Development of central projections of lumbosacral sensory neurons in the chick

The development of central projections of sensory neurons in lumbosacral dorsal root ganglia (DRGs) was examined by using horseradish peroxidase labeling techniques in chick embryos from stage 23 (E4) to stage 39 (E13). Our results show that primary afferents reach the spinal cord by stage 23. Afferent axons extend in the primordium of the dorsal funiculus for several segments rostral and caudal to their segment of entry for over 24 hours before invading the gray matter at stage 28 (E6). Sensory fibers grow into the vicinity of motoneuron dendrites by stage 32 (E7.5), about the time that reflexes and apparent monosynaptic EPSPs can first be elicited. Dense projections into the dorsal laminae of the spinal cord, presumably representing cutaneous afferents, appear somewhat later, at about stage 39 (E13), when the segmental projection pattern begins to resemble the mature pattern.