Degenerative changes in the distal stump of the severed human facial nerve

Specimens of distal segments of the human facial nerve obtained at reinnervation surgery (hypoglossal-facial anastomosis) were studied by light and electron microscopy. Severance of the facial nerve led to the Wallerian degeneration in the distal segment. When the lesion was made at the intracranial level of the nerve, the sensory bundle remained intact. In a specimen 17 days after section, motor fibers either had disappeared or had loss of axons and advanced stage of myelin breakdown. Sensory fibers appeared intact. In two specimens three months after severance, many Schwann cells of degenerated fibers still possessed degraded myelin; most Schwann cells had ingested myelin and formed endoneurial tubes (bands of Büngner). Endoneurium showed proliferation of fibroblasts and increased collagen deposition. A specimen obtained seven months after section showed fibrosis only. The fifth specimen obtained 30 months after severance of the tympanic segment showed only mild collagenization and well formed, not appreciably shrunken Schwann tubes. The distal stump of human facial nerve appears to undergo less denervation atrophy than demonstrated in the peripheral nerves of experimental animals. The collagenization process appears to be essentially completed already after three months denervation with little change after that. The distal stump of the human facial nerve appears to provide a good structural basis for reinnervation even after prolonged denervation.