cell replacing

When I was young, it seems that I was taught that the every cell in the human body is replaced after about 7 years. Is that true? Got any reference sites?

I’ll give you a bump and WAG. I think the seven year thing is only for skin. IICR, some tissues, brain for example, don’t replace that quickly.

Try the Google search site, it’s quite good. http://www.google.com/

That might be the average for all cell types or something. Some cells have a much mroe rapid turnover (IIRC a few days for skin and red blood cells). In adults, I don’t think neurones (nerve cells) in the brain are replaced at all.

My own WAG is that many of our cells are never replaced through our whole lifetimes. Bone cells, corneal cells and cartilage cells, for example.

But I’d further guess that quite a few of the individual molecules contained in and used by those cells are replaced. Like the gas tank of your car: it’s always the same tank, but the contents change regularly.

I do not have my texts here, so this is best effort from memory & a tiny bit of web searching, but I did study biology (but not specifically cell biology) at university & taught it for two years.

All cells can be replaced, but at varying rates depending on the function of the cell & wear and tear likely to be experienced by it. For example, the average life span of a red blood cell is (iirc) 8 weeks. They are simple cells to make and likely to be damaged in circulation, lost in haemorrhaging (internal/external bleeding) etc.

I don’t recall specific times for skin cells, I’m afraid, but I am sure that except for injuries, when rapid cell division is triggered by healing mechanisms, it would not be a few days. You have a couple of square meters of surface area of skin. Since the surface skin cells are effectively dead and get sloughed off rather than the nutrients reabsorbed as in the internal cell replacement, if you replaced all of that every few days, your diet/metabolism would not be able to keep up.

Bone cells are replaced slower - there is a poorer blood supply, and the formation of bone (cells in an inorganic matrix, the protein providing flexibility and the calcium matrix providing strength) is therefore a slow process. But - if you couldn’t replace bone cells, broken bones would never heal. More on bone cells is available here…

Corneal cells also replace themselves…otherwise the eye operations where a flap is cut in the cornea to gain access to the lens would never heal. However, due to the very poor blood supply to the outside of the eye, this would normally be a slow process. When my dog had a deep ulcer in the centre of his eye last year, they actually sewed a flap of inner eye across to both improve blood supply and to protect it more thoroughly during healing.

Even cartilage cells replace themselves - it may seem like they don’t because after some specific injuries, the weight is distributed unevenly on the joint and this causes new damage to be done faster than the cells replace themselves, so the injury never fixes itself.

Even ignoring the effects of accident and disease, cells such as bone and cartilage have to be designed to replace themselves, since they are all equipped with self destruct mechanisms…if the cell function starts to deteriorate, or go badly wrong, autolytic reactions will be started by enzymes inside the cell to break the cell down. Overtime, this would leave the body weakened and lacking tissue to do vital jobs if there weren’t a corresponding repair/replace mechanism of new cell growth.

In addition, the myth of nerve tissue has recently been proven to be just that - recent studies have shown that brain cells can grow in adult mammals. One slightly elderly cite is http://www.wave-guide.org/archives/emf-l/Jun1998/(Maxey)-emf-&-cell-replacement--(fwd).html but I’m sure that a more specific google search could turn up more up to date material on this, if you are interested.

Actually, I believe Five was on the right track. It’s the actual atoms and molecules of which you are composed that get replaced, yeilding a you made of entirely new particles every seven years or so.

Proteins have half-lives and degrade periodically; membranes are in constant motion and the individual lipid molecules are continually being altered; nucleic acids are continually replecating, being repaired, and degrading; and carbohydrates probably have the highest turnover rate of all.

So yes, by the time seven years come and go, you’ll still be thinking and feeling like the same person, but every part of you will be new. It’s amazing to think that the complex information system can persist despite the complete replacement of all the individual components.