When axons in the brain and spinal cord of a mammal such as a mouse or a human have been damaged, they don't regenerate on their own. (The only known exception is olfactory sensory nerve cells.) The ...

A false-colored confocal microscope image (1500x) of the Drosophila melanogaster visual system halfway through pupal development, showing the retina (gold), photoreceptor axons (blue), and brain ...