Nicholas Wade of the New York Times reports on how the use of carbon-14 dating of cellular DNA by Jonas Frisen shows that most cells in the body are less than 10 years old. (same article here and here)
But Frisen, a stem cell biologist at the Karolinska Institute in Stockholm, Sweden, 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.
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, 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. 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.
Having validated the method with various tests, he and his colleagues reported the results of their first tests with a few body tissues in the July 15 issue of Cell. They say cells from the muscles of the ribs, taken from people in their late 30s, have an average age of 15.1 years.
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, Frisen found.
Read the full article for more details on the average age of various types of cells. Note that the vast majority of neurons have existed since childhood. The need to rejuvenate existing neural cells makes brain rejuvenation by far the hardest part of the total rejuvenation therapy development puzzle.
While the researchers found that in some parts of the brain the average cell age was less than the age of the person in the visual cortex the brain was about the same age as the person.
They found that all of the samples taken from the visual cortex, the region of the brain responsible for processing sight, were as old as the subjects themselves, supporting the idea that these cells do not regenerate. "The reason these cells live so long is probably that they need to be wired in a very stable way," Frisén speculates.
Keep in mind that just because a cell divided, say, 7 years ago that doesn't make it youthful. The duration of time since a cell was created from mitotic division is not a measure of the cell's functional age. The damage done to parent cell DNA is inherited by the two cells that are produced when a cell divides. Therefore newly created cells in older organisms will function more like old cells. Also, chromosome telomere caps get shorter each time cells divide and this limits how many times cells can divide. A 7 year old stem cell is of no use if it can no longer divide when damage occurs in joints, muscles, blood vessels, or other components of the body.
The fact that on-going cellular division makes most cells chronologically young and that old cells divide less well actually presents an opportunity for the development of rejuvenation therapies. The development of technologies for producing youthful adult stem cells will provide sources of youthful and healthier stem cells cells to replace the older and less healthy cells that accumulate in our bodies as we age. Since older stem cells divide more slowly rejuvenated stem cells introduced into various parts of the body would out-compete and gradually displace the older cells. Then since most cells are, as reported above, not all that old gradually over a period of several years many more specialized cells (again, blood vessel lining, skin cells, gut cells, etc) would get produced from the healthier introduced stem cells. So gradually a larger fraction of our bodies would become young again.
This latest result supports arguments for an acceleration of the development of stem cell therapies. But we still need to develop gene therapies and other therapies aimed at repairing existing aging neurons and glial support cells in situ, meaning right in the brain. Stem cell therapies and gene therapies are probably the two most important Strategies for Engineered Negligible Senescence (SENS).
|Share |||Randall Parker, 2005 August 03 12:37 PM Aging Studies|