Ultrastructural studies of normal and degenerating mouse neuromuscular junctions.

A quantitative study of the ultrastructural changes occurring at degenerating motor axon terminals of the mouse hemidiaphragm in the first 26 h following unilateral phrenicotomy has been made. Several ultrastructural characteristics of axon terminals from normal diaphragms and phrenicotomized and control hemidiaphragms of phrenicotomized preparations were analyzed. Considerable swelling and disruption of mitochondria occurred in terminals of control hemidiaphragms at 3, 6 and 9 h postphrenicotomy after which no more damage than that seen in normal terminals after fixation for electron microscopy was observed. At terminals of the phrenicotomized hemidiaphragm, much mitochondrial damage was observed from 3 h post-phrenicotomy onwards. In phrenicotomized hemidiaphragms all terminals had completely degenerated by 26 h post-phrenicotomy. Reduced synaptic vesicle population densities occurred during degeneration. At many axon terminals on phrenicotomized hemidiaphragms the population densities of synaptic vesicles were reduced compared with controls and vesicle aggregates were noted in many engulfed or partially engulfed nerve terminals. These results are discussed with respect to the vesicle hypothesis for nerve-muscle transmission. The mechnisms underlying Schwann cell hyperactivity are also considered.