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. Author manuscript; available in PMC: 2014 Jun 19.
Published in final edited form as: Nat Rev Neurosci. 2014 Jan;15(1):7–18. doi: 10.1038/nrn3650

Figure 1. Stages of axon branching in developing CNS pathways.

Figure 1

In the mammalian corpus callosum and thalamocortical pathway (a) and in the chick retinotectal pathway (b), axons initially extend past their eventual terminal regions (1). After a delay, branches extend interstitially from the axon shaft (2) followed by elimination of the distal axon (3) and formation of terminal arbors in topographically correct target regions (4). Figure c depicts interstitial branching from an individual layer 5 sensorimotor cortical axon into the pons and spinal cord (adapted from 9). In the retinotectal system (b), opposing gradients of EPHA–ephrin repellents inhibit retinal axon branching anterior and posterior to the correct target, whereas positive BDNF cues along the tectum promote branching at appropriate topographic positions7, 42. Callosal axons connect homotypic regions of the cortical hemispheres but can also branch to other cortical areas11,12 (not shown). Thalamocortical axons form branches that arborize topographically in layer 4 of somatosensory cortex 23 but other thalamocortical axons can branch into multiple cortical areas19 (not shown).