The phrase "mouse heart" refers to the heart of a mouse, which is a small mammal. It is often used metaphorically to describe something or someone that is weak, fragile, or lacking strength. Full definition
They studied what happened in mouse hearts when they artificially removed that gene, either by editing it out of the genome or by giving mice a drug to block its effects. (scientificamerican.com)
They compared the immune response to cardiac damage in neonatal and adult mouse hearts. (sciencedaily.com)
He showed that, when the factors are injected into a damaged mouse heart, they convert scar tissue into healthy beating muscle cells, improving heart function. (gladstone.org)