Transitional epithelium (urothelium)
Urothelium; epithelium transitionale
Also known as: urothelium, umbrella cells, uroplakins
A stratified epithelium unique to the urinary tract whose large dome-shaped surface "umbrella" cells flatten as the organ fills, forming a stretchable, urine-tight barrier.
Overview
When the bladder is empty the urothelium is ≈5–7 cells thick: basal cuboidal cells, several layers of polyhedral intermediate cells and a surface layer of large, rounded umbrella (dome, superficial) cells, some binucleate. When the bladder fills, cells slide past one another and the umbrella cells flatten, so the epithelium thins to ≈2–3 layers and resembles stratified squamous epithelium.
Umbrella cells provide the blood–urine barrier: their apical membrane is covered by rigid plaques of uroplakins (the "asymmetric unit membrane"), which are reversibly internalised into fusiform vesicles as the bladder empties and re-inserted as it fills, and their tight junctions are very tight. The barrier prevents water and toxic solutes in hypertonic urine from leaking into the tissues.
- Location
Renal calyces and pelvis, ureters, urinary bladder, proximal urethra (prostatic urethra in males).
Anatomy
- Histology
Many layers when relaxed; large pale dome-shaped surface cells, sometimes binucleate, bulging into the lumen; no keratin.
Function
- Distensible lining that accommodates large volume changes.
- Impermeable barrier to urine.
Clinical significance
Urothelial carcinoma
The commonest bladder cancer, associated with smoking (most important), aromatic amines (aniline dyes, rubber industry), cyclophosphamide and phenacetin; presents with painless haematuria. Multifocal recurrence ("field effect") throughout the urothelium is typical.
Squamous metaplasia and carcinoma
Chronic irritation — Schistosoma haematobium infection, long-term catheters, stones — can induce squamous metaplasia and squamous cell carcinoma of the bladder. Cyclophosphamide also causes haemorrhagic cystitis (acrolein; prevented by mesna).
Test yourself
The epithelium lining the urinary bladder, which can change from about six cell layers to two or three as it fills, is:
- Stratified squamous non-keratinised
- Transitional epithelium (urothelium)
- Pseudostratified columnar
- Stratified cuboidal
Answer: Transitional epithelium (urothelium)
Urothelium has large dome-shaped umbrella cells with uroplakin plaques and tight junctions that make an impermeable blood–urine barrier and flatten when the bladder is distended.
Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.