Urothelium - an overview | ScienceDirect Topics (2023)

Sensor-Transducer Function of the Urothelium

Whereas the urothelium has historically been viewed primarily as a barrier, there is increasing evidence that urothelial cells display a number of properties similar to sensory neurons (nociceptors and mechanoreceptors) and that both types of cells use diverse signal-transduction mechanisms to detect physiologic stimuli. Examples of “sensor molecules” (i.e., receptors and ion channels) associated with neurons that have been identified in urothelium include receptors for bradykinin (Chopra et al., 2005), neurotrophins (TrkA and p75) (Murray et al., 2004), purines (P2X and P2Y) (Birder et al., 2004;Chopra et al., 2008;Hu et al., 2002;Lee et al., 2000;Sun and Chai, 2004;Tempest et al., 2004), norepinephrine (α and β) (Birder and de Groat, 1998;Birder et al., 1998, 2002), ACh (nicotinic and muscarinic) (Beckel et al., 2006;Chess-Williams, 2002;Kullmann et al., 2008b) protease-activated receptors, amiloride-mechanosensitive Na⁺ channels such as ENaC (Araki et al., 2004;Smith et al., 1998;Wang et al., 2003), and a number of transient receptor potential (TRP) channels (TRPV1, TRPV2, TRPV4, TRPM8) (Birder and de Groat, 1998;Birder et al., 2001, 2002, 2007a, 2007b;Gevaert et al., 2007;Stein et al., 2004).

When urothelial cells are activated through these receptors and ion channels in response to mechanical as well as chemical stimuli, they can, in turn, release chemical mediators such as NO, ATP, ACh, and substance P (SP) (Birder and de Groat, 1998;Birder et al., 1998, 2003;Burnstock, 2001a, 2001b;Chess-Williams, 2004;Ferguson et al., 1997). These agents are known to have excitatory and inhibitory actions on afferent nerves that are close to or in the urothelium (Bean et al., 1990;Birder et al., 2001;Yoshimura et al., 2008).Video 110.1

Chemicals released from urothelial cells may act directly on afferent nerves or indirectly through an action on suburothelial interstitial cells (also referred to asmyofibroblasts) that lie in close proximity to afferent nerves. Myofibroblasts are extensively linked by gap junctions and can respond to chemicals that in turn modulate afferent nerves (Fowler et al., 2008). Thus it is believed that urothelial cells and myofibroblasts can participate in sensory mechanisms in the urinary tract by chemical coupling to the adjacent sensory nerves.

NO can be released by the urothelium, particularly during inflammation (Birder and de Groat, 1998;Birder et al., 1998). The release of NO may be evoked by the calcium ionophore A-23187, norepinephrine, and capsaicin. SP also acts on receptors on urothelial cells to release NO. The adrenergic release of NO from bladder strips was reduced by 85% after removal of the urothelium. Denervation of the bladder did not completely block the release of capsaicin-induced NO production, suggesting other sites of production. This is consistent with the observations that capsaicin released NO from cultured rat, cat, rabbit, and human urothelial cells and that the TRPV1 capsaicin receptor is expressed in cultured urothelial cells. NOS expression in afferent neurons is also increased in chronic bladder inflammation. Given that NO does not have much effect on the detrusor muscle but does inhibit Ca²⁺ channels in rat bladder afferent neurons (Yoshimura et al., 2001),the role of NO in the urothelium has still to be clarified. However,NO released locally in the bladder appears to have an inhibitory effect on afferent activity in the bladder because suppression of endogenous NO by intravesical oxyhemoglobin, an NO scavenger, or L-NAME, a NOS inhibitor, enhances bladder activity in rats (Masuda et al., 2007;Pandita et al., 2000).

2.5 Urothelium

The urothelium is a unique, highly specialized epithelium lining the lower urinary tract. It has a variable number of cell layers, but in the rodent urinary bladder it is usually three cell layers thick in the urinary bladder and more in the ureter. In larger mammals, such as dogs, monkeys, and humans, the urothelium frequently has more than three layers, occasionally up to ten or more (Figure48.3). There are distinct cell types in the urothelium, with the basal cell layer composed of cuboidal cells resting on a basement membrane attached via hemidesmosomes. Intermediate cells tend to be somewhat larger, and if the urothelium has more than three cell layers it is because of multiple layers of intermediate cells.

