[Development of the cutaneous nervous system]

Skin of vertebrates is richly innervated, mainly by sensory nerve fibres which form a well organized pattern, particularly around phaners. This innervation develops segmentally (dermatomes) from cutaneous branches 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 characteristic pattern around feathers, and can be visualized on whole mounts. Its development can be traced from 6 days of development in relation to feather morphogenesis. Experiments producing non formation of spinal ganglia (X-ray irradiation or neural tube ablation) or production of neoapteria (hydrocortisone treatment) or ectopic feathers on scales (retinoic acid treatment) show there is a close link between feather development and nerve pattern formation. In vitro co-cultures of dorsal root ganglia and epidermis combined with the use of synthesis inhibitors and antibodies, showed that epidermis has a repulsive effect on nerve fibres mediated, at least in part, by chondroitin sulphate proteoglycans. These compounds have been localized, using antibodies mainly at the base of the feather buds and seem to play a key role in the construction of the fine nerve pattern around feather follicles. In conclusion, the specific nerve patterns are the final result of selective responses of growing nerve endings to unique combinations of local cues and conflicting interactions which are developmentally regulated in parallel with the morphogenesis of phaners.