Number of deaths in a life?

October 5, 2006 11:44 AM

How often is a human body completely "new"? Or: No cell composing my body existed a month, a year, [the time I'm looking for] ago.

I think rates of reproduction and death differ for certain cells/areas. Differences between, say...neurons and my liver cells. This complicates things.

posted by Korou to science & nature (22 comments total)

Nerve cells stop dividing at age 6 months. All of your nerve cells are the same ones you had when you were 6 months old, and that will remain the case until you die, no matter when.
posted by Steven C. Den Beste at 11:47 AM on October 5, 2006

I wonder if the molecules and atoms in those unchanging nerve cells stay the same, too -- interesting from a persistence of self-identity standpoint.
posted by hodyoaten at 11:49 AM on October 5, 2006

Steve C. Den Beste is kind of right. Certainly that is true for non-vascular spinal chord neurons. However, tactile neurons and even some brain neurons are capable of regenerating to some extent. I do not know what their normal turnover is (if any) without sustaining injury though.
posted by dendrite at 11:52 AM on October 5, 2006

(some areas of the brain keep making new cells throughout adulthood -- it's a reasonably new finding though).
posted by gaspode at 11:54 AM on October 5, 2006

hadyoaten, those molecules do not stay the same. Cells are constantly synthesizing new proteins and tagging old proteins for degradation. Many of the molecules in the cell are reused, but some of them are shunted out/excreted as other compounds.

In summary: Molecularly,we are almost entirely new. Cellularly, a significant part of our nervous system is old.
posted by dendrite at 12:04 PM on October 5, 2006

Just to make sure the obvious is not passed over, the calcium in your skeleton does not go through a renewal process. That you're pretty much stuck with.
posted by tkolar at 12:08 PM on October 5, 2006

According to this article in Cell (which was summarised in a New Scientist article (subscribers only) earlier this year), the average age of various types of cells are as follows:

Cerebral cortex: As old as you are
Visual cortex: As old as you are
Cerebellum: Slightly younger than you are
Intercostal muscle: 15.1 years
Gut (not the lining): 15.9 years
Gut epithelium: 5 days
Skin (outer layer): 2 weeks
Red blood cells: 120 days
Bone: 10 years

They are still investigating the average age of the cells in your eye lens, heart, liver, pancreas, fat and bone marrow.

This study was based on figuring out the age of the DNA in the cells. Most of the rest of the cell goes through constant change at the molecular level, but the DNA essentially stays put for the life of the cell.

They did carbon-14 dating on tissues from cadavers of people who were alive between 1955 and 1963, and used the fact that there was a huge spike in the levels of carbon-14 in the atmosphere during this period, due to all the above-ground nuclear tests.
posted by chrismear at 12:31 PM on October 5, 2006 [5 favorites]

I was told that the nerves in my arm (which were crushed but not severed) regrew at a rate of about 1 inch/month. True to prediction, I regained feeling (motor control was ok after two months) down the arm at about that rate.
posted by notsnot at 1:12 PM on October 5, 2006

dendrite's right about being molecularly entirely new. You cannot step into the same river twice, indeed; you are not the same person as the one who stepped into the river the first time.

Identity, which isn't a fixed thing anyway but a "reciprocal fluxion" (in Patrick O'Brian's words), is organization of matter (i.e. form), not matter itself. That organization also determines how we respond to the environment. This is true of the cell or of the entire organism.
posted by ObeyScient at 1:14 PM on October 5, 2006

This is from "A Short History of Nearly Everything" by Bill Bryson:

"Most living cells seldom last more than a month or so, but there are some notable exceptions. Liver cells can survive for years, though the components within them may be renewed every few days. Brain cells last as long as you do. You are issued a hundred billion or so at birth, and that is all you are ever going to get. It has been estimated that you lose five hundred of them an hour, so if you have any serious thinking to do there really isn’t a moment to waste. The good news is that the individual components of your brain cells are constantly renewed so that, as with the liver cells, no part of them is actually likely to be more than about a month old. Indeed, it has been suggested that there isn’t a single bit of any of us—not so much as a stray molecule—that was part of us nine years ago. It may not feel like it, but at the cellular level we are all youngsters."
posted by Jesco at 1:24 PM on October 5, 2006

New is a misleading term, because every new cell is just half of a bulked-up old one. When a cell divides, which one is new and which one is old? Are they both new?

Since every cell you have is derived from one progenitor cell, (created from the fusion of sperm and egg) no cell is ever really 'new'.

