Formation of Neuromuscular Junctions in Transplanted Peroneus Longus Muscles in the Rat: A Quantitative Comparison with Reinnervation of the Muscle in situ

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Abstract
The formation of neuromuscular junctions in free heterotopic muscle transplants in the rat was studied quantitatively and compared with the reinnervation of the muscle in situ. The AChE[acetylcholinesterase]-containing area and the nerve terminal were stained in the same longitudinal section and the length of end-plate, the average number of nerve terminal branch points within the end-plate and the terminal innervation ratio (TIR) were determined. In denervated muscles, early disappearance of nerves was noticed, whereas the AChE-stained end-plates were visible after 8 wk. The reinnervation of the muscle in situ and after transplantation showed considerable similarities. Early during reinnervation the number of AChE positive areas increased and many of them were innervated by > 1 axon. Terminal axons were also seen innervating > 1 end-plate situated on the same or on different muscle fibers. Later, the number of end-plates decreased and they were innervated by only 1 axon. The end-plate length reached normal value in the reinnervated muscle in situ but remained smaller in the transplanted muscle. In all reinnervated muscles, ultraterminal sprouting was found as indicated by an increased number of nerve terminal branch points within the end-plate area. The TIR was increased in all reinnervated muscles due to terminal and ultraterminal sprouting of the axon. No significant difference in reinnervation was noted between normal and predenervated transplants. The reinnervation of transplated muscles obviously has similarities not only with the reinnervation of normal muscles but also with the development of muscular innervation during early postnatal growth. In spite of the plasticity of the peripheral nerve, the transplanted muscles underwent considerable atrophy which may have been enhanced by the unphysiological placement of the muscles in the abdomen. This experimental model made it possible to study reinnervation of muscle fibers only originating from the graft.