|MadSci Network: Cell Biology|
Hi Braden, A very interesting question, and it has lots of medical and health implications. The answer is, it depends on which cells you’re talking about. About 1% of your red blood cells are replaced every day, for example, giving a turnover time of about 100 days for a completely new set of red cells. Other cells are never replaced, like muscle cells (that is, “skeletal” muscle, the kind that move your limbs) or neurons in the brain or spinal cord, that last your whole life once you've grown to adult size. In between you have lots of other turnover rates. A disease like polio, which kills muscle cells, can leave people paralyzed or crippled for life, and brain damage can only be helped if other parts of the brain can be “trained” by therapy to take over the jobs of some of the brain cells that are damaged. Ditto for spinal cord injuries. In between those two extremes, you have just about every possible rate. The lining of your gut from mouth to the exit turns over pretty fast, and hair cells in the roots of your hair are always growing. This explains why treating cancer with chemotherapy, which targets dividing cells, also causes hair loss and digestive system problems. Bone marrow, where your blood cells, red and white, are made are also killed by cancer therapies. This is a serious and dose-limiting side-effect of chemotherapy because it causes anemia and loss of white cells and platelets, which are needed to fight infection and to help your blood clot, respectively. The lining of your airway also replenishes itself, as does you skin and the lining or your urinary tract. These turn over more slowly than skin or blood. Liver cells die and are replaced, too. Your skeleton also has its own turnover rate. When you reach adult size, about 10% of your skeleton is resorbed, that is, eaten, by special cells. New bone is then formed to replace it, so you basically get a new skeleton about every ten years. So, you’re really a mixture of old and new cells. Some cells are brand new, and others last your whole life. Hope you didn’t have a big wager on this! Paul Odgren, Ph.D. Department of Cell Biology University of Massachusetts Medical School Worcester
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