A surge of thyroid hormone just before adolescence causes mouse hearts to grow drastically, suggesting that the organ may be easier to regenerate than previously thought.

Credit: Nawazish Naqvi

Ahsan Husain of Emory University School of Medicine in Atlanta, Georgia, and Robert Graham of the Victor Chang Cardiac Research Institute in Sydney, Australia, and their colleagues labelled heart muscle cells of baby mice with a chemical. When the mice were 15 days old, the number of cardiomyocytes (pictured, red) increased by about 40%.

It had previously been thought that cardiomyocytes stopped replicating just after birth. The findings suggest that giving thyroid hormone to babies with heart defects might help to repair the organ.

Cell 157, 795–807 (2014)