By Bernadine Healy, M.D.
It's humbling to see medical dogma overturned, but that is exactly what happened when, contrary to deeply embedded thought, scientists led by Jonas Frisen from the Karolinska Institute in Stockholm reported in Science today that the heart can grow new muscle cells, and does so regularly, albeit slowly, in the course of a lifetime.
To cardiologists, this is a blockbuster discovery, since the heart has been pegged as a disadvantaged organ in terms of injury, healing, and repair. Susceptible to coronary blockages that can cut off blood and destroy major hunks of heart muscle at one time in a heart attack, the heart can only heal itself slowly, often leaving behind thinned and baggy scar tissue devoid of healthy, beating muscle. And the distortion and remodeling of the heart that comes with this muscle loss sets the patient up for cardiac failure, blood clots, and nasty heart rhythms. It was always assumed the heart could do no better. But that does not seem to be so.
The clever piece of work from Sweden used carbon dating to figure out the age of human heart cells. The spike in concentration of atmospheric radioactive carbon-14 triggered by above-ground Cold War nuclear tests between 1955 and 1963 allowed the researchers (with the help of physicists and sophisticated mass spectrometry from the Lawrence Livermore National Laboratory in California) to discover that, lo and behold, the heart has slow and silent regenerative abilities. The evidence: the many heart cells whose nuclei—which last the life of the cell—had radioactive carbon levels that coincided with the atmospheric spikes, occurring many years after the person was born. The study found that younger adults renew about 1 percent of their heart cells per year. The growth falls off to roughly half of that in the elderly.
This is no abstract, ho-hum science factoid. It makes incredible sense of something that has always puzzled me: If hearts can't make new heart tissue, why did ever efficient Mother Nature give them stem cells? Yes, for years, scientists have known that adult stem cells can be found in the heart. This has prompted numerous centers in many countries to pursue stem cell therapeutics in patients with heart attacks, heart failure, and even severe angina to repair muscle and improve blood supply.
The work looks more than promising. In several studies, using cocktails of patients' own bone marrow stem cells, which can be sifted out of the bloodstream and infused back into the patients in a concentrated and enriched form, has produced better-than-expected heart function and blood flow. (Adult stem cells circulating in the blood are known repairmen that can hone in on injured tissue anywhere in the body.) Recent studies in rats have gone so far as to create a matrix for these cells to grow on that can become a healthy looking, growing and beating tissue graft after being implanted in damaged heart wall.
The work is moving fast and furiously to make stem cell technology a standard part of cardiac care. Even the greatest skeptics have taken note. The Cochrane Collaboration, a well-respected international group that assesses the latest technology with a very tough eye, concluded late last year that, based on its review of reports involving over 800 patients from several centers, stem cell infusions after heart attacks have shown some definite benefits. To be sure, more work needs to be done, though.
Smart medicine honors the rules of the human body as best they can be determined. For example, a sturdy immune system fights off most microbes we encounter; and vaccines and antibiotics work because they complement that already finely tuned inborn system. The discovery that the wounded heart can renew itself over time, giving reason for the naturally occurring stem cells found in its muscle, provides great encouragement that harnessing and accelerating the body's own regenerative capacity will become a powerful technology in the not-too-distant future.