No doubt you know that your skin cells are constantly falling off. Hopefully you have arrived at the conclusion that, since this has never caused your muscles to show, your skin cells are dividing to make new skin cells to replace those that are lost overboard. You will never run out of skin because it replenishes itself. No doubt you have also heard that your brain cells are not so fecund, although the old trope that you are born with all the brain cells you will ever have isn’t quite true. All of the cells in your body from your liver to your heart can at some point wear out and need to be replaced, but the speed at which that happens depends on the organ… some are as fast as the skin, others slow like the brain. So, some of the cells in your body were born recently and others quite a while ago. Your birthday as an organism then, is not the same as the birthday of your various cells. When were their birthdays? How can we possibly determine the birthday of a cell? For one cell its impossible, but if you can get a whole bunch of the same type of cell (as in a chunk of tissue), you can get a fairly accurate estimate of the average birthday of those cells. But how?
I imagine that “I sure am glad that nuclear weapons were tested in the atmosphere, spewing radioactive compounds that would one day end up in my food.” is rarely uttered. In fact, I am reasonably confident that I might just have been the first one to type it out. But from the 1950s to the early 1960s that is exactly what happened. Very smart people with college degrees thought it a great idea to study nuclear bomb designs by blasting them above ground. Thanks guys. Without these knuckleheads though, cellular birthdays would be a lot harder to determine.
Some chemistry that I barely understand: when you detonate a nuclear device in the atmosphere, “thermal neutrons” are generated. I could figure out exactly what those are from the Wikipedia entry and summarize that here, or you could just read it yourself; I would prefer the latter. Apparently when a thermal neutron collides with a molecule of nitrogen in the atmosphere, it produces Carbon-14, a radioactive form of carbon that reacts with other atoms exactly like non-radioactive carbon. Being radioactive, however, means that these Carbon-14 atoms occasionally fall apart and emit a bit of energy that we call radioactivity and THAT will be key to measuring cellular birthdays. One final bit of chemistry – unless you are talking about diamonds or pencil lead, carbon generally doesn’t exist by itself. For the purposes of this sermon, imagine that Carbon-14 as carbon dioxide. That is one atom of Carbon-14 bonded to two different oxygen atoms. Carbon dioxide gets us into the carbon cycle which eventually ends up in your food. [ * (big fat asterisk)]
In the classroom I work hard to draw out my students into revealing their misperceptions (and getting them to see that we ALL have them); one of the most prevalent ones is that diffusion of molecules in the air, while good enough to move perfume or body odor through out a room, can not act over great distances. So when I show them the Keeling Curve (below), for the most part they refuse to believe that carbon dioxide produced in North America or Asia could make it all the way to Hawaii.
If the graph above doesn’t convince you that carbon dioxide and other gasses move large distances around our globe, stop reading now because none of this will make much sense to you. Besides, when I bring the Keeling Curve in class its in reference to global warming and climate change. Maybe nuclear fallout in their food would be more engaging.
Back to Carbon-14. Carbon gets around, whether its radioactive or not. It cycles. As carbon dioxide, it is taken up by photosynthetic organisms and is transformed into their bodies. Inevitably, every organism is food for another, so the carbon travels through the food chain, occasionally popping out again as carbon dioxide to re-enter the cycle again (think about it, this explains the Keeling Curve). Eventually, some of this 14C made its way into a corn plant in Iowa, which is fed to a cow in Illinois, which is processed and becomes part of your cells via a drive thru window.
We are almost there. As you pull out of the parking lot and back onto the freeway, hamburger in hand, your intestines are sucking up the nutrients** and sending them through your bloodstream for use by your cells. Some cells will take in the nutrients and suck out the energy they contain – a process which turns the carbon in those nutrients back into carbon dioxide for you to breathe out. Some cells will store those nutrients for later use. But some cells will be repairing tissue and will use those nutrients to make more cells; if some of the carbon in those nutrients was Carbon-14, then those new cells will be have some of that as their permanent structure.
Key point: since atmospheric levels of C-14 have been steadily decreasing since the Partial Treaty in 1963, the amount of C-14 in the carbon cycle and thus the food supply is steadily decreasing too. Cells that were born in in 1984 have less C-14 than cells that were born in 1970. By removing tissue and measuring the amount of C-14 in it, scientists can match the amount for that tissue up to the graph of historical atmospheric levels and tell you when the average cell in that tissue was born.
As a simple example, look at how the C-14 levels in the rings of a tree decrease with the age of the ring (remember, older rings on the outside):
Now back to humans. When 3 tissues were compared (see below), some of the data was not surprising – the cells of your intestines are younger than those of your brain. An age of 15 years for your intestinal cells might at first seem surprising – the cells that line this organ system are constantly being rubbed off by you food and last only about 5 days (known from animal studies). The difference is that in this study they grabbed more than that outer layer of cells so the 15 year quote is for the organ as a whole (probably includes some connective tissue and smooth muscle in addition to the outer epithelial layer).
Interesting too that the neurons in your cerebellum, which controls motor coordination are older than those in your cortex, which is the part you “think” with. I have no idea what that means, other than it looks very doubtful that I will ever have the coordination to play any sport involving a ball and my hands.
In a more recent study, this technique was used to show that the cells of the heart regenerate over our lives too. So there is hope for the broken heart. Except that sadness is not experienced by the heart but the brain, and those fix themselves too.
* (big fat asterisk) – Don’t freak out here, Carbon-14 is also produced naturally when cosmic radiation strikes nitrogen molecules in the upper atmosphere, its just that the “natural” levels are lower than the “Man made” levels. There are radioactive compounds in nuclear fallout (like Iodine isotopes) that can end up getting in or on food that I am not comfortable with, but Carbon-14 is not one of them. To explain that I would need to explain my experience using different isotopes in the research lab, and how comparatively weak the radiation coming off of carbon-14 is. I am not thrilled about the increased amount of 14C in my food, but I am not all that worried about it. I am be more worried about the proven carcinogenicity of charred food (see third paragraph) or dying in a car wreck.
** – debatable
Spalding KL, Bhardwaj RD, Buchholz BA, Druid H, Frisén J. (2005). Retrospective Birth Dating of Cells in Humans. Cell. Jul 15;122(1):133-43.
Kajstura J, Rota M, Cappetta D, Ogórek B, Arranto C, Bai Y, Ferreira-Martins J, Signore S, Sanada F, Matsuda A, Kostyla J, Caballero MV, Fiorini C, D’Alessandro DA, Michler RE, del Monte F, Hosoda T, Perrella MA, Leri A, Buchholz BA, Loscalzo J, Anversa P. (2012). Cardiomyogenesis in the aging and failing human heart. Circulation. Oct 9;126(15):1869-81