Heart Cells Can Regenerate, Nuclear-Bomb Evidence Shows

Christine Dell'Amore
National Geographic News
April 2, 2009
They may not be able to mend a broken heart, but our heart cells are replaced over our lifetimes, a new study says.

The discovery, made by analyzing heart cells irradiated by 1950s nuclear bomb tests, might lead to new heart treatments, scientists say.

It's long been a mystery whether human adults have a set number of heart cells or whether cells are added to replace old ones over time.

As it turns out, heart muscle cells, which are responsible for the contractions that pump blood through our bodies, can in fact renew themselves. The cells regrow at an annual rate of 1 percent at age 25, slowing to about 0.45 percent by age 75, the new study says.

This slow turnover has made it "very difficult" to prove that heart muscle regenerates, said study co-author Jonas Frisén, a cell biologist at Sweden's Karolinska Institutet medical school.

"It's easy to study renewal of short-lived cells," Frisén said, but "if the [cell] exchange rate is low, you cannot use the regular tricks."

So in the new study, researchers employed a "very clever use" of history.

By identifying traces of radiation from aboveground 1950s nuclear bomb tests in people born before the tests, the team determined the irradiated cells must have appeared sometime after birth.

"The bottom line is that, despite the fact we [had] some evidence there is turnover in heart cells, there's been a bit of controversy," Tomaselli said.

"Now this data suggest there is a renewal of heart cells within the heart."

Carbon Spike

During the nuclear bomb tests of the 1950s, the radioactive carbon isotope carbon-14 dramatically increased in the atmosphere. As a result, everyone alive at the time has traces of carbon-14 in their tissues.

That means the amount of carbon-14 in DNA can serve as a date mark for when a cell was born.

Frisén and colleagues extracted DNA from the nuclei of heart cells in 14 patients who had died while at the Karolinska Institute, as well as from samples from the U.K. Human Tissue Bank, according to the study, to be published tomorrow in the journal Science.

All the tissues sampled were from people born up to 22 years before the onset of the 1950s nuclear tests.

The results showed that some heart cells had higher carbon-14 concentrations, suggesting that the cells had been created any years after birth.

(See heart photos.)

Other organs in the body also regenerate, most of them faster, study co-author Frisén added. For instance, all of a person's white blood cells regenerate over the course of a year.

Beyond Stem Cells

The finding may be promising for creating new drugs or other medical treatments that could boost heart cell growth in people with heart troubles, Frisén said.

For instance, people with heart injuries recover very slowly, probably because their heart cells are not regenerating fast enough.

Currently, regenerative treatments involve taking cells from other organs or bone marrow and injecting them into the heart, said the American Heart Association's Tomaselli, who also works at Johns Hopkins University in Baltimore.

Stem cell treatments are another option, but just getting the cells and preparing them is "complicated and expensive and fraught with a lot of side effects," Tomaselli said.

(Related: "'Stem Cell Tourists' Go Abroad for Unproven Treatments.")

The new discovery may allow treatments that can "marshal forces and try to get cells to turn over in a more rapid rate in situ," Tomaselli said.

Meanwhile, study co-author Frisén and his team have already taken up a new project: Looking at whether a slower-than-normal regrowth rate of heart cells could portend disease.

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