Histological study of the human external anal sphincter.

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A histological and histochemical study of the normal external anal sphincter was peformed in 4 men and 4 women. The mean percentages of each fiber type were 85.85% for type 1 fibers, 6.15% for type 2A fibers, 7.95% for type 2B fibers and 0.05% for type 2C fibers. Type 1 fiber predominance was seen in all cases. The mean diameters of types 1, 2A and 2B fibers were 28.4±8.96μm, 40.2±9.21μm and 39.6±8.85μm, respectively. The mean diameter of type 1 fibers was significantly smaller than that of the other types (p<0.01). Ragged red fibers which were stained reddish by the modified Gomori trichrome method were found in all cases. The appearance rate of these ragged red fibers was 0.65% (297 of 45, 449 fibers). Some of these fibers had cytochrome c oxidase deficiency. All these changes were found only in type 1 fibers. Type 1 fiber predominance is thought to be in keeping with the function of the external anal sphincter which always holds tonus and generates resting potentials. The mechanism of the appearance of the ragged red fibers and partial deficiency of cytochrome c oxidase is not clear, but it is thought to be one of the peculiarities of the external anal sphincter, and further study is necessary.
日本大腸肛門病学会の論文