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Re: [ontolog-forum] Two ontologies that are inconsistent but bothneeded

To: "[ontolog-forum] " <ontolog-forum@xxxxxxxxxxxxxxxx>
From: "Azamat" <abdoul@xxxxxxxxxxxxxx>
Date: Sat, 9 Jun 2007 23:02:52 +0300
Message-id: <005f01c7aad1$2a1933d0$a70c7d0a@homepc>
BS: Perhaps someone can even look at Fritz's body and see it as a life, 
though I find it hard to do so.    (01)

Don't remember the URL. Below some interesting empirical findings made by a 
stem cell biologist shedding light on the issue: what is a human body, a 
process or a substance. Enjoy and surpise.
Your Body Is Younger Than You Think    (02)

Top of Form    (03)


Published: August 2, 2005    (05)

Whatever your age, your body is many years younger. In fact, even if you're 
middle aged, most of you may be just 10 years old or less.    (06)

This heartening truth, which arises from the fact that most of the body's 
tissues are under constant renewal, has been underlined by a novel method of 
estimating the age of human cells. Its inventor, Jonas Frisen, believes the 
average age of all the cells in an adult's body may turn out to be as young 
as 7 to 10 years.    (07)

An Eye is Forever, but Is a Liver?    (08)

But Dr. Frisen, a stem cell biologist at the Karolinska Institute in 
Stockholm, has also discovered a fact that explains why people behave their 
birth age, not the physical age of their cells: a few of the body's cell 
types endure from birth to death without renewal, and this special minority 
includes some or all of the cells of the cerebral cortex.    (09)

It was a dispute over whether the cortex ever makes any new cells that got 
Dr. Frisen looking for a new way of figuring out how old human cells really 
are. Existing techniques depend on tagging DNA with chemicals but are far 
from perfect. Wondering if some natural tag might already be in place, Dr. 
Frisen recalled that the nuclear weapons tested above ground until 1963 had 
injected a pulse of radioactive carbon 14 into the atmosphere.    (010)

Breathed in by plants worldwide and eaten by animals and people, the carbon 
14 gets incorporated into the DNA of cells each time the cell divides and 
the DNA is duplicated.    (011)

Most molecules in a cell are constantly being replaced but the DNA is not. 
All the carbon 14 in a cell's DNA is acquired on the cell's birth date, the 
day its parent cell divided. Hence the extent of carbon 14 enrichment could 
be used to figure out the cell's age, Dr. Frisen surmised. In practice, the 
method has to be performed on tissues, not individual cells, because not 
enough carbon 14 gets into any single cell to signal its age. Dr. Frisen 
then worked out a scale for converting carbon 14 enrichment into calendar 
dates by measuring the carbon 14 incorporated into individual tree rings in 
Swedish pine trees.    (012)

Having validated the method with various tests, he and his colleagues have 
reported in the July 15 issue of Cell the results of their first tests with 
a few body tissues. Cells from the muscles of the ribs, taken from people in 
their late 30's, have an average age of 15.1 years, they say.    (013)

The epithelial cells that line the surface of the gut have a rough life and 
are known by other methods to last only five days. Ignoring these surface 
cells, the average age of those in the main body of the gut is 15.9 years, 
Dr. Frisen found.    (014)

The Karolinska team then turned to the brain, the renewal of whose cells has 
been a matter of much contention. Prevailing belief, by and large, is that 
the brain does not generate new neurons after its structure is complete, 
except in two specific regions, the olfactory bulb that mediates the sense 
of smell, and the hippocampus, where initial memories of faces and places 
are laid down.    (015)

This consensus view was challenged a few years ago by Elizabeth Gould of 
Princeton, who reported finding new neurons in the cerebral cortex, along 
with the elegant idea that each day's memories might be recorded in the 
neurons generated that day.    (016)

Dr. Frisen's method will enable all regions of the brain to be dated to see 
if any new neurons are generated. So far he has tested only cells from the 
visual cortex. He finds these are exactly the same age as the individual, 
showing that new neurons are not generated after birth in this region of the 
cerebral cortex, or at least not in significant numbers. Cells of the 
cerebellum are slightly younger than those of the cortex, which fits with 
the idea that the cerebellum continues developing after birth.    (017)

Another contentious issue is whether the heart generates new muscle cells 
after birth. The conventional view that it does not has recently been 
challenged by Dr. Piero Anversa of the New York Medical College in Valhalla. 
Dr. Frisen has found the heart as a whole is generating new cells, but he 
has not yet measured the turnover rate of the heart's muscle cells.    (018)

Although people may think of their body as a fairly permanent structure, 
most of it is in a state of constant flux as old cells are discarded and new 
ones generated in their place. Each kind of tissue has its own turnover 
time, depending in part on the workload endured by its cells. The cells 
lining the stomach, as mentioned, last only five days. The red blood cells, 
bruised and battered after traveling nearly 1,000 miles through the maze of 
the body's circulatory system, last only 120 days or so on average before 
being dispatched to their graveyard in the spleen.    (019)

The epidermis, or surface layer of the skin, is recycled every two weeks or 
so. The reason for the quick replacement is that "this is the body's saran 
wrap, and it can be easily damaged by scratching, solvents, wear and tear," 
said Elaine Fuchs, an expert on the skin's stem cells at the Rockefeller 
University.    (020)

As for the liver, the detoxifier of all the natural plant poisons and drugs 
that pass a person's lips, its life on the chemical-warfare front is quite 
short. An adult human liver probably has a turnover time of 300 to 500 days, 
said Markus Grompe, an expert on the liver's stem cells at the Oregon Health 
& Science University.    (021)

Other tissues have lifetimes measured in years, not days, but are still far 
from permanent. Even the bones endure nonstop makeover. The entire human 
skeleton is thought to be replaced every 10 years or so in adults, as twin 
construction crews of bone-dissolving and bone-rebuilding cells combine to 
remodel it.    (022)

About the only pieces of the body that last a lifetime, on present evidence, 
seem to be the neurons of the cerebral cortex, the inner lens cells of the 
eye and perhaps the muscle cells of the heart. The inner lens cells form in 
the embryo and then lapse into such inertness for the rest of their owner's 
lifetime that they dispense altogether with their nucleus and other cellular 
organelles.    (023)

But if the body remains so perpetually youthful and vigorous, and so 
eminently capable of renewing its tissues, why doesn't the regeneration 
continue forever?    (024)

Some experts believe the root cause is that the DNA accumulates mutations 
and its information is gradually degraded. Others blame the DNA of the 
mitochondria, which lack the repair mechanisms available for the 
chromosomes. A third theory is that the stem cells that are the source of 
new cells in each tissue eventually grow feeble with age.    (025)

"The notion that stem cells themselves age and become less capable of 
generating progeny is gaining increasing support," Dr. Frisen said. He hopes 
to see if the rate of a tissue's regeneration slows as a person ages, which 
might point to the stem cells as being what one unwetted heel was to 
Achilles, the single impediment to immortality.    (026)

As reality is in the state of constant flux, so our body is in constant 
change, renewal. It appears that there is no permanent structure in the 
world barring ontological truths: fundamental principles, essential 
invariants, objective patterns, underlying laws of unvarying validity.    (027)

Azamat    (028)

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