My understanding of this (non scientist view) is that they found stem cells grown in isolation of other cells tended to become tumors. But when placed in the presence of similar cells or with cells of the areas of the body where they needed to grow, they were more likely to behave as hoped and turned into the needed cells. Maybe some kind of cellular chemical signature guidance to the stem cell of what to do. But what a gamble, you have to put the stem cells right into the affected area where if things go wrong, it would be very hard to fix. This is why I thought they were working towards using stem cells to grow new teeth since they could implant them and then if necessary pull them out if things went wrong. Teeth are apparently similar in many ways to more sophisticated internal organs, so a lot would be learned by perfecting it. Plus, it would totally be marketable if you could grow people brand new perfect teeth.