Target dependency of developing motoneurons in Xenopus laevis.

The effect of complete and partial limb bud removal on motoneuron survival was studied in Xenopus laevis tadpoles. Amputation of both hind limb buds at stage 49 before limb innervation begins caused the subsequent death of all motoneurons. This result confirms that there are no exceptions to the rule that developing motoneurons must contact the limb to survive. Partial removals of the limb at early stages of innervation caused the subsequent death of motoneurons that normally project to the deleted segments while motoneurons for the remaining segments survived. Before dying, the motoneurons deprived of their normal targets invaded the remaining limb segments thereby forming erroneous projections. Since the motoneurons died despite contacting limb tissue, it is concluded that they must be dependent on contact with specific limb regions and that errors of projection lead to death. Since large-scale errors of the type induced by this experiment affect only a minority of developing motoneurons in normal embryos, the possibility of small-scale errors is discussed in an attempt to equate all naturally occurring motoneuron death with error correction.