Expert: Walter Hintz - 9/30/2005

Question
I have heard that after a certain number of years, I think I remember seven, all the cells in the human body have been replaced by new ones, so in seven years you would have a completely different body, physically. Does this include organ transplants? For example, if you had a kidney transplant,  over the next several years your kidney would be a mixture made up of two different genetic types, and I wonder if they would be able to work together properly. I know transplant recipients receive drugs to suppress their immune systems so I wonder if they would fight the replacement process.
Thank You

Answer
Hi Paul:
 This is a question that I have never considered before.
First of all the seven year idea is a supposition based upon cellular reproduction. In youth this may occur even faster and in the aging process it slows down. It is true that all living tissues except muscles and nerves are replaced by new cells. The brain is not replaced with new cells.  If it did you would have to learn everything all over again every seven years.  Worn out cells are recycled by a process called apoptosis.
 Transplanted tissue must also undergo cellular division but they still contain the DNA of the organ donor and therefore are still subject to rejection. The foreign kidney remains a foreign kidney. The organ recipient must still take anti rejection drugs.