What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Transitional Epithelium: Structure and Function

Name: Transitional Epithelium: Structure and Function
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Transitional epithelium, also called urothelium, is a stratified epithelium unique to the urinary tract whose superficial cells change shape as the organ fills and empties. It lines the renal pelvis, ureter, urinary bladder, and proximal urethra, forming the blood-urine barrier that keeps concentrated urine from leaking into surrounding tissue.

Why this tissue matters in veterinary practice goes beyond histology slides. The urothelium is the main site of bladder cancer in animals and people, it is the first line of defense against ascending bacterial infection, and it is the layer that must regenerate after cystotomy or bladder wall reconstruction. Understanding its layered architecture explains why urine cytology looks the way it does, why urothelial tumors behave aggressively, and why a bladder can hold a large volume without its lining tearing.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

What Transitional Epithelium Is and Where It Is Found

Transitional epithelium is a stratified epithelial tissue in which the number of cell layers and the shape of the surface cells vary with the degree of distension of the organ. In a relaxed (contracted) bladder, the epithelium appears thick with many layers and a dome-shaped apical surface. In a distended bladder, the same epithelium appears thinner, with fewer apparent layers and flattened surface cells. This plasticity is the defining feature that separates transitional epithelia from every other epithelial type.

The transitional epithelial tissue lines a continuous surface from the kidney to the lower urinary tract:

Renal pelvis and major calyces
Ureter
Urinary bladder
Proximal urethra

Below the proximal urethra, the lining changes to stratified squamous epithelium, which is why the distal urethra is not considered urothelial. This transition matters in biopsy interpretation, because a pathologist sampling the urethra can recover two different epithelial types from the same organ.

The urothelium sits on a basement membrane and rests on a lamina propria of loose connective tissue. Beneath that lies the muscularis (the detrusor in the bladder). The epithelium, basement membrane, and lamina propria together form the urothelial mucosa, and the whole structure is built to tolerate extreme mechanical cycling.

The Three Layers of the Urothelium

The stratified urothelium is conventionally divided into three zones: basal, intermediate, and superficial. Spatial transcriptomic mapping of the mouse bladder at near single-cell resolution has confirmed these three populations as distinct clusters within the urothelium, with umbrella cells showing polarized messenger RNA distribution [1]. That molecular evidence supports what histologists have described for decades.

Basal Layer

The basal layer is a single row of small, cuboidal to low columnar cells resting directly on the basement membrane. These cells are the proliferative reservoir of the urothelium. They are relatively undifferentiated, contain few specialized organelles, and divide to replace cells lost from the surface. In normal urothelium, mitotic figures are uncommon and are found almost exclusively in this layer.

Intermediate Layer

The intermediate layer consists of several tiers of pear-shaped or fusiform cells that sit above the basal cells. These cells are connected to one another and to the basal and superficial layers by intercellular junctions. The intermediate cells are sometimes called "middle" or "wing" cells in older texts. They contribute to the barrier and act as a transit population between the proliferative basal cells and the terminally differentiated umbrella cells.

Superficial Umbrella Cells

Umbrella cells are the largest and most distinctive cells of the urothelium. They are large, often binucleate, and cover the underlying intermediate cells like a canopy, which is the origin of the name. They are the only urothelial cells in direct contact with urine, and they carry the specialized machinery that makes the blood-urine barrier work.

Umbrella cells are also the cells most often seen in urine sediment when the urothelium is irritated or shedding. In cytology, they appear as large, sometimes multinucleated cells with abundant cytoplasm. Their presence in urine is normal in low numbers and increases with inflammation, urolithiasis, or neoplasia.

The Blood-Urine Barrier: Uroplakin Plaques and the GAG Layer

The barrier function of transitional epithelium depends on two apical specializations of the umbrella cell: uroplakin plaques and a glycosaminoglycan (GAG) coating.

