why are elephants bigger than mice ?
suih...@usa1.com
Why are elephants bigger than mice ?
Do they have more cells, or are their cells bigger ?
Assume both factors, (probably the first one more) contribute
to the larger size of elefants.
Thus they must generate more cells during ontogeny to produce
more body mass. This could be achieved by a faster cell cycle,
a longer live span of individual cells (more divisions until
cell senescence) or a lower apopotosis rate.
One had to assume that elefant cells can remain longer
differentiated, i.e. have a more stable genome and tolerate a
longer span of being "normal" through multiple divisions. This
leads to following questions:
1. How long lasts the cell cycle in elefants ? Is it longer
(since more cells needed) or shorter (since probably cells are
bigger) ?
2. Do elefant cells have more telomerase activity ?
3. Do elephants have a significantly lower or higher incidence
of neoplasia than mice ? (I know for mice it is quite high,
(probably even taking into account the fact that we only study
_inbred_ mice)
Any comments, suggestions or even attempts of an answer
appreciated.
Sui Huang
Surgical Research
Children's Hospital/Harvard Medical School
Boston
e-mail: suih...@usa1.com
Joachim Dagg
bigger! By having more cells, or by having bigger cells, by faster
cell cycle, etc..
The "how-question" is the typical physiological question. And you'll
get typical physiological answers. The "why-question" can be asked as
a historical question. Then it's the typical phylogenetic
question. But you'll only get phylogenetic answers. If you ask the
"why-question" in a heuristic sense, you'll ask the typical ecological
question and get typical ecological answers.
But asking why and exspecting physiological answers does not seem very
promising to me.
----
Mark Ion Quest
Carlisle Landel
suih...@usa1.com <suih...@usa1.com> wrote:
>
>Why are elephants bigger than mice ?
>
>Do they have more cells, or are their cells bigger ?
[snip]
Elephants have more cells. The cell cycle should be about the same
length in both.
(Remember that you get tremendous size variations even within species,
without changes in cell cycle length.)
In principle, making a bigger animal doesn't require much more cell
division, since every division *doubles* the size of the animal.
(Yeah, I know, it is a gross oversimplification, but you get the idea.)
>3. Do elephants have a significantly lower or higher incidence
>of neoplasia than mice ? (I know for mice it is quite high,
>(probably even taking into account the fact that we only study
>_inbred_ mice)
The tumor rates in big vs small animals is an interesting question, but
I would think that more important than the extra divisions involved in
making a bigger animal is the relative susceptibility due to the way the
genes are used to make an animal. (ooooo--now *that's* a clear statement!
Let's see if I can rephrase it...) That is to say, species probably have
inherent susceptibilities to cancers that are the result of the genetic
program used to make those animals (eg, the spectrum of tumors that result
from p53 knockouts in humans are different from those in mice), and these
probably mask any effects due to size differences. But I could be wrong.
That's why I think it is an interesting question.
Finally, let me say that on a deeper level, this is a pretty cool problem.
Basically, all us mammals share pretty much the same set of structural
genes. I mean, hemoglobin is pretty much hemoglobin, whether it comes
from mice or elephants or humans. What differentiates the myriad body
plans of mammals (or all life forms, for that matter) is the way those
genes are used during the developmental program. Thinking about how those
control circuits came about is a very interesting question in evolutionary
and developmental biology.
Which poses an interesting question: If we developmental biologists ever
figure it all out, does this mean that we ought to be able to not only
clone any existing animal, but indeed we ought to design any animal we want?
(That was one of my problems with Jurassic Park: if those guys could clone
dinosaurs, they could clone anything, so if a velociraptor chewed your arm
off, you should be able to grow a new one.)
Back to my real job!
Later,
Carlisle
R M Bernstein
<helios!lan...@uunet.uu.net> wrote:
>>3. Do elephants have a significantly lower or higher incidence
>>of neoplasia than mice ? (I know for mice it is quite high,
>>(probably even taking into account the fact that we only study
>>_inbred_ mice)
>
>The tumor rates in big vs small animals is an interesting question, but
>I would think that more important than the extra divisions involved in
>making a bigger animal is the relative susceptibility due to the way the
>genes are used to make an animal. (ooooo--now *that's* a clear statement!
