Positional Cues in the Developing Eyebud of Positional Cues in the Developing Eyebud of Xenopus

Abstract

The formation of spatial patterns in the nervous system appears to require a complex series of interactions. In the visual system, this not only involves the patterning of neurons and glia within the eye, but also the patterning of the spatially ordered optic projections in visual centers of the brain. An ideal system for studying patterning in the optic projections is the retinotectal projection which forms the main visual pathway in lower vertebrates. In the retinotectal projection, the retinal ganglion cells in the eye project along the optic nerve and into the midbrain where their connections form a topographic map of the retina over the surface of the contralateral optic tectum. In this well ordered pattern 1) neurons in a particular part of the retina consistently project to a particular part of the tectum, and 2) neurons in neighboring positions in the retina project to neighboring positions in the tectum giving the projection a smooth internal order. Lower vertebrates are capable of regenerating their optic nerve after injury and eventually reforming this same pattern of connections. A multitude of studies has examined the ability of optic nerve fibers to consistently find their correct target region in the tectum both during development and regeneration.