biology question #2753
Mateusz, a 13 year old male from New York asks on May 6, 2005,Q:
Why does it take nerves so long to grow back, more than for skin cells?
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Short answer: skin cells are made to be disposable, so there is a system to replace them regularly with fresh ones. Nerve cells are designed to form connections that last a lifetime so they will reliably give the same signal when stimulated.
By now you have probably heard about stem cells. They are cells in your body that have the unique potential to grow into many different types of cells. We say they have not yet been differentiated. That is, they have not turned into their final shape and function. We have many kinds of stem cells and even if you are a kid, we would call yours "adult" stem cells. That is because your stem cells sort of already know what they will be when they grow up or become fully differentiated. For example, you have bone marrow stem cells that can become red blood cells or white blood cells depending on what kind you need, but these will not ever become nerve cells in your body. They are committed to becoming blood cells, but not committed to one type. When we need blood cells, the stem cells divide into daughter cells, one which stays as a stem cell and the other goes on to make many more divisions into the various types of blood cells. Once a cell decides what kind of cell it will be, it can't go back.
Your skin is subject to stress all day long--clothes and places where we don't wear clothes cause the skin to be shed, thus we need skin stem cells to make new skin. Nerves, on the other hand, have a very defined place to be, and very defined connections they must make with their neighbours and your brain, and are not required (or probably even permitted) to grow after you reach adulthood. Therefore we don't need nerve stem cells on a daily basis like we do skin cells or blood cells. That is partly why we heal skin faster than nerves. And perhaps you can also see why it is easier for a young child to repair a damaged nerve than an adult.
We do know that brain and nerve stem cells exist though. At the University of Calgary where I work, one researcher is finding brain stem cells and getting them to produce new brain cells when someone has a stroke. Others are looking for ways to take bone marrow stem cells and try to tell them to back up and please become nerve cells. These are all very difficult to do, but some progress is being made.
The easiest cells to tell, "please be a nerve cell", are the stem cells from the cord blood of a new born or those from an embryo. That is because these stem cells are pluripotent stem cells which means that they can become bone marrow OR nerve OR skin stem cells given the right signals.