Muscularis mucosae of the urinary bladder revisited with emphasis on its hyperplastic patterns: a study of a large series of cystectomy specimens

Usefulness of COL11A1 as a Prognostic Marker of Tumor Infiltration

BACKGROUND

Determining the infiltration of carcinomas is essential for the proper follow-up and treatment of cancer patients. However, it continues to be a diagnostic challenge for pathologists in multiple types of tumors. In previous studies (carried out in surgical specimens), the protein COL11A1 has been postulated as an infiltration marker mainly expressed in the extracellular matrix (ECM). We hypothesized that a differential expression of COL11A1 may exist in the peritumoral stroma of tumors that have acquired infiltrating properties and that it may be detected in the small biopsies usually available in normal clinical practice.

MATERIAL AND METHODS

In our study, we performed immunohistochemical staining in more than 350 invasive and noninvasive small samples obtained via core needle biopsy (CNB), colonoscopy, or transurethral resection of bladder tumor (TURBT) of breast, colorectal, bladder, and ovarian cancer.

RESULTS

Our results revealed that COL11A1 immunostaining had a sensitivity to classify the samples into infiltrative vs. noninfiltrative tumors of 94% (breast), 97% (colorectal), >90% (bladder), and 74% (ovarian); and a specificity of 97% (breast), 100% (colorectal), and >90% (bladder). In ovarian cancer, the negative predictive value (0.59) did not present improvement over the usual histopathological markers. In all samples tested, the cumulative sensitivity was 86% and the specificity 96% (p < 0.0001).

CONCLUSIONS

COL11A1-positive immunostaining in small biopsies of breast, colon, bladder and ovarian cancer is an accurate predictive marker of tumor infiltration that can be easily implemented in daily clinical practice.

Proteomics as a Complementary Technique to Characterize Bladder Cancer

Simple Summary

Although immunohistochemistry is a routine technique in clinics, and genomics has been rapidly incorporated, proteomics is a step behind. This general situation is also the norm in bladder cancer research. This review shows the contributions of proteomics to the molecular classification of bladder cancer, and to the study of histopathology due to tissue insults caused by tumors. Furthermore, the importance of proteomics for understanding the cellular and molecular changes as a consequence of the therapy of bladder cancer cannot be neglected.

Abstract

Bladder cancer (BC) is the most common tumor of the urinary tract and is conventionally classified as either non-muscle invasive or muscle invasive. In addition, histological variants exist, as organized by the WHO-2016 classification. However, innovations in next-generation sequencing have led to molecular classifications of BC. These innovations have also allowed for the tracing of major tumorigenic pathways and, therefore, are positioned as strong supporters of precision medicine. In parallel, immunohistochemistry is still the clinical reference to discriminate histological layers and to stage BC. Key contributions have been made to enlarge the panel of protein immunomarkers. Moreover, the analysis of proteins in liquid biopsy has also provided potential markers. Notwithstanding, their clinical adoption is still low, with very few approved tests. In this context, mass spectrometry-based proteomics has remained a step behind; hence, we aimed to develop them in the community. Herein, the authors introduce the epidemiology and the conventional classifications to review the molecular classification of BC, highlighting the contributions of proteomics. Then, the advances in mass spectrometry techniques focusing on maintaining the integrity of the biological structures are presented, a milestone for the emergence of histoproteomics. Within this field, the review then discusses selected proteins for the comprehension of the pathophysiological mechanisms of BC. Finally, because there is still insufficient knowledge, this review considers proteomics as an important source for the development of BC therapies.

Updated pathology reporting standards for bladder cancer: biopsies, transurethral resections and radical cystectomies

Aim

Optimal management of bladder cancer requires an accurate, standardised and timely pathological diagnosis, and close communication between surgeons and pathologists. Here, we provide an update on pathology reporting standards of transurethral resections of the bladder and cystectomies.

Methods

We reviewed recent literature, focusing on developments between 2013 and 2021.

Results

Published reporting standards developed by pathology organizations have improved diagnosis and treatment. Tumor sub-staging and subtyping has gained increased attention. Lymph nodes continue to be an area of debate, and their staging has seen minor modifications. Several tasks, particularly regarding specimen preparation (“grossing”), are not yet standardized and offer opportunity for improvement. Molecular classification is rapidly evolving, but currently has only limited impact on management.

