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

Although the presence of the muscularis mucosae (MM) in the urinary bladder is well known, hyperplastic MM (HMM) has not been well established in the English literature. In this study, we analyzed the presence and distribution of the MM and HMM in 102 consecutive cystectomy specimens and 17 bladders obtained at autopsy. Hyperplastic MM, defined as MM composed of more than 3 layers of muscle fibers appearing either as fibers parallel to the surface mucosa or rounded bundles, was found in all of the cystectomy specimens. Hyperplasia was subclassified as mild (4 to 6 layers of smooth muscle) and prominent (more than 6 layers). Hyperplastic MM was found in the dome, trigone, and anterior, posterior, left lateral, and right lateral walls. The incidence was relatively similar among these locations. The 3 patterns of HMM (continuous, interrupted, scattered) were similar to those of MM. Hyperplastic MM appeared most frequently in the scattered pattern and was always adjacent to large-caliber blood vessels. The hyperplasia is usually mild and is occasionally prominent. The presence of HMM did not appear to be related to the patient's age and occurs slightly more often in bladders from cystectomy for neoplasms than bladders obtained from autopsy. Although the MM has been used along with the muscularis propria as a histological landmark for staging of bladder cancer, little is known about HMM. Pathologists should be aware of its occurrence and morphologic patterns to accurately assess the depth of invasion of bladder tumors and avoid misinterpreting HMM as the muscularis propria.