Regeneration in liver and pancreas: time to cut the umbilical cord?

Organisms that are capable of robust tissue regeneration, including the urodele amphibians, use mechanisms that recapitulate embryonic development to regrow organs. Although mammals are not so adept at regeneration, several adult tissues exhibit partial or complete regrowth after injury. An ability to influence growth in mammalian tissues has become more imperative with the emergence of "regenerative medicine" as a discipline. For this field to fulfill its promise of providing functional tissues for clinical use, a more detailed picture will be required of how adult human tissues maintain mass during normal homeostasis and after injury. Studies of developing and regenerating liver and pancreas now suggest that mammals use distinct programs to regulate tissue growth during embryogenesis and adulthood.