Synaptic takeover at developing neuromuscular junctions following laser-induced damage of an axonal input.

*t = 17 h. (A) At 0 h the larger diameter (“Thick”) input was removed by laser illumination. In one case (**) the smaller diameter (“Thin”) input was removed. Axon caliber was correlated with territory occupied at the junction. In general the “Thin” input occupied less territory than the “Thick” input. The territory occupied by the remaining input (% occupancy at 0 h) was measured relative to the territory occupied by the two together (100%). In every case the remaining input grew and took over the synapse. In most cases (n = 14/15) takeover was complete (100% occupancy) by 24 h. Takeover occurred even when the input was very small (1%–5% occupancy at 0 h). (B) Synaptic takeover also occurred at junctions that were singly innervated and had a retracting axon nearby, that is, where synapse elimination had recently been completed. In this case, one input occupied 100% of the junction, and the other occupied 0% at the time of laser irradiation. In these cases, removal of the innervating input often resulted in the retracting axon growing back to occupy the junction (55%, n = 10/18). In the remaining cases the retracting axon did not grow back and/or had disappeared when the junction area was viewed the next day. The distance of retracted axons from the junctions at the first time point (0 h) is indicated in the far right column.