Conclusion

Pathological reporting of bladder cancer is continuously evolving and remains challenging in some areas. This review provides an overview of recent major developments, with a particular focus on published reporting standards.

Prognosis of patients with T1 bladder cancer after en bloc transurethral resection of bladder tumor stratified by invasion to the level of the muscularis mucosa

PURPOSE

To evaluate the prognosis of patients with pT1 bladder cancer who underwent en bloc resection of bladder tumors (ERBTs), stratified by invasion to the muscularis mucosa (MM) level.

METHODS

Among 64 specimens obtained by ERBT with bipolar energy from patients with pT1 bladder cancer, MM was detected in 61 specimens. Thus, 61 specimens were included in this retrospective study. Patients were stratified by invasion to the MM level (pT1a, invasion above the MM level; pT1b, invasion within the MM level; and pT1c, invasion beyond the MM level). In specimens with discontinuous MM, invasion to the MM level was predicted from the dispersed MM in the specimen. The primary endpoints were progression-free survival (PFS) and cancer-specific survival (CSS).

RESULTS

Progression occurred in 2/39 patients with pT1a (5.1%), 1/6 patients with pT1b (16.7%), and 6/16 patients with pT1c cancer (37.5%). Cancer death occurred in 1/39 patients with pT1a (2.6%), 0/7 patients with pT1b, and 3/16 patients with pT1c cancer (18.8%). Patients with pT1a or pT1b cancer had a significantly better prognosis than those with pT1c cancer. On univariate analysis, tumor size ≥ 3 cm and pT1c were significantly associated with shorter PFS. On multivariate analysis, only pT1c was independently associated with shorter PFS.

CONCLUSION

This is the first study evaluating the prognosis by T1 substaging based on invasion to the MM level using ERBT specimens. ERBT provided high-quality specimens for diagnosing the MM and showed poor prognosis in pT1c bladder cancer. ERBT could be an appropriate surgical approach for an accurate diagnosis and prognosis of the T1 bladder cancer substage.

Bicentric Retrospective Analysis of en Bloc Resection and Muscularis Mucosae Detection Rate in Non-Muscle Invasive Bladder Tumors: A Real-World Scenario

Introduction

For risk stratification of non-muscle invasive bladder cancer (NMIBC), the depth of stromal invasion can be further classified, where the lamina muscularis mucosae (MM) serves as a reference structure. While the overall identifiability of MM in standard transurethral specimens is low, en bloc resection may help in identification and overall orientation. The aims of this study were to report the detection rate of MM in en bloc resected bladder tumors (ERBT) and to provide real-world information on tissue stability and preservation of en bloc architecture during recovery and processing for histopathologic evaluation.

Methods

Thirty-four ERBT specimens were histologically re-evaluated with regard to MM detectability and structure as well as the presence of en bloc architecture and further histologic features. Associations with tumor size and energy source and within histologic parameters were assessed by standard Pearson's chi-squared analyses and Cramér’s V effect size testing (V).

Results

The first parameter assessed was MM detection rate. In 19 out of 34 samples (56%) MM was detectable: scattered in 9 cases (26%), interrupted in 8 cases (24%) and continuous in 2 cases (6%). The second parameter assessed was preservation of en bloc architecture. In 11 out of 34 samples (32%), en bloc architecture could not be confirmed, and these samples served as a reference group for the detection of MM. Preservation of en bloc architecture was associated with an increased MM detection rate (MM in en bloc preserved 16/23, 70% vs. non-preserved 3/11, 27%; p = 0.020; V = 0.398) and with tumor size (p = 0.005; V = 0.595). Medium-sized tumors (1.1–2 cm) were best preserved. The choice of energy source did not show relevant association with en bloc architecture (p = n.s.).

Conclusions

In line with recent publications, ERBT increases the MM detection rate considerably. However, a third of the ERBT specimens lost en bloc architecture during sample recovery and processing. Tumor size is a relevant factor, with optimal architecture preservation between 1 and 2 cm. Optimizing resection techniques, recovery, transport, and diagnostic processing of ERBT samples is warranted to verify the diagnostic value of MM-based substaging.

pT1 high-grade bladder cancer: histologic criteria, pitfalls in the assessment of invasion, and substaging

