Question

Describe the structure and function of different types of epithelial tissues.Draw the diagram for each type of epithelial tissues

Answer 1

Epithelial tissue is the thin protective layer of cells which covers the surface of the body and lines the internal organs. The cells of this tissue are generally packed close together. The shape of the cells depends on the location and function of the tissue. Epithelial tissue originates from the ectoderm. But, epithelial tissue lining the intestine originates from the endoderm. Epithelial tissue may be simple, i.e., composed of single layer of cells or stratified, i.e., made up of several layers of cells. Depending upon the shape and function of cells epithelial tissues are classified as (i) Squamous epithelial tissue This tissue is composed of a single layer of thin and fiat, plate-like cells. The cells fit closely, like the bricks in a wall, to form a smooth membrane. It is also known as tessellated and pavement epithelium. It is found in the outer layer of the skin, and covers internal cavities and ducts. Tongue, esophagus and the lining of the mouth are made up of squamous epithelium. It is also found in blood vessels and alveoli. It protects the underlying parts of body from mechanical injury, entry of germs, chemicals and drying. It also forms a selectively permeable surface through which filtration occurs. (ii) Cuboidal epithelial tissue This tissue is composed of cube-like cells that fit closely. The cells look like squares in section, but the free surface appears hexagonal. This tissue lines the inside of the kidney tubules (the tubes leading from the cups of nephrons) thyroid vesicles and in glands like sweat glands, exocrine pancreas and the salivary glands. It forms germinal epithelium of gonads (testes and ovaries). It helps in absorption, excretion and secretion. It also provides mechanical support,  (iii) Columnar ciliated epithelial tissue This tissue is generally composed of a single layer of column like cells. The presence of a conspicuous striated border of microvillus at the free surface end of each cell increases the surface area of the cell for absorption and secretion, It is generally found in the inner lining of the alimentary canal. It also forms the lining of gall bladder and oviducts. The major functions of this tissue includes secretion (e.g., mucus of goblet cells) and absorption (e.g., stomach and intestine). In some parts of the body, columnar epithelium develops protoplasmic outgrowths called cilia. The constant lashing movements of the cilia help to move substances. It is found in the sperm ducts. It also lines the trachea (wind-pipe), bronchi (lungs), kidney tubules and oviducts (Fallopian tubes). Ciliated epithelium helps the movement of ova in the fallopian tubes and the movement of mucus in the respiratory tract.  (iv) Stratified squamous epithelial tissue This tissue is found in skin and covers the external dry surface of the skin. Cells of this tissue are arranged in many layers, but the cells forming different layers of this epithelium are not similar. Deeper layers of the tissue have cuboidal cells which become polygonal and finally flattened (squamous) towards the free surface. The flattened cells of superficial layer may contain a fibrous protein, the keratin and become dead cells and are called keratinized stratified squamous epithelium. This epithelium is water proof and highly resistant to mechanical injury. (v) Glandular epithelial tissue Epithelial tissue often acquire additional specialization as gland cells, which can secrete substances at the epithelial surface. Sometimes, a portion of epithelial tissue folds in wards and a multicellular gland is formed. This is called glandular epithelium. 

Answer 2

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$\Huge{\mathfrak{\blue{\underline{ANSWER}}}}$

⭕ Epithelial tissue lines the surface of a body and forms a protective covering. Epithelium cells are packed tightly together with little intercellular matrix.

⭕ Epithelial tissue in the body is of two types.

$\mathcal\red{Simple}$ $\mathcal\red{epithelium}$

• It consists of a single layer of cells where cells are in direct contact with the basement membrane. It is further sub-divided into the following types:

✔️ Simple squamous epithelium:

•It consists of a single layer of flat cells with irregular boundaries.

•It is found in the walls of the blood vessels and in the lining of alveoli.

✔️ Simple cuboidal epithelium:

•It consists of a single layer of cube-like cells.

•It is present in regions where secretion and absorption of substances takes place such as the proximal convoluted tubule region of the nephron.

✔️ Simple columnar epithelium:

•It consists of a single layer of tall, slender cells with their nuclei present at the base of the cells.

•They may bear micro-villi on the free surfaces.

•Columnar epithelium forms the lining of the stomach and intestines, and is involved in the function of secretion and absorption.

✔️ Ciliated epithelium:

• It consists of columnar or cuboidal cells with cilia on their free surfaces.

• They are present in bronchioles and oviducts from where they direct mucus and eggs in specific directions.

✔️ Glandular epithelium:

•It consists of columnar or cuboidal cells involved in the secretion of substances.

•Glands are of two types, unicellular glands (goblet cells of the alimentary canal) and multicellular glands (salivary glands).

•They can be classified as exocrine (ductless glands) and endocrine glands (duct glands) by the method through which they release enzymes.

$\mathcal\red{Compound}$ $\mathcal\red{epithelium}$

• It consists of many layers of cells. It is involved mainly in the function of providing protection and has a limited role in secretion and absorption.

• Dry surface of the skin or moist inner lining of the buccal cavity, pharynx, pancreatic ducts, and the inner lining of ducts of salivary glands are some of the examples of compound epithelium

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