Cell size and succinate dehydrogenase activity of different types of fibers in different regions of the tibialis anterior muscle in mice and rats

Cross-sectional areas (CSAs) and succinate dehydrogenase (SDH) activities of different types of fibers in deep, middle, and superficial regions of the tibialis anterior muscle in mice and rats were determined using quantitative histochemistry. In both mice and rats,mean CSAs of fibers were in the rank order of type IIa <type IIx <type IIb, while mean SDH activities of fibers were in the rank order of type IIa>type IIx >type IIb, irrespective of the muscle region. In mice, mean CSAs of the corresponding type of fibers were in the rank order of deep<middle<superficial, while there were no differences in the mean SDH activity of the fibers among the three muscle regions. In rats, type IIa and type IIx fibers in the deep region had smaller CSAs and higher SDH activities than the corresponding type of fibers in the middle region, while mean CSAs of type IIb fibers were in the rank order of deep<middle=superficial and mean SDH activities of the fibers were in the rank order of deep>middle=superficial. Muscle fibers in mice had smaller CSAs and higher SDH activities than the corresponding type of fibers in rats. These results indicate species-specific patterns and regional differences in the CSA and the SDH activity of fibers in the muscle even within the same fiber type defined by myosin heavy chain isoform expressions.