Partial denervation of the whiskerpad in adult mice: pattern and origin ofreinnervation

We studied sensory organ reinnervation after nerve transection in the mouse whisker-to-barrel pathway. In one set of adult mice, we determined at ligh t microscopy level the number of fibres reaching the caudal whisker follicl es 5, 15, 20, 60, 100 days and 1 year after transection of the sensory nerv e of row C. Regenerated fibres were first detected 15 days post lesionem (p .I.) and myelin first observed at 20 days. Between 60 and 100 days, the num ber of fibres stayed at approximate to 80% of the values obtained in contro l animals. At that time, myelinated fibres reached only 58% of their number in controls. At the electron microscopy level, these fibres differ from co ntrol ones by a smaller fibre diameter. The innervation of follicles of adj acent rows was not modified, indicating that follicular reinnervation is ro w specific. We checked this feature by injecting in another set of mice the denervated follicles and the adjacent ones with distinct retrograde tracer s 45 days and 1 year after nerve transection. The percentage of double-labe lled neurons in the Gasserian ganglion did not increase in experimental ani mals. This confirms the absence of colonization of intact follicles by rege nerating fibres and indicates that reinnervation of the whisker follicles t akes place by regeneration of the degenerated axons without collateral rein nervation. The companion paper describes the pattern of activation of the b arrel cortex relative to the present findings.