Most patients with bladder carcinoma are diagnosed with non-muscle-invasive disease, stage Ta, and pT1. Stage remains as the single most important prognostic indicator in urothelial carcinoma. Among the pT1 bladder cancer patients, recurrence and progression of disease occur in 50% and 10%, respectively. The identification of high-risk patients within the pT1 subgroup remains an important clinical goal and an active field of research. Substaging of pT1 disease has been claimed as important histologic discriminator by the 2016 World Health Organization (WHO) classification of the genitourinary tract tumors and by the 8th edition of the American Joint Committee on Cancer (AJCC) staging manual supporting its implementation in clinical practice. Interobserver variation in pT1 diagnosis and the associated pitfalls in pT1 assessment are the critical pathological issues. The aim of this review paper is to provide the practicing pathologist with the state of the art of morphological and immunohistochemical features useful for the diagnosis of early invasive bladder carcinomas, including practical clues on how to avoid relevant interpretative pitfalls, and to summarize the current status of pT1 substaging.

Clinical significance of urothelial carcinoma ambiguous for muscularis propria invasion on initial transurethral resection of bladder tumor

PURPOSE

To evaluate the clinical significance of invasive urothelial carcinoma that is ambiguous for muscularis propria invasion on initial transurethral resection of bladder tumor (TURBT).

METHODS

All consecutive in-house TURBTs with invasive urothelial carcinoma from 1999 to 2017 that underwent radical cystectomy (RC) were grouped as follows: invasion of the lamina propria (INLP; n = 102; 24%), invasion of muscularis propria (INMP; n = 296; 69%) and ambiguous for muscularis propria invasion (AMP; n = 30; 7%). AMP was defined as extensive invasive carcinoma displaying thin muscle bundles where it is difficult to determine with certainty if those muscle bundles represent muscularis mucosae or muscularis propria (detrusor). Cases with any amount of small cell carcinoma or prior therapy were excluded.

RESULTS

The average age was 66 years in INLP, 67 years in INMP, and 65 years in AMP. RC showed invasive carcinoma stage pT2 or above in 50/102 (49%) of INLP vs. 255/296 (86%) of INMP (P ≤ 001) vs. 25/30 (83.33%) of AMP (P = 0.002). Lymph nodes showed metastatic carcinoma in 18/98 (18.36%) of INLP vs. 96/272 (35.29%) of INMP (P = 0.002), and 6/25 (24%) in AMP (P = 0.729). The average follow-up was 48 months (range 0-192). Survival of AMP patients was similar to INLP and both were significantly better than INMP (P = 0.002 and P = 0.016).

CONCLUSION

The great majority of patients with AMP on initial TURBT have advanced disease on RC and emphasizes the need for early repeat TURBT or even consideration of early cystectomy to lower the risk of worse pathological findings and to prolong survival.

Development of the human bladder and ureterovesical junction

The urinary bladder collects urine from the kidneys and stores it until the appropriate moment for voiding. The trigone and ureterovesical junctions are key to bladder function, by allowing one-way passage of urine into the bladder without obstruction. Embryological development of these structures has been studied in multiple animal models as well as humans. In this report we review the existing literature on bladder development and cellular signalling with particular focus on bladder development in humans. The bladder and ureterovesical junction form primarily during the fourth to eighth weeks of gestation, and arise from the primitive urogenital sinus following subdivision of the cloaca. The bladder develops through mesenchymal-epithelial interactions between the endoderm of the urogenital sinus and mesodermal mesenchyme. Key signalling factors in bladder development include shh, TGF-β, Bmp4, and Fgfr2. A concentration gradient of shh is particularly important in development of bladder musculature, which is vital to bladder function. The ureterovesical junction forms from the interaction between the Wolffian duct and the bladder. The ureteric bud arises from the Wolffian duct and is incorporated into the developing bladder at the trigone. It was previously thought that the trigonal musculature developed primarily from the Wolffian duct, but it has been shown to develop primarily from bladder mesenchyme. Following emergence of the ureters from the Wolffian ducts, extensive epithelial remodelling brings the ureters to their final trigonal positions via vitamin A-induced apoptosis. Perturbation of this process is implicated in clinical obstruction or urine reflux. Congenital malformations include ureteric duplication and bladder exstrophy.

Diagnostic Value of Smoothelin and Vimentin in Differentiating Muscularis Propria from Muscularis Mucosa of Bladder Carcinoma

Background

Determination of the extent of involvement or pathological staging is one of the requirements for adequate evaluation of bladder cancer specimens. Therefore, the differentiation between MP and MM is essential for proper treatment and avoiding over or under staging.

