Mouse heart re-grows when cut

The conditions are quite specific, but this could be significant.

BBC News:

The researchers at the University of Texas Southwestern Medical Center surgically removed what is known as the left ventricular apex of the heart (about 15% of the heart muscle) from mice just a day after birth.

The heart was then quickly seen to regenerate and was fully restored after 21 days. After two months, the organ still appeared to be functioning normally.

But when the same procedure was tested on mice aged one week, the heart failed to regenerate, suggesting this power of self-repair is extremely short-lived in mice.