The spatiotemporal characterization of endplate reoccupation, with special reference to the superposition patterns of the presynaptic elements and the postsynaptic receptor regions during muscle reinnervation

Abstract

In this study, an immunohistochemical investigation was carried out to define spatiotemporal characteristics of superposition patterns of the presynaptic elements and the postsynaptic acetylcholine receptor (AChR) sites during the period of endplate regeneration after sciatic nerve crush. The extent of close correspondence of terminal Schwann cell (TSC)-, or axon terminal-, apposing AChR sites was quantitated with three-dimensional images of neuromuscular junctions (NMJs) taken under confocal laser-scanning microscopy. After 3-weeks post-crush (wpc), reoccupation of regenerating TSCs and later arriving axon terminals proceeded within the scope of previously denervated AChR plaques. During this period, the areas of presynaptic elements and the areas of postsynaptic elements were highly correlated. TSCs rapidly reoccupied a greater part of the postsynaptic receptors. In contrast, there was a slower increase of the contact areas of AChR sites overlapped by the axon terminals. Reoccupation by the presynaptic elements at 20 wpc was almost completed in a majority of NMJs, but some anomalous changes still continued to occur in a small proportion of the NMJs (20-30%). Our results suggest that: (a) with gradual increase of the contact areas between presynaptic and postsynaptic elements, imperfect reinnervation and regeneration, due to spatial mismatching or unbalanced growth between presynaptic and postsynaptic elements, result in sporadic remodeling; (b) the difference in superposition patterns between TSCs and axon terminals depends on the ability of making alignment to the endplate gutters in regenerating NMJs; and (c) a complex set of anatomical relationships among the three endplate components affects the process of endplate reoccupation synthetically.