Injury-stimulated neurogenesis in sensory systems.

Injury-induced repair of sensory cells does occur in mammals in the sense that partial replacement can transpire in sensory ganglion cells and in visual photoreceptors. In the somatosensory system, functional replacement of the peripheral processes of sensory cells can occur when the cell body is not severely damaged. In the visual system, the outer segments of photoreceptors are replaced under physiologic conditions, but complete replacement after injury occurs only in cold-blooded animals. In the olfactory system, complete replacement of sensory nerve cells has evolved as the means by which animals, including mammals, can retain olfactory function with a receptor apparatus that is highly vulnerable to injury. In the taste system, the sensory cells are also disposable and are completely replaced within less than 2 weeks. In the auditory and vestibular systems, replacement is not physiologic and when it does occur, appears to be injury-induced.