DEAR DOCTOR K:
I see advertisements about various treatments that stop your cells from aging. Is there anything to that? What happens to our cells as we age?
There are no treatments that can stop our cells from aging. But in the past 10 years scientists have made giant steps in understanding what causes cells to age. That knowledge could lead to true “anti-aging” treatments.
As for what happens to our cells as we age, I’d rephrase the question: What happens to us as our cells age?
Our bodies are made up of trillions of cells. As we get older, so too do our cells. And like us, our cells do not live forever. Most of the cells in our bodies today were not with us when we were born.
From birth through adulthood, new cells are constantly forming; that’s how our bodies grow larger. Cells “grow” not by getting bigger, but by dividing in two. This process is called mitosis. Normally, the two cells that result from mitosis are identical right down to the last bit of genetic information.
Once we reach adulthood, our cells don’t need to divide to help our bodies grow bigger. They divide only to replace cells that have died or been damaged. This happens differently in different organs. Liver cells, for example, multiply only in response to injury. But skin cells continue to divide regularly, even if the skin hasn’t been injured, though at a pace that slows over time.
Cells can split only a limited number of times. For example, fibroblasts, the collagen-producing skin cells, typically divide about 50 times. Once a cell reaches this end point, it enters a stage in which it no longer divides and finally dies. When the cell receives certain chemical signals that its time is up, the cell switches on a program called apoptosis that leads to the cell’s death. In other words, a cell that learns its time has come commits suicide.
Each time a cell divides, a little bit of the telomere — DNA at the tip of each chromosome — is lost. Thus, telomeres of young cells are longer than the telomeres of middle-aged cells, which in turn are longer than the telomeres of old cells. When the telomeres become very short, the cell can no longer divide, and it dies. (See illustration.)
The life and death of a cell
Chromosomes are long strings of DNA with proteins wrapped around them tucked inside the nucleus (the center) of every cell. The chromosomes contain almost all of a cell’s genes. Each human cell has 23 pairs of chromosomes.
Before a cell divides, each chromosome makes a complete copy of itself. Then, when the cell divides into two, one chromosome goes to one cell and the copy goes to the other cell. At the tip of every chromosome are telomeres. They protect the ends of the chromosome from damage just as do the aglets at the tips of shoelaces.
Each time a cell divides, a little bit of the telomere at the tip of the chromosome is lost, shortening it. Thus, telomeres of young cells are longer than the telomeres of middle-aged cells which, in turn, are longer than the telomeres of old cells. When the telomeres become very short, the cell can no longer divide, and it dies.
Along with telomeres that get shorter, older cells also suffer increasing damage to their DNA. In addition, the mitochondria — the little “batteries” inside each cell that supply its energy — start to become less efficient.
The bottom line is this: We get older because our cells get older. What makes a cell age, and what could help keep a cell young, were nearly total mysteries just 25 years ago. Since then, medical research has provided many answers. Because of that research, I believe that someday we will be able to slow aging.