Uroplakin Plaques

The apical surface of mammalian bladder urothelium is covered by large two-dimensional crystals of hexagonally packed 16-nanometer uroplakin particles, called urothelial plaques [2]. These plaques are 500 to 1,000 nanometers across and are the structural basis of the permeability barrier. Uroplakins are major differentiation products of umbrella cells and also serve as receptors for uropathogenic Escherichia coli [3]. The asymmetric unit membrane (AUM) that forms the outer leaflet of the plaque is the ultrastructural hallmark of urothelium, and its appearance during development marks the transition from simple cuboidal to mature stratified transitional epithelium [4].

Glycosaminoglycan Layer

Overlying the plaques is a layer of glycosaminoglycans that coats the luminal surface. This GAG layer is hydrophilic and negatively charged, and it adds a second barrier against ions, solutes, and bacterial adhesion. Together, the uroplakin plaques and the GAG layer prevent urine from diffusing back into the bloodstream and prevent bacteria from gaining a foothold on the epithelial surface.

The barrier is not passive. The urothelium actively responds to infection. In a mouse model of cystitis caused by uropathogenic E. coli, orally administered gallium maltolate reached the transitional epithelium, the potential site of infection, and distributed similarly to iron in the tissue [5]. That study illustrates how the barrier and its surface chemistry are relevant to antimicrobial therapy.

Stretch Adaptation: How the Urothelium Changes Shape

The function of transitional epithelium is inseparable from its ability to accommodate volume. The bladder can expand many times its contracted size, and the epithelium must increase its surface area without rupturing or losing barrier integrity.

Folded Apical Membrane and Plaque Unfolding

In the relaxed bladder, the apical membrane of umbrella cells is thrown into folds, and the cytoplasm contains an abundant pool of discoidal and fusiform vesicles (DFVs) whose membranes are already loaded with uroplakin plaques [6]. As the bladder fills, the apical membrane unfolds and the DFVs fuse with the apical surface by exocytosis, inserting new plaque-bearing membrane and increasing surface area. When the bladder empties, the membrane is retrieved by endocytosis and the vesicles are rebuilt.

This cycle is regulated by a defined molecular machinery. RAB27B is required for stretch-induced exocytosis of DFVs, and knockdown of RAB27B significantly inhibits filling-induced exocytosis [6]. The apical delivery of fusiform vesicles involves a sequential and compartmentalized action of Rabs, SNAREs, and the protein MAL, with keratin 20 forming a subapical network that positions vesicles for fusion [3]. MAL facilitates incorporation of uroplakin-delivering vesicles into the apical membrane, and MAL knockout in vivo leads to accumulation of fusiform vesicles in umbrella cells [2]. Dynamin-2 is also expressed in umbrella cells and is involved in formation of discoid vesicles, and treatment of bladders with the dynamin inhibitor dynasore markedly reduces the number of these vesicles [7].

Mechanosensing

Umbrella cells sense mechanical stretch directly. Piezo1 channels in the apical membrane mediate calcium entry when the bladder stretches, and the resulting calcium signal regulates exocytosis, ATP release, and downstream purinergic signaling [8]. Mathematical modeling of stretch-induced membrane traffic in umbrella cells has been used to calibrate vesicle trafficking rates and to investigate how abnormalities in this machinery could contribute to bladder pathology [9]. In other words, the urothelium is not just a passive bag lining. It is a mechanosensory epithelium that reports fullness to the nervous system.

Comparison With Other Epithelial Types

Transitional epithelium is frequently confused with other stratified and pseudostratified epithelia. The table below summarizes the key differences in location, layering, and function.

Epithelium	Typical location	Layers	Surface cells	Main function
Transitional (urothelium)	Renal pelvis, ureter, bladder, proximal urethra	Stratified, 3 zones (basal, intermediate, umbrella)	Large, often binucleate umbrella cells with uroplakin plaques	Stretch-adapted barrier between urine and blood
Simple cuboidal	Kidney tubules, many ducts, glandular acini	Single layer	Cuboidal	Absorption and secretion
Stratified squamous	Skin, oral cavity, esophagus, vagina, distal urethra	Stratified, many layers	Flattened squamous cells, often keratinized	Protection against abrasion
Pseudostratified columnar	Trachea, bronchi, epididymis	Single layer, nuclei at different heights	Columnar, often ciliated	Mucus transport and protection

The distinction that matters most in practice is between transitional epithelium and stratified squamous epithelium. Both are stratified, but only transitional epithelium has umbrella cells and uroplakin plaques, and only transitional epithelium changes its apparent layering with distension.

