This chapter discusses the development of retinal projections. It presents new insights into the interrelation between target influences and other factors directing axonal outgrowth and the development of orderly retinotectal connections. During maturation of the mammalian retinotectal projection, it appears that optic axons are not initially influenced by target-derived cues but instead follow substrate cues close to the subpial margin of the rostral brainstem. Directed by a polarized distribution of “pathway markers,” these axons make their way toward the dorsolateral midbrain, and thereby reach the tectum. A number of cell-surface and extracellular molecules have been co-localized along the subpial margin and one or more of these may be important components in tectopetal growth along the subpial pathway. The retinal transplant studies also reveal an evidence of a target-directed growth pattern effective over limited distances that depend on the prior optic innervation of the superior colliculus. The establishment of an orderly retinotectal projection is not a simple process and may depend upon an elaborate interaction between substrate-dependent events and target-derived cues.
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