Histology and enzymatic activity in the postnatal development of limb muscles in rodents.

The present work examines how increases in spontaneous motor capabilities during postnatal development are reflected in enzymatic activity and the histology of hindlimb muscles of the dormouse (Eliomys melanurus), the jird (Meriones tristrami), the vole (Microtus socialis), and the spiny mouse (Acomys cahirinus). The precocial neonate of the spiny mouse had the most advanced developmental state of young myofibers with striations as early as 1 week after delivery. At the same age, the altricial neonate vole had less developed muscles compared to the spiny mouse, but was more mature compared to other altricial species. The dormouse was the least developed, with numerous myoblasts and few myotubes at 1 week after delivery. These differences in myogenic development were conspicuous throughout postnatal development. Similar differences between the species were also evident at the biochemical level, as measured in the kinetics of activity of the enzyme creatine-phosphokinase immediately after delivery. On postnatal day 7, the creatine-phosphokinase level in the spiny mouse was fourfold higher than in the dormouse or vole. The enzymatic activity of acid phosphatase decreased during the first week postdelivery in the spiny mouse while peaking in the first, second, and third week in the jird, vole, and dormouse, respectively. These results support the notion that precocial species undergo certain developmental stages in utero, whereas, the same stages commence in altricials only postnatally. For the tested altricial species, the results illustrate that limb muscles in the vole, which displays more basic gaits, mature before limb muscles of the jird and dormouse, which display more specialized gaits.