The axonal projections of the Hofmann nuclei in the spinal cord of the late stage chicken embryo.

The projection pattern of the neurons of the paragriseal Hofmann nuclei was mapped in the chicken embryo using the lipophilic tracer DiI. This report focuses on the pattern of projection from the Hofmann nuclei major observed 1-4 days prior to hatching, at which time the projection appears to be substantially developed. (1) Each neuron extends a commissural axon through the ventral gray matter and across the midline in the ventral commissure. The axons originating from a single Hofmann nucleus cross within a stretch of the cord equivalent to about one spinal segment. There is a small overlap of the axon populations originating from adjacent Hofmann nuclei. After reaching the contralateral ventral columns the individual axons bifurcate and extend rostrally and caudally up to 5 spinal segments in each direction. The rostral and caudal trajectories differ; the rostral axons shift progressively more laterally while the caudal axons tend not to deviate from their initial course. (2) Throughout their longitudinal course the axons give rise to terminal collaterals that are concentrated in lamina 8. Rostrally and caudally the terminals decrease in density and become increasingly scattered. (3) Hofmann neurons are multipolar with 4-5 laterally directed primary dendrites whose arbors are restricted to the Hofmann nucleus major within which the neurons reside. (4) Hofmann neurons receive afferent terminals from a longitudinal column of commissural interneurons located contralaterally in close approximation to the central canal. Each Hofmann nucleus major is innervated by a rostrocaudally restricted subset of these presynaptic neurons. The axon trajectories of the presynaptic neurons are similar to those of the Hofmann neurons. (5) Paragriseal neurons that are not located within Hofmann nuclei major are also commissural intersegmental interneurons and tend to be clustered segmentally. The segmentation is clearest for the Hofmann nuclei minor, which are clusters of neurons iterated along the ventrolateral margin of the thoracic spinal cord but not organized in protruding lobes.