Cells that last a lifetime
Q: If I am 95 years old, do I have any cells in my body that
were present when I was 3 years old? What about at conception? Also, could you expand on your earlier
statement, "stem cells live a lifetime?" In a nutshell, I am curious if I
am a completely "new" person or if there is anything "in" me that has been with
me since birth...or conception.
A: Nope, we're not completely new people. In fact, most of our cells are a
good seven to ten years old, as biologist
Jonas Frisén,
professor at the Karolinska Institute in Stockholm,
reported in July 2005. For instance, we grow a new skeleton over
about a 10-year period.
 Lifetime
cells: The baby has cortex neurons, heart muscle cells
and eye lens cells he will still have the day he dies. Photo courtesy of Carin Araujo
and Wikipedia.
Moreover, at 95 we will have three kinds of cells we had since three years
of age and, indeed, since before birth. They last a lifetime:
- cerebral cortex neurons
- heart muscle cells
- cells in the inner portion of the crystalline lens of our eyes.
"I wouldn't necessarily single out cerebral cortex neurons as lasting a
lifetime because it's probably true that most neurons in the entire adult brain
were generated during development," emails neuroscientist
Elizabeth Gould professor of psychology at Princeton.
The brain can repair and renew itself, but most of the cells in the cerebral
cortex and the brain last a lifetime. We used to think that the cerebral cortex
nerve cells remained unchanged. Now we know better. Brain cells, even in the
cerebral cortex, renew themselves. In 1999, Frisén
startled the neuroscience community by announcing he had found stem cells in the
brain. Stem cells create new cells and, in this case, new brain cells.
Now, however, Frisén seems to have changed his thinking. "My estimate
is that 100% of cortical neurons stay with us from birth," he emails.
That's probably true, but doesn't rule out the possibility that the brain
also makes new cells. "Two very different types of cells
live a life time. Hence, your brain has both fully differentiated neurons
that live a lifetime, and 'stem cells' that continue to make new brain cells,"
clarifies researcher Gail W. Sullivan (retired assistant professor at the
Cardiovascular Research Center in Charlottesville, Virginia).
The heart is still more controversial. Most heart muscle cells probably stay with us, from birth. Again, like the cerebral cortex, recently we believed heart muscle remained
unchanged throughout a lifetime. However,
cardiovascular researcher
Piero Anversa
has found stem cells in rat hearts and new heart muscle cells.
The crystalline lens in our eyes has innermost cells that last a lifetime. Lens cells
are like skin cells in structure, but are totally unlike skin in lifespan.
The skin renews itself daily; whereas the lens never loses cells. The lens
does add more cells to the outside of the lens, but the oldest, inside ones have
been there since before birth.
A fertilized egg divides to form inside eggs, which can create any cell, for example, heart, brain or T-cell.
Diagram courtesy of Mike Jones and Wikipedia, modified by author.
Finally, there's the story of the immortal stem-cell DNA, which begins with the fertilized human egg,
at conception.
The fertilized egg divides repeatedly to form ten cells: the inside
and outside cells. The inside cells divide more, and become the embryo.
Each inside cell is a stem cell that can form any of the 220 kinds
of cells in the body — heart, brain,
T-cell — whatever. These remarkable
embryonic stem cells don't last a lifetime, but their DNA does.
An ordinary cell splits to form two daughter cells, and duplicates its DNA so
each daughter cell gets a copy. But over a lifetime, as a
cell reproduces perhaps thousands of times, duplication errors can build up, and
cause trouble. The embryonic stem cell, consequently, doesn't do it this
way, as
Shahragim Tajbakhsh of the Pasteur Institute in Paris recently
discovered.
The stem cell splits into two daughter cells (as does an
ordinary cell), but instead of creating two ordinary cells, it forms
another stem cell and a specialized cell (for example, a heart cell). Now,
the crucial difference — the stem cell retains the original DNA strands, and
makes a DNA copy only for the specialized cell. So, the DNA of the
original fertilized egg lasts from conception to death.
Click for an
animated demonstration of how an animal cell splits to form two daughter
cells, courtesy of Cells Alive.
Further Reading:
Live
and death of cells, WonderQuest
Cells alive
The reinvention of self by Jonah Lehrer, SeedMagazine.com, February 2006
Muscle cells in hearts may divide by J. Brainard, Science News Online, July
1998
Heart rehab, ScienCentral News, February 2007
Ultrastructure of vertebrate lenses by Pertti Malkki and Ronald Körger,
Vision Group of Lund University
Molecular expressions galleria by Michael Davidson, Florida State University
Immortal DNA in skeletal muscle stem cells, InformationHospitaliere.com,
June 2006.
Human embryonic stem cells: a primer by Scott Gilbert, Swarthmore College.
Your body is younger than you think by Nicholas Wade, New York Times, August
2005
(Answered April 2, 2007)
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