Progressive restriction of synaptic vesicle protein to the nerve terminal during development of the neuromuscular junction

Antibodies to synaptic vesicle (SV) proteins and to neurofilament (NF) proteins were used to investigate presynaptic differentiation and its relation to the formation of acetylcholine receptor (AChR) clusters at developing mouse neuromuscular junctions. At all times during development, SV proteins and NF proteins were segregated into neighboring, but separate regions of the presynaptic neurite. At embryonic day (ED) 14 SV proteins were present throughout preterminal neurites but at later ages became progressively restricted to the distal parts of the neurites. NF proteins occupied a more proximal region that extended distally during development until NF proteins occupied the entire axon up to the terminal. The restriction of SV proteins exclusively to the terminal did not occur until the second postnatal week. At the time of their first appearance (ED 14), up to 50% of AChR clusters were not associated with neurites; precise colocalization required 12-36 hr to develop. These findings demonstrate a progressive restriction of both pre- and postsynaptic components to the synapse during development.