Axonal regeneration into chronically denervated distal stump. 2. Active expression of type I collagen mRNA in epineurium.

During the first 2 weeks after an injury to peripheral nerve, endoneurial cells proliferate and express integrin beta 1 and mRNA for collagen types I and III. Clinical results for surgical repair within this time are clearly better than those obtained after delayed (months after original injury) surgery. The question of whether this is due to changes in the proliferative capacity of endoneurial cells or to changes in expression of mRNA for collagen types I and III or integrin beta 1 was studied using rats. The left common peroneal nerve was transected and allowed to degenerate for 3 and 6 months. After these times, the tibial nerve of the same animals were transected, and the fresh proximal stump of the transected tibial nerve was sutured into the chronically denervated distal stump of the common peroneal nerve. At 3 and 6 weeks after the reoperation, samples were collected from the distal stump for morphometry, immunohistochemistry and in situ hybridization. Proliferating cells and Schwann cells were identified by immunohistochemistry. These cells increased markedly in number during the axonal reinnervation. In situ hybridization revealed that in the epineurium and perineurium, which were fibrotic, especially type I but also type III collagen mRNA were highly expressed. The amount of type I collagen mRNA in the endoneurium seemed to increase with progressing axonal reinnervation. Immunostaining for integrin beta 1 was negative in these distal stumps. In the present study the proliferation of endoneurial cells and expression of type I collagen mRNA in the endoneurium were similar to those found after immediate regeneration of transected peripheral nerve.(ABSTRACT TRUNCATED AT 250 WORDS)