h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Is it true that all the material in the body is replaced over a period of 7 years, so that the body you have today is not, so to speak, the same body you had 7 years ago?

Pretty much.

The cells are constantly breaking down old material and making new.

I want the body I had seven yesrs ago - can I find it somewhere? Do they get recycled?

A modest enough wish Magwitch. I'd have to go back at least 14 to re-establish smooth running and comfort

Thanks Orcus. Does that include the brain and eggs? The eggs in the ovaries must be the same cells a female is born with I suppose. But has the actual material of those cells changed - even though the eggs are never replaced?

That's an interesting question. I guess they're not 'exchanged', though, and they're too old at some point - isn't it the reason why the chances to have a healthy baby get less the older the mother is? But maybe something is exchanged, and that is what causes the risk in the first place? Because the exchanged material isn't as good as the original one?

In relation to the brain, it is in a state of constant change and much of what you had as a newborn is not what you have now, although many of the neurones will be the same cells connections are made, changed and remade as we learn and grow. From about the age of 21/22 the brain starts to loose cells faster than it can replace them but the number and quality of connection as well as their permanence increases for many more years.

As for eggs in the ovaries, girls are born with all the eggs they will ever have and don't use more than a fraction of what they have. Those egg cells develop in utero and never change except to deteriorate.

t.

The number and quality of connections... increases for many more years.

Hmmm, that's not been my... er... ummm... what was the question again?

You need to be careful about what you mean by a cell (or half cell such as an egg) not ever being replace.

That is true of some cells such as eggs and neurons but that doesn't mean that individual bits of those cells are not worn out and replaced.

Just as your car brakes eventually wear out - so do the part of a cell as is respires, functions and generally stays alive. So a protein here, a bit of DNA there etc. are continually being maintained by the old one being destroyed and a new identical bit being synthesised.

Even eggs must respire to stay alive and so although one can say that the cell has not divided and the old one dies (as is the case for most cells) the consituent parts of it will eventually be replaced one by one.

It's the old philosophical problem of a ship that has systematically had planks replaced over the years so that is no longer made of any of the original bits. Is it still the same ship? If so, there must be something about that ship's identity that is not intrinsic in the materials it is made of.

I'm not sure how quickly bone and teeth material are reformed by the body mind you.

Bone is a very living tissue and is constantly remodelled. The balance between osteoclasts (bone eaters) and osteoblasts (bone creators) is very delicate.

Thank you folks

That's what I like about h2g2: you can get the answer to just about anything. Now that you mention it, teeth are interesting too, aren't they? They come along, usually some time after birth, drop out, get replaced once and most people lose some or all of them long before they die, as if the body hadn't quite cracked the problem of maintaining them. And more generally, with all the constant maintenance and replacement of cells, dna etc, the body still shrivels up and dies. Fascinating!

Our cells are programmed to only replicate around 50 times before simply dying off. This is presumed to be a defense against that scourge - the immortal cell (one that replicates forever given enough food, water and oxygen). That is a cell called a cancer cell.

One of the ways people are looking into prevent aging is by stopping cells' programmed cell death (a process called apoptosis). Each method of doing this of course removes one part of the natural process of preventing malignant cells from forming if you ask me.

>>
It's the old philosophical problem of a ship that has systematically had planks replaced over the years so that is no longer made of any of the original bits. Is it still the same ship? If so, there must be something about that ship's identity that is not intrinsic in the materials it is made of.
<<

That's like my granddad's axe - it's had three new shafts and a new head, but it's still Granddad's axe.

Hence, presumably, increasing chance of procreating imperfections with age.

Even worse with the ship, because all the planks have washed away onto a nearby seashore, where somebody rebuilt a ship out of them

The 7-year thing is a complete myth though. Some types of cells last a lifetime; some are replaced in a matter of days. There was a very good NS acticle on this a while back, which I'll try to dig out.

There's a deeply philosophical matter here.

I saw a juggler on TV who was about to juggle an axe, a meat cleaver and a chainsaw (the chainsaw was running). Before he went off on this madness he had a bit of spiel that included: "This is the very axe that Washington used to cut down the cherry tree. I've had to replace the handle... and the blade.

It occupies the same space."

>>Now that you mention it, teeth are interesting too, aren't they? They come along, usually some time after birth, drop out, get replaced once and most people lose some or all of them long before they die, as if the body hadn't quite cracked the problem of maintaining them.<<

The body can maintain the teeth perfectly well ; it's our taste for sweet foods and drinks that causes their destruction

In addition, many perfectly sound teeth are lost because of periodontal disease; there's nothing wrong with the tooth itself but the supporting tissues can no longer provide an adequate anchor.

Odontology is fascinating

If one is not consuming enough dietary calcium, the body makes use of whatever calcium it can find, including the teeth.

>>The 7-year thing is a complete myth though. Some types of cells last a lifetime; some are replaced in a matter of days. There was a very good NS acticle on this a while back, which I'll try to dig out.<<

Three steps forward and two steps back
Good luck with the dig. In the meantime, these cells that last a lifetime: are they *just* the eggs, and made up of a completely different set of atoms to the set they started out with? Or are there other such cells? And do they hang on to any of their original material?

>>The body can maintain the teeth perfectly well ; it's our taste for sweet foods and drinks that causes their destruction<<

Stephen Fry on QI (I think) said that the worst enemy of teeth is stuff like potato crisps that can lurk in the gaps, harbouring acid generating bacteria that eat away at the enamel. Doesn't seem unlikely. But on the subject of sweet food, it seems unhelpful (to say the least) that the very food we've evolved to most desire, is the very food our teeth can't cope with.

>>Odontology is fascinating<<

Certainly is!

>>If one is not consuming enough dietary calcium, the body makes use of whatever calcium it can find, including the teeth.<<

I read somewhere that the placenta fights the pregnant woman for all sorts of resources, including calcium, to supply the growing baby's needs. Not absolutely sure, but I think pregnant women in Britain get free dental care to help them keep their teeth.

People here say, that you lose one tooth for/with every pregnancy, so you're certainly right. It doesn't need to be like that nowadays, though.

... so after your 32nd your all toothless then?

Hmm, may be that's why my grandfather had all false teeth ... I might have 29 unknown aunts/uncles