The tissue types that can replace themselves usually have stem cells present, unspecialized self-renewing cells found in specialized tissues. Stem cells are not differentiated. That is: they are still not commited to being adult cells. Stem cells can, when stimulated, divide and provide cells that replace the damaged cells with the appropriatecell type. Continuous renewal of the outermost layer of the skin, the epidermis, is maintained by epidermal stem cells.
The nervous system does not have many stem cells, thus it is not very good at replacement of damage. Researchers today are now learning how to change other common types of stem cells into nerve stem cells and early experiments are beginning that place these cells into people with brain damage or spinal damage, with the rare result that a few are coming out of comas or are regaining movement in their fingers or legs.