Smooth muscle distribution in the extrahepatic bile duct: Histologic and immunohistochemical studies of 122 cases

The distribution of smooth muscle fibers in the extrahepatic bile duct (EBD) wall is not well characterized. We analyzed 101 consecutive Whipple's operation specimens and 21 autopsy specimens for the pattern of smooth muscle distribution in EBD using the Masson-trichrome stain and the desmin immunohistochemical stain. The patterns were categorized as continuous, interrupted, scattered, and no muscle layer. EBDs were divided into lower, middle, and upper portions, and the distribution pattern of smooth muscle fibers was analyzed separately in each portion. Because most surgically resected specimens contained the middle and lower EBDs with only a portion of the upper EBD, only the length of the middle and lower EBDs (common bile duct, CBD) was measured. The mean length of CBD in surgically resected specimens was 6.4 ± 1.4 cm (men, 6.6 ± 1.3 cm; women, 6.1 ± 1.5 cm). The mean length of CBD in autopsy specimens was 6.8 ± 1.0 cm. The predominant patterns of the lower third of the EBD were interrupted (49%) and continuous (43%). The predominant patterns of the middle third of the EBD were scattered (63%) and interrupted (23%). Those of the upper third of the EBD were no muscle fiber (58%) and scattered (39%). In conclusion, different patterns of smooth muscle distribution were observed in different portions of the EBD. Because scattered muscle fibers or no muscle fibers were the main features of the upper third of the EBD, understanding of this pattern may be helpful for assessment of the depth of invasion or staging of carcinoma of the upper third of the EBD.