Presumably, the eventual goal is to grow full-sized organs inside some larger animal. Then the organ can be harvested and transplanted to a human who needs it. Per my understanding, medicine does a lot of work with rats because they're cheap and easy to work with, so any new technology of this type would be tried on rats first.
By the way, my neutral explanation above doesn't mean that I approve of doing such things. Honestly, I'm quite torn about the issue.
By the way, my neutral explanation above doesn't mean that I approve of doing such things. Honestly, I'm quite torn about the issue.