Heart muscles, like skeletal muscles, are made up of two major types of muscle fibers: fast twitch and slow twitch. Fast twitch fibers move quickly but tire easily, while slow twitch fibers move slower but last longer. Both serve important functions in different circumstances. For example, marathon runners tend to have a predominance of slow twitch fibers in their skeletal muscle; the opposite is true for sprinters.
It was the variability that intrigued pediatric cardiologist William Pu, MD, about his patient with heart failure. The boy suffered from a rare genetic mitochondrial disorder called Barth syndrome. While he ultimately needed a heart transplant, his heart function seemed to vary day-to-day, consistent with reports in the medical literature.
“Often patients present in infancy with severe heart failure, then in childhood it gets much better, and in the teen years, much worse,” says Pu, of the Cardiology Research Center at Boston Children’s Hospital. “This reversibility suggests that this is a disease we should really be able to fix.”
Though it needs much more testing, a potential fix may now be in sight for Barth syndrome, which has no specific treatment and also causes skeletal muscle weakness and low white-blood-cell counts. It’s taken the work of multiple labs collaborating across institutional lines. …
For more than 100 years, people have been debating whether human hearts can grow after birth by generating new contractile muscle cells, known as cardiomyocytes. Recently, Bernhard Kühn, MD, at Boston Children’s Hospital and his colleagues added fuel to the debate—and hope for regenerative therapies for diseased hearts—with their findings that infants, children and adolescents are indeed capable of generating new cardiomyocytes.
Research in the 1930s and 1940s suggested that cardiomyocyte division may continue after birth, and recent investigations in zebrafish and newborn mice presented the possibility that some young animals can regenerate heart muscle through muscle cell division. Still, for many years, the accepted dogma among physicians and researchers was that human hearts grow after birth only through existing cells growing larger.
“This is a very sticky subject in cardiology,” says Kühn. Not only do long-held scientific beliefs die hard, but the ability to directly study heart cell growth in humans has been limited. “Healthy human hearts are hard to come by,” he says. …