Epithelial Tissue: Its Characteristics, Simple Epithelium, Compound Epithelium and Intercellular Junctions

You know we all are multicellular organisms and we have lots of organs and organ systems in our body.  Have you ever wondered how multicellular organisms are formed ? 

Animals are multicellular organisms in which cells form the basic structural and functional unit of the body. A group of cells having the same structure, function and origin together form a tissue. The functioning in the body of simple animals like Hydra is organised at the tissue level.

However in complex animals like human beings, tissues, organs and organ systems work together to make the animal a functional living organism.

Now you heard about tissues. To understand the levels of organisation in a multicellular organism, first you must study about tissues. Let’s start from the outer covering of the body, that is skin. 

It is the protective covering of the body. The tissues in animals which comprise the outer layers of body structures are commonly referred to as the epithelial tissue. It is the constituent of epithelium. Let’s understand more about epithelial tissue in this article. 

Table of Contents:

• Characteristics of epithelial tissue
• Types of epithelial tissues
• Simple epithelium
• Compound epithelium
• Classification of epithelium based on function
• Intercellular junctions
• Practice Problems
• FAQs

Characteristics of Epithelial Tissue

The epithelial tissue acts as a protective covering or lining for all the surfaces in the animal body. The cells of epithelial tissue lie on a basement membrane and have a free surface on the other end.

• The free surface of the epithelial tissue faces the body fluids when the epithelium lines the inner surface of organs. 
• The outside environment, epithelium faces when it covers the outer surface of body parts. 

  

• The epithelial tissues possess very little intercellular matrix as the cells are very tightly packed.
• This tissue is devoid of blood vessels and is said to be avascular. Hence it receives the nutrients from capillaries that are present in the underlying connective tissue.
• In certain areas like the linings of hollow organs or cavities epithelial tissues secrete mucus. 
• They can be formed from all the primary germ layers like ectoderm, endoderm and mesoderm.

Types of Epithelial Tissue

Epithelial tissue is divided into two categories based on the number of layers of cells it possesses. These are as follows:

• Simple epithelium
• Compound epithelium

Simple Epithelium

A single layer of cells only makes up this epithelium. The main function of simple epithelium is to cover the body cavities, ducts, and tubes. 

Types of Simple Epithelium

Simple epithelium is subdivided into the following categories based on structural modifications as follows:

• Simple ciliated epithelium
• Pseudostratified epithelium

Simple Ciliated Epithelium

Simple ciliated epithelium is formed when the cells of simple columnar or cuboidal epithelium possess cilia on their free surface. This tissue is present in the inner lining of  hollow organs, such as bronchioles and fallopian tubes. The cilia help in the specific directional movement of mucus or particles over the epithelium.

Pseudostratified Epithelium

This epithelial tissue is composed of columnar cells arranged in a single layer. However, it appears double-layered due to differences in the size of the cells and the level at which their nuclei are located. Pseudostratified epithelium may or may not bear cilia. Ciliated pseudostratified epithelium is present in the trachea whereas non-ciliated one can be found in the auditory canal.

Compound or Stratified Epithelium

In compound epithelial tissue, cells are arranged in multiple layers or stratas. Thus, it is also known as stratified epithelium. 

Functions of Compound Epithelium

This tissue has a negligible role in secretion and absorption. The primary function of compound epithelium is to protect the organs from chemical and mechanical stress. It is present in the lining of the ducts arising from the pancreas, on the dry surface of the skin, moist surface of the cavity of the mouth and pharynx.

Types of Compound Epithelium

Compound epithelium is classified into following categories on the basis of types of cells and number of layers:

• Stratified squamous epithelium
• Stratified cuboidal epithelium
• Stratified columnar epithelium
• Stratified columnar ciliated epithelium

Stratified Squamous Epithelium

It is composed of multiple layers of cells and the apical surface contains squamous cells. The cells of the deeper layers are columnar or cuboidal. The primary function of stratified squamous epithelium is protection. It is present in the areas that are susceptible to abrasions, such as mouth, oesophagus and skin. 

