Peripheral nerve injury and regeneration.

The process of nerve regeneration has been studied extensively by traditional morphological methods, but it is only recently that has been possible to identify more precisely the contribution of different nerve subpopulations. By studying different models of nerve repair and regeneration, it is becoming apparent that other tissue components are contributing to the overall process. When muscle grafting is carried out to repair an injured nerve, the regenerating axons are migrating in parallel with Schwann cells to bridge the nerve gap. The presence of Schwann cells is essential for a successful nerve regeneration, most probably because their production of different neuronal trophic factors. This pattern is also repeated when fibronectin mats are used for nerve repair, indicating the possibility to use this new synthetic matrix for clinical application. If the target organ is analysed after nerve repair, the recovery of all nerve components is evident. However, the process occurs at different times in separate skin compartments, and the regeneration of the autonomic innervation appears to be preceded by that of the sensory nerves. When looking at cutaneous nerve regeneration following different type of injury, a common pattern of events becomes apparent. In skin flaps, nerve regeneration begins from the skin surrounding the wound edge, or from the pedicle, and sensory nerves are the first to penetrate into the flap. Angiogenesis precedes reinnervation of the flap, and initially regenerating fibres appear to be associated with newly formed blood vessels. This pattern is evident also in full-thickness wounds and in suction blisters, where only the more superficial cutaneous layer is disrupted. Furthermore, the presence of keratinocytes appears to exert a directional influence on both regenerating blood vessels and nerves, which follow the regenerating keratinocytes when reepidermalisation is taking place. These results would indicate that there is a close relationship between nerve fibres and blood vessels during regeneration, with a substantial contribution to the process from other tissue components and soluble factors from the surrounding environment.