Ectopic endplates induce localized changes in skeletal muscle ultrastructure

To investigate the processes by which motoneurons control protein synthesis, and thus the ultrastructure of the muscle fibers they innervate, ectopic endplates were induced to form on adult mouse skeletal muscle fibers by transplantation of a foreign nerve onto the muscle. In the dually innervated muscle fibers thus created, we examined two ultrastructural parameters that correlate with the expression of distinct isoforms of the myofibrillar proteins alpha-actinin and titin, specifically, Z-line width and sarcomere length. It was found that Z-lines were significantly thinner (98 vs. 128 nm) and sarcomeres were significantly shorter (1.69 vs. 2.06 microm) near the ectopic than near the original endplates. Thus, ectopic endplate formation on adult skeletal muscle fibers induces a localized alteration in myofibrillar morphology. These results may help to elucidate the role played by motoneurons in the determination and maintenance of muscle fiber properties and the processes that occur following muscle reinnervation after nerve injury.