Postnatal growth of motor nerve terminals in muscles of the mouse

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Abstract
A reduced silver stain was used to examine the development of complexity of motor nerve terminals in the postnatal period. Terminals in three histochemically different muscles were examined in mice aged 12 days to 3 years. The total number and total length of intraterminal axon branches increases with age, but only until animals are 3 months old. Terminals become longest and most branched in the histochemically glycolytic tensor fascia latae (TFL) muscle, shortest and least branched in the oxidative diaphragm, and intermediate in the histochemically mixed gluteus muscle. In addition, myelinated terminal branches develop in TFL and to a lesser extent in the gluteus between 1 and 3 months of age. These myelinated branches appear to be produced by nodal sprouting from the penultimate node of Ranvier of the terminal axon, and also by myelination of pre-existing terminal branches. The diameter of muscle fibres also increases until animals are 3 months old, and there is a good correlation between mean fibre diameter and either mean terminal length or mean number of terminal branches when all muscles at all ages are compared. This suggests that terminal growth could be determined by muscle fibre growth; however, within any given muscle there is little or no correlation between the diameter of a muscle fibre and either the length or number of branches of its nerve terminal, suggesting that terminal morphology is not controlled solely by muscle fibre growth. The presence of a myelinated branch in a nerve terminal is also unrelated to fibre diameter within a given muscle, but again when means are compared there are good, but significantly different correlations for the three different muscles. Thus some kind of muscle or nerve type-specific property additional to a general effect of muscle fibre size influences the development of myelinated terminal branches. Between 3 and 12 months of age terminal complexity remains constant or may decrease slightly. At 19 months or older, when mice are becoming senile, a large proportion of synapses have terminal sprouts and muscle fibres become innervated by two or more distinct axons. These changes can be attributed to the death of some motor neurons and sprouting of the remaining axons.