Dr. Jonas Frisen, a stem cell biologist at the Karolinska Institute in Stockholm, has developed a new method of estimating the age of human cells, and in doing so made an interesting discovery.

You're Under 10, Except for Your Brain

Many of the cells of your body renew themselves constantly, meaning that while you may be 40, most of the cells of your body may be less than 10 years old. Dr. Frisen's procedure revealed that most cells in an adult's body are as young as 7 to 10 years old. However, some of the body's cells remain unrenewed from birth to death, including many or all of the cells in the cerebral cortex.

Dr. Frisen used the carbon-14 content of tissues to measure their age. Although most molecules in a cell are replaced constantly, all of the carbon-14 in a cell dates from the moment the cell is created, when it is absorbed from the environment into the unchanging DNA. Nuclear weapons tests dramatically affect environmental carbon-14, so the atmospheric content has changed in measurable, known ways since 1963. Since the carbon-14 content of a cell will vary in the same way, it can be used to determine the cell's year of birth.

This will, alongside many other uses, allow researchers to answer long-standing questions about whether any cells in the brain are renewed, or whether new brain cells simply stop growing at some point in human development.

Why do we Grow Old?

Human cells can have a lifetime measured in days, as with skin cells and red blood cells, or years, as with the cells of your bones. From what is currently known, the only cells that last a lifetime are those of your cerebral cortex, the inner lens cells of your eye, and possibly the muscle cells of your heart.

This raises the question of why, if renewal is constant everywhere else, does your body age and degrade?

Some experts believe that cell DNA information gradually degrades; others blame mitochondrial DNA, which lacks repair mechanisms. Another theory is that stem cells, the source of new cells, eventually wear out.

If Dr. Frisen's investigations show tissue regeneration slowing as age increases, it could point to stem cell wear as the ultimate source of age and death.