I think the regeneration power do not come from any external source, it's one power. At least it explain why the master when run out of regenerations was griping to his decaying body and trying to get a new ways to do the process!
When the 12 cycle come out, no one know that the show would reach the XXI century and 13 incarnations were a good number. And with the Sara Jane Adventures, the Doctor can live forever!
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