Q.Distinguish between holocrine and apocrine glands.

Epithelial Tissue Types – A First Look

Imagine you are building a house. The first thing you need is a covering — a roof to keep the rain out, walls to hold the structure together, and a floor to walk on. In your body, epithelial tissue does exactly that: it covers every surface, lines every cavity, and forms the inner lining of every organ, blood vessel, and duct. It is the body's universal wrapping paper.

The key intuition is this: epithelial tissue is a sheet of cells with almost no extracellular material between them. They sit on a thin, non-living basement membrane (like a foundation) and have one free surface facing the outside world or a hollow space (the apical side) and one attached surface (the basal side). Because they cover and line, they are always exposed to wear and tear — so they regenerate rapidly.

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The Two Big Categories: Simple vs. Compound

The first split is based on how many layers of cells are present.

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The Three Shapes: Squamous, Cuboidal, Columnar

Now, within each category, cells come in three basic shapes. Think of them like building blocks:

• Squamous – flat, like a fried egg or a floor tile. Thin, perfect for diffusion.
• Cuboidal – cube-shaped, like dice. Good for secretion and absorption.
• Columnar – tall, like a pillar or a brick standing on end. Excellent for absorption and secretion, often with microvilli or cilia on top.

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Simple Epithelium – One Layer, Three Shapes

1. Simple Squamous Epithelium

Structure: Single layer of extremely flat cells. Nucleus looks like a bulge in the centre.

Function: Filtration, diffusion, osmosis — anything that needs to pass through quickly.

Locations: Lining of blood vessels (endothelium), lining of body cavities (mesothelium), air sacs of lungs (alveoli).

2. Simple Cuboidal Epithelium

Structure: Single layer of cube-shaped cells. Nucleus is round and central.

Function: Secretion and absorption.

Locations: Kidney tubules, thyroid follicles, salivary gland ducts, surface of ovaries.

3. Simple Columnar Epithelium

Structure: Single layer of tall, pillar-like cells. Nucleus is oval and usually near the base.

Function: Absorption and secretion. Often has microvilli (brush border) to increase surface area, or cilia to move mucus.

Locations: Stomach lining, intestinal lining (with microvilli), fallopian tubes (with cilia).

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Compound (Stratified) Epithelium – Multiple Layers, Same Shapes

1. Stratified Squamous Epithelium

Structure: Many layers; the topmost cells are flat (squamous), while deeper layers are cuboidal or columnar. The deepest layer (basal layer) keeps dividing to replace worn-out surface cells.

Function: Protection against abrasion, friction, and pathogens.

Two types:

• Keratinised – dry, tough, dead surface cells filled with keratin. Found in skin.
• Non-keratinised – moist, living surface cells. Found in mouth, oesophagus, vagina.

2. Stratified Cuboidal Epithelium

Structure: Two or more layers of cube-shaped cells. Rare.

Function: Protection and secretion.

Locations: Sweat gland ducts, developing ovarian follicles.

3. Stratified Columnar Epithelium

Structure: Multiple layers; top layer is columnar, deeper layers are cuboidal. Also rare.

Function: Protection and secretion. …