Retinogeniculate projections in albino and ocularly hypopigmented rats

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Abstract
The retinogeniculate fiber projections were studied by degeneration methods in several strains of rats with pigmentation in their eyes and pelts ranging from the intensely pigmented self phenotype to the albino. The ipsilateral retinogeniculate input in the self, Irish, and hooded rats, and rats with “bicolor fundus” is located medially within the dorsal lateral geniculate nucleus (LGd) and is seen as a single lamina of moderately dense degeneration. The contralateral retinogeniculate input in these ocularly pigmented rats is a dense network of degeneration covering all of the LGd except for the region receiving the ipsilateral input which is an area of sparse degeneration forming a “pocket.” There is a general paucity of ipsilateral fibers in albino rats. The ipsilateral projections in albino, light-eyed cream-pelted rats, and the rats with heterochromia irides is seen as several fragmented laminae. The contralateral input to the LGd is evenly distributed, is dense throughout and lacks any discernible “pocket” of sparse degeneration. Only the presence of melanin pigment of optic cup origin appears to correlate positively with the development of normal retinogeniculate projections. Variability was found between animals regardless of the degree of pigmentation shown by the animal. There would appear to be enough genetic modifying factors inducing variation of these fiber projections that the appearance of a particular pattern of input to the LGd should be considered as having only a statistical probability of occurrence.