Pattern formation in the retinal ganglion cell layer and visual brain centres.
Development of adult patterns of cell distribution in the retinal ganglion cell layer from an approximately even initial topography are described for frogs and mammals. An area centralis and visual streak form in the frog only after metamorphosis, probably by the greater addition of cells to the ciliary margin nasally and temporally than dorsally and ventrally. By contrast, in mammals all cells destined to occupy the ganglion cell layer are generated before the area centralis and visual streak are formed. Patterns of cell death seem unlikely to explain changing cells distributions within the ganglion cell population and we are currently documenting patterns of cell migration to address this issue. The formation of specific connections between retinal ganglion cells and their partner cells in the brains of lower vertebrates is also discussed. At stages before optic axon outgrowth the eye already contains positional information to determine formation of appropriate central connections. Optic nerve regeneration studies in frogs indicate that positional cues are available in visual centres. However, interactions between fibres can overwhelm any such cues, allowing an orderly shift in connections to accommodate changes in the relative numbers of ganglion cells and their partner cells in the brain. Thus in the frog, retinotopicity is maintained during the postmetamorphic increase in ganglion cell number associated with area centralis and visual streak formation, despite an absence of central cell division. Similarly, an entire visual field in time becomes represented in an orderly fashion within the remaining portion of a partially ablated mature visual centre. Interactions between fibres are also considered responsible for the formation of ocular dominance columns, such as those found in binocular regions of the mammalian visual cortex. The role of visual experience is usually to refine and maintain the precise patterns of connectivity established by ontogenetic mechanisms.