Simple or stratified, then squamous, cuboidal or columnar
Mnemonic
Classified by shape and by number of layers:
Layers: simple (one), stratified (many), pseudostratified (one layer that looks like several), transitional (urothelium).
Shape: squamous (flat), cuboidal, columnar.
Then each combination has its site:
- Simple squamous - alveoli, endothelium, mesothelium. Diffusion
- Simple cuboidal - kidney tubules, thyroid follicles, ducts
- Simple columnar - gut, gallbladder. Absorption and secretion
- Pseudostratified ciliated columnar - respiratory tract. Mucociliary clearance
- Stratified squamous keratinised - skin. Protection against abrasion and drying
- Stratified squamous non-keratinised - mouth, oesophagus, vagina
- Transitional (urothelium) - bladder and ureters. Stretch
“Thin for diffusion, tall for absorption, layered for protection, transitional for stretch.” Structure follows function in every case.
Expansion
Two-part naming: number of layers, then shape of the surface cells.
| Type | Location | Function |
|---|---|---|
| Simple squamous | Alveoli, endothelium, mesothelium | Diffusion, filtration |
| Simple cuboidal | Thyroid follicles, renal tubules, ducts | Secretion, absorption |
| Simple columnar | Stomach, intestine, gallbladder | Absorption, secretion |
| Pseudostratified columnar ciliated | Trachea and bronchi, epididymis | Mucociliary clearance |
| Stratified squamous, non-keratinised | Oesophagus, mouth, vagina, anal canal below pectinate line | Protection from abrasion |
| Stratified squamous, keratinised | Skin | Protection, waterproofing |
| Transitional (urothelium) | Bladder, ureter, renal pelvis | Distension |
| Stratified cuboidal | Sweat gland ducts | Ducts |
Pseudostratified epithelium looks layered because nuclei sit at different heights, but every cell touches the basement membrane, which is the defining feature.
Transitional epithelium has dome-shaped umbrella cells when relaxed that flatten when the bladder fills.
Every epithelium is avascular and sits on a basement membrane, receiving nutrition by diffusion from the underlying connective tissue, which is why breach of the basement membrane defines invasion in carcinoma.