Structure and Function of Epithelial Tissues

What is Epithelial Tissue?

Epithelial tissue is a sheet of tightly packed cells that covers the body surface and lines internal organs, cavities, and ducts. It also forms glands and serves as a protective barrier.

Structure of Epithelial Tissue

🔹 Key Structural Features:

1. Cellularity: Made up of closely packed cells with minimal intercellular space.

2. Polarity: Epithelial cells have two surfaces Apical Surface – faces the lumen or external environment and Basal Surface – rests on a basement membrane (a thin, non-cellular layer of protein fibers).

3. Avascular: Lacks blood vessels; nutrients and oxygen diffuse from nearby connective tissues.

4. Innervation: Contains nerve endings (sensory receptors).

5. Regenerative: High mitotic activity enables quick repair and renewal.

Classification of Epithelial Tissue

Epithelial tissue is classified based on two main criteria:

1. Number of Cell Layers

2. Shape of the Cells

1. Based on the Number of Cell Layers

a) Simple Epithelium

1. Structure: Single layer of cells

2. Function: Involved in absorption, secretion, and filtration

3. Location: Lining of blood vessels, alveoli of lungs, kidney tubules

b) Stratified Epithelium

1. Structure: Multiple layers of cells

2. Function: Provides protection against mechanical and chemical stress

3. Location: Skin surface, lining of the mouth, esophagus

c) Pseudostratified Epithelium

1. Structure: Appears multilayered, but all cells touch the basement membrane

2. Function: Secretion and movement of mucus

3. Location: Respiratory tract (trachea, bronchi)

2. Based on the Shape of Cells

a) Squamous Epithelium

1. Shape: Flat and thin cells

2. Function: Allows diffusion and filtration

3. Location: Lining of blood vessels (endothelium), alveoli

b) Cuboidal Epithelium

1. Shape: Cube-like cells with a central nucleus

2. Function: Secretion and absorption

3. Location: Kidney tubules, ducts of glands

c) Columnar Epithelium

1. Shape: Tall, column-shaped cells

2. Function: Absorption and secretion

3. Location: Lining of stomach, intestine, and gallbladder

d) Ciliated Epithelium

1. Special Feature: Has cilia on the surface

2. Function: Moves particles or fluids over the epithelial surface

3. Location: Trachea (respiratory tract), fallopian tubes

A. Summery table based on number of Cell Layers

Type	Description	Example
Simple Epithelium	Single layer of cells	Lining of alveoli
Stratified Epithelium	Two or more layers	Epidermis of skin
Pseudostratified Epithelium	Appears layered, but all cells touch basement membrane	Trachea lining

B. Summery table based on Cell Shape

Shape	Description	Example
Squamous	Flat and thin	Lung alveoli
Cuboidal	Cube-shaped	Kidney tubules
Columnar	Tall, column-like	Intestine lining
Ciliated	Has hair-like projections (cilia)	Respiratory tract
Glandular	Specialized for secretion	Salivary glands
Type	Description	Example
Simple Epithelium	Single layer of cells	Lining of alveoli
Stratified Epithelium	Two or more layers	Epidermis of skin
Pseudostratified Epithelium	Appears layered, but all cells touch basement membrane	Trachea lining

Function of Epithelial Tissue

Epithelial tissue performs a variety of important functions in the body. These tissues form the lining of internal and external body surfaces, including the skin, organs, and cavities. Here are the main functions:

1. Protection

Epithelial tissues act as a protective barrier against mechanical injury, harmful chemicals, invading microbes, and excessive water loss. For example, the skin epithelium protects the body from environmental damage.

2. Absorption

Some epithelial cells, especially those lining the small intestine, are specialized for absorption. They help absorb nutrients from digested food into the bloodstream.

3. Secretion

Epithelial tissues form glands that produce and secrete enzymes, hormones, mucus, sweat, and other substances. For instance, glandular epithelium in the pancreas secretes digestive enzymes.

4. Excretion

Epithelial tissues also help in the removal of waste products. For example, the epithelial lining of the kidney tubules plays a role in filtering and excreting waste materials in urine.

5. Filtration

In organs like the kidneys, epithelial cells filter substances from the blood to form urine, playing a crucial role in maintaining internal balance.

6. Sensory Reception

Certain epithelial cells are specialized for sensory functions. They are found in sensory organs like the nose, eyes, and ears, helping detect stimuli like smell, light, and sound.

These functions make epithelial tissue essential for maintaining homeostasis and protecting the body’s internal environment.

Summery table of Function of Epithelial tissues

Function	Description	Example
Protection	Shields underlying tissues from injury and pathogens	Skin epithelium
Absorption	Absorbs nutrients and fluids	Intestinal lining
Secretion	Produces enzymes, hormones, mucus, etc.	Glands (exocrine & endocrine)
Excretion	Eliminates waste	Kidney tubules
Filtration	Filters blood plasma	Glomeruli of kidneys
Sensory Reception	Detects changes in environment	Taste buds, retina

Examples and Locations

Type	Location
Simple squamous	Alveoli of lungs, blood vessels
Simple cuboidal	Kidney tubules, glands
Simple columnar	Lining of stomach and intestines
Stratified squamous	Skin, mouth, esophagus
Ciliated epithelium	Trachea, fallopian tubes
Glandular epithelium	Salivary glands, endocrine glands

Summary

1. Epithelial tissue is essential for protection, secretion, absorption, and sensory reception.

2. It is highly specialized in structure to match its functions.

3. Found covering all body surfaces, lining hollow organs, and forming glands.

📚 References

1. Tortora, G.J., & Derrickson, B. (2020). Principles of Anatomy and Physiology. Wiley.

2. Ross, M.H., & Pawlina, W. (2018). Histology: A Text and Atlas. Wolters Kluwer.

3. Ncert.nic.in - Class 11 Biology Textbook, Chapter 7: Structural Organisation in Animals.

4. Guyton, A.C., & Hall, J.E. (2021). Textbook of Medical Physiology. Elsevier.