Reinnervation of motor endplate-containing and motor endplate-less muscle grafts.

Damaged muscle fibers degenerate and are phagocytosed, but their connective tissue sheaths and myogenic cells survive. New myofibers regenerate within the connective tissue sheath and functional reinnervation is restored. The present study compared regenerating myofibers and their reinnervation in muscle grafts where the original motor endplates were removed and when they were present. In regenerating muscles, where the endplates were present, axons made neuromuscular connections with the myofibers. High levels of CAT activity, characteristic of cholinergic innervation, were reached and maintained in these muscles. Also, motor endplate-containing grafts developed characteristic muscle fiber types and nerve-evoked muscle contraction was observed. In preparations lacking the original endplate region, CAT activity initially rose to high levels, but with time diminished to 17% of control, and muscle fiber types did not develop. Muscle contraction after nerve stimulation was not observed in these grafts. It was concluded that, although a small number of motor endplates are formed in grafts devoid of original motor endplates, the presence of the motor endplate region in regenerating muscle is critical for development and maintenance of myofibers and the axons that contact them.