- Also known as "deep muscle" or "detrusor muscle" (image 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).
- 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.