Enlarge Diagram courtesy of Mike Jones and Wikipedia, modified by author A fertilized egg divides to form inside eggs, which can create any cell, for example, heart, brain or T-cell.

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.Jonathan, Los Angeles, California, USA

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 example, we grow a new skeleton over about a 10-year period.

Moreover, at 95 we will have three kinds of cells we've 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," e-mails 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 — in this case, new brain cells.

However, Frisén seems to have changed his thinking. "My estimate is that 100% of cortical neurons stay with us from birth," he e-mails.

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 lifetime. 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).

Heart cells are even 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 rats' 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 ones inside have been there since before birth.

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)

April Holladay, science journalist for USATODAY.com, lives in Albuquerque, New Mexico. A few years ago Holladay retired early from computer engineering to canoe the flood-swollen Mackenzie, Canada's largest river. Now she writes a column about nature and science, which appears Fridays at USATODAY.com. To read April's past WonderQuest columns, please check out her site. If you have a question for April, visit this informational page.

Enlarge By Henning Kaiser, AFP/Getty Images Newborn baby Raphael, born in Cologne, Germany, around half past midnight on Jan. 1. A newborn has cortex neurons, heart muscle cells and eye lens cells he will still have the day he dies.