Regeneration in the optic nerve of adult rats: influences of cultured astrocytes and optic nerve grafts of different ontogenetic stages.

We have studied the effects of transplanted optic nerves of different ontogenetic stages (E19 to adult), and cultured astrocytes from P2 cerebral cortex on the regeneration of axons in the optic nerve of adult rats. Regeneration was visualized by anterograde tracing with rhodamine-iso-thiocyanate. Grafts were identified with Nuclear Yellow. Astroglia within both the cut optic nerve and the transplants were detected by anti-glial fibrillary acidic protein staining. In control animals (cut optic nerve, 2-3 mm behind the optic disc), only a few neurites were found 15 days after the operation which grew randomly for short distances into the surrounding meningeal sheaths. Perinatal (E19 to P2) optic nerves induced a massive outgrowth of RITC-filled axons from the host optic nerve. The regenerating fibres grew for up to 3 mm towards the graft, ahead of glial fibrillary acidic protein-positive astroglia emanating from the host optic nerve that seemed to follow them. Although the regenerating fibres reached the grafts, they did not penetrate them. Optic nerve grafts of increasing age elicited smaller growth responses; e.g. grafts from P8 promoted only a very limited (several 100 microns) growth response, grafts from P12 and later induced outgrowth comparable with that of control animals. Grafted astrocytes from P2 donors that had previously been grown in culture, were also capable of promoting outgrowth of rhodamine-iso-thiocyanate-filled axons from the host optic nerve. These findings suggest that only astrocytes at an immature stage of differentiation are capable of inducing axon growth from the adult optic nerve. Furthermore, the absence of an obvious cellular bridge between host and graft suggests that the graft effect is probably mediated by the release of astroglia-derived diffusible neurite growth promoting factors.