GRADUAL LOSS OF SYNAPTIC CARTELS PRECEDES AXON WITHDRAWAL AT DEVELOPING NEUROMUSCULAR-JUNCTIONS

We have studied the spatial deployment of synapses arising from differ ent axons that converge on the same developing neuromuscular junctions . Labeling the competing synaptic ''cartels'' with different dyes in m ouse muscle showed that, perinatally, each axon adds similar terminal areas, whereas later, areas occupied by the competing cartels diverged by gradual elimination of one axon's synapses and ongoing addition of synaptic area by the other. Activity-dependent labeling of synapses c apable of vesicle recycling in snake muscle also revealed a gradual ch ange in territories occupied by competing inputs, implying that an axo n maintained some functional synapses even as others in its cartel wer e being eliminated. Thus the process of synapse elimination is gradual , with loss of one viable synapse after another, until an axon is left with no synaptic territory and withdraws.