A team of researchers from several institutions in Spain has found that heart muscle regeneration in neonatal mice only occurs during the first day after birth. In their paper published on the open access site Science Advances, the group explains their study of regeneration of heart muscle in newborn mice and what they found.

Back in 2011, a team of researchers claimed to have seen heart muscle regeneration in newborn mice. They reported that after clipping off a piece from a tiny heart, the muscle would regenerate, a form of self-repair. The report caused a stir, because no other researchers had ever seen such heart muscle rejuvenation in mammals. It also caused some controversy, as some researchers were unable to replicate the experiments while others succeeded. In this new effort, the researchers sought not only to replicate those results, but to better understand the variable efforts at replication.

The experiments by the team were straightforward and simple. They opened up several healthy, newly born mice, and cut off a piece of the heart muscle in a non-lethal procedure. The procedure was performed on mice that were one through four days old, and nine days old. They then closed up the mice and allowed them to heal for three weeks. At that point, they opened them up again to see if the heart muscle had regenerated. The researchers report that their findings were not so simple. They report that the mice who had lost heart muscle on day one showed minimal signs of scarring. All of the others had significant scarring where the heart muscle had been snipped. The researchers suggest that the heart muscle on the day one mice regenerated, but as yet, they cannot prove it. All they can say for sure is that there was little scarring and that the hearts of those mice were the same size and weight as unaltered control mice.

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