Proteoglycans as cues for axonal guidance in formation of retinotectal or retinocollicular projections.

Understanding the formation of neuronal circuits has long been one of the basic problems in developmental neurobiology. Projections from the retina to their higher center, the optic tectum in nonmammalian vertebrates and the superior colliculus in mammals, are most amenable to experimental approaches; thus, much information has been accumulated about the mechanisms of axonal guidance. The retinal axons navigate along the appropriate pathway with the help of a series of guidance cues. Although much of the work has focused on proteinaceous factors, proteoglycans have been identified as playing important roles in retinal axon guidance. Chondroitin sulfate proteoglycans and heparan sulfate proteoglycans are involved in essential decisions of axon steering along the pathway. However, it has not been determined whether diversity of the carbohydrate chains results in differential effects and how their diversity is recognized by growth cones, which represent an important area of future research.