An ultrastructural study of Schwann cell response to axonal degeneration.

Adult Sprague-Dawley rats were used to prepare isolated and distal segments of ovarian, vagus and saphenous nerves. Isolated segments were prepared by cutting between double ligatures followed by retracting and anchoring the proximal and distal segments 0.5 cm or more away from the isolated piece. After various intervals between 3 and 123 days and rats were perfused with buffered glutaraldehyde and nerves were processed for electron microscopic examination. Cross sections of isolated and distal segments of all nerves displayed an abundance of mostly circular profiles containing microtubules and filaments that were indistinguishable from those in unmyelinated nerve fibers. Many such profiles were nested against perinuclear Schwann cell cytoplasm, others were present in isolated clusters enclosed only by basement membrane. Examples of continuity between the nerve-like profiles and perinuclear cytoplasm of Schwann cells were found in longitudinal sections. In time, the number of such processes gradually diminished, but 123 days after transection they still could be found. It was concluded that Schwann cells in isolated and distal segments of transected nerves develop great numbers of small cylindrical processes that are arranged in bundles parallel to the long axis of the degenerated nerve. These processes had an arrangement of cytoplasmic organelles similar to axons of the proximal segment and the contralateral nerve. However the processes differed from most mature axons in that they were not ensheathed by Schwann cell cytoplasm with a mesaxon.