Per Zheng Cui, the chief obstacle to fundraising for development was that he didn't have a molecular mechanism of operation in hand and explained. I have no reference to hand for that; it might have been in email.
The funded work looking into this, where it exists at all, is presently largely focused on finding that mechanism and thus doing away with the need for donor cells. e.g.
"Per Zheng Cui, the chief obstacle to fundraising for development was that he didn't have a molecular mechanism of operation in hand and explained."
When you hear people complaining about this, it's a good sign that they're on the extreme margins of whatever field of study they're in, or that they're getting too far ahead of themselves (i.e. trying to pursue clinical studies before basic scientific facts are known). The "mechanism of action" becomes much more important to reviewers when your evidence for efficacy is weak and/or the thing you're advocating tends to go against established scientific theory.
Sometimes the medical establishment is conservative, but this is a feature, not a bug.
The funded work looking into this, where it exists at all, is presently largely focused on finding that mechanism and thus doing away with the need for donor cells. e.g.
http://www.fightaging.org/archives/2009/12/seeking-funding-t...