richard lee is antoinette owens heart failure.her for diastolic heart failure. >> her heart doesn't fill properly. >> he and his colleague joined forces in their labs to tackle this problem. what they found was astonishing. >> you found basically when you joined an old and a young mouse together, the sizes and the structure of the hearts changed. how dramatic was this change? >> it was amazing enough that even i could see it looking at the heart as a whole. and even more amazing when you started digging more deepg into the layers -- deeply into the cells, as a result of this exposure to young blood in the old animal. >> that something was growth differentiation factor 11. gdf 11. >> science in action here. >> now, mice don't get a heart disease like humans do but they might give us clues how to fight it. here at harvard, researchers are trying to follow these clues, trying to fight a cardiac cries i, maybe young ones will heal old ones. >> the blood has access to all these different cells and tissues and may be able to communicate