Stratified Epithelium

Object: Examination of Stratified Epithelium

General Principle

Stratified epithelium has multiple cell layers — only the basal layer contacts the basement membrane. The surface cell shape defines the type. Primary function: protection from mechanical abrasion, chemical injury, and desiccation.

A — Stratified Squamous Epithelium

Non-Keratinised

Surface cells are flattened but retain their nuclei and remain viable. Found where a moist, smooth surface is needed: oral cavity, pharynx, oesophagus, vagina, ectocervix, conjunctiva. Layers:

• Stratum basale: columnar/cuboidal, mitotically active
• Stratum spinosum: polygonal cells with desmosomes
• Surface: flat nucleated squames

Keratinised (Epidermis)

Found only in skin. Surface cells are anucleate, filled with hard keratin. Five layers: stratum basale, spinosum, granulosum, lucidum (thick skin only), corneum. Provides a complete waterproof barrier.

B — Stratified Cuboidal Epithelium

Two or more layers of cuboidal cells. Rare — found in large ducts of exocrine glands (sweat gland ducts, salivary gland ducts). Provides mechanical protection for duct walls while allowing secretion transport.

C — Stratified Columnar Epithelium

Multiple layers; surface cells are columnar. Very rare — found in largest ducts of mammary glands, male urethra (membranous), and conjunctiva near lid margin. Often transitional between other epithelial types.

D — Transitional Epithelium (Urothelium)

Structure

Unique to the urinary tract (renal pelvis, ureter, urinary bladder, proximal urethra). Designed to accommodate stretch and recoil. Three layers:

• Basal layer: small cuboidal cells on basement membrane
• Intermediate layer: polygonal cells, 1–3 layers depending on distension
• Superficial layer (umbrella/facet cells): large, dome-shaped cells with a thick apical plasma membrane (asymmetric unit membrane — AUM) resistant to hypertonic urine; may be binucleate

Relaxed vs Distended States

• Relaxed (empty bladder): appears thick — 6–8 cell layers; cells rounded; umbrella cells dome-shaped
• Distended (full bladder): cells flatten; appears only 2–3 layers; umbrella cells spread flat

Key Histological Features

• Dome-shaped umbrella cells — most distinctive feature
• Thick apical membrane (AUM) visible at EM
• Underlying lamina propria rich in elastic fibres (allows recoil)
• No goblet cells, no brush border