At the time the nervous system forms, when an embryo is developing, nerve growth factors (also called neurotrophic factors) stimulate the cells that are programmed to form neurons to divide and grow. Growth factors do this by interacting with specialized regions (called growth factor receptors) on the surface of each developing neuron. This interaction goes on to produce changes inside the neuron, which causes it to grow.

Several growth factors also attract neurons to areas that contain the highest concentrations of growth factor, while others have the opposite effect of repelling growing neurons. Therefore, as well as stimulating neuronal growth, growth factors also direct neurons to their appropriate target regions.

Many different growth factors have been identified and it is essential to identify which are most effective in spinal cord repair. For example, BDNF, NT-3, NGF and TGF have each been shown to promote the growth of neurons in the spinal cord. However, not all are likely to be equally beneficial – there is evidence that some selectively target pain-transmitting fibres, adding to the pain associated with spinal cord injury. It is essential that any proposed treatments do not do more harm than good.

In addition to exploring which is the most effective growth factor (or combination of factors), scientists are investigating how best they might be administered to the spinal cord. Purified growth factors degrade easily, which means that their effects are quickly lost after being administered.

Some researchers overcome this by genetically engineering special cells to become ‘growth factor factories’ that make and continually pump out specific growth factors. Once implanted at the injury site, they provide a continuous source of growth factors. Others are investigating ways to manipulate the cells in the spinal cord itself, so that they become the source of growth factors. This would provide a source of growth factors in exactly the right place and the right time.