Persistence of nerve sprouting with features of synapse remodelling in soleus muscles of adult mice.

An increase in the number of nerve branches of the unmyelinated axon terminals with increasing age was observed in normal adult mouse motor endplates. In addition, ultrastructural investigation revealed signs of nerve retraction. A combined light and electron microscopic investigation was performed on zinc-iodine-osmium stained endplates in soleus muscles. The number of branch points in a synapse, endplate length and muscle fiber diameter were evaluated in "young adult" (3 months) and adult (6 and 11 months) mice. For all 3 parameters, 3-month-old animals had the lowest values. Eleven-month-old animals had more branch points and larger endplate lengths than 6-month-old animals while there was no significant difference in fiber diameters. Branch point numbers and endplate length were correlated in each muscle while fiber diameters did not correlate with any of the other parameters. The ultrastructure of 15 thin nerve branches--likely candidates for new branches--was investigated in serial section and in 14 of them synaptic contacts were found. Near such contacts, empty gutters, possibly abandoned former synaptic sites, were present in several cases. It is concluded that there is continual nerve sprouting in synapses of adult mice and that sprouts form synaptic contacts. The possible signs of nerve retraction observed indicate that, as in the frog, synaptic contacts in mouse muscles undergo some continual remodeling.