Flat Urothelial Lesions With Atypia

Historically, flat urothelial lesions with atypia were classified at different centers using variable terminology, but the WHO/ISUP 2016 classification (based on modifications from WHO/ISUP 1998)²¹² is now more uniformly utilized throughout the world. The diagnostic categories includereactive urothelial atypia,urothelial atypia of unknown significance,urothelial dysplasia, andurothelial carcinoma in situ (CIS). The most important diagnostic thresholds are those for definitive reactive changes and definitive CIS.

Reactive urothelial atypia is frequently associated with acute and chronic inflammation from a variety of causes (e.g., infection, indwelling catheter, calculi).²⁴¹ The inflammatory cell infiltrate may involve the lamina propria, but intraurothelial inflammation is most common. Urothelium with reactive atypia often has a basophilic appearance; their nuclei are enlarged and may be somewhat rounded, but they are typically monomorphic without significant variation in size and shape. In general, the nuclei are no larger than the size of approximately three lymphocytes. Importantly, the nuclear chromatin remains fine, but prominent nucleoli (sometimes multiple) are not infrequent. Mitoses may be frequent and can involve the upper levels of the urothelium, especially in patients with recent indwelling catheters.

Urothelial CIS is histologically heterogeneous, but there are general over-reaching features (Fig. 25.13).²⁴² Compared with normal urothelium, nuclei are often more rounded and enlarged, often greater than 4–5 times the size of a lymphocyte.²⁴³ The urothelial cells, in contrast to reactive atypia, become disorganized, losing their orientation perpendicular to the basement membrane. The nuclear chromatin becomes coarse, nuclear membranes are often irregular, and nucleoli may be present or absent. Mitotic figures are often easily seen and may be atypical. Unlike many other anatomic sites, the nuclear-to-cytoplasmic ratio of the neoplastic cells may not be increased as abundant cytoplasm is often preserved. Diverse patterns of CIS may be seen and include undermining growth, pagetoid spread, and denuding/clinging CIS (Fig. 25.14). Rare cases may show glandular differentiation.²⁴⁴ It is important for the term CIS not to be equated with “superficial carcinoma”; the latter is used by urologists for tumors that have not invaded into the MP regardless of their type and grade, and therefore it does not represent a pathologic entity.²¹¹ Similarly, one should realize that while papillary urothelial carcinomas that do not invade the stroma could be conceptually considered “carcinomas in situ” (pTa), they are not designated as such to avoid lumping together two conditions with markedly different morphologic features, natural history, and probably molecular pathways.^245,246

11.1.1 The urothelium

The urothelium is a transitional epithelium, classified as such because its properties lie between stratified squamous and simple non-stratified epithelia. Although urothelium is mitotically quiescent, with a very low constitutive rate of cell turnover, it has a high regenerative capacity in response to damage (Varley et al., 2005). this regenerative property, which is important for conserving barrier function, also makes it an ideal target for tissue engineering and regenerative strategies. As discussed below, the urothelium not only plays an essential role in providing a urine-proof lining, but also plays an important role in tissue accommodation by contributing to the changes in luminal surface area.

Histologically, the urothelium is stratified into basal and superficial cell zones, interposed by three to six intermediate cell layers that vary according to the degree of distension of the bladder. The superficial cells are highly specialised and provide the primary urinary barrier through expression of urothelium-restricted uroplakin proteins in the apical membrane (Hu et al., 2000; Olsburgh et al., 2003) and well-developed intercellular tight junctions (Varley et al., 2006). These molecular features contribute to transcellular and paracellular barrier functions, with the urothelium recognised as the ‘tightest’ epithelium in the body (Acharya et al., 2004, Slobodov et al., 2004). The uroplakins also contribute to a vesicular mechanism of apical membrane insertion and turnover that regulates changes in luminal surface area during bladder accommodation (Truschel et al., 2002).

