Regrowth of PNS axons through grafts of the optic nerve of the Browman-Wyse (BW) mutant rat.

We have examined the behaviour in vivo of regenerating PNS axons in the presence of grafts of optic nerve taken from the Browman-Wyse mutant rat. Browman-Wyse optic nerves are unusual because a 2-4 mm length of the proximal (retinal) end of the nerve lacks oligodendrocytes and CNS myelin and therefore retinal ganglion cell axons lying within the proximal segment are unmyelinated and ensheathed by processes of astrocyte cytoplasm. Schwann cells may also be present within some proximal segments. Distally, Browman-Wyse optic nerves are morphologically and immunohistochemically indistinguishable from control optic nerves. When we grafted intact Browman-Wyse optic nerves or 'triplets' consisting of proximal, junctional and distal segments of Browman-Wyse optic nerve between the stumps of freshly transected sciatic nerves, we found that regenerating axons avoided all the grafts which did not contain Schwann cells, i.e., proximal segments which contained only astrocytes; regions of Schwann cell-bearing proximal segments which did not contain Schwann cells; junctional and distal segments (which contained astrocytes, oligodendrocytes and CNS myelin debris). However, axons did enter and grow through proximal segments which contained Schwann cells in addition to astrocytes. Schwann cells were seen within grafts even after mitomycin C pretreatment of sciatic proximal nerve stumps had delayed outgrowth of Schwann cells from the host nerves; we therefore conclude that the Schwann cells which became associated with regenerating axons within the grafts of Browman-Wyse optic nerve were derived from an endogenous population. Our findings indicate that astrocytes may be capable of supporting axonal regeneration in the presence of Schwann cells.