Our adult cells communicate with their immediate neighbors through some pretty complex chemical signalling, and stem cells are receptive to that. It's one of the reasons stem cells are able to cooperatively develop from one single cell into a complex multi-cellular organism. Each stem cell carries the complete blueprints for a human being, it just needs to access and activate them based on the signals from it's neighbors.
In fact, pregnant mothers have been found to have fetal stem cells in their blood stream, which target areas of injury and are thought to be repairing them, even within the brain.
I in no way intend this to be an argument against embryonic stem cells. I just find it to be extraordinarily interesting and an intriguing avenue for further development.
More info here: https://en.wikipedia.org/wiki/Induced_pluripotent_stem_cell