Genetic Alterations in Normal and Benign Bladder Urothelium

Given bladder cancer's propensity to recur, plus the fact that it is often multifocal, it has been proposed that there may be genetic changes in broad areas of the urothelium. Such a hypothesis would be in keeping with the “field cancerization” concept (also known as “field effect”), first devised bySlaughter et al. in 1953 to help explain the multifocal nature and high local recurrence rates of cancers of the oral cavity, as well as the finding of histologically abnormal epithelium in areas adjacent to cancer. Slaughter et al. proposed that multiple cancer foci arose within a wider field of abnormal epithelium that had been preconditioned by some prior carcinogenic insult(s). Genetic changes have been detected in samples of histologically normal-appearing urothelium obtained from surgical samples from cancer patients. For example,Muto et al. (2000) found shared instances of LOH as well as promoter hypermethylation of theINK4A gene between normal-appearing and tumor areas from the same case. Likewise,Stoehr et al. (2005) performed LOH analyses on a large number of cases in which normal-appearing epithelium was isolated by laser capture microdissection. They also reported cancer-associated genetic changes in the normal-appearing urothelium, which, in some cases, matched the changes found in concurrent cancers in the same case. However, caution is warranted when assessing such results, given the possibility of contamination of the normal areas sampled by small multifocal cancer lesions or by pagetoid spread of tumor cells (Junker et al., 2003). On the other hand, a study by Obermann et al. using interphase FISH in tissue sections detected losses involving chromosome 9 in normal-appearing cells, which, from a technical standpoint, should be effective at excluding possible confounding microscopic foci of cancer cells (Obermann et al., 2004).

Inverted papillomas of the urinary bladder are considered benign entities. In keeping with this, they exhibit infrequent LOH at cancer-associated chromosomal loci, as well as infrequent (less than 10%) mutations ofFGFR3 (Eiber et al., 2007;Sung et al., 2006). In contrast to inverted papillomas, papillary urothelial neoplasia of low malignant potential (PUN-LMP) exhibit high rates (85%) ofFGFR3 mutation (van Rhijn et al., 2002), a genetic alteration that, as described previously, is strongly associated with bladder tumors of low stage and low grade. However, a study by Cheng et al. found frequent LOH at several loci that typically undergo LOH in advanced bladder carcinoma (greater than or equal to pT2) (Cheng et al., 2004).

METAPLASIA

Urothelium frequently undergoes squamous or glandular metaplasia, most likely in response to chronic inflammatory stimuli, such as urinary tract infections, calculi, diverticula, or frequent catheterizations.^365,366 Squamous metaplasia, especially in the area of the trigone, is common in women; it is thought to occur in response to estrogen and is not considered preneoplastic. In the setting of repeated chronic inflammation, such as schistosomal infection or bladder diverticula, squamous cell carcinoma occasionally may arise from squamous metaplasia, particularly when the metaplasia involves keratinization.³⁶⁷ Metaplastic squamous cells are histologically similar to cervicovaginal mucosa with intercellular bridges and surface keratin. Glandular metaplasia most commonly occurs in the form of cystitis glandularis described earlier. Surface metaplasia can be seen and can be extensive (see Fig. 31-24). This common finding is important to recognize because when the intestinal type of mucinous epithelium is seen, it can be associated with prominent mucin extravasation into the stroma and should not lead to the misdiagnosis of adenocarcinoma.³⁶⁸

Characterization of urothelial cells

Urothelium displays distinctive features associated with its specialized role as a permeability barrier. The progression from basal to superficial cells is accompanied by an ordered succession of phenotypic changes that can be detected with immunocytochemical staining. A group of urothelial umbrella cell-specific markers called ’uroplakins’ (UPs) consists of four major uroplakin proteins (UP Ia, Ib, II, and III) that form UPIa/UPII and UPIb/UPIII pairs. UPIa/UPII are confined to superficial cells and UPIb alone is expressed by intermediate cells.³⁶ These proteins are synthesized abundantly by normal urothelial cells. Therefore, UPs are the characteristic integral membrane proteins in the terminally differentiated, superficial urothelial asymmetric unit membrane. The other cell markers for urothelial cells are cytokeratin proteins. The cytokeratin family (CK) is generally expressed as a correlate of all epithelial tissue development and is commonly used as a bladder epithelial cell marker to identify urothelial cells. On histological sections of human bladder tissue, monoclonal antibodies to CK 7, 8, 17, 18, and 19 react with all cell layers of bladder mucosa. Antibodies to CK 13 react with the basal and intermediate cells, but not with superficial cells.³³ Antibodies to CK 20 react only with superficial cells. Cultured urothelial cells maintain the expression pattern of almost all keratin isotypes present in the normal mucosa tissue. Anti-pancytokeratins, AE1/ AE3s, are better overall cytokeratin markers, and are made up of a cocktail of both high and low molecular weight cytokeratins. AE1 contains CK 10, 13, 14, 15, 16, and 19; and AE3 contains CK 1, 2, 3, 4, 5, 6, 7, and 8. AE1/ AE3 is one of the most commonly used epithelial cell markers for identifing urothelial cells.^37,38

