Ultraterminal sprouting in innervated and partially denervated adult and aged rat muscle.

End plates from the extensor digitorum longus, soleus and diaphragm muscles of adult (10-month-old) and aged (25-month-old) rats were examined to determine whether aging affects the frequency of occurrence of ultraterminal sprouting, a form of terminal sprouting commonly associated with muscle denervation. There was a significant increase in the fraction of end plates exhibiting ultraterminal sprouts in the extensor digitorum longus muscle only; the number increased from 5.5% in 10-month-old animals to 23.0% in 25-month-old animals. To see whether age also influences the sprouting response to denervation, extensor digitorum longus muscles from 10-and 25-month-old animals were partially denervated by severing spinal nerve L5, and end plates were examined 4-7 and 10-14 days following denervation. Denervation induced a more profound increase in the percentage of end plates exhibiting ultraterminal sprouting in the 10- compared with 25-month-old animals. However, sprouting remained significantly greater in the 25- compared with the 10-month-old animals. Furthermore, following denervation the average ultraterminal sprout length was significantly greater in the 25- compared with the 10-month-old animals. There was no correlation between the extent of muscle denervation and the percentage of end plates exhibiting ultraterminal sprouts. End plates with ultraterminal sprouts had larger areas and contained fewer nerve terminal branches than end plates without these sprouts. It is suggested that the limited response to partial denervation in the 25-month-old extensor digitorum longus muscles may indicate that the aged extensor digitorum longus has already approached the maximum capacity for sprouting.