Epithelial Tissue

EPITHELIAL TISSUE

Epithelial tissue covers:

• Body surfaces
• Internal organs
• Body cavities
• Blood vessels
• Glandular structures

Thus, epithelial tissue acts as a protective covering and also forms the functional units of glands.

CHARACTERISTICS OF EPITHELIAL TISSUE

1. Cells form continuous sheets
   – Tightly packed like tiles, with very little space between them.
2. Apical Surface
   – The top (free) surface faces a cavity or the outside of the body.
3. Basement Membrane
   – The lower surface attaches to underlying connective tissue.
4. Avascular
   – Contains no blood vessels; nutrients diffuse from nearby connective tissue.
5. Rapid regeneration
   – Repairs and replaces cells quickly due to constant wear and tear.

FUNCTIONS OF EPITHELIAL TISSUE

• Protects the body from friction, dehydration, and injury
• Allows selective exchange of chemicals
• Secretes hormones into the bloodstream
• Produces sweat, mucus, enzymes, and other secretions

TYPES OF EPITHELIAL TISSUE

Epithelial tissues are classified based on:

1. Based on Arrangement of Layers

i)        Simple epithelium

ii)      Stratified epithelium

i)                  Simple Epithelium – Types

• Simple squamous epithelium
• Simple cuboidal epithelium
• Simple columnar epithelium
• Simple ciliated epithelium
• Glandular epithelium

Glandular Epithelium – Types

·         Unicellular glands

·         Multicellular glands

o    Exocrine glands

o    Endocrine glands

ii)               Stratified Epithelium – Types

• Stratified squamous epithelium
• Keratinised stratified squamous epithelium
• Non-keratinised stratified squamous epithelium
• Stratified cuboidal epithelium
• Stratified columnar epithelium
• Transitional epithelium
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2. Based on Shape of Cells

• Squamous epithelium (flat)
• Cuboidal epithelium (cube-like)
• Columnar epithelium (tall)
• Ciliated epithelium
• Glandular epithelium

SIMPLE EPITHELIUM

Cells are arranged in a single layer.
Their thinness makes them ideal for absorption, filtration, secretion, and diffusion.

Types of Simple Epithelium

1. Simple Squamous Epithelium

• Single layer of flat, thin cells
• Allows easy diffusion of gases and nutrients

Location:

• Air sacs of lungs
• Lining of blood & lymph vessels
• Heart lining

Function:

• Lubrication
• Diffusion
• Filtration

2. Simple Cuboidal Epithelium

• Single layer of cube-shaped cells
• Often involved in secretion and absorption

Location:

• Kidney tubules
• Small gland ducts

Function:

• Secretion
• Absorption

3. Simple Columnar Epithelium

• Tall, pillar-shaped cells
• May contain microvilli or goblet cells (mucus-secreting)

Location:

• Digestive tract lining (non-ciliated)
• Uterine tubes, uterus, bronchi (ciliated)

Function:

• Absorption
• Secretion of mucus and enzymes

4. Simple Ciliated Epithelium

• Columnar cells with cilia on the free surface
• Cilia move substances like mucus or ovum

Location:

• Trachea, upper respiratory tract
• Fallopian tubes
• Spinal cord canal

Function:

• Moves mucus or reproductive cells

5. Glandular Epithelium

• Specialized for secretion
• Made of cuboidal or columnar cells containing secretory granules

Types of Glands:

Exocrine glands: Secrete enzymes, sweat, saliva
Endocrine glands: Secrete hormones directly into blood

STRATIFIED EPITHELIUM

Also called compound epithelium.
It has many layers of cells and mainly provides protection.

It usually lacks a distinct basement membrane due to multiple layers.

Types of Stratified Epithelium

1. Stratified Squamous Epithelium

Has many layers:

• Bottom layers → cuboidal/columnar
• Top layers → flattened (squamous)

Two forms:

a) Keratinised Stratified Squamous Epithelium

• Contains keratin, a tough waterproof protein
• Found on dry surfaces exposed to friction

Location: Skin, hair, nails

b) Non-keratinised Stratified Squamous Epithelium

• Moist surfaces
• Protects from friction and prevents drying out

Location:

• Mouth
• Pharynx
• Esophagus
• Vagina
• Conjunctiva

Function (both forms):

• Protection from wear and tear

2. Stratified Cuboidal Epithelium

• Two or more layers
• Apical layer contains cuboidal cells

Location:

• Sweat gland ducts
• Male urethra
• Uterus and anus

Function:

• Protection
• Secretion
• Absorption

3. Stratified Columnar Epithelium

• Several layers
• Apical layer contains columnar cells

Location:

• Large excretory gland ducts
• Conjunctiva of the eye
• Parts of urethra

Function:

• Protection
• Secretion

4. Transitional Epithelium

• Cells are pear-shaped and stretchable
• Looks cuboidal when relaxed but squamous when stretched

Location:

• Urinary bladder
• Ureters

Function:

• Allows stretching
• Protects underlying tissues