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Figure 2: Glia-axon interactions in the development of callosal axons. (a) and (B) Axon guidance by long-range molecules, attractive (a) or repulsive (b) signals. Glial populations (gray and dotted circles in (a) and (b), resp.) secrete guidance molecules, forming a concentration gradient, which navigates the callosal axons during their development (c). (d) Axon guidance by short-range molecules. Repulsive molecules expressed on the cell membranes navigate callosal axons through repulsive and bidirectional cell-cell contact functions.