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Pathology for Urologists

Urinary Bladder: Muscularis Propria (MP)

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Image B
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  • Also known as "deep muscle" or "detrusor muscle" (images A).
  • Forms the bulk of bladder wall and consists of 3 differently oriented layers of smooth muscles that contract during micturition.
  • An important histo-anatomic landmark in staging invasive bladder cancer.
    • Identification and status of involvement of MP is essential in TURBT containing high grade or invasive carcinoma for staging adequacy.
    • Muscularis mucosae (MM) when hyperplastic can mimic MP (tumor in MM is pT1 and tumor in MP is pT2*).
  • MP consists of large bundles of regularly oriented muscle fibers with regular outline that are typically closely packed together (image B).

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  • Variations at trigone, where MP bundles are nearer to the suburothelium and become smaller in caliber towards the surface (image C).
  • Adipose tissues can be seen in between MP muscle bundles, which may continue extravesically or extend superficially to lamina propria (image D).
  • Outer boundary of MP with perivesical soft tissues (a landmark for staging extravesical invasion) is irregular and not well defined.

* Non-specific terms in pathology reporting such as "smooth muscle" or "muscularis" must be clarified as MP or not.


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