Cool. We've known about the whole chemical signals part for quite awhile. If you can release the right signal at the right time, you can cause a cascade effect for the rest of the cells in the area. The most famous, that I'm familiar with, of these effects has been observed in D. Melanogaster, the common fruit fly. We figured out the proteins to make them grow their eyes and their gonads. Then, because all of us scientist-types are bascially sick in the head, we'd cause them to grow eyes on their wings, ass, and the like. Then, we caused them to grow testicles on their antennae, which as the scientist who explained rather glibly said, "as you might suspect, this must be rather socially embarrassing for the flies." Therefore, if we put stem cells in the right spot, and then managed to trigger the MCG to kick off, we could certainly get around many birth defects - assuming we know there's a defect present, we know the triggers, etc - which is, as you said, a far from ready for human tests.