The skin is an excellent example of an organ that heals and regrows, or regenerates to be like new.
Unfortunately, many cells of the CNS, especially neurons, are so specialized that they have lost this ability to divide and generate new cells.
As a result, injury to the brain or spinal cord isnt easily repaired by replacing the cells that have been injured or have died with new ones.
Cells are connected together so intricately, making the right connections between new cells (or between new cells and the older ones that survive the injury) is a huge challenge for the brain and spinal cord.
Challenge for the scientists trying to find way to help an injured spinal cord heal.