I suspect that looking at the question in terms of molecular turnover is slightly more interesting, but exceedingly difficult to quantify.

In areas exposed to the outside world, like your gut and your skin, the distinction can often be made a little more clearly. When the progenitors of skin cells divide, one gets pushed upwards towards the surface and differentiates, while the other remains behind and continues to grow and divide.
posted by chrisamiller at 1:26 PM on October 5, 2006

You know, when I was about 16 I went through the usual adolescent growth spurt and lost a lot of puppy fat I'd been carrying for at least ten years. Digging around in my belly button, I pulled out a small dark thing covered in what must have been years of skin cells, quadrupling its size.

The thought that struck me at the time was "holy shit, the molecules in this little piece of filth are older than than I am".

So I kept it on my windowsill to marvel at biology.
posted by claudius at 2:18 PM on October 5, 2006

Molecularly,we are almost entirely new.

Dendrite, are you sure about that?

An AskMe question that I've had on queue for a while is how many pounds of material in my current (42 year-old) body were delivered to me through the body of my mother, who gestated me for nine months and then nursed me for another six. I would expect it's at least a couple.
posted by alms at 2:19 PM on October 5, 2006

Note that the numbers in Chrismear's list are averages, likely medians. The replacement process, whether of cells or of individual atoms within the cells, is statistical and there's no question whatever that there are still cells in your body, and atoms within cells, left over from the womb.

Take a five gallon jug and fill it with water. Pour out pint and toss it, and then pour in a new pint to replace it and stir well. Keep doing that pint-out-pint-in over and over. How soon before there are no water atoms left from the original five gallons?

Well, you won't live that long.
posted by Steven C. Den Beste at 2:20 PM on October 5, 2006

Notsnot, nerve cells are not being created in your arm. What's happening is that existing nerve dendrites are growing new axons to connect to other nerve cells down in your hand.

It's true that it's been discovered recently that some nerve cells in the brain do divide. But most do not, and there's a good reason why.

Brain tissue is the most complex substance known to us per unit mass, and what makes it different from any other is that nerve cells have to work cooperatively in order for the organ to perform its function. It is the interconnection between nerves which is critical, not the characteristics of individual cells themselves.

So if a nerve cell divides, what happens to the connections it had, both in and out. Hard to say, and apparently evolution has decided that it's not worth finding out, which is why the vast majority of nerves stop dividing before the brain begins to self-organize and begin to function as an information processing organ.

Some nerves can partially regenerate. Peripheral nerves have what's called a "myelin sheath" around the axon, and if the axon is severed, the dendrite can grow a new axon which will channel down the existing myelin sheath and reconnect.

But nerves in the brain don't have that. And even peripheral nerves don't generally replicate after age 6 months.

By the way, that's why there's no such thing as "nerve cancer". Brain tumors are made up of berserk epithelial cells, not of neurons.
posted by Steven C. Den Beste at 2:32 PM on October 5, 2006

Sorry, I'm using a word incorrectly. For "dendrite" read "nerve cell body".
posted by Steven C. Den Beste at 2:37 PM on October 5, 2006

Jesco quotes Bill Bryson "Indeed, it has been suggested that there isn’t a single bit of any of us—not so much as a stray molecule—that was part of us nine years ago. It may not feel like it, but at the cellular level we are all youngsters.'"

Bullshit, Bill Bryson.

What about the calcium and strontium molecules in bones and teeth? Then how can paleontologists tell a skeleton's birthplace from the radioactive strontium in the teeth?
posted by orthogonality at 3:03 PM on October 5, 2006

It's exciting to think that you might not be in any way physically the same being as you were seven years ago (that's the one I always heard). But look at it from the other direction: with any luck, when you die you might become fossilized: and then those bits of bone and tissue could conveivably continue to be "you" until the sun annihilates your final permineralized form as it expands, dying, wiping out the solar system. Now that's longevity.
posted by vraxoin at 3:08 PM on October 5, 2006

Teeth are old, bones are new. Bones are dissolved and rebuilt regularly. How do you think fractures heal?
posted by NortonDC at 3:48 PM on October 5, 2006

Yep.
posted by Phred182 at 8:23 PM on October 5, 2006

I 've also heard seven years as the traditional answer to this question. The new details up-thread are fascinating. My question is, does the enamel on our teeth (or really, any part of the tooth) regenerate?
posted by Rash at 9:28 AM on October 6, 2006

Rash, no, the enamel on our teeth is dead material and it does not regenerate.
posted by Steven C. Den Beste at 3:33 PM on October 6, 2006