[Analyses on the fiber compositions of the lumbar back muscles in mammals]

Monkeys, being close to the human, have a lot of opportunities to sustain the lumbar spine in the extended or upright position. So it is interesting to compare the histochemical profiles of the fiber types in the lumbar back muscles of monkey with those of the other tetrapod animals. An attempt was made to analyze the muscle fiber compositions of the whole epaxial muscles from 32 mice, 28 rats, 18 cats, 27 dogs and 8 monkeys, using the Sudan black B staining. In the present study, the epaxial muscular systems in cross section were divided into three compartments in mouse and rat: transversospinalis (TS), medial component of longissimus (McL) and lateral longissimus (LL), or into four compartments in cat, dog and monkey: TS, McL, longissimus (LG) and iliocostalis (IC). Fiber counting was undertaken by the light microscope widely through the each compartment. Results were as follows. The mean percentages of the white fibers progressively increased towards the laterally located muscle compartments of the epaxial muscles in every animal except monkey. As the most noteworthy difference from the other animals, a large dark area was found in the monkey longissimus muscle. This area was composed solely or predominantly (85-90%) of "non white fibers" which could not be referred to whether the red or the intermediate fiber. Consequently, it caused marked decrease of the white fibers, whereas a mosaic pattern of fibers was presented in the whole LG compartments of the other animals. The "non white fibers", which seem to take part in the tonic movement of the muscle, appear to be the resultant differentiation as an adaptation for keeping the vertebral column in the upright position in monkey. Concerning the sizes of the red fibers in the epaxial muscles and the m. rectus abdominis (RA), those of the RA were markedly smaller than those of the epaxial muscles in mouse, rat, cat and dog except monkey. But the size of the red fibers in the RA of monkey was as large as that in the epaxial muscle of monkey. From these findings, it is likely to understand that the epaxial muscle in monkey, in contrast to the other tetrapod animals, functions antagonistically with the rectus abdominis muscle. The total numbers and the sizes of the muscle fibers in the RA and the epaxial muscles for both sexes of the mouse and rat during growth were also investigated.