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Abstract

One basic feature of the central nervous system is the presence of functional organizations containing ordered representations of stimulus features relevant for sensory processing. In the mammalian visual system there is a high topographic order with respect to the location of receptive fields (retinotopy), the ocularity of neuronal responses (ocular dominance hyers/stripes), and the orientation selectivity of conical cells (orientation columns). The neuronal response specificities rely on very accurate axonul conrtectivities which elaborate in an activity-dependant manner from exuberant axunal ~xborizations during a restricted 'critical period' in postnatal development. Experimental evidence will be summarized indicating that adaptive plasticity is due to rapid morphological changes including axon- and synapse-elimination and axon growth. Fmlhennore, it will be shown that morphological, axonal plasticity in the developing visual pathway can be influenced by the glial environment. The expression of the astrogliul derived matrix molecule tenascin reveals a close co~espondenea with the developing layering of the thalamic visual relay, the dorsal lateral geulculate nucleus. This is compatible with an instructive role of this molecule on layer formation by an influence on axonal growth. Evidence will be presented showing that an oligodendroglial protein contributes to the end of the critical period for cortical plasticity by inhibiting axonal growth. The datawill be discussed with respect to the role of axonal growth and retraction in the fonundon of functional topopgraphies in the CNS.