Axonal guidance during development of the optic nerve: The role of pigmented epithelia and other extrinsic factors

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Abstract
It is well established that a congenital lack of ocular melanin (albinism) can lead to developmental abnormalities of the central visual pathways. However, it is yet unknown how the pigmentation per se acts to influence formation of the optic projection. In order to study the possible interaction between eye pigment and optic axons during development, we have examined, with the use of serial section techniques, a series of timed embryos at stages when the ocular pigment and outgrowing axons first become apparent. Our results have demonstrated that, in mice and rats, the upper wall of the distal half of the primitive eye stalk (a region which lies along the potential route to be taken by the earliest developing nerve fibers) is transiently pigmented prior to and during the migration of the pioneer optic axons. All outgrowing neurites avoid this stretch of melanotic tissue and instead grow preferentially through a system of extracellular tunnels in the ventral, pigment-free zones of the distal eye stalk. The stalk remains unpigmented from about its midpoint and continuing toward the brain. At the pigment/pigment-free interface many of the axons shift upward from their ventral positions, forming a marginal annulus. In the chick, on the contrary, pigmentation of the stalk does not occur and as the optic axons exit the globe they grow immediately in an annulus configuration. In Xenopus, the entire stalk becomes pigmented and the optic fibers congregate in one discrete bundle of fascicles along the length of the stalk's most ventral margin. These observations suggest that melanin-producing stalk cells may play a role in controlling the topographic patterning of optic fibers within the developing nerve by inhibiting the lateral spread of axonal growth cones into or within their territory. To test this hypothesis we have charted the distribution of optic fibers in the developing optic stalks of timed albino rat embryos. Indeed, as fibers leave the mutant eye, it was found that a small but consistent number of pioneering axons (day E15) become ectopic and immediataely invade nonpigmented regions (those normally pigmented and axon-free) in the distal optic stalk. Thus, the usual topographic arrangement of the collection of pioneer optic fibers is altered in the albino.