SITNFlash

Hello Science in the News members!

It is almost time for this year's lecture series and we wanted to let you know that we have posted our topics and schedule on our website at www.SITNboston.org. While you anxiously await this season's talks, satisfy your appetite for demystified science with this explanation of how scientists are able to determine the age of human cells and what this means for the future health of your body's organs and tissues.

Hope to see you in September!

Sincerely,

The SITN staff

Have you ever wondered how old your cells are? At first glace this may seem like an odd question. Of course the cells that make up your body have been around for as long as you have been alive. It is not so simple however. Our bodies are made up of millions of cells, but a cell in one area of the body may have a very different lifespan from a cell in another part of the body. For example, millions of skin cells die and are sloughed off each day and are replaced with new cells. The fact that skin cells can be constantly rejuvenated explains why when we get a cut our bodies can usually repair what has been damaged. But what about other areas of the body such as the intestines, muscles or brain? Are cells in other areas of the body replaced when they die? Inspired by archeologist’s ability to date objects using levels of the radioactive C-14 (Carbon dating), researchers have found a unique way to measure the age of human cells. Understanding how human cells age and to what extent they are replaced throughout a person’s lifetime will help researchers to better understand aging and potentially suggest new therapies for treating degenerative diseases.

How is it possible to measure the age of a cell? A cell’s lifespan begins when it is “born” (after cell division) and ends when it next divides into two new cells (or dies, whichever comes first). When a cell is getting ready to divide, it copies its DNA in order to pass on equal amounts of DNA to each new cell it will divide into. Because DNA molecules contain carbon, it is possible to “label” the DNA of a cell using radioactive elements like carbon-14 (C-14). The C-14 will only be incorporated into the DNA during replication, when new DNA is made. If after being labeled a cell divides rapidly, the label will be spread among many cells and be effectively diluted. However, if the amount of label is constant, than the cell is not dividing and is therefore said to have a longer lifespan.

So how does the radioactive C-14 get into the human body? Well, under normal circumstances a very small percentage of carbon molecules in the environment are radioactive (not to the extent that they are harmful to humans). And carbon atoms, including the radioactive ones, are absorbed into the human body in the foods we eat. It happens that the amount of radioactive C-14 in the environment has remained relatively constant for centuries with the exception of the decade from the mid 1950s to the mid 1960s due to above-ground nuclear testing. During this time, the levels of C-14 in the environment increased dramatically and then dropped off again in 1963 when testing stopped. Because of this, during that decade the amount of C-14 incorporated into the DNA of dividing cells was higher than during other times. This unique circumstance allows scientists now many decades later to examine the cells of a person who has died and see how much C-14 is present in the cells of various body tissues. In this way, scientists found that cells from muscles were found to have a lifespan of 15 years while cells from the intestine had a lifespan of 38 years. Interestingly, they found that in parts of the brain, the cells had never been replaced. This C-14 label was never diluted out by cell division, and therefore it appears that the neurons in these brain regions are used for the entire lifetime of the person.

This study gives insight into how cells age in different parts of the body and will be important in developing therapies to treat damaged and diseased tissues by understanding how natural regeneration, at least in certain tissues, occurs. In addition, as the population ages, these studies can be expanded to include the cells of people who suffered from dementia to determine if the ageing process of these cells differs from unaffected individuals. There is a special interest in the ageing of brain cells due to the ongoing controversy of whether or not new neurons are born in adulthood. Although the researchers of this study found no evidence for adult neurogenesis more exhaustive studies need to be done before a conclusion can be reached. However, the C-14 dating technique described in this article is exciting because it has the potential to answer this important and elusive question as well as many others.

For more information:

http://www.boston.com/news/science/articles/2005/07/18/carbon_dating_may_work_for_human_cells_too/