The liver is a vital organ that helps to eliminate toxins from our systems. It is likely to be hurt on a frequent basis as it deals with harmful substances on a daily basis.
To combat this, the liver is one of the few organs that can regenerate after being damaged. Because the body’s ability to heal and regenerate reduces as we get older, experts wondered if the liver’s ability to replenish itself did as well.
The nature of liver regeneration in humans was likewise unknown. Animal models gave us inconsistent results.
According to Dr. Olaf Bergmann, study group head, “some studies pointed to the possibility that liver cells are long-lived while others showed a constant turnover. It was clear to us that if we want to know what happens in humans, we need to find a way to directly assess the age of human liver cells.”
Human Liver Continues to Be a Young Organ
Dr. Bergmann headed an interdisciplinary team of biologists, physicists, mathematicians, and doctors who examined the livers of various people who died between the ages of 20 and 84. Surprisingly, the researchers discovered that all of the participants’ liver cells were almost the same age.
“No matter if you are 20 or 84, your liver stays on average just under three years old,” Dr. Bergmann adds.
The findings reveal that the ongoing replacement of liver cells regulates the adjustment of liver mass to the needs of the body, and that this process is maintained even in elderly persons. This continuous liver cell replacement is critical for liver regeneration and cancer growth in many ways.
DNA-rich liver cells regenerate less
Not all of the cells in our liver, however, are so young. A small percentage of cells can live for up to ten years before needing to be replaced. This subset of liver cells has more DNA than the rest of the cells.
“Most of our cells have two sets of chromosomes, but some cells accumulate more DNA as they age. In the end, such cells can carry four, eight, or even more sets of chromosomes,” adds the author.
“When we compared typical liver cells with the cells richer in DNA,” Dr. Bergmann adds, they “found fundamental differences in their renewal. Typical cells renew approximately once a year, while the cells richer in DNA can reside in the liver for up to a decade.”
“As this fraction gradually increases with age, this could be a protective mechanism that safeguards us from accumulating harmful mutations. We need to find out if there are similar mechanisms in chronic liver disease, which in some cases can turn into cancer.”
Nuclear Fallout Has Taught Us a Few Things
Figuring out the biological age of a human cell is a very hard technical problem, because the methods that are usually used for animal models can’t be used for people.
Dr. Bergmann’s team specializes in retrospective radiocarbon birth dating, which is used to determine the biological age of human tissues. Carbon is a ubiquitous chemical element that is essential to life on Earth.
One of the many kinds of carbon is radiocarbon. It can be found in the atmosphere naturally. It is absorbed by plants through photosynthesis in the same way as carbon is absorbed by plants, and it is passed on to animals and humans.
Radiocarbon is a radioactive isotope of carbon that is both weakly radioactive and unstable. In archeology, these traits are used to assess the age of ancient samples.
“Archeologists have used the decay of radiocarbon successfully for many years to assess the age of specimens,” adds Dr. Bergmann, citing the dating of the Shroud of Turin as an example.
“The radioactive decay of radiocarbon is very slow. It provides enough resolution for archeologists but it is not useful for determining the age of human cells. Nevertheless, we can still take advantage of the radiocarbon in our research.”
In the 1950s, nuclear tests that were done above ground put huge amounts of radiocarbon into the air, the plants, and the animals. As a result, radiocarbon levels in DNA in cells created during this time period are greater.
Following the official prohibition of aboveground nuclear testing in 1963, the amount of radiocarbon in the atmosphere began to decline, as did the amount of radiocarbon integrated into animal DNA. The values of atmospheric and cellular radiocarbon are very closely related.
“Even though these are negligible amounts that are not harmful, we can detect and measure them in tissue samples. By comparing the values to the levels of atmospheric radiocarbon, we can retrospectively establish the age of the cells,” Dr. Bergmann explains.
Direct From the Source, Unparalleled Insights
The Bergmann group also looks at the mechanisms that drive the regeneration of other, more static tissues including the brain and heart.
The team has previously demonstrated that the production of new brain and heart cells is not confined to prenatal time but continues throughout life using their expertise in retrospective radiocarbon birth dates.
The research is currently investigating whether or whether new human heart muscle cells may be created in persons with chronic heart disease.
“Our research shows that studying cell renewal directly in humans is technically very challenging but it can provide unparalleled insights into the underlying cellular and molecular mechanisms of human organ regeneration,” Dr. Bergmann adds.
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