In a number of mammalian species, fibre outgrowth in the developing retinofugal pathway is coincident with the presence of melanin in the retinal part of the optic stalk. The presence of melanin is transient in this developing system and has been proposed to play a role in the guidance of retinofugal fibres. Further, it has been suggested that this stalk melanin accounts for the differences between the size of the uncrossed retinal component in pigmented and nonpigmented strains. However, a recent study showed that there is no melanin in the optic stalk of Manchester rats during fibre outgrowth. Since such rats supposedly have a normal pigment distribution and a normal pattern of decussation at the optic chiasm, this finding appears to undermine the suggested role played by stalk melanin in establishing the laterality of retinal fibre projections in other mammalian species. The aim of this study was to re-evaluate the relationship between melanin in the stalk and the development of the retinofugal pathway in three strains of rat: the Wild type, Long Evans Hooded, and the Albino. The Albino rat, which lacks melanin-bearing cells entirely, was shown to have the smallest uncrossed projection, approximately 1,340 ipsilaterally projecting cells (ipc), whereas the Long Evans (2,760 ipc) and the Wild-type strain (2425 ipc) were found to have a larger uncrossed retinal component. In both pigmented strains, melanin was restricted to the eye cup and absent from the optic stalk throughout all stages of development. These results extend the finding that in the rat no correlation can be found between the size of the adult uncrossed retinofugal pathway and the existence of stalk melanin during development. Further, it was found that during initial outgrowth the distribution of fibres in the distal stalk was similar for all three strains, providing evidence that fibre topography does not account for the differences in the number of ipsilaterally projecting ganglion cell fibres. Finally, the position of earliest uncrossed fibres in the stalk was determined in the mouse, an animal with melanin-bearing cells in this region. It was found that these fibres intermingled with crossed fibres throughout the length of the stalk and that their position within the stalk was similar in both pigmented and albino strains. These observations suggest that stalk pigment plays no specific role in the development of the chiasmatic pathway of retinofugal fibres in either the rat or the mouse.
