Abstract
Extirpation of the wing bud in 2-day chick embryos results in a conspicuous degeneration of neurons in both populations of brachial dorsal root ganglia (DRG). Daily injections of 1 to 6 micrograms of nerve growth factor (NGF), beginning at 4 1/2 days of incubation, rescued all small, late differentiating (DM) neurons and approximately 50% of large, early differentiating (VL) neurons, which would have died otherwise. The fact that NGF is an effective substitute for the hypothetical trophic maintenance factor for DRG which is normally produced by limb tissues strengthens our belief that NGF is identical with this factor. The control experiment, i.e., wing extirpation without NGF injections, revealed an inconsistency with previous data. Experiments on a number of different neuronal units had shown rather consistently that the period of experimentally induced neuron degeneration, caused by removal of the target, is synchronous with the period of normally occurring neuronal death in the same neuronal unit. This synchrony rule is violated by the VL population of brachial DRG. In this unit, the peak of degeneration resulting from wing bud extirpation occurs considerably earlier than the peak of normally occurring neuronal death. The competition hypothesis for the explanation of neuronal death had been based, in part, on the synchrony rule. We discuss the question of whether the deviation from the synchrony rule observed in our material represents a serious challenge to the competition hypothesis.