>Let's see if I can rephrase it...) That is to say, species probably have
>inherent susceptibilities to cancers that are the result of the genetic
>program used to make those animals (eg, the spectrum of tumors that result
>from p53 knockouts in humans are different from those in mice), and these
>probably mask any effects due to size differences. But I could be wrong.
>That's why I think it is an interesting question.
there are two nifty points to be made from this question,
1) tumour size is relative. since cells are pretty much the same size
from one animal to the next, it its the palpability of the tumour which
allows us to realise the presence of a tumour. as a 1x10^6 tumour is
palpable in a mouse, in a human it is too small to be noticed, imagine in an
elephant.
2) as to mice having higher tumour rates, "even though" they are imbred,
well most of the imbred mice have restricted numbers of MHC molecule types,
which doesn't allow for them to respond to some antigens, in effect these
mice often have huge "holes in their repitoire."
so, elephants could have tumourous growths quite frequently and they
wouldn't be noticed or fatal to the organism, as they are surveyed by the
immune system-as occurs in humans.
regards, ralph
Ralph M. Bernstein
Dept of Micro/Immuno
University of Arizona
Ph: 602 626 2585
Fx: 602 626 2100
url: http://lamprey.medmicro.arizona.edu
Shane McKee
>3. Do elephants have a significantly lower or higher incidence
>of neoplasia than mice ? (I know for mice it is quite high,
>(probably even taking into account the fact that we only study
>_inbred_ mice)
"Why is it that everything I like doing causes cancer in mice?"
Shane McKee (SHO, RVH, Belfast) | / Art becomes science when
Sh...@reservoir.win-uk.net --O-- you start trying to figure
/ | out what the heck you're doing
Richard Johnston
On 12 Sep 1995, Carlisle Landel wrote:
> The tumor rates in big vs small animals is an interesting question, but
> I would think that more important than the extra divisions involved in
>making a bigger animal is the relative susceptibility due to the way the
>genes are used to make an animal. (ooooo--now *that's* a clear statement!
> Let's see if I can rephrase it...) That is to say, species probably have
> inherent susceptibilities to cancers that are the result of the genetic
> program used to make those animals (eg, the spectrum of tumors that result
> from p53 knockouts in humans are different from those in mice), and these
> probably mask any effects due to size differences. But I couldbe wrong.
> That's why I think it is an interesting question.
>
Shouldn't a species' average life span have more to do with tumor
susceptibility than size? If a given species has a short reproductive
cycle, it can tolerate a greater susceptibility to tumors (assuming of
course, that the average member of that species reproduces before tumors
become life-threatening). Larger animals do tend to live longer than
small ones, but at any particular size there can a fairly broad range of
life spans. For example, birds and rodents have similar body weights, but
birds tend to live much longer. I would assume that the quicker a species
reproduces itself the more susceptible it should be to not just tumors,
but other genetic defects and diseases.
p-m agapow
>On 12 Sep 1995, Carlisle Landel wrote:
>
>> The tumor rates in big vs small animals is an interesting question, but
>> I would think that more important than the extra divisions involved in
>>making a bigger animal is the relative susceptibility due to the way the
>>genes are used to make an animal. (ooooo--now *that's* a clear statement!
>> Let's see if I can rephrase it...) That is to say, species probably have
>> inherent susceptibilities to cancers that are the result of the genetic
>> program used to make those animals (eg, the spectrum of tumors that result
>> from p53 knockouts in humans are different from those in mice), and these
>> probably mask any effects due to size differences. But I couldbe wrong.
>> That's why I think it is an interesting question.
>Shouldn't a species' average life span have more to do with tumor
>susceptibility than size? If a given species has a short reproductive
>cycle, it can tolerate a greater susceptibility to tumors (assuming of
>course, that the average member of that species reproduces before tumors
>From memory, there's a fairly good correlation between efficiency of DNA
repair and lifespan. Note that this tells us nothing about cause and
effect - a longer life could necessitate better DNA repair, or better
repair could enable longer life. (Or they need not be directly related at
all ...)
In any event, this tells us nothing about the relative sizes of elephants
and mice.
p-m
paul-michael agapow (aga...@latcs1.oz.au), LaTrobe Uni, Infocalypse