How Transitional Epithelium Is Studied and Observed

Histology

On hematoxylin and eosin stained sections, relaxed urothelium shows a thick epithelium with a scalloped or dome-shaped apical border created by the umbrella cells. Distended urothelium looks thinner and flatter. The basement membrane is thin and the lamina propria is loose. Histology reference collections provide labeled micrographs of transitional epithelia for comparison, including the characteristic umbrella cell layer [10].

Electron Microscopy

Transmission electron microscopy reveals the asymmetric unit membrane of the plaques and the fusiform vesicles in the apical cytoplasm. Scanning electron microscopy shows the microridges and ropy surface patterns that develop as the urothelium differentiates. During rat development, the urothelium is simple cuboidal from gestational day 15 to 17, and small discoid vesicles lined with AUM appear in the apical cytoplasm of surface cells at day 17, followed by increasing density of microridges and larger fusiform vesicles [4].

Cell Harvest and Cytology

Because the transitional epithelium is only a small fraction of the whole bladder, selective harvest techniques have been developed for research. An enzymatic stripping method using trypsin-EDTA incubation plus gentle scraping selectively harvests transitional cells from rat bladders, and cells from a single bladder are sufficient for multiple Comet assays to measure DNA damage [11]. In clinical cytology, transitional epithelial cells in urine are identified by their large size, abundant cytoplasm, and sometimes binucleate appearance.

Immunohistochemistry

Markers such as 14-3-3σ, p53, vimentin, COX-1, COX-2, and LOX-5 have been used to characterize normal and neoplastic urothelium. Normal urinary bladder epithelium shows high levels of 14-3-3σ, and expression is decreased in a majority of canine transitional cell carcinomas [12]. COX-1 is constitutively expressed in normal canine bladder epithelium, while COX-2 is not expressed in normal epithelium but is expressed in neoplastic epithelium in primary and metastatic lesions [13]. LOX-5 is expressed in 95% of canine transitional cell carcinomas, 23% of cystitis cases, and 10% of controls [14]. These patterns are used in research and increasingly in diagnostic pathology.

Comparative and Species Notes

Urothelial thickness varies across species and with the degree of bladder distension. The general architecture, basal, intermediate, and umbrella layers with uroplakin plaques, is conserved across mammals, which is why rodent and canine models are used to study human urothelial disease. Canine transitional cell carcinoma is recognized as a relevant model of human urothelial carcinoma [12]. The World Health Organization and International Society of Urological Pathology consensus classification of urothelial tumors has been applied directly to the dog, although canine tumors differ from human tumors in that most show extensive glandular differentiation or metaplasia, and hyperplastic lesions tend to be more florid than in people [15].

Species differences in absolute urothelial thickness are documented in the comparative histology literature, but the functional organization is consistent. What varies more than thickness is the tendency toward metaplastic change. In dogs, squamous metaplasia of the urothelium can occur with chronic irritation, and it must not be mistaken for normal transitional epithelium on biopsy.

Clinical Relevance, Limitations and Common Mistakes

Transitional Cell Carcinoma

Transitional cell carcinoma (TCC), also called urothelial carcinoma, is the most common canine urinary tract tumor and mimics human invasive TCC [14]. The urothelium is the main site of bladder cancer, and transitional cells are recommended over whole bladder tissue for carcinogenesis and genotoxicity studies [11]. In F344 rats, dimethylarsinic acid in drinking water changes the morphology of bladder transitional epithelium, with swollen mitochondria appearing to be the main source of vacuoles, although messenger RNA levels of several DNA repair genes were not altered [16]. This illustrates how the urothelium can be a target of environmental carcinogens.

