There are trillions of cells in your body, but the cells that you have today are not all the exact same cells that you had yesterday. Over time, cells age and become damaged, so your body's cells are constantly replicating, creating their own replacements.
This constant cellular activity has sparked a popular idea (opens in new tab): Every seven years or so, your cells have been so productive that your body has replaced every part of itself — from your eyelashes to your esophagus. In other words, after about seven years of cellular replication, you're an entirely new collection of cells, inside and out.
But is that true? Not exactly. Certain cells in some organs and systems in your body are totally replaced in a matter of months, but others remain much the same as they were on the day you were born.
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"Most of the skin and gut are replaced very fast, most likely within months," Olaf Bergmann, a principal researcher in the Department of Cell and Molecular Biology at the Karolinska Institute in Stockholm, Sweden, told Live Science in an email. Cells in the liver regenerate at a somewhat slower pace, Bergmann and his colleagues reported June 15 in the journal Cell Systems (opens in new tab). For the study, the authors analyzed liver tissue using radiocarbon dating and found that most liver cells are replaced within three years.
However, cells in other organs and systems are even slower to replicate and lag behind the seven-year cut-off.
For example, "the human heart renews at a rather low rate, with only 40% of all cardiomyocytes [the cells responsible for the contracting force in the heart] exchanged throughout life," Bergmann said. Skeletal cells, meanwhile, need around 10 years to replicate a skeleton in its entirety, according to the New York Times (opens in new tab).
In the brain, cell renewal can be even more leisurely. Scientists have uncovered evidence showing that some neurons in the hippocampus are renewed, but only at a rate of 1.75% annually, according to a 2013 study in Cell (opens in new tab). And some types of neurons within the striatum also regenerate, according to a 2014 study in Cell (opens in new tab). But other types of neurons stay with a person for their entire lifetime, Bergmann said. And even the distinct cell populations that can rejuvenate are not replaced entirely, but only partly over a lifetime, he said.
Regardless of how "young" our skin, guts and liver may be, we feel older as years go by because of our biological age, Bergmann explained. Even if a person's cells are relatively young, their biological age reflects how their body responds to the passage of time. As organs renew their cells, the organs still age due to changes in the replicating cells, such as mutations, Bergmann said. As cells replicate, the DNA continually divides and copies; and over time, mistakes are made. Mutations can thereby accumulate and affect the life of the cell (opens in new tab) or the expression of certain genes.
So even if the cells in parts of our bodies are relatively new, our aging, much-copied DNA makes us feel the weight of all those years that have passed.
This article was originally published on Live Science on April 4, 2011 and rewritten on June 28, 2022.