Development of cutaneous and proprioceptive afferent projections in the chick spinal cord.

Muscle and cutaneous nerves were individually labeled with DiI in chick embryos to examine the development of sensory afferent arborizations in the spinal cord. Initially, cutaneous and muscle arbors were similar; both types first entered the spinal gray matter at stage 28-29 (embryonic day (E) 6). Differences in projections were first observed by late stage 34 (E8.5): muscle afferent collaterals extended almost unbranched to the level of motoneuronal dendrites while cutaneous afferents branched frequently and remained within the dorsal horn. Projections of putative small caliber axons into laminae 1 and 2, located laterally in the chick, did not develop until E13-14.