Relationship of retinotopic ordering of axons in the optic pathway to the formation of visual maps in central targets

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Abstract
We examined in rats the relationship between the ordering of retinal axons in the optic pathway and the formation of a retinotopically organized projection to their primary target, the contralateral superior colliculus (SC). We have previously found that axons labeled by focal injections of 1, 1′‐dioctadecyl 3, 3, 3, ′3′ ‐tetramethylindocarbocyanine perchlorate (DiI) made in temporal or nasal retina of perinatal rats commonly mistarget along the medial‐lateral and rostral‐caudal axes of the SC. By postnatal day (P) 11–12, the retinocollicular projection attains an adult‐like topography. Incorrectly targeted axons or axon segments are removed; axons that persist terminate in the topographically appropriate part of the SC (Simon and O'Leary: Dev Biol 137:125, 1990).In the present study, we made similar DiI injections, covering less than 2% of the retinal area, in peripheral temporal, nasal, superior, or inferior retina, in rats of two age groups, embryonic day (E) 21 to P (postnatal day) 2 and P11‐P17. Whole mounts of retina, optic nerve and tract, and SC, and cross sections of the optic nerve, were examined. In E21‐P2 rats, retinal axons labeled from each retinal site are diffusely distributed in the SC, and poorly ordered in the optic pathway. In retina, labeled axons travel in fascicles directly from the injection site to the optic disc, but neighbor relationships begin to degrade as fascicles split and mix. Retinotopic order is virtually lost in the optic nerve; axons labeled from each injection site disperse throughout its cross‐sectional area, but the labeled axons tend to be concentrated toward a specific half of the nerve depending upon their retinal origin. This slight tendency toward retinotopic order increases in the optic tract, but axons are still poorly ordered as they leave the tract and enter the SC. Targeting errors along the medial‐lateral axis of the SC, but apparently not along its rostral‐caudal axis, are related to the positioning of axons across the width of the optic tract.In P11–P17 rats, axons labeled from each injection site arborize only in a small, topographically correct part of the SC. However, the distributions of labeled retinal axons observed in whole mounts of the retina and optic pathway have a degree of disorder similar to those in E21‐P2 rats. Further, the scatter of labeled axons in optic nerve cross sections is comparable in both age groups. Therefore, the emergence of topographic order in the retinocollicular projection is not accompanied by an emergence of a retinotopic ordering of axons in the optic nerve. This indicates that axons positioned anywhere within the optic nerve are equally likely to establish a correctly positioned arbor, and survive the phase of map remodeling. Thus, a retinotopic organization of the rat optic nerve at perinatal ages is neither essential nor important for the subsequent development of topographic order in retinal projections.

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