Histology: Epithelium & Glands Classification

Epithelium is the tissue that covers surfaces, lines cavities and forms glands - from your skin to the lining of your gut. Its cells sit packed edge-to-edge on a basement membrane, with almost no space between them.

It matters because nearly every barrier and secretion in the body is epithelial: skin keeps germs out, the gut lining absorbs food, and glands release everything from sweat to hormones.

Key Concepts

• Simple epithelium: a single cell layer, built for absorption, filtration and secretion (gut lining, kidney tubules).
• Stratified epithelium: multiple layers, built for protection against abrasion (skin, oesophagus).
• Cell shapes: squamous (flat), cuboidal (cube-like), columnar (tall); transitional epithelium stretches (bladder).
• Keratinised vs non-keratinised: skin's surface is keratinised and waterproof; mouth and oesophagus are non-keratinised and moist.
• Basement membrane: the sheet anchoring epithelium to underlying connective tissue.
• Exocrine glands: release via ducts (sweat, salivary). Endocrine glands: ductless, secreting into blood (thyroid, pituitary).
• Secretion modes: merocrine (exocytosis), apocrine (pinched-off cell apex), holocrine (whole cell disintegrates, as in sebaceous glands).

How It Works

• Absorption: microvilli on simple columnar cells of the small intestine multiply the surface area for nutrient uptake; the gut lining renews every 3-5 days.
• Protection: stratified squamous epithelium sheds its surface cells as new ones rise from below, so abrasion never reaches living tissue.
• Secretion: gland cells package products in vesicles and release them by merocrine, apocrine or holocrine mechanisms.
• Filtration: simple squamous endothelium lines blood vessels and the kidney's Bowman's capsule, letting fluids and small solutes pass.

Worked Example

You are shown an unlabeled slide and must identify the epithelium.

1. Step 1: the cells are tall with oval nuclei near their bases - columnar.
2. Step 2: there is only one cell layer sitting on the basement membrane - simple.
3. Step 3: pale mucus-filled goblet cells are scattered among the tall cells.
4. Conclusion: simple columnar epithelium with goblet cells - the lining of the small intestine, built for absorption plus lubrication. Structure predicts function.

Common Misconceptions

• Epithelium has its own blood vessels. It is avascular; nutrients diffuse from the underlying connective tissue, which is why deep ulcers heal slowly.
• All glands have ducts. Endocrine glands lost theirs during development and secrete directly into the bloodstream.
• Keratinised means the whole layer is dead. Only the surface stratum corneum is dead; the deeper layers are alive and constantly dividing.

Real-World Applications

• Pathology: carcinomas are cancers of epithelium - the commonest human cancers (skin, breast, lung, colon).
• Pap smears screen cervical epithelium for precancerous changes.
• Burn depth is judged by which epithelial layers survive, deciding grafting.
• Wound healing and skin grafts depend on epithelial regeneration from wound edges.

Learn to read epithelium - its layers, shapes and glands - and you can predict what a tissue does, how it heals, and how it fails in disease.