Pathway specificity of reticulospinal and vestibulospinal projections in the 11-day chicken embryo.

The organization of the axonal pathways of reticulospinal and vestibulospinal projections in the 11-day chicken embryo was ascertained through retrograde tracing experiments. An in vitro preparation of the brainstem and cervical spinal cord facilitated precisely localized tracer applications. Single- and double-labelling experiments involving high cervical injections of tracers in combination with selective lesions defined the specific pathways by which different brainstem neurons project to the spinal cord. Coherent, and in many cases distinct, groups of reticulospinal and vestibulospinal neurons could thus be identified on the basis of their position and projection pathway. The organization of these groups and their projections in the 11-day chicken embryo is similar to that in avian and other vertebrate adults and therefore serves as a reference point for studies of pathfinding by bulbospinal axons during early development.