The answer to one of the most rudimentary questions about the heart remains deeply enigmatic: can our hearts repair themselves? The answer has been steeped in controversy for over a century, but recent findings are helping to resolve the issue.

In 2009, a paper in the journal Science gave definitive evidence that the human heart is capable of self-renewal. Somewhat surprisingly, nuclear bomb tests during the Cold War had made this discovery possible; these detonations caused a sharp increase in the levels of radioactive carbon-14 in the atmosphere, which after being absorbed by plants in the form of carbon dioxide, entered the human food chain. By using a technique called radiocarbon dating, the Swedish research team were able to use the elevated levels of carbon-14 in human DNA to retrospectively calculate the age of heart cells. They found that 1% are renewed every year at the age of 25, and that this rate decreases to 0.45% by the age of 75.

This landmark study set the stage for an ongoing worldwide effort to exploit the heart's innate regenerative capacity to tackle disease. Back in 2011, to promote this research, the British Heart Foundation released evocative television adverts which depicted an animated talking zebra fish who explains how they can already repair their own hearts. Initially, it was thought that these fascinating creatures used a specialized source of stem cells to achieve such a feat. As it turns out though, it is the cells they are already endowed with, their pre-existing heart cells, which divide and repopulate the organ in times of trouble. This raised another question: where do new cells in the human heart come from? Again, stem cells seemed intuitively to be the most reasonable candidates – they are self-renewing, and have the power to become any one of the major cell types in the heart.

Recently however, a group at Harvard Medical School published a paper in the journal Nature giving strong evidence against this idea. They showed that, like the zebra fish, we too generate new heart cells from pre-existing ones, and that this phenomenon increases during a heart attack. This discovery is potentially a game changer for the field at large, for it suggests that our hearts possess zebra fish-like regenerative mechanisms – only on a much more diminished scale. Many researchers now believe that if we can figure out why this is, we may eventually be able to safely boost the regenerative capacity of our hearts for therapeutic gain.

1. The primary purpose of the passage is to

A. recognize the similarities in the functioning of the hearts of humans and zebra fish
B. chronicle recent research findings about a capability of a human organ
C. evaluate the studies conducted about a physiological phenomenon
D. identify how to boost the functioning of a human organ for therapeutic gain
E. offer various explanations for the self-regenerative ability of the human heart



2. According to the passage, the research group at Harvard Medical School would be likely to disagree with which of the following?

A. Humans are similar to zebra fish in that human heart cells are generated from pre-existing ones
B. Pre-existing heart cells are responsible for the renewal of heart cells in humans
C. Stem cells have the potential for self-renewal and can become any of the major types of heart cells
D. Self-renewing stem cells are responsible for the regeneration of heart cells in humans
E. The scale of cell regeneration in humans is much more diminished as compared to that of zebra fish



3. The author of the passage refers to the contents of the 2009 paper published in Science magazine as “somewhat surprising” most probably because:

A. The sharp increase in the levels of radioactive carbon-14 in the atmosphere due to the nuclear tests was unexpected
B. Until the Cold War period, radiocarbon dating had not been used to study human organs and physiological processes
C. The primary objective of nuclear bomb tests during the Cold War had not been to study self-renewal of the human heart
The Swedish research team had not anticipated that the radioactive carbon-14 would enter the human food chain
E. The radiocarbon dating techniques available during the Cold War period had not been advanced enough to be used to study the age of heart cells