Cutaneous nerves of the embryonic chick wing do not develop in regions denuded of ectoderm.

Peripheral nerves travel to their targets along precise routes, and it is likely that different cues provide guidance at different stages of the journey. In a developing chick limb, the cutaneous nerve fibres follow at first deep mixed nerve trunks, in company with motor axons; they branch from these trunks at predictable points and approach the skin; they then ramify profusely to form a plexus at a precisely defined depth beneath the ectoderm, at exactly the same level as the blood vascular plexus. To analyse the role of signals from the target patch of skin in regulating cutaneous nerve development, we have ablated patches of dorsal wing ectoderm using short-wave ultraviolet irradiation at E4 (embryonic day 4), approximately one day before nerves grow into the limb bud. The irradiated patches remain denuded of ectoderm for more than a week, by which time the cutaneous nerve plexus on the contralateral control side is well developed and can be revealed by whole-mount silver staining. Where the ectoderm has been ablated, no cutaneous nerve plexus forms, and the nerve branches that normally would have diverged from the neighbouring mixed nerve trunk to innervate the missing patch of skin are absent - ab initio, apparently. The routes of the mixed nerve trunks are not affected. Partial ablation of the territory of a cutaneous nerve branch often leads to loss of the whole nerve branch; the intact skin territory thus left vacant is invaded by ramifications from the remaining cutaneous branches, as expected if the normal extent of a cutaneous nerve's territory is regulated by competition. Where there is an ectodermal lesion, cutaneous innervation stops precisely at its boundary, even though the vascular plexus extends for some distance beyond this margin, beneath the denuded surface. The data suggest that the embryonic skin is required firstly to trigger divergence of cutaneous nerve branches from the mixed nerve trunks, and secondly, once the nerve fibres have reached the skin, to supply a trophic cue (probably NGF) encouraging growth of a plexus; at the same time, the embryonic skin generates a signal inhibiting nerves from approaching closer than about 70 microns to the surface.