2) Replacing lost neurons from stem cells. Stem cells are undifferentiated cells that have the capability to divide and produce many other types of cell. A neural stem cell has the ability to differentiate into any type of cell in the brain. It has recently become clear that the adult brain contains stem cells in some regions, and that these cells can be proliferated in tissue culture. Moreover after some types of brain or spinal cord injury stem cells appear in the injury site. Scientists need to understand how to direct stem cells to differentiate into the types of neuron that are lost after damage. While some of the signals that direct stem cells to differentiate are known, much remains to be worked out. Since stem cells can be grown in tissue culture to produce large numbers of cells, stem cell technology is probably the way forward for neuron replacement in the future. At present the main effort is directed towards understanding how to control their differentiation.
Regeneration of nerve fibres
When nerve fibres are cut in the brian or spinal cord they do not regenerate. There are three reasons for this:
- The scar tissue that forms where the brain or spinal cord is injured produces several molecules that inhibit the growth of nerve fibres
- The oligodendrocytes that insulate nerve fibres produce molecules that inhibit nerve fibre growth
- The nerve fibres, while capable of regrowing through a helpful environment, mount a very feeble regenerative response.
In recent years great progress has been made in finding methods to promote nerve fibre regeneration. These methods are:
- Block the inhibitory molecules made by oligodendrocytes
- Digest the inhibitory molecules produced in the scar tissue surrounding injuries
- Enhance the vigour with which nerve fibres try to regenerate using growth factors and other treatments
- Transplant cells into the damaged brain or spinal cord that have the ability to encourage nerve fibre regeneration, and which can form a bridge taking nerve fibres through the scar tissue around an injury