Today's body part is the anal sphincter, that handy little ring of muscle that maintains the separation between your insides and your outsides. Researchers grew them from cells, implanted them in mice, and compared the new sphincters' function with the animals', ah, native orifices. And apparently, they were quite satisfactory.
The major step forward made here is that these sphincters, which were grown in a circular mold from human muscle biopsy cells and mouse nerve cells, could, by virtue of those nerve cells, communicate with the animal's nervous system, a level of functionality that had not previously been reached. The big-picture goal is to build new sphincters from patients' own cells, which could then be implanted without the immune system kicking up a fuss.
Have you seen a better headline in weeks? I didn't think so.