Proximo-distal organization and fibre type regionalization in rat hindlimb muscles

Five muscles of the rat's lower hindlimb were compared with regard to their histochemical fibre type distribution at seven different proximo-distal levels. The muscles were: extensor digitorum longus (ED), flexor digitorum and hallucis longus (FD), gastrocnemius medialis (GM), peroneus longus (PE) and tibialis anterior (TA). In all the five muscles, the relative density of the 'slow' type I fibres showed a striking and similar decrease from proximal toward more distal levels. In addition, the type I fibres were concentrated within smaller and more eccentrically placed regions at distal than at more proximal levels. As a background for the further analysis of these lengthwise aspects of type I fibre regionalization, architectural features of the muscles were determined. Pinnation angles and the position of major tendons and tendon sheets were assessed in fresh specimens. Muscle fibre lengths were measured for single fibres dissected from different regions of macerated muscles. In all cases, fibre length was much shorter than muscle length (mean fraction ranging from 21 to 55%), implying that the proximo-distal changes in histochemical fibre properties were indeed explainable as being due to gradual lengthwise changes in fibre type populations. The similarity of these lengthwise changes across the muscles was in contrast to their differences in other aspects of functional organization, such as the average density of type I fibres and architectural features determining their relative capacities for shortening and force generation. The possible functional role of the proximal accumulation of type I fibres was discussed in relation to circulation and thermal balance; besides, the proximo-distal regionalization of type I fibres might (partly) reflect processes that had been associated with early stages of muscle differentiation. Furthermore, the results underline that, when determining the fibre type composition of rat hindlimb muscles, identifying the proximo-distal level of sampling is a matter of great importance.