MadSci Network: Cell Biology

Re: How long does it take for every cell in a human body to be replaced?

Date: Tue Dec 21 08:20:27 2004
Posted By: Paul Odgren, Assistant Professor, Dept. of Cell Biology
Area of science: Cell Biology
ID: 1097769194.Cb

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

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