Ultrastructural evidence for axon retraction during the spontaneous elimination of polyneuronal innervation of the rat soleus muscle

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Abstract
This study provides morphological evidence for axon retraction during the elimination of polyneuronal innervation in postnatal rat soleus muscle. The motor innervation of muscles from animals 7–15 days and two months of age was examined in silver-stained preparations at the light microscopic level and in serial sections with the electron microscope. Transection of the soleus nerve in immature muscles revealed that the pattern of terminal and preterminal degeneration was similar to, but much more rapid than, that reported for adult muscles. During normal development, less than 1% of the preterminal axons sampled exhibited a denervation-like morphology and none of the neuromuscular junctions exhibited dense terminal degeneration. This indicates that autotomy and phagocytosis of the excess axons play a minor role in the development of connections. From estimates of the number of axons lost per day, about 20% of the endplates in 14-day-old muscles would be expected to have involuting axons associated with them. Slender axons terminating in vesicle-laden enlargements were associated with 18% of the endplates examined ultrastructurally which is consistent with the predicted rate of axon removal. These structures are interpreted as morphological manifestations of axon retraction because they were not present in adult material and they were not produced by nerve transection. Furthermore, they are comparable in shape and location to the putative retraction fibres identified in silver-stained sections at the light microscopic level. The present results indicate that retraction is the major means by which all but one of the multiple inputs per endplate are removed during the establishment of the adult pattern of motor innervation.