Wiring up the visual system

1. In this review we describe some of our recent studies on the developing marsupial visual pathway. The description focuses on retinal ganglion cells, considering the formation of their dendritic trees, the outgrowth of axons and the formation of connections within the brain. 2. Both dendritic trees and outgrowing axons undergo a period of exuberance, followed by one of refinement. The dendritic tree transiently develops a more complex branching pattern than is found in adults. Short side branches, referred to as spines, are a feature of immature dendrites and, to a lesser extent, of axons. These structures are mostly lost as development proceeds. However, they are retained on the dendritic trees of small-field ganglion cells and, for a proportion of axons, on that part within the nerve fibre layer of the retina. Although most axons navigate fairly direct routes towards their targets, a minority follow inappropriate courses, such as doubling back towards the eye or entering the opposite optic nerve at the chiasm. As such errant axons are not seen in the adult, we assume that their parent cell bodies die during development. 3. Throughout development, optic axons are arranged in an approximate retinotopic order along the length of the visual pathway; as a result, axons approach the visual centres aligned to form, at least, a crude retinotopic map. Axons from dorsal and ventral retina exchange locations along the optic nerve and in this way correct for the inversion of the image brought about by the lens.(ABSTRACT TRUNCATED AT 250 WORDS)