Neurochemistry of Urothelium and Bladder Pharmacology

The urothelium (the bladder epithelium) has specialized sensory and signaling properties that allow the bladder to respond to chemical and mechanical stimuli and to engage in reciprocal chemical communication with nerves in the bladder wall.¹⁴ The urothelium expresses nicotinic, muscarinic, tachykinin, adrenergic, bradykinin, and transient-receptor-potential vanilloid receptors.¹⁵ It has the ability to release chemical mediators such as adenosine triphosphate (ATP), acetylcholine, and nitric oxide, which can regulate the activity of adjacent nerves and trigger local vascular changes or reflex bladder contractions.

The presence of muscarinic and nicotinic receptors in the urothelium has focused attention on the role of acetylcholine as a chemical mediator of neural–urothelial interactions.¹⁶ Acetylcholine is released from the urothelium in response to chemical or mechanical stimuli. Thus, the clinical effect of antimuscarinic agents in overactive bladder conditions might not only lead to a motor response, but also influence the afferent pathway.

Various neurotransmitters have been implicated in the central control of the lower urinary tract. Putative excitatory transmitters include glutamate, tachykinins, pituitary-adenylate-cyclase-activating polypeptide, nitric oxide, and ATP.¹⁷ Glutamate seems to be the essential transmitter in spinal and supraspinal reflex pathways that control the bladder and the external urethral sphincter.¹⁸ Inhibitory amino acids (γ-aminobutyric acid and glycine) and opioid peptides (enkephalins) exert a tonic inhibitory control in the pontine micturition center and regulate bladder capacity.¹⁹ These substances also have inhibitory actions in the spinal cord. Drugs used in management of the bladder symptoms have mainly developed in accordance with this neurochemistry. Although antimuscarinics are the mainstay of therapy, other agents have been developed that influence the vanilloid, cannabinoid, and β3 receptors, which are discussed further in the section on treatment.

Urothelium

The urothelium is stratified and displays a regular, polarized architecture of increasing morphologic complexity and differentiation from base to surface.⁶⁶ It is composed of basal, intermediate, and superficial cells. Basal cells are small (approximately 10 μm in diameter), form a single layer, and serve as precursors for the other cell layers. Intermediate cells are piriform in shape, are 10 to 25 μm in diameter, and form a layer of variable thickness. The highly specialized superficial or “umbrella” layer comprises very large, hexagonally shaped cells with diameters of 25 to 250 μm. The umbrella cells have a long half-life; however, they are rapidly regenerated when the urothelium is damaged. This regeneration can result from cell division within any of the three cell layers, and generation of the multinucleate umbrella cells is likely the result of intermediate cell-cell fusion.⁶⁷

The umbrella cells have a unique, highly specialized surface membrane containing thickened plaques of asymmetric unit membrane that represent the primary transcellular urinary barrier. The asymmetric unit membrane plaques are formed by the interactions of four membrane proteins—uroplakins Ia, Ib, II, and III—which are useful markers of terminal urothelial cytodifferentiation.⁶⁸ Urothelial cell layers can be characterized by staining against markers of epithelial differentiation. Symplekin (a marker of tight junctions), uroplakins, and cytokeratin 20 are found exclusively in superficial cells, whereas other cytokeratins, such as CK7, CK8, CK13, CK17, CK18, and CK19, are expressed in basal and intermediate urothelium.⁶⁹

E. UROTHELIUM