djoslin@bbn.com (David Joslin) (12/04/89)
A philosophy paper that I was reading yesterday makes this statement: "With the exception of some brain cells, the cells in our bodies are replaced with new cells several times in our lives." Unfortunately no reference is given. I've heard a stronger version of this that says that over a period of ten (?) years of so, the normal process of cell repair causes every atom in our bodies to be replaced. I don't have the foggiest idea where I read it, though, and I haven't been able to find it anywhere. Are either or both of these statements true? I'm writing a paper for a philosophy of mind class, and it would help me greatly if I could find a reliable reference to either, especially the latter. David djoslin@bbn.com POBox 1592, Cambridge MA 02238
houpt@husc8.HARVARD.EDU (Thomas Houpt) (12/05/89)
There are two separate processes you're talking about. One is the lifetime of particular cells (In this case nerve cells) and the other is the turnover of the actual molecules which make up the cell. There is quite a range of timescales for both processes. Most cells in the body (like skin and soft tissues like liver cells) die quite frequently and are replaced rapidly, even after injury. However, brain cells are not replaced, and if one dies, that's it. Generally, the more highly differentiated the cell type (and nerve cells and muscle cells are the most complex, arguably, in the body) the less capable of regeneration or reproduction they are. Hence the interest in compounds such as nerve growth factor which appear to promote nerve regeneration, and the transplanting of fetal tissue into adult brains (since fetal brain cells obviously are still capable of repoduction. On the other hand, you're right that there is near complete turnover of the constituent chemicals that make up the cells. The rate of turnover depends on how tightly bound the stuff is, how freely it diffuses, etc. Water freely moves and is exchanged rapidly (on the order of days? weeks?) whereas something like lead can take years. A few molecules don't turnover at all, in particular the proteins in the lenses of the eye; Hence if the proteins start to oxidize (causing cataracts) you're in trouble because the damaged proteins won't get replaced, they're so tightly packed/bound. In terms of your philosophy paper, its like replcing the pieces of wood in a wooden boat; the boat stays the same (until its destroyed by attrition) but its wood is constantly being replaced. A Reference: I used Guyton, A textbook of medical physiology,1986, but any decent neuro/physiol. text should sya something under cell life cycle.
warg6606@uxa.cso.uiuc.edu (12/06/89)
/* Written 4:30 pm Dec 3, 1989 by djoslin@bbn.com in uxa.cso.uiuc.edu:sci.bio */ /* ---------- "cell regeneration" ---------- */ a philosophy of mind class, and it would help me greatly if I could find a reliable reference to either, especially the latter. If this is a philosophy class, then why do you need a reference? Anyhow, how can anyone measure the displacment of every atom in the body-impossible! So if you're looking for a scientific measurement to demonstrate you're argum- ment, good luck. W. R. (a biochemist) /* End of text from uxa.cso.uiuc.edu:sci.bio */