Classification of stratified squamous epithelium

It is classified into two types based on the presence or absence of protein as follows:

• Keratinised stratified epithelium
• Non-keratinised stratified epithelium

Keratinised Stratified Epithelium

The uppermost layer of the skin is composed of stratified squamous epithelium and it contains large quantities of keratin protein. Keratin is a fibrous and strong protein that protects against abrasion and water loss. The new cells are produced in the basement membrane which is then pushed towards the apical surface. As these cells move upwards, they fill with keratin protein and eventually die. This forms a layer of dead cells that is present on the upper surface of the epidermis. 

Non-keratinised stratified epithelium

This layer contains living squamous cells and is mainly found in the moist surface, such as buccal cavity, oesophagus and vagina. The function of non-keratinised stratified epithelium is to protect the organs from mechanical damage.

Stratified Cuboidal Epithelium

It is composed of multiple layers of cells in which the outermost layer is of cuboidal cells. This type of layer is found in the conjunctiva of eyes, lining of ducts of sweat glands, salivary gland, mammary glands and urethra. It protects the organ from mechanical and chemical stress.

Stratified Columnar Epithelium

It is composed of multiple layers of cells in which the outermost layer is of columnar cells and the middle layer is of cuboidal cells. This type of layer forms a lining of vasa-deferentia, respiratory tract and mammary gland. It is primarily involved in the secretion of fluids and protection from mechanical and chemical stress.

Stratified Ciliated Columnar Epithelium

It is composed of multiple layers in which the superficial layer is ciliated and columnar. This type of layer lines the respiratory passages, the vas deferens and the epididymis. 

Classification of Epithelial Tissue-based on Function

Based on function the epithelial tissue is mainly divided into two types as follows:

• Transitional epithelium
• Glandular epithelium

Transitional Epithelium

Transitional epithelium is a type of stratified epithelium in which the cells are more elastic but are less flattened. The tissue is generally 4–6 layers thick. The innermost layer consists of cuboidal or columnar cells. The middle layer is made up of  pear-shaped cells. The surface is covered with large, broad, rectangular or oval-shaped cells. This tissue is located in the renal pelvis, urinary bladder, ureters and parts of the Urethra. Its main function is to help in distension of the organs.

Glandular Epithelium

This tissue is composed of columnar or cuboidal cells that are specialised for secretion. 

Types of Glandular Epithelium

There are two types of glandular epithelial tissue based on the number of cells present in it. They are as follows:

Unicellular 

These are isolated glandular cells. For example, goblet cells present in the alimentary canal which secrete mucus.

Multicellular 

These are clusters of cells. They are formed by the dipping down of glandular epithelium into connective tissues. For example, salivary glands.

Types of Glands based on Ducts

Glands are divided into two types based on the presence or absence of ducts as follows: 

• Exocrine glands
• Endocrine glands

Exocrine glands

They have ducts through which they pour their secretions. They secrete saliva, mucus, earwax, digestive enzymes, milk, oil, and other cell products. Examples include salivary glands, intestinal glands etc. 

Endocrine Glands

These are ductless glands. These glands secrete hormones directly into the circulating fluid.

Types of Glands based on Modes of Secretion

Glands are of three types based on modes of secretion. They are as follows:

• Merocrine glands
• Apocrine glands
• Holocrine glands 

Merocrine Glands

In these glands the secretion occurs by simple diffusion. There is no loss of cells or their parts in these glands during secretion. Examples include goblet cells, salivary glands, intestinal glands and most of the sweat glands.

Apocrine Glands

In these glands the cell normally loses a part of its cytoplasm during secretion. These secretions the cells store in the apical part and release by bursting the contents. Examples include mammary glands, sweat glands etc.

Holocrine glands

In these glands, an entire cell is filled with secretory products. It disintegrates and discharges the secretions. Examples include sebaceous glands. 

Intercellular Junctions

Specialised junctions called intercellular junctions are present in animal tissues that serve as functional and structural links between the cells. 

