🧫 Epithelial Tissues

Epithelial tissue forms the protective covering of the body and the inner lining of organs, tubes, and cavities. Think of epithelium as the body's “surface + lining sheet”.

✅ Where is it found?

  • Outer surface: skin
  • Inner lining: mouth, oesophagus, stomach, intestine, trachea, urinary bladder
  • Covers organs: liver surface, kidneys surface
  • Forms glands: sweat glands, salivary glands, thyroid, etc.

🌿 1️⃣ Characteristics of Epithelial Tissue

(A) Cells are tightly packed

  • Cells are very close to each other.
  • Intercellular space is minimal, and there is very little matrix.

(B) Basement membrane (very important concept)

  • Every epithelium rests on a thin, supportive sheet called the basement membrane.
  • It attaches epithelium to connective tissue below.
  • Helps in support, filtration (kidney), and regeneration.

(C) Avascular but nourished

  • Epithelium has no blood vessels.
  • It gets nutrition by diffusion from the connective tissue below.

(D) Polarity

  • Epithelial cells have:
  • Apical surface (towards lumen/outside)
  • Basal surface (towards basement membrane)
  • This polarity helps in functions like absorption and secretion.

(E) Regeneration

  • Epithelial cells divide frequently → heals quickly.
  • Example: mouth lining heals faster than muscle.

🌱 2️⃣ Classification of Epithelium

Epithelial tissue is mainly classified into:

  1. Simple epithelium → single layer (absorption, diffusion, secretion)
  2. Compound/Stratified epithelium → many layers (protection)

(A) Simple Epithelium (Single Layer)

Best for exchange, absorption, secretion because substances can pass easily.

1. Simple Squamous Epithelium

  • Flat, tile-like cells.
  • Works like a thin membrane.

✅ Functions:

  • Diffusion (gases)
  • Filtration (fluid movement)

✅ Locations:

  • Alveoli of lungs (gas exchange)
  • Capillaries (endothelium)
  • Bowman's capsule in kidney

2. Simple Cuboidal Epithelium

  • Cube-shaped cells, round nucleus.

✅ Functions:

  • Secretion + absorption

✅ Locations:

  • Kidney tubules
  • Ducts of glands

3. Simple Columnar Epithelium

  • Tall cells, nucleus usually near the base.

✅ Functions:

  • Absorption (especially in intestine)
  • Secretion (mucus, enzymes)

✅ Special feature:

  • Often has microvilli (brush border) to increase absorption.

4. Ciliated Epithelium (Usually ciliated columnar)

  • Columnar cells with cilia on the apical surface.

✅ Function:

  • Moves substances in one direction.

✅ Locations:

  • Trachea: moves mucus + trapped dust upwards
  • Oviduct: helps movement of ovum

5. Glandular Epithelium

  • Epithelium specialized for secretion.

✅ Types:

  • Unicellular gland: Goblet cell (mucus in intestine)
  • Multicellular glands: Salivary, sweat, gastric glands

(B) Compound/Stratified Epithelium (Many layers)

  • Main function: Protection from wear and tear.

Stratified Squamous Epithelium

✅ Two types:

  1. Keratinized → skin (dry surface)
  2. Non-keratinized → buccal cavity, oesophagus (moist)

🌸 3️⃣ Special Surface Modifications

✅ Microvilli

  • Tiny finger-like folds
  • Increase surface area
  • Seen in intestine and kidney tubules

✅ Cilia

  • Hair-like projections
  • Help in movement of mucus/ovum

✅ Goblet cells

  • Unicellular glands
  • Secrete mucus for lubrication and protection

🌾 4️⃣ Summary of Functions

  • Protection: skin, mouth lining
  • Absorption: intestine
  • Secretion: glands
  • Filtration: kidney
  • Transport/Movement: ciliated epithelium
  • Sensory function: some epithelium has sensory receptors

🧠 Quick Revision Table

Type Key feature Main function Common location
Simple squamous flat cells diffusion/filtration alveoli, capillaries
Simple cuboidal cube cells secretion/absorption kidney tubules
Simple columnar tall cells absorption/secretion intestine
Ciliated cilia present movement trachea, oviduct
Stratified squamous many layers protection skin, mouth

💡 Questions and Answers

Q1. What is epithelial tissue?

A1.

Epithelial tissue is a sheet of tightly packed cells that covers the body surface and lines internal organs, cavities, and ducts. It mainly provides protection, absorption, secretion, filtration, and transport.

Q2. What is the basement membrane and why is it important?

A2.

The basement membrane is a thin, supportive layer between epithelium and connective tissue. It helps in anchoring epithelium, acts as a filter (kidney), and supports regeneration of epithelial cells.

Q3. Why do intestinal epithelial cells have microvilli?

A3.

Microvilli increase the surface area of the intestinal lining, which greatly improves absorption of digested nutrients.

Q4. Where is ciliated epithelium found and what is its function?

A4.

Ciliated epithelium is found in the trachea and oviduct. In the trachea it moves mucus and dust upward; in the oviduct it helps move the ovum toward the uterus.

Q5. What is keratinization and why is it useful?

A5.

Keratinization is the deposition of keratin protein in surface epithelial cells of the skin. It makes the surface tough, waterproof, and protective, preventing dehydration and mechanical injury.

Q6. Why are epithelial tissues called avascular?

A6.

Epithelial tissues do not contain blood vessels. They receive oxygen and nutrients by diffusion from the underlying connective tissue through the basement membrane. This is why damage to deeper connective tissue can slow epithelial healing.

Q7. Name the epithelial tissue found in alveoli of lungs and explain why it is suitable.

A7.

Simple squamous epithelium lines the alveoli. It is extremely thin (single layer of flat cells), so gases like oxygen and carbon dioxide can diffuse quickly across it during respiration.

Q8. Differentiate between ciliated epithelium and glandular epithelium with examples.

A8.

  • Ciliated epithelium has cilia that beat rhythmically to move substances (e.g., mucus in trachea, ovum in oviduct).
  • Glandular epithelium is specialized for secretion (e.g., salivary glands secrete saliva; goblet cells secrete mucus).