Programmed death of peripheral pioneer neurons in the grasshopper embryo.

The Ti1 pioneer neurons arise at the distal tip of the metathoracic leg in the grasshopper embryo, and are the first neurons in the limb bud to extend axons to the central nervous system (C. M. Bate (1976) Nature (London) 260, 54-56; H. Keshishian (1980) Dev. Biol. 80, 388-397). By providing a neural pathway along which growth cones of later arising neurons migrate, these pioneer axons establish the route of one of the major nerve trunks in the leg (Keshishian, 1980; R. K. Ho and C. S. Goodman (1982) Nature (London) 297, 404-406; D. Bentley and H. Keshishian (1982) Science 218, 1082-1088). Here, we demonstrate that at the 55-59% stage of development, the two Ti1 pioneer neurons undergo programmed death. The role which these pioneers serve in establishing a nerve route appears to be their only function, and may be important for the normal development of the peripheral nervous system. The Ti1 pioneers provide an example of a previously hypothesized class (J. W. Truman (1984) Annu. Rev. Neurosci. 7, 171-188) of programmed neuron death: obsolete neurons whose function was developmental rather than behavioral.