Types of Intercellular Junctions

There are three types of intercellular cell junctions as follows:

• Tight junctions
• Adhering junctions
• Gap junctions

  

Types of Adhering Junctions

There are three types of adhering junctions as follows: 

• Zona adherens
• Desmosomes
• Hemidesmosomes

Zona Adherens

They are dense plaque-like structures lying on the cytoplasmic side of the plasma membrane from which actin microfilaments extend into the cytoplasm. Intercellular filaments are absent. 

Desmosomes

They are also called macula adherens. They have intercellular proteins which form thick plaque-like structures called protein plates. The microfilaments made up of keratin-like proteins extend from these plaque-like structures into the cytoplasm. They help in anchoring.

Hemidesmosomes

They are single sided desmosomes. They are similar to desmosomes, but the thickening of cell membrane is present only on one side. They join the epithelial cells with the basal lamina. 

Practice Problems of Epithelial Tissue

Question 1. Identify the given figure and find out the correct labelling:

a. ‘A’- Unicellular glandular epithelium like goblet cells in the alimentary canal
    ‘B’- Multicellular glandular epithelium like salivary glands

b. ‘A’- Unicellular glandular epithelium like salivary gland
    ‘B’- Multicellular glandular epithelium like goblet cells of alimentary canal

c. ‘A’- Multicellular glandular epithelium like salivary glands
    ‘B’- Unicellular glandular epithelium like goblet cells in the alimentary canal

d. ‘A’- Multicellular glandular epithelium like goblet cells in the alimentary canal
    ‘B’- Unicellular glandular epithelium like salivary gland

Answer: In the given figure, ‘a’ represents the Multicellular glandular epithelium of salivary glands and ‘b’ represents the unicellular glandular epithelium like goblet cells of the alimentary canal. Hence, the correct option is c.

Question 2. Which of the following epithelial cells can be ciliated?

a. Squamous epithelium
b. Cuboidal epithelium
c. Columnar epithelium
d. Both b and c

Answer: Ciliated epithelial cells are columnar or cuboidal cells with cilia on their surface. The cilia carry particles or mucus over the epithelium in a certain direction. The inner surface of bronchioles and fallopian tubes show this. Hence, the correct option is d.

 Question  3. Which of the following is not an exocrine gland?

a. Salivary glands
b. Mammary glands
c. Thyroid gland
d. Goblet cells

Answer: Exocrine glands have ducts through which they secrete their products. The exocrine glands in the human body include salivary glands, mammary glands, sweat glands, and tear glands. Endocrine glands, on the other hand, include the thyroid, pituitary, and adrenal glands. Hence, the correct option is c.

Question 4. Which type of epithelium is found predominantly towards the ovarian end of fallopian tubes?

a. Ciliated columnar
b. Squamous epithelium
c. Glandular epithelium
d. Stratified squamous

Answer: The fallopian tube cells near the ovarian end are mostly columnar cells with cilia that help the ovum travel towards the uterus. Hence, the correct option is a.

Question 5. What is the main difference between simple and compound epithelium?

Answer: The main difference between simple and compound epithelium is that the simple epithelium consists of a single layer of cells, whereas compound epithelium consists of multiple layers of cells. Moreover, simple epithelium is involved in the absorption and excretion of substances through diffusion, whereas the compound epithelium is involved in the protection of internal organs.

FAQs of Epithelial Tissue

Question 1. Which types of cells are normally seen in the epithelial tissue?

Answer: In epithelial tissue, three different types of cells are involved. These are squamous cells, cuboidal cells and columnar cells.

Question 2. Which epithelium is also known as the pavement epithelium?

Answer: Simple squamous epithelium is also known as pavement epithelium because it is composed of a single layer of flat and thin cells which look like tiles on the floor.

Question 3. Why does the outer layer of skin contain dead cells?

Answer: The outer layer of the skin consists of dead cells because they are filled with keratin protein that eventually die. The dead cells protect the skin from abrasions.

Question 4. Which is considered as the largest organ in the human body?

Answer: The largest organ in the human body is your skin. It is made up of epithelial tissue. It is about 3.6 kilograms and covers 2 square metres. 

Related Topics to Epithelial Tissues in Biology

• Connective Tissues
• Connective Tissue Blood
• Connective Tissue Bone
• Connective Tissue Cartilage

NCERT Class 11 Biology Chapters