| The chick retinotectal System
During development of the chick, retinal ganglion cells (RGCs) form specific connections with their target the optic tectum in a highly organized topographic manner. The retinotectal map refers to the coordinate maps of RGC cell bodies across the retinae and their terminations across the tecta. In higher vertebrate, a substantial number of RGC axons project to the ipsilateral optic tecta. In embryonic chick ipsilateral retinotectal projections persists for only approximately one week. The final retinotectal projection in adult chick is almost exclusively contralateral. Nasal RGCs (red, dark blue) will map to the posterior part of the tectum and temporal RGCs (yellow, bright blue) will map to the anterior part. Specific molecules are responsible for the guidance of temporal and nasal fibers across the tectum. Two repellent molecular guidance cues expressed in a gradient across the optic tecta have been determined to be ligands for ephrin tyrosine kinase receptors. Many other factors such as electrical activity might play an important role in axonal pathfinding and much research still needs to be done to elucidate the mechanisms that lead to development of the retinotectal map.
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