Aim

The present study aimed at evaluation of diagnostic value of smoothelin and vimentin expression both singly and in combination for differentiation between MM and MP.

Methods

This study was carried out on 59 cystectomy specimens of primary bladder carcinoma and eleven cystoscopic biopsies for non-neoplastic bladder lesions (cystitis).

Results

Histologically, MM was identified clearly in 40 cases and MP was identified in all 70 examined cases. The cases were immunostained for smoothelin and vimentin. Intensity of smoothelin expression showed significant difference (P = 0.001) between MM and MP with 97.5% sensitivity and 95% specificity and the percentage of smoothelin expression was significantly higher in MP compared to MM (P = 0.001) with 95.7% sensitivity and 85% specificity (using 65% as cut-off point). Vimentin was negative in MP and showed positive expression in 32 cases (80%) of MM with a statistical significant difference (P = 0.001) providing 80% sensitivity and 100% specificity. Combined moderate to strong smoothelin and negative vimentin offered 100% sensitivity and 100% specificity towards the identification of MP.

Conclusions

Differentiation of MM from MP can be made based on histopathological criteria, which are unfortunately overlapping in many cases. Moderate to strong smoothelin expression with negative vimentin could be very helpful procedure in difficult and overlapping cases with a high diagnostic validity.

Challenges in Pathologic Staging of Bladder Cancer: Proposals for Fresh Approaches of Assessing Pathologic Stage in Light of Recent Studies and Observations Pertaining to Bladder Histoanatomic Variances

The paradigm of pathologic stage (pT) categorization in bladder cancer remains the depth of invasion into the different histologic layers of the bladder wall. However, the approaches to assigning pT stage category toward an enhanced outcome stratification have been marked by challenges and innovations, due in part to our growing appreciation of the surprisingly perplexing bladder histoanatomy. Upstaging of pT1 tumors after radical cystectomy is substantial and underscores the potential value of pT1 substaging in transurethral resection (TUR) specimens. The 2017 American Joint Committee on Cancer tumor-node-metastasis system recommends pT1 substaging but recognizes the need to optimize the approach. Over the years, the cut-off for microinvasion has been significantly lowered to 0.5 mm and is now a promising scheme for pT1 (micrometric) substaging. Unlike the micrometric approach, histoanatomic substaging using muscularis mucosae (MM) and vascular plexus as landmarks is less feasible in TUR specimens and inconsistent in stratifying the outcome of pT1 tumors. The lamina propria possesses inherent variations in depth, MM, and vascular plexus dispositions that should be factored in future pT1 substaging proposals. Histoanatomic variations among the bladder regions also occur, and studies suggest that trigone and bladder neck cancers may have more adverse outcomes. The muscularis propria (MP), besides being the essential histologic landmark for assigning pT2 stage category, is also considered a surrogate for the adequacy of TUR, furthering the importance of identifying its presence in TUR specimens. MP, however, may be mimicked by hyperplastic or isolated MP-like MM muscle bundles in the lamina propria with overstaging implications, and caution should be exercised in distinguishing these 2 muscle types morphologically and immunohistochemically. Presence of additional superficial MP unique from the detrusor muscle proper may also complicate staging at the trigone and ureter insertion sites. With regard to the depth of MP invasion, large and multicenter studies have reaffirmed the prognostic significance of pT2a/b subcategories. It is revealed that there are at least 3 ways used to demarcate the irregular MP to perivesical soft tissue junction, and use of a common criterion indicates improvement in pT2b/pT3a staging reproducibility. Although studies have shown significantly poorer outcome in pT3b compared with pT3a tumors, this designation has a substantial reliance on the prosector's gross assessment of perivesical soft tissue invasion which if performed incorrectly may lead to staging inaccuracy of pT3 tumors. The 8th edition of the American Joint Committee on Cancer has updated the staging schema for bladder cancers with concomitant prostatic stromal invasion and cancers within bladder diverticula. Because of 2 possible pT designations, prostatic stromal invasion in TUR specimens should not be automatically staged as either pT4a or pT2 (urethral). Recent data support that bladder cancer invading into the seminal vesicle has comparable outcome to pT4b tumors. Interestingly, several studies in pT4a tumors, which are staged based on sex-specific organs, have shown poorer outcome in females than males after radical cystectomy, and while there are possibly several reasons, they may also include anatomic factors. Despite the progress has been made, work remains to be done to inform future bladder cancer pT category definitions and their reproducibility in application and prognostication.

