid salivary glands and the urethra of the human male.
   b) Pseudostratified Ciliated Columnar Epithelium: It has columnar cells that bear cilia at the surfaces. It lines the trachea and large bronchi.
   Function: Protection, movement of secretions from glands, urine, and semen in urethra and mucus loaded with dust particles and bacteria in the trachea.

Fig. Types of Epithelial Tissue

2. Compound Epithelial Tissue

1. It is formed of more than one layer of cells.
2. They are present on surfaces where constant replacement of cells is required due to rapid wear and tear by friction.
3. It is divided into two types:
   A. Stratified Compound Epithelium

It consists of many layers of cells and has been divided into the following four types on the basis of the shape of the cells present in the superficial layers:
 i) Stratified Squamous Epithelium
The cells in the basal layer are columnar or cuboidal with oval nuclei, and also called the germinal epithelium. The cells keep dividing by mitosis to form new cells. It is further subdivided into two types:
a) Keratinised Stratified Squamous Epithelium: This epithelium contains a hard waterproof protein called keratin or horn which is highly resistant to mechanical abrasions. It forms the epidermis of the skin in land vertebrates.
b) Non-Keratinised Stratified Squamous Epithelium: This epithelium does not contain keratin and is unable to check water loss. It provides moderate protection against abrasion. It lines the buccal cavity, pharynx, oesophagus, alimentary canal, the lower part of the urethra, vocal cord, vagina, cervix, conjunctiva, the cornea of the eye, and the inner surface of the eyelids.

Fig: (a) Keratinised Epithelium, (b) Non-Keratinised Epithelium

ii) Stratified Cuboidal Epithelium
It has an outer layer of cuboidal cells and a basal layer of columnar cells. It forms the epidermis of many fishes and tailed amphibians like salamanders. It also lines larger salivary and pancreatic ducts as well as the sweat glands also.
iii) Stratified Ciliated Columnar Epithelium
Its basal layer consists of columnar cells and the outer layer consists of ciliated columnar cells. It lines the larynx and upper part of the soft palate.
iv) Stratified Columnar Epithelium
It is formed of columnar cells in both superficial and basal layers. It covers the epiglottis and lines mammary gland ducts and parts of the urethra.
B. Transitional Compound Epithelium
It is formed of more than one layer of cells but is much thinner and more stretchable than the stratified epithelium. It contains cuboidal cells at the base, two or three layers of large polygonal or pear-shaped cells in the middle, and a superficial layer of large, broad, rectangular, or oval cells.
Location: It occurs in the inner surface of the urinary bladder, ureter, and renal pelvis.

3. Specialised Epithelial Tissue

This epithelial tissue is specialised to perform specific functions, so they are specialised in structures also. They are as follows:
a) Glandular Epithelium
1. Some of the columnar or cuboidal cells get specialized for secretion and form the glandular epithelium.
2. It is divided into two types:
 i) Unicellular Glandular Epithelium: It consists of isolated glandular epithelial cells called intraepithelial cells, e.g., goblet cells of the alimentary canal.
ii) Multicellular Glandular Epithelium: It consists of a cluster of epithelial cells called extra epithelial cells which unite to make up one gland, e.g., salivary gland.

Fig: (a)Unicellular Glandular Epithelium, (b) Multicellular Glandular Epithelium

b) Germinal Epithelium
This epithelium consists of specialised cuboidal cells capable of producing gametes (sperms and ova) from the gonads (i.e., testes and ovaries).
c) Sensory Epithelium
a. This epithelium contains neurosensory cells (singly or in groups) scattered among the epithelial cells.
b. This epithelium is present in olfactory mucosa, the retina of the eyes, and the epithelial covering of the tongue containing taste buds.
d) Pigmented Epithelium
This epithelium is present in the basal layer of the retina, iris, and skin that contains pigment.

Fig. Flowchart of Epithelial Tissue and its Types

Functions of Epithelial Tissue

1. It helps to protect the underlying tissues from mechanical injury, entry of germs, harmful chemicals, and drying up.
2. The epithelium of nephrons is specialised for urine secretion.
3. It helps to check the absorption of harmful or unnecessary materials.
4. It helps to receive various stimuli and convey them to the brain by the help of sensory epithelia of sense organs.
5. The epithelium of alveoli brings about the exchange of gases between the blood and air.
6. The cavity of the eyeball darkens due to the presence of pigmented epithelium of the retina.
7. It also forms the glands that secrete secretions such as mucus, gastric juice, and intestinal juice.
8. The germinal epithelium of the ovaries and seminiferous tubules of the testes produce ova and sperms respectively.
9. It also produces exoskeletal structures like scales, feathers, hair, nails, claws, horns, and hoofs.
10. Ciliated epithelia help to conduct the mucus and other fluids in the ducts they lie.

Do you know?

1. The term ‘Epithelium’ was coined by Ruysch.
2. The pavement epithelium becomes wavy in a few regions and is called tessellated epithelium.

Summary

The epithelial tissue forms the outer covering of our body which acts as a physical barrier, thus preventing the entry of germs or foreign particles into our body. It has a surface, which faces either a body fluid or the outside environment and thus, provides a covering or lining for most of the parts of the body.

Frequently Asked Questions (FAQs) on Epithelial Tissues

Q.1. How to identify epithelial tissue under the microscope?
Ans: Under the microscope, a squamous epithelial cell looks flat, a cuboidal epithelial cell looks like a square, a columnar epithelial cell looks like a rectangle and a few epithelial cells have transitional shapes.

Q.2. What are epithelia?
Ans: Epithelia are one of the animal tissues that covers both external and internal surfaces of the animal body.

Q.3. What are the two examples of Epithelial Tissue?
Ans: The two examples of epithelial tissue are simple squamous epithelium and simple cuboidal epithelium.

Q.4. What are the three types of epithelial tissue?
Ans: The three types of epithelial tissue are:
1. Simple Epithelial Tissue
2. Compound Epithelial Tissue
3. Glandular Epithelial Tissue

Q.5. What are the functions of epithelial tissue?
Ans: The functions of epithelial tissue are as follows:
1. It helps to protect the underlying tissues from mechanical injury, entry of germs, harmful chemicals, and drying up.
2. The epithelium of nephrons is specialised for urine secretion.
3. It helps to check the absorption of harmful or unnecessary materials.
4. It helps to receive various stimuli and convey them to the brain by the help of sensory epithelia of sense organs.
5. The epithelium of alveoli brings about the exchange of gases between the blood and air.
6. The cavity of the eyeball darkens due to the presence of pigmented epithelium of the retina.
7. It also forms the glands that secrete secretions such as mucus, gastric juice, and intestinal juice.
8. The germinal epithelium of the ovaries and seminiferous tubules of the testes produce ova and sperms respectively.
9. It also produces exoskeletal structures like scales, feathers, hair, nails, claws, horns, and hoofs.
10. Ciliated epithelia help to conduct the mucus and other fluids in the ducts they lie.

Q.6. What are the characteristics of epithelial tissue?
Ans: The characteristics of epithelial tissue are as follows:
1. The cells are tightly packed.
2. Intercellular spaces are narrow.
3. Adjacent cells are held together by intercellular junctions.
4. This tissue lies on a thin, non-cellular basement membrane.
5. Blood vessels are absent.