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  3. Human Heart Can Make New Ceils Solving a longstanding (为时甚久的) mystery, scientists have found that the human heart continues

Human Heart Can Make New Ceils Solving a longstanding (为时甚久的) mystery, scientists have found that the human heart continues

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问题                        Human Heart Can Make New Ceils
    Solving a longstanding (为时甚久的) mystery, scientists have found that the human heart continues to generate new cardiac (心脏的) cells throughout the life span, although the rate of new cell production slows with age.
    The finding, published in the April 3 issue of Science, could open a new path for the treatment of heart diseases such as heart failure and heart attack, experts say.
    "We find that the beating cells in the heart, cardiomyocytes (心肌细胞), are renewed," said lead researcher Dr. Jonas Frisen, a professor of stem cell research at the Karolinska Institute in Stockholm, Sweden. "It has previously not been known whether we were limited to the cardiomyocytes we are born with or if they could be renewed," he said.
    The process of renewing these cells changes over time, Frisen added. In a 20-year-old, about 1 percent of cardiomyocytes are exchanged each year, but the turnover (更替) rate decreases with age to only 0.45 percent by age 75.
    "If we can understand how the generation of new cardiomyocytes is regulated, it may be potentially possible to develop pharmaceuticals (药物) that promote this process to stimulate regeneration after, for example, a heart attack," Frisen said.
    That could lead to treatment that helps restore damaged hearts.
    "A lot of people suffer from chronic heart failure," noted co-author Dr. Ratan Bhardwaj, also from the Karolinska Institute. "Chronic heart failure arises from heart cells dying," he said.
    With this finding, scientists are "opening the door to potential therapies (疗法) to having ourselves heal ourselves," Bhardwaj said. "Maybe one could devise a pharmaceutical agent that would make heart cells make new and more cells to overcome the problem they are facing. "
    But barriers remain. According to Bhardwaj, scientists do not yet know how to increase heart cell production to a rate that would replace cells faster than they are dying off, especially in older patients with heart failure. In addition, the number of new cells the heart produces was estimated using healthy hearts -- whether the rate of cell turnover in diseased hearts is the same remains unknown.
In people in their mid-70s, only 0.45 percent of cardiomyocytes
选项 A、are still functional.
B、are reduced each year.
C、are replaced each year.
D、are damaged each year.
答案C
解析 短文第四段第二句提到“到了75岁。心肌细胞的更替率就降到每年0.45%了”，由此可知，到了75岁时，每年只有0.45%的心肌细胞被替换，故选C。
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