Clinicopathologic characterization of intradiverticular carcinoma of urinary bladder - a study of 22 cases from a single cancer center

Background

To examine histopathologic features and clinical outcomes of intradiverticular bladder carcinomas.

Methods

Twenty-two consecutive patients with intradiverticular bladder carcinoma treated with either endoscopic transurethral resection or partial or radical cystectomy at a single institution between years of 1995 to 2011. Clinicopathologic characteristics and oncologic outcomes of patients were retrospectively analyzed, including tissue histology re-review by genitourinary pathologists.

Results

Histologically, 9 cases (41%) were non-invasive papillary urothelial carcinoma, 13 cases (59%) were invasive urothelial carcinoma, including three cases of small cell carcinoma. Final pathology revealed synchronous extradiverticular urothelial carcinomas in 6 out of 9 cases (67%) of non-invasive and 2 out of 10 cases (20%) invasive intradiverticular urothelial carcinoma, respectively. More than half of cases (13/22, 59%) showed a distinctive hypertrophic layer of muscularis mucosae. There was no statistical difference in disease free survival or overall survival between non-invasive and invasive tumors within approximately 3 years of follow up (mean 38 months, median 32 months). While stage T3 patients generally did poorly, oncologic outcomes for stage T1 patients were no different than those of stage Ta.

Conclusion

Intradiverticular carcinomas are often associated with a hypertrophic layer of muscularis mucosae that can potentially confound tumor staging. Non-invasive intradiverticular urothelial carcinomas are more likely to have coexisting synchronous extradiverticular lesions. The absence of a muscularis propria layer may not necessarily predispose T1 tumors to more aggressive disease.

Virtual Slides

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/13000_2014_222

Unique properties of muscularis mucosae smooth muscle in guinea pig urinary bladder

The muscularis mucosae, a type of smooth muscle located between the urothelium and the urinary bladder detrusor, has been described, although its properties and role in bladder function have not been characterized. Here, using mucosal tissue strips isolated from guinea pig urinary bladders, we identified spontaneous phasic contractions (SPCs) that appear to originate in the muscularis mucosae. This smooth muscle layer exhibited Ca(2+) waves and flashes, but localized Ca(2+) events (Ca(2+) sparks, purinergic receptor-mediated transients) were not detected. Ca(2+) flashes, often in bursts, occurred with a frequency (∼5.7/min) similar to that of SPCs (∼4/min), suggesting that SPCs are triggered by bursts of Ca(2+) flashes. The force generated by a single mucosal SPC represented the maximal force of the strip, whereas a single detrusor SPC was ∼3% of maximal force of the detrusor strip. Electrical field stimulation (0.5-50 Hz) evoked force transients in isolated detrusor and mucosal strips. Inhibition of cholinergic receptors significantly decreased force in detrusor and mucosal strips (at higher frequencies). Concurrent inhibition of purinergic and cholinergic receptors nearly abolished evoked responses in detrusor and mucosae. Mucosal SPCs were unaffected by blocking small-conductance Ca(2+)-activated K(+) (SK) channels with apamin and were unchanged by blocking large-conductance Ca(2+)-activated K(+) (BK) channels with iberiotoxin (IbTX), indicating that SK and BK channels play a much smaller role in regulating muscularis mucosae SPCs than they do in regulating detrusor SPCs. Consistent with this, BK channel current density in myocytes from muscularis mucosae was ∼20% of that in detrusor myocytes. These findings indicate that the muscularis mucosae in guinea pig represents a second smooth muscle compartment that is physiologically and pharmacologically distinct from the detrusor and may contribute to the overall contractile properties of the urinary bladder.

