Order in the initial retinotectal map in Xenopus: a new technique for labelling growing nerve fibres.

Retinal nerve fibres form an orderly map of visual space in several centres in the vertebrate brain. Such topographic maps are a common feature of central nervous system organization, yet the way in which they develop is poorly understood. Early nerve projections in the fetal and neonatal mammalian brain have been found in several cases to be less restricted than those in the adult, suggesting that nerve fibres may initially form a diffuse set of connections in their target structure from which the adult map is sculpted by the elimination of terminals. Indeed, previous electrophysiological data indicate that the retinotectal map in Xenopus laevis might be initially disorganized. We report here, however, that the retinotectal projection is ordered from the beginning of tectal innervation (stage 39/40). We demonstrate this first autoradiographically by tracing groups of growing ganglion cell axons which we labelled by incubating sectors of eye rudiments, before axonal outgrowth, in 3H-proline and replacing them orthotopically. Separate labelling of dorsal and ventral parts of the initial projection showed that retinal fibres are organized topographically, as in the adult, in the tectal rudiment and throughout much of the pathway. Second, we show that visual responses are ordered in the tectum from the first stage that they can be mapped (stage 40). We conclude that the topographic ordering of retinotectal connections develops as a result of directed axonal outgrowth.