Abnormal pigmentation and unusual morphogenesis of the optic stalk may be correlated with retinal axon misguidance in embryonic Siamese cats.

Studies of albino rodents have shown that an absence of pigment in the developing optic stalk may alter the position of the first retinal fibers that grow toward the brain, thereby disrupting the gross topographic relationship of fibers in the nerve (Silver and Sapiro: J. Comp. Neurol. 202:521-538, '81). The abnormalities associated with albinism are more extensive in the Siamese cat than-in previously studied species. Therefore, any abnormalities in differentiation of the stalk and axon guidance may be more readily detected. To investigate the guidance and/or misguidance of optic axons, light and electron microscope analyses were made of serial sections through the optic stalk in normally pigmented and Siamese fetal cats. On E20, before axons enter the optic stalk, the only clear morphological distinction between Siamese and normal cats is the distribution of pigment in the stalk. Pigment is found in the dorsal stalk cells of the normal cat for 200 microns from the optic disc. Although the retinal pigment epithelium of the Siamese optic stalk. By E23 axons invade the ventral optic stalk in both strains. Concurrent with the early stages of axonal exit from the retina, there is complete separation of the stalk's dorsal and ventral tiers. As the cleavage occurs, basal lamina invaginates into the zone of separation following along the plane of the old lumen. The ventral stalk fills with axons while the dorsal tier is shed gradually. In contrast, in the Siamese cat, dorsal stalk cells are not sloughed off properly and instead are incorporated ectopically into the nerve. Basal lamina invagination is irregular. Axons do not fill the Siamese stalk symmetrically but enter the region of ectopic cells, which in turn disrupts gross fiber position. Usually, in the mutant, axons originating from the retina temporal to the optic fissure are those that invade the dorsal tier of ectopic cells. The altered position of optic axons in the mutant stalk may provide an explanation for the chiasmatic misrouting of optic axons in this species.