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:
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.
Epithelial tissue is divided into two categories based on the number of layers of cells it possesses. These are as follows:
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.
Simple epithelium is subdivided into the following categories based on structural modifications as follows:
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.
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.
In compound epithelial tissue, cells are arranged in multiple layers or stratas. Thus, it is also known as stratified 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.
Compound epithelium is classified into following categories on the basis of types of cells and number of layers:
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.
It is classified into two types based on the presence or absence of protein as follows:
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.
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.
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.
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.
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.
Based on function the epithelial tissue is mainly divided into two types as follows:
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.
This tissue is composed of columnar or cuboidal cells that are specialised for secretion.
There are two types of glandular epithelial tissue based on the number of cells present in it. They are as follows:
These are isolated glandular cells. For example, goblet cells present in the alimentary canal which secrete mucus.
These are clusters of cells. They are formed by the dipping down of glandular epithelium into connective tissues. For example, salivary glands.
Glands are divided into two types based on the presence or absence of ducts as follows:
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.
These are ductless glands. These glands secrete hormones directly into the circulating fluid.
Glands are of three types based on modes of secretion. They are as follows:
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.
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.
In these glands, an entire cell is filled with secretory products. It disintegrates and discharges the secretions. Examples include sebaceous glands.
Specialised junctions called intercellular junctions are present in animal tissues that serve as functional and structural links between the cells.
There are three types of intercellular cell junctions as follows:
In a tight junction
The plasma membranes of the adjacent cells are held tightly together.
Tight junctions prevent the leakage of substances across the tissue.
Adhering junctions help in cementing adjacent cells together.
Cell communication occurs through gap junctions between neighbouring cells.
It occurs due to movement of particles (ions and molecules)
There are three types of adhering junctions as follows:
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.
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.
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.
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.
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.
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