The death of large numbers of young neurones is a striking feature in the developing nervous system. Many authors have postulated that the neurones are produced in excess of the numbers that their target tissues can support, forcing them to compete for survival1–3. Observations on developing limb motoneurones (ventral horn cells) in both frogs and chicks seem to support this hypothesis. Normally, one-half to three-quarters of ventral horn cells die as limb movements begin4,5, but some of these (up to 25% in chicks, 7% in frogs) can be rescued by the provision of supernumerary limb buds before motor axon outgrowth begins6–10. Conversely, amputation of the limb bud causes an increased death rate approaching 100% (refs 4,11). The crucial test of the hypothesis depends on its prediction that the limb should limit the number of surviving motoneurones, and here I have attempted to make the test. For this, both sides of the spinal cord were forced to project to a single hind limb bud well before the onset of ventral horn cell death. It was found that the combined total of surviving motoneurones (right plus left sides) projecting to the single limb exceeded the number that projects to one limb in normal animals by up to ∼100%. It is concluded that the limb does not limit the number of survivors, and therefore the hypothesis is refuted, at least for ventral horn cells. It now seems more likely that motoneurone numbers are determined either by the need to eliminate ventral horn cells not connected with appropriate limb regions, or by factors operating within the central nervous system.
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