Fibre type regionalization of forelimb muscles in two mammalian species, Galea musteloides [Rodentia, Caviidae) and Tupaia belangeri (Scandentia, Tupaiidae), with comments on postnatal myogenesis

To investigate structural differences between propulsory and antigravity mu scles, the spatial distribution of slow (type I) and fast (type II) muscle fibres in forelimb muscles of two species of small mammals was studied, Gal ea musteloides and Tupaia belangeri. Serial sections through complete forel imbs were prepared. Following histochemical fibre typing, the forelimbs wer e reconstructed three-dimensionally using product design software. Most for elimb muscles of both species showed a homogenous distribution of type I fi bres. In the supraspinatus and triceps brachii (capita longum et laterale) muscles, however, a segregation of fibre types into "fast" superficial area s and "slow" deep regions was observed. Slow regions contained at least 60% type I fibres and were positioned along intramuscular extensions of the te ndons of insertion. The functional implications of fibre type regionalizati on are discussed. An analysis of intramuscular fibre type distribution duri ng postnatal myogenesis revealed no significant differences in muscle fibre differentiation between altricial and precocial juveniles. Differences in locomotor ability probably arise from heterochronic development of connecti ve tissue components (endo- and perimysium).