Retinal ganglion cells projecting to the optic tectum and visual thalamus of lizards

Retinal ganglion cells projecting to the optic tectum and visual thalamus have been investigated in the lizard, Podarcis hispanica. Injections of biotinylated dextran-amine in the optic tectum reveal seven morphological cell varieties including one displaced ganglion cell type. Injections in the visual thalamus yield similar ganglion cell classes plus four giant ganglion cells, including two displaced ganglion cell types. The present study constitutes the first comparison of tectal versus thalamic ganglion cell types in reptiles. The situation found in lizards is similar to that reported in mammals and birds where some cell types projecting to the thalamus are larger than those projecting to the mesencephalic roof. The presence of giant retino-thalamic ganglion cells with specific dendritic arborizations in sublaminae A and B of the inner plexiform layer suggests that parts of the visual thalamus of lizards could be implicated in movement detection, a role that might be played by the ventral lateral geniculate nucleus, which is involved in our tracer injections.