Histochemical alterations of re‐innervated rat extensor digitorum longus muscle after end‐to‐end or graft repair: a comparative histomorphological study

Changes in the histochemical profile of 43 rat extensor digitorum longus muscles undergoing de‐innervation and re‐innervation were recorded. Assessment of fibre type composition and muscle fibre cross‐sectional area was performed at 15, 30, 90 and 180 days post operative (p.o.) after either primary end‐to‐end repair or autologous graft repair of the common peroneal nerve ( n = 5 per time point and type of repair). The size and histochemical profile of single muscle fibres were analysed by computer‐assisted quantification on the basis of their myofibrillar ATPase (pH 4.3) and succinate dehydrogenase (SDH) activities in serial, whole‐muscle cross‐sections. Accordingly, four muscle‐fibre types could be functionally identified: (1) slow oxidative (SO, type I); (2) fast‐oxidative glycolytic (FOG, type IIA); (3) fast glycolytic (FG, type IIB); and (4) succinate dehydrogenase intermediate (SDH‐INT). At 15 days following end‐to‐end repair, the SDH‐INT muscle fibre type was observed. By contrast, 15 days following graft repair, no changes in fibre type composition were observed (vs. control). At 30 days p.o. in the group that received end‐to‐end repair, type SDH‐INT reached its maximum and was significantly higher than in the group that underwent graft repair. At 90 days p.o., the amount of SDH‐INT fibres declined after end‐to‐end repair, but it was still significantly higher than in the group treated with a nerve graft. The increase of the SDH‐INT fibre type was mirrored by a proportional disappearance of FG and FOG fibres. These changes were time‐dependent, not reversible at 180 days p.o and largely blunted after nerve graft. Muscle‐fibre size decreased at 15 and 30 days after both types of nerve repair. This decrease was transient and reversible within 90 days p.o. These findings reflect the fact that the reorganization of the histochemical profile in re‐innervated muscles is both time dependent and long lasting. The degree of this reorganization is significantly higher after end‐to‐end repair than after graft repair.