Urinary Tract Infection

The urothelium is the first barrier against ascending bacterial infection. Uropathogenic E. coli can bind uroplakin UPIa, which serves as a receptor [3], and can be encased into umbrella cells during infection, with dynamin-2 enriched in the bacteria-encasing compartments [7]. This intracellular niche helps explain why urinary tract infections can persist or recur.

Regeneration After Surgery

The urothelium has strong regenerative capacity. In dogs undergoing partial bladder wall substitution with autologous tunica vaginalis, regenerated transitional epithelium completely covered the substituted portion and smooth muscle regeneration was present at 6 weeks [17]. This is clinically relevant for surgeons planning bladder reconstruction.

Common Mistakes

The most frequent student error is confusing transitional epithelium with stratified squamous epithelium. Both are stratified, but only transitional epithelium has umbrella cells and uroplakin plaques, and only transitional epithelium changes its apparent layering with distension. A second error is confusing normal urothelium with squamous metaplasia. Squamous metaplasia is a pathological change in which the urothelium is replaced by squamous epithelium, often in response to chronic irritation. It is not a normal variant and it does not have umbrella cells. A third error is assuming that transitional epithelial cells in urine always indicate disease. Small numbers are normal, and increased numbers can reflect inflammation, calculi, catheterization, or neoplasia. A fourth error is treating the urothelium as a passive barrier. It is an active, mechanosensory, secretory, and regenerative tissue.

Individual cases require veterinary assessment. This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Review

Transitional epithelium (urothelium) is a stratified epithelium unique to the renal pelvis, ureter, bladder, and proximal urethra.
It has three zones: basal (proliferative), intermediate (transit), and superficial umbrella cells (barrier).
Umbrella cells are large, often binucleate, and carry uroplakin plaques and a GAG layer that form the blood-urine barrier.
The apical membrane unfolds and fusiform vesicles fuse with the surface as the bladder fills, increasing surface area without losing barrier integrity.
RAB27B, RAB11A, RAB8A, MAL, dynamin-2, and keratin 20 coordinate vesicle trafficking in umbrella cells.
Piezo1 channels transduce stretch into calcium signals that regulate exocytosis and ATP release.
Transitional cell carcinoma is the most common canine urinary tract tumor, and the urothelium is its site of origin.

Frequently Asked Questions

What is transitional epithelium?

Transitional epithelium is a stratified, stretch-adapted epithelium that lines the urinary tract from the renal pelvis to the proximal urethra. Its superficial umbrella cells carry uroplakin plaques and a glycosaminoglycan layer that form the blood-urine barrier.

What is the function of transitional epithelium?

Its main function is to form a distensible, impermeable barrier that separates urine from the bloodstream while accommodating large changes in organ volume. It also participates in mechanosensing, innate defense against bacteria, and regeneration after injury.

Why are transitional epithelial cells found in urine?

Small numbers of transitional epithelial cells are normally shed into urine as the urothelium turns over. Increased numbers can occur with inflammation, urolithiasis, catheterization, or urothelial neoplasia, so the finding is interpreted alongside the clinical picture.

How does transitional epithelium stretch?

In the relaxed state, the apical membrane of umbrella cells is folded and the cytoplasm holds a pool of uroplakin-loaded fusiform vesicles. As the bladder fills, the membrane unfolds and the vesicles fuse with the apical surface, adding membrane area. When the bladder empties, the membrane is retrieved by endocytosis.

What are umbrella cells?

Umbrella cells are the large, often binucleate superficial cells of the urothelium. They are the only urothelial cells in contact with urine and they carry the uroplakin plaques and GAG layer that create the permeability barrier.

Is transitional epithelium the same as squamous epithelium?

No. Both are stratified, but transitional epithelium has umbrella cells and uroplakin plaques and changes its apparent layering with distension. Squamous epithelium has flattened surface cells, is often keratinized, and lines sites such as skin, oral cavity, and vagina.