Muscularis mucosae versus muscularis propria in gallbladder, cystic duct, and common bile duct: smoothelin and desmin immunohistochemical study.

The muscle layer in the cystic duct and common bile duct is not well defined, and it is unresolved whether it represents muscularis mucosae or muscularis propria. Smoothelin is a novel smooth muscle-specific contractile protein expressed only in fully differentiated smooth muscle cells of the muscularis propria and not in proliferative or noncontractile smooth muscle cells of the muscularis mucosae. In this study, we characterize the histologic aspects of the muscle layer in gallbladder, cystic duct, and common bile duct by evaluation of routine histologic sections and the utilization of immunohistochemistry using desmin and smoothelin. Formalin-fixed, paraffin-embedded sections of the gallbladder (15 cases), cystic duct (11 cases), and common bile duct (10 cases) were stained for smoothelin and desmin. Staining intensity was evaluated as weak or strong. The staining pattern score was evaluated as follows: 0 or negative = less than or equal to 5% positivity, +1 or focal = 6% to 10% positivity, +2 or moderate = 11% to 50% positivity, and +3 = greater than 50% muscle cells positivity. With desmin, strong and diffuse (+3) staining was observed in all gallbladder cases (15/15, 100%), highlighting one continuous muscle layer. The muscle layer was discontinuous and interrupted in all cystic duct cases and in most common bile ducts, highlighted by the desmin stain. Smoothelin intensely stained (at least +2) muscle fibers in the gallbladder in 11 (73%) of 15 cases similar to that observed with desmin staining. In contrast, common bile ducts predominantly had absent or weak and focal immunostaining (0 or +1 staining) with smoothelin (7/10, 70%), with only a few cases (3/10, 30%) having +2 staining (no cases with +3). Cystic ducts also showed absent or weak and focal immunostaining with smoothelin, with 5 (44%) of 11 cases showing 2+ immunostaining with smoothelin (no cases with 3+). Based on our findings, we conclude that, in the gallbladder wall, the muscle layer is muscularis propria and there is no muscularis mucosae present. In the cystic duct and common bile duct, only an attenuated and incomplete muscle layer of muscularis mucosae is present; because there is no muscularis propria, there probably is limited contractile function. Differentiating these anatomical muscle structures may be important for the pathologic staging of carcinoma in these organs.