David Bailey asked:
Why can't the brain and nerve cells heal and other tissue can. There must be an evolutionary disadvantage?
We posed this question to the brain panel.
Bill - Skin cells replace themselves all the time. So, weíre always making new skin cells. Most of our brain cells we are given just one of each and they have to last our lifetime. There's not a lot of cell replacement in the brain. So, the question then becomes, if a cell dies, it doesnít get replaced because we have no mechanisms to replace it in our brains whereas we have mechanisms to replace some skin cells or muscle cells, there's something intrinsic about the nerve cells that doesnít let them re-grow so well. The distances are a lot longer in an adult than they were when they were connecting up as an embryo. When you get a cut in the central nervous system, there's a reaction, an environment that makes it difficult for cells to re-grow through that injured environment and connect that properly. So, brains have a couple of challenges in the healing.
Katie - I think itís important also that the brain can adapt to induce to a certain extent. So, with other parts, the brain is working to take over lost functions from the damaged area even if thatís not sufficiently as it happened before. And also, that the brain does have some fairly strong defence mechanism. So, when you make a cut in your skin, it might happen in everyday life Ė you're doing some cooking, slip with the knife. But the brain, itís protected. Itís in the skull. There's two membranes beneath the skull and between that, there's a cushion layer so to speak of cerebrospinal fluid. And then even beyond that, youíve got what's called the blood-brain barrier which helps to control some substances that can enter the brain. The brain isnít exposed to harm than in quite the same way as an organ like the skin in everyday life.
One should note that in many senses, the spinal cord is considered to be an extension of the brain and doesn't regrow. And, as mentioned, if it did regrow, making the appropriate connections would be complicated.