DEVELOPMENT OF CUTANEOUS NERVOUS-SYSTEM

Skin of vertebrates is richly innervated, mainly by sensory nerve fibr es which form a well organized pattern, particularly around phaners. T his innervation develops segmentally (dermatomes) from cutaneous branc hes provided by spinal nerves. The innervation begins at 13 days (E 13 ) in the mouse embryo and, although hair buds form at E 16, follicles are only innervated from 5 days postnatally being complete at about 20 days. In the chick skin, innervation forms a regular and characterist ic pattern around feathers, and can be visualized on whole mounts. Its development can be traced from 6 days of development in relation to f eather morphogenesis. Experiments producing non formation of spinal ga nglia (X-ray irradiation or neural tube ablation) or production of neo apteria (hydrocortisone treatment) or ectopic feathers on scales (reti noic acid treatment) show there is a close link between feather develo pment and nerve pattern formation. In vitro co-cultures of dorsal root ganglia and epidermis combined with the use of synthesis inhibitors a nd antibodies, showed that epidermis has a repulsive effect on nerve f ibres mediated, at least in part, by chondroitin sulphate proteoglycan s. These compounds have been localized, using antibodies, mainly at th e base of the feather buds and seem to play a key role in the construc tion of the fine nerve pattern around feather follicles. In conclusion , the specific nerve patterns are the final result of selective respon ses of growing nerve endings to unique combinations of local cues and conflicting interactions which are developmentally regulated in parall el with the morphogenesis of phaners.