Every Surface Is Lined
Epithelium covers every body surface, lines every cavity, and forms every gland. Its defining features follow from that job: cells packed tightly with almost no matrix between them, resting on a basement membrane, avascular so nutrients must diffuse from the tissue below, and polarised with a distinct apical and basal face.
Epithelia are classified twice over, and the classification predicts function.
By layers: simple is one cell thick and suits exchange; stratified is many cells thick and suits protection; pseudostratified is one layer that looks like several because the nuclei sit at different heights.
By cell shape: squamous cells are flat, cuboidal are cube-shaped, columnar are tall.
| Epithelium | Where | Why |
|---|---|---|
| Simple squamous | Alveoli, capillaries | Thin, so diffusion is fast |
| Simple cuboidal | Kidney tubules, glands | Secretion and absorption |
| Simple columnar | Gut lining | Absorption, with room for organelles |
| Stratified squamous | Skin, oesophagus | Withstands abrasion |
| Pseudostratified ciliated | Airways | Moves mucus by ciliary beating |
The pattern is consistent: thin where exchange matters, thick where protection matters. An alveolus one cell thick allows oxygen across in fractions of a second; skin many cells thick sacrifices exchange entirely for durability.
Apical surfaces are specialised. Microvilli multiply absorptive area, the intestinal brush border increases it around 20-fold, and combined with villi and folds the small intestine achieves roughly 200 m² from a tube a few centimetres wide. Cilia move material along a surface. Tight junctions seal neighbouring cells so nothing leaks between them.
Glands are epithelium that has sunk inward and specialised in secretion. Exocrine glands keep a duct and secrete onto a surface, sweat, saliva, digestive enzymes. Endocrine glands lose their duct and secrete hormones into the blood, which is why they are richly vascularised while the epithelium they came from is not.
Skin combines the two tissue types: a stratified squamous epidermis for protection over a connective-tissue dermis carrying blood vessels, nerves and glands.
Common pitfall: forgetting that epithelium is avascular. Every nutrient reaches it by diffusion from the connective tissue beneath, which puts a hard ceiling on how thick a living epithelium can be, and is why the outer layers of skin are dead.