Transitional epithelium with umbrella cells and a specialised apical membrane
Expansion
Layers: basal, intermediate, and superficial umbrella (facet) cells. Appears 5 to 6 cells thick when relaxed and 2 to 3 when distended, although no cells are lost.
Umbrella cells are the specialisation that makes the urinary tract possible:
- Dome shaped when relaxed, flattening on distension
- Often binucleate
- Apical membrane of uroplakin plaques joined by hinges, forming a rigid, highly impermeable barrier
- Extensive tight junctions
- Discoid vesicles that fuse with the apical membrane to add surface area as the bladder fills
The result is a barrier that withstands the extreme osmolality, acidity and toxicity of urine while accommodating large volume change.
Extent: renal calyces, pelvis, ureters, bladder and proximal urethra. This entire surface shares the same carcinogen exposure, which is the basis of field change and explains why urothelial carcinoma is frequently multifocal and recurrent, and why follow-up cystoscopy is required.
Squamous metaplasia follows chronic irritation from stones, catheters or schistosomiasis, and predisposes to squamous carcinoma.