Scientists have discovered that fat, bone marrow and other parts of the body contain stem cells, immature cells that can rejuvenate themselves, at least in the tissue they are naturally found. But it has yet to be proved that these cells can regenerate no matter where they are placed, or under what conditions this might occur. Moreover, questions about safety remain unanswered. These sober realities do not appear to have slowed the rise of an international industry catering to customers who may pay tens of thousands of dollars in cash for their shot at a personal miracle. (Some foreign operators offer creative variations on the theme, like cells from sharks and sheep.)