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Name: Jason M.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: March 2004


Question:
I have been wondering how long the cells in our bodies stay with us. I mean, I know that our cells are dividing and eventually are passed from our bodies. I found that red blood cells have a life span of 120 days, but what about the rest of our cells. How often do we (basically) get a new body? I have also learned that nerve cells don't come and go like red blood cells. Is this true and are other cells the same?



Replies:
Very good questions! I mammals we probably know the most about blood cells ( a type of connective tissue) since they have been studied so much. It turns out that there is a complete scale of life-times for cells. Some cells are in what we call G-0 because they seldom if ever divide ( some nerve cells are like this). Cells that are dividing can produce more of themselves or can divide and mature (differentiate) into another cell type. Cells that can consistently produce both themselves and produce cells that differentiate are often called "stem cells". Under favorable stimuli, a population of hundreds of stem cells can give rise to billions of mature functional cells...like red cells which last about 115-120 days. Some stem cell lines are more "committed" to making a certain line of cells.
White blood cells are produced from a certain "cell line" of stem cells in a mature person. Cells of a very early embryo are capable of producing any cell in your body. Stem cells of a mature person typically produce a certain line of cells. Cells that line the inside and outside of our bodies are often rapidly dividing every 20 or so hours. So your skin cells, the lining of your intestine, and your blood cells are very actively living dying and being replaced....As to the question of what you are...think of this...about 75% of your body is water and every month or so almost all the water in your body is replaced, about 7% of your body weight is blood fluid and blood cells, most of which are replaced in three months. And the mitochondria that provide most of the energy in most of the cells of our body are actually bacterial descendents that live in perfect harmony. Then subtract the non-cellular bone matrix and there isn't much left of us to wonder about.

By the way...it is my belief that any nucleated cell can divide...and even dedifferentiate under the correct circumstances...but no one I know of has done so with animal cells...plants can be stimulated to do so but not yet animals.

PF


Different cell types have different longevities. The cells on your skin and in your mouth for example have a very fast rate of mitosis. The cells lining your stomach also divide to replace the ones that are eaten away by stomach acid. The cells in your clavicle (collar bone) have a very low rate of mitosis. Your bone marrow does replenish your red blood cells every 3 months. But some cells never divide again once they are made. Some of the cells of the nervous system fit that bill. Recently however, nervous system stem cells have been discovered in an adult, so there must be some new nervous tissue cells made. So, there is not a consistent rate of replacement. Your body doesn't completely replace itself, although most of it does.

vanhoeck



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