Retinal projections in two Australian polyprotodont marsupials: kowari, Dasyuroides byrnei, and fat-tailed dunnart, Sminthopsis crassicaudata (Dasyuridae).

Retinal projections were examined in two small dasyurids, the kowari and the fat-tailed dunnart, following injections of 3H-proline into one eye. In both animals retinal fibres terminate in the dorsal and ventral lateral geniculate nuclei (LGd, LGv), the lateral posterior nuclear complex, the pretectum, the superior colliculus, the suprachiasmatic nucleus of the hypothalamus and the nuclei of the accessory optic system. The lateroposterior thalamic complex and the accessory optic nuclei receive projections from the contralateral eye only; the remaining centres receive bilateral inputs. Both LGd contain an undifferentiated beta or medial segment and an alpha or lateral segment that comprises further cellular sublaminae, 4 in the kowari and 3 in the dunnart. There is substantial overlap of crossed and uncrossed terminals in both segments, though in each animal a narrow cell lamina next to the optic tract receives only crossed projections and the lateral part of the beta segment receives only uncrossed projections. There is a cell-sparse zone within the alpha segment that receives a predominately uncrossed projection in the kowari and a crossed projection in the dunnart. In both marsupials the density of crossed and uncrossed terminals is equal, a feature of dasyurid quolls but not of another dasyurid, the Tasmanian devil. Additionally, retinal terminals do not form dense clusters within the LGd neuropil. This feature is characteristic of quolls, but not of other mammals, marsupial or placental, all of which display LGd terminal clusters. These findings suggest that the functional organisation of the LGd in these dasyurids may differ from that found in other marsupials.