EVIDENCE THAT SOME DEVELOPING LIMB MOTO-NEURONS DIE FOR REASONS OTHER THAN PERIPHERAL COMPETITION

In vertebrate embryos up to 75% of lateral motor column (LMC) cells die soon after innervating the limb bud. The hypothesis was tested that competition for unknown limb factors may decide which cells will survive. Removal of the future knee flexor motoneurons before the onset of cell death was attempted with varying success in Xenopus laevis tadpoles by removing a piece of spinal cord containing the rostral part of the left lumbar LMC. In normal tadpoles, hundreds of cells in the caudal part of the LMC temporarily project to the presumptive knee flexors and are among the first to die. The competition hypothesis predicts that they should remain alive after a successful operation. After maturation the most successful operations were found to have resulted in paralysis and hypoplasia of the knee flexors. Horseradish peroxidase tracing techniques confirmed that the knee flexors were not innervated. However, ankle and foot movements were normal indicating that the remaining caudal LMC cells had developed their normal projections to the distal limb. The failure to survive of the caudal LMC cells projecting to the knee flexors, despite the absence of rostral LMC cell innervation, shows that factors other than competition must control at least some LMC cell deaths. © 1979.