Cell death of motoneurons in the chick embryo spinal cord. III. The differentiation of motoneurons prior to their induced degeneration following limb‐bud removal

The differentiation of motoneurons following early limb‐bud ablation was studied in chick embryos from four days to hatching. Following the removal of the normal targets of these cells about 90% of the neurons in the lateral motor column (LMC) of segments 23–29 (lumbar) were found to disappear. By counting degenerating cells it was shown that virtually all of the cell loss could be accounted for by cell death, rather than impaired proliferation or enhanced migration away from the LMC. Quantitative comparisons of cell death between the peripherally deprived and the control, non‐deprived side demonstrated that limb‐bud removal not only enhanced the 50% natural cell death known to occur in this system, but also greatly accelerated the whole process. By stage 30 (6.5‐7 days) 75% of the final cell loss had occurred on the deprived side, whereas only 40% of the final cell loss had occurred on the control side. In both cases, however, cell death was confined to the period of limb innervation. Axon counts of the peripherally deprived ventral root showed that all the deprived neurons initially had sent an axon out of the spinal cord. Most of these, however, became caught in a neuroma before reaching the site of limb attachment. Though no synapses were found in the neuroma the axons were shown to be able to transport HRP back to the spinal cord. Before they began to degenerate, the deprived LMC motoneurons developed dendritic processes and these were able to form synapses with axons in the prospective lateral white matter. In early stages, frequent axo‐glial “synapses” were observed in the prospective lateral white matter of both deprived and control sides of the spinal cord. Since by stage 36 (day 10) these had virtually all disappeared, it was suggested that synapse formation in this region of the spinal cord may initially be under few constraints. In late stages (i.e., after day 8) it was noted that there were frequently signs of axonal degeneration in the lateral white matter on both sides of the spinal cord, suggesting a retrograde transneuronal degeneration initiated by the earlier cell death of motoneurons. Electron microscopic examination of the deprived LMC cells at different stages prior to degeneration failed to uncover any obvious differences between them and control cells on the non‐deprived side of the spinal cord. By histochemical and neurochemical methods the cholinergic enzymes acetylcholinesterase and choline acetyltransferase were found to develop normally up until the onset of frank degeneration in the deprived motoneurons, on day 5 or 6. After this the enzymes decreased at a rate comparableto the morphological loss of motoneurons by cell death. On the basis of these various lines of evidence, it is argued that all the motoneurons in the LMC have a remarkable intrinsic capacity to initiate differentiation and that neurons experimentallydeprived of their normal target are no different in this respect.