Differential expression of immunohistochemical markers in bladder smooth muscle and myofibroblasts, and the potential utility of desmin, smoothelin, and vimentin in staging of bladder carcinoma

Distinguishing bladder muscularis propria from muscularis mucosae can be problematic especially in transurethral resection specimens performed for bladder carcinoma. Moreover, bladder carcinoma can be associated with a proliferative/desmoplastic myofibroblastic response that can resemble smooth muscle and potentially lead to overdiagnosis of muscularis propria invasion. The aim of this study was to investigate the potential role of immunohistochemistry in staging bladder carcinoma by evaluating the expression of different markers in myofibroblasts and nonvascular smooth muscle cells in 15 cases of invasive bladder carcinoma. Reactive myofibroblasts were consistently positive for vimentin and smooth muscle actin, consistently negative for caldesmon, desmin, and smoothelin, and had variable expression of actin and CD10. Nonvascular smooth muscle cells of the bladder were consistently positive for smooth muscle actin, actin, desmin, and caldesmon, and consistently negative for CD10. In contrast to smooth muscle cells of the muscularis propria, which displayed strong smoothelin expression in all 15 cases, the smooth muscle cells of the muscularis mucosae displayed moderate smoothelin expression in only 1 (9%) of 11 cases (P=10(-7)). Surprisingly, although strongly highlighting endothelial and endomysial cells, the smooth muscle cells of the muscularis propria weakly expressed vimentin in only 1 (7%) of 15 cases, whereas smooth muscle cells of the muscularis mucosae had moderate or strong expression in 9 (82%) of 11 cases (P=0.00016). The sensitivity and specificity of desmin or caldesmon expression for smooth muscle cells were 100%. The sensitivity and specificity of strong smoothelin expression for muscularis propria were 100%, whereas those of absent vimentin expression were 93 and 82%, respectively. Although morphology remains the gold standard, the findings suggest that immunohistochemistry, using a panel composed of desmin, smoothelin, and vimentin, may be potentially useful for staging of bladder carcinoma. Confirmatory larger-scale studies, especially on transurethral resection specimens, are warranted.

Diagnostic utility of antibody to smoothelin in the distinction of muscularis propria from muscularis mucosae of the urinary bladder: a potential ancillary tool in the pathologic staging of invasive urothelial carcinoma

Accurate recognition of urinary bladder muscularis propria (MP) invasion by urothelial carcinoma is crucial as it is the critical crossroad between conservative and aggressive management for the patient. It is now widely known that an inconsistent layer of muscularis mucosae (MM) muscle exists in the lamina propria, which can mimic the MP muscle, particularly when hyperplastic, making staging extremely challenging in some limited, unoriented, or highly cauterized specimens. Smoothelin is a novel smooth muscle-specific contractile protein expressed only by fully differentiated smooth muscle cells, and not by proliferative or noncontractile smooth muscle cells and myofibroblasts. We performed immunohistochemical staining in the bladder for smoothelin to: (a) evaluate its expression in MM and MP muscle in cystectomy specimens and by comparing the staining pattern with smooth muscle actin (SMA), (b) study MP variations in the bladder trigone and at the ureteric insertion in the bladder wall, and (c) assess the staining pattern of MM and MP in a representative group of transurethral resection of bladder tumor specimens. In contrast to SMA, which equitably stained both types of muscle fibers, smoothelin displayed striking differential immunoreactivity between MM and MP muscle. With smoothelin, the MM muscle (including hyperplastic forms) typically showed absent (19/42, 45%) or weak and focal (18/42, 43%) staining, whereas the MP muscle typically showed strong and diffuse staining (36/42, 86%). Smoothelin accentuated individual muscle fibers within groups of MP bundles only, a feature which was evident in both MM and MP stained by SMA. When only strong and diffuse immunoreactivity in muscle was set as a threshold for positivity, 100% specificity and positive predictive value of smoothelin for MP (vs. MM) was achieved in our study. Smoothelin staining confirmed the morphologic variations in MP muscle in the bladder wall of the trigone and at the ureteric insertion. In addition to the well-defined muscle layers of MM and MP, SMA staining revealed a continuous band of ill-defined haphazardly oriented compact spindle cells that were immediately subjacent to the urothelium in all cases. These spindle cells blended with the morphologically recognizable thin slender fascicles of the MM muscle. We designate this hitherto uncharacterized thin layer of SMA-positive [muscle-specific actin positive (6/6), Masson trichrome stain predominantly blue (5/6)] and smoothelin-negative cells as suburothelial band of myofibroblasts. In all 10 transurethral resection of bladder tumor sections, smoothelin staining was in agreement with the routine light microscopic presence and absence of MP muscle. In conclusion, the relatively distinct immunohistochemical staining pattern of smoothelin between MP and MM (including its hyperplastic forms) makes it a robust and attractive marker to be incorporated in the contemporary diagnostic armamentarium for the sometimes difficult area of staging bladder urothelial carcinoma.