| Abstract: | Postnatal changes in histochemical properties of the soleus muscles were examined in male and female mice aged 5 to 40 weeks. The fiber type composition did not significantly differ between sexes 5 weeks after birth. In males the percentage of type I fibers increased from 5 to 22 weeks of age but did not change thereafter. In females the percentage of type I fibers increased from 5 to 40 weeks of age. As a result, females had significantly higher percentage of type I fibers than males 30 and 40 weeks after birth. The smaller increase in the percentage of type I fibers during postnatal growth in males is suggested to be attributable to the higher testosterone level. In males the cross-sectional area of both type I and type II fibers increased with age. In females, however, the cross-sectional area of type II fibers increased with age whereas that of type I fibers increased from 5 to 10 weeks of age but thereafter decreased gradually. The ratio of mean fiber cross-sectional area of type I fibers to that of type II fibers decreased slightly from 5 to 10 weeks of age but did not change thereafter in males. In females the ratio increased from 5 to 10 weeks but thereafter decreased gradually. The ratio was significantly higher in females than in males in all age groups. The percentage area occupied by type I fibers increased with age in both sexes. The increase was greater in females than in males, however. Furthermore, females had significantly higher value than males in all age groups. No significant age difference and sex difference were observed in the total number of muscle fibers. |
