The development of brain stem projections to the spinal cord in the chicken embryo.

Recent studies of the development of brain stem projections to the spinal cord in the chicken embryo, with an emphasis on axon pathway selection, are reviewed. Neurons from medullary to mesencephalic levels project to the spinal cord along specific fiber tracts. Coherent, segregated neuron groups can be defined on the basis of which tract and which side of the brain stem they project on. The choice of axon trajectory is, therefore, correlated with neuron position. During development, these trajectory-defined brain stem groups project to the spinal cord in a stereotyped sequence. Early stages of this sequence reveal a potential homology between the reticulospinal systems of avians and lower vertebrates. The possibility that neuron position may be involved in determining axon pathway choice of brain stem projections is supported by complementary studies on vestibuloocular projections. The boundaries between vestibuloocular, vestibulospinal, and reticulospinal neuron groups coincide with rhombomere boundaries and boundaries between longitudinal cell columns. Axon trajectory-specific domains are, therefore, correlated with segmental and mediolateral patterns of differential gene expression.