[A comparative electron microscope study on the fine structure of the muscular coat in the common bile duct]

Little is known about the architecture of common bile duct, especially smooth muscle, except for Oddi's sphincter. The purpose of this study is to reveal the ultrastructural architecture of the upper part of the common bile duct from the viewpoint of comparative anatomy. Animals used in this study were the hamster, rat, dog, monkey and rabbit, and the results obtained were as follows. There were considerable differences in the structural appearance and distribution of smooth muscle cells in the upper part of the common bile duct among the animals examined. In the rat, a small number of smooth muscle cells were sparsely distributed in the subepithelial connective tissue of the duct. In the dog, smooth muscle cells showed a star-shaped appearance with many cytoplasmic processes, by which networks were formed. In the monkey, smooth muscle cells with fibroblast-like appearance were circularly distributed. In the monkey and rabbit, muscular coats consisting of typical smooth muscle cells were observed in the outermost layer of the duct. The muscular coat in the monkey was arranged longitudinally, and in the rabbit circularly. The muscular coat in the rabbit was thicker than that in the monkey. In the hamster, however, the duct wall was devoid of smooth muscle cells. The present study suggests that the occurrence of smooth muscular coat is not essential for transport of the bile.