Implications of cells only dividing 50 times

[Helen_s_Eidolon]

I was taught in high school, and again in university, that cells can only divide 50 times in the human body (whether this istrue for other cases, I’m not sure). The exception to this rule is cancer.

Okay, this is fine, but this has implications! Since the fertilized ovum is a cell (assuming that it’s considered a “new” cell) does that mean that any given human can have a maximum of 2[sup]50[/sup] cells in their entire life?

[x-ray_vision]

You are probably thinking of the Hayflick limit. This is roughly the amount of times that a human cell can double cultured in a lab (not in the human body).

[GreyWanderer]

@x-ray vision

You are probably thinking of the Hayflick limit. This is roughly the amount of times that a human cell can double cultured in a lab (not in the human body).

Why is the number of times it can divide limited in a lab?

[Qadgop_the_Mercotan]

@GreyWanderer

Why is the number of times it can divide limited in a lab?

Good question. Figure that out and you’ll get some major kudos!

Where’s apoptosis when you need him?

[mnemosyne]

No, no, no! Apoptosis KILLS cells! That won’t help answer this question at all!

(sorry!)

[Epimetheus]

@x-ray vision

You are probably thinking of the Hayflick limit. This is roughly the amount of times that a human cell can double cultured in a lab (not in the human body).

No, cells have a fixed number of times they can divide, skin cells, apparently, can only divide 50 times.

http://www.exn.ca/Stories/1998/05/12/52.asp

In the same way the reproduction of audio cassettes yields progressively lower sound quality, there is only a certain amount of times the cell can divide before its telomeres are so degraded the cell lapses into a state of senescence, and divides no more. Different cells are designed with different potentials for division - for example, brain cells don’t divide at all, skin cells divide a good 50 times, and sperm-producing cells never exhibit senescence.

[viking]

well, keep in mind that 2^50 is a truly, colossally, huge number. 1.26x10^15, in fact. 1,260,000,000,000,000, in other words. http://science.howstuffworks.com/cell1.htm suggests that there are in fact 10 trillion cells in a human body, which is 10^12, or one one-thousandth of that number…

[Epimetheus]

Ahh, your question has already been answered, I found it for ya: Scroll downs a ways to find the original.

http://www.pbs.org/safarchive/3_ask/archive/qna/32103_harley.html

**
I am amazed by the leaps and bounds that the field of biotechnology has taken during the last ten years. During the PBS show, a comment was made that a cell can only divide approximately 50 times before the telomeres “wear out.” This number seems quite small considering the amount of dead skin that falls off a normal human. The only explanation that I can derive is that the number is really 2 (two) to the 50 (fiftieth) power number of cells or 1,125,900,000,000,000. This would be the total number of cells if one cell divided into two and all subsequent generations also divided into two with fifty iterations of the process. Please advise on my assumptions and conclusions. Thanks. (Question from Ross)**

*Your math is correct, assuming cells divide exponentially. In some cases they do, in some cases they don’t. Your number of 1.1259 x 10^15 cells for only 50 doublings translates to about 10,000 kilograms of cells from EACH starting cell. This is more than enough for a lifespan, given that we are born with BILLIONS AND BILLIONS (a Sagan Unit) of cells in every organ. In most cases, tissues do not use full exponential growth, and very likely highly proliferative tissues like skin and blood need to use transient telomerase expression to slow telomere loss and extend the lifespan of clonal populations of cells. *

[Helen_s_Eidolon]

Thanks, Epimetheus! However, I notice that that answer still brings up another question which I skirted around in my OP, what counts as the start of the 50? That paragraph says that we are born with a certain number of cells in every organ. Shouldn’t that already have necessitated a number of divisions?

[Epimetheus]

@LaurAnge

Thanks, Epimetheus! However, I notice that that answer still brings up another question which I skirted around in my OP, what counts as the start of the 50? That paragraph says that we are born with a certain number of cells in every organ. Shouldn’t that already have necessitated a number of divisions?

I am not sure what he is getting to tell you the truth. I believe he is saying that past a certain number of divisions, certain cells (some organs, for example) stop dividing expotentially.(sp)

As for the born part. Well, when a person is BORN they are not one cell, so yeah… that does suggest that some division has occured already. The part that counts as the 50 is from the first division on. One cell divides to create two, the one has 49 division left, and the new cell has 50, they both divide, and the first cell has 48, the second cell has 49, and the other two have 50. (as they have not divided yet)

Repeat ad nauseum. Not sure if that repeats your question- not quite sure if that was what you were getting at…

[mischievous]

Telomeres exist as “caps” on the end of chromosomes. They get shorter every time a cell divides, and when they get too short the cell generally dies due to chromosomal abnormalities. Hence the estimate that an adult cell can only divide around 50 times.

So, you’re thinking, why haven’t we all died? The answer is that cells in the early embryo have an enzyme called telomerase, which re-lengthens the telomeres to the appropriate length at the begining of gestation - so your parent’s cell divisions are cancelled out. Cancer cells, which are dividing wildly, often re-activate this enzyme, and some other cell types may do so under normal conditions.

While telomerase is obviously necessary in the long run, shortening telomeres don’t seem to play a role in any one lifespan. Researchers have mutated the telomerase gene in mice, thereby stopping ANY cells (including embryonic) from re-lengthening their telomeres. These mice were perfectly happy and fertile through the fifth generation, when their telomeres finally ran out. So, in mice at least, there is more than enough room in the telomeres for all the cell divisions in the adult animal.

mischievous

[Epimetheus]

Good post, but I hate to bother you with this, but do you have a link to the study with those rats? Can’t find anything myself…

[mischievous]

@Epimetheus

Good post, but I hate to bother you with this, but do you have a link to the study with those rats? Can’t find anything myself…

The mouse (not rat) mutation was published in Cell, 1997, volume 91, pp25-34. I think that the site won’t let you get more than the abstract without a subscription.

(telomerase-deficient)mice lacked detectable telomerase activity yet were viable for the six generations analyzed. Telomerase-deficient cells could be immortalized in culture, transformed by viral oncogenes, and generated tumors in nude mice following transformation.

Of course, later studies did show defects in sixth-generation mice, particularly in sperm production and in an inability to form skin cancers, which is consistent with telomerase being required eventually, but not within a single lifespan.

mischievous

[Epimetheus]

@mischievous

The mouse (not rat) mutation was published in Cell, 1997, volume 91, pp25-34. I think that the site won’t let you get more than the abstract without a subscription.

Of course, later studies did show defects in sixth-generation mice, particularly in sperm production and in an inability to form skin cancers, which is consistent with telomerase being required eventually, but not within a single lifespan.

mischievous

Thanks, so basically that route is not the route towards longer life spans, and telomerase is a dead end? Or is it saying that we have a ways to go before the lifespans reach a point at which telamerase is going to be ncessary to supplant?

[mischievous]

@Epimetheus

Thanks, so basically that route is not the route towards longer life spans, and telomerase is a dead end? Or is it saying that we have a ways to go before the lifespans reach a point at which telamerase is going to be ncessary to supplant?

Telomerase is not the limiting factor for aging, anyway. I suppose that if we managed to extend our lifespans to 200-300 years, we might need to figure out a way to reactivate telomerase in proliferative tissues (such as blood precursors, gut, skin, and hair). Right now it’s not an issue.

mischievous