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Ultrastructure of early axonal regeneration in an end-to-side neurorrhaphy model.

The ultrastructure of the early regenerative response in an end-to-side neurorrhaphy rat model was studied using transmission electron microscopy. The ipsilateral saphenous nerve was grafted to the sciatic nerve under the following conditions: Group 1, the epineurium and perineurium of the sciatic nerve remained intact; Group 2, an epineurial and perineurial window was created at the site of the lateral neurorrhaphy; Group 3, the same as in Group 2 and, in addition, the sciatic nerve sustained a partial neurectomy. Rats were perfused through the heart with fixative containing 2 percent paraformaldehyde and 2.5 percent glutaraldehyde in 0.1 M cacodylate buffer (pH 7.4) at 4, 8, 12, 24 and 48 hr after surgery. In Group 1, no regenerating axons were observed and the myelin sheath in the donor nerve did not demonstrate any degenerative changes through 48 hr. In Group 2, an increased diameter of the unmyelinated axons and growth cones was observed in the donor nerve proximal to the coaptation site after 12 hr. Degenerative changes in the myelin sheath were observed after 12 hr within the several layers under the coaptation site. In Group 3, many growth cone-like structures were observed in the area proximal to the coaptation site after 12 hr. After 24 hr, proximal regenerating axons elongated to the coaptation site and, at 48 hr, many regenerating nerves grew inside the Schwann cell basement membrane of the graft nerve. These results indicate that the perineurial window and nerve graft are the critical conditions for inciting nerve regeneration in the donor nerve.