PROGRAMMED CELL-DEATH AND THE CONTROL OF CELL-SURVIVAL - LESSONS FROMTHE NERVOUS-SYSTEM

During the development of the vertebrate nervous system, up to 50 perc ent or more of many types of neurons normally die soon after they form synaptic connections with their target cells. This massive cell death is thought to reflect the failure of these neurons to obtain adequate amounts of specific neurotrophic factors that are produced by the tar get cells and that are required for the neurons to survive. This neuro trophic strategy for the regulation of neuronal numbers may be only on e example of a general mechanism that helps to regulate the numbers of many other vertebrate cell types, which also require signals from oth er cells to survive. These survival signals seem to act by suppressing an intrinsic cell suicide program, the protein components of which ar e apparently expressed constitutively in most cell types.