ABSTRACT
The presence of progenitor cells in organisms that contribute to renewal of differentiated tissues has been understood for many years.13-15 Throughout the life of the organism, repair of skin, remodeling of bone, renewal of blood, and regenerative healing of organs, such as the liver, represents normal physiology. The existence of less-restricted progenitor cells in higher organisms in sufficient numbers for isolation and culture was less well established. Verification that stem cells exist at multiple sites immediately generated hope that cell-based approaches could be used to treat damaged organs or create artificial organs.16,17 Variability in competency of the different types of stem cells implied that exogenous stimulation could selectively drive the cells down specific