OPTIC-NERVE REGENERATES BUT DOES NOT RESTORE TOPOGRAPHIC PROJECTIONS IN THE LIZARD CTENOPHORUS-ORNATUS

In adult fish and amphibians, the severed optic nerve regenerates and visual behaviour is restored. By contrast, optic axons do not regenera te in the more recently evolved birds and mammals. Here we have invest igated optic nerve regeneration in a member of the class Reptilia, phy logenetically intermediate between the fish and amphibians and the bir ds and mammals. We assessed visual recovery anatomically and behaviour ally one year after unilateral optic nerve crush in the adult ornate d ragon lizard, Ctenophorus ornatus. Ganglion cell densities and numbers of axons in the optic nerve on either side of the crush site indicate d that two-thirds of ganglion cells survived axotomy and regrew their axons. However, myelination fell from a mean of 21% in normals to 5.5% and 3%, proximal and distal to the crush, respectively. Anterograde l abelling of the entire optic nerve showed that axons regenerated along essentially normal pathways and that the major projection, as in norm als, was to the superficial one-third of the contralateral optic tectu m. However, localised retinal injections indicated that regenerated pr ojections lacked retinotopic order. Any one retinal region projected t o the entire tectum. This feature presumably explains why the experime ntal lizards consistently appeared blind to stimuli via the regenerate d nerve. Our findings indicate that although axons regenerate along es sentially normal pathways in adult Lizards, conditions within the visu al centres do not allow regenerating optic axons to select appropriate central connections. In a wider context, the result suggests that the ability for regenerating central axons to form topographic maps may a lso have been lost in the more recently evolved vertebrate classes. (C ) 1997 Wiley-Liss, Inc.