Although adult muscle cells become inflexible after differentiation, these cells temporarily loosen the structure to divide in fetal development. Mark T. Keating found that in some lower vertebrates, heart tissue regenerates without the scarring seen in mammals. This seems to occur by proliferation of existing cardiomyocytes, not stem cells.

Felix Engel, an HMS research fellow in Keating’s lab, set out to accomplish this in mammals. Engel first identified a growth factor that could specifically stimulate cardiomyocytes. To probe its activity, Engel began combining the factor with inhibitors of signaling pathways. Originally he used an enzyme inhibitor as a control, but the rise in DNA synthesis in these control cells leaped, and the cell populations looked denser.

The team examined rat hearts to study the enzyme’s role in cardiomyocyte development; they found that inhibitor activity was lowest during heart growth and highest during slow muscle cell growth and in adulthood when growth had stopped. The team analyzed cells treated with the growth factor and the inhibitor alone and combined, but only treatment with both factors combined stimulated genes involved in cell cycle progression.

The team then used immunofluorescent stains to prove their cells had undergone three stages of cell division. The number of cells continued to grow with repeated treatment.