Expert: John Locke - 1/1/2009

Question
I realize, being in the medical field, that the life span of all the cells that exist in humans are subject to a number of stresses that can control the growth and health of a cell. My question is simply; do you have a table that lists the average life of each cell group ... skin, bone, nerve, etc.? and how many times they can actually reproduce in a life time?

The discussion arose regarding how frequently the cells of the entire human are able to regenerate. This is subject to a number a variables. The brain cells, I understand remain under investigation and that some feel these never regenerate. We are just extremely curious and would appreciate your thoughts on this.

Thank you

Answer
Thanks for using AllExperts. First, it's important to realize that any description that you find of average cell lifespan makes the assumption that the cells are not experiencing environmental stress--this will decrease their survival time significantly. Sources generally agree on the lifespan of major cell types (e.g., red blood cells, mucosal cells of the stomach, etc.); a brief description of some lifetimes can be found here: http://www.timeshighereducation.co.uk/story.asp?storyCode=198208§ioncode=26
Unfortunately, I don't have a comprehensive resource on the subject online; I would recommend any good histology text on the subject (Bloom and Fawcett's Textbook of Histology is good, as is Ross's Histology: A Text and Atlas, as is Wheater's Functional Histology: A Text and Colour Atlas). They will give a description of the different cell types in the different tissues of the body.

As to the question of neuron regeneration, there are several points that must be made. First, central nervous system neurons are end-differentiated, meaning that they can no longer undergo mitosis to produce additional neurons; this occurs because mature neurons have their reproductive ability deactivated by growth inhibiting factors. Brain and spinal cord injuries are generally permanent because of this inability to regrow or repair damaged neurons. If the inhibitory substances were removed, there is strong reason to believe that CNS neurons could divide and therefore regenerate after injury. Some of the research on neuron regeneration has now focused on doing exactly that, with some success achieved in a mouse model (http://www.sciencemag.org/cgi/content/full/322/5903/963). Neurons in the peripheral nervous system can naturally regenerate--they can regenerate their axons, in particular, with no outside influence required, because they do not have their mitotic pathways inhibited as the CNS neurons do.

This website provides a good overview of the issues involved in neuron damage and regeneration: http://biomed.brown.edu/Courses/BI108/BI108_2001_Groups/Nerve_Regeneration/Intro...

The key takeaway from this description is that both the CNS and PNS have the intrinsic ability to regenerate damaged neurons, but this ability is deactivated in the CNS. This is generally accepted to be irreversible in the CNS, but it is the target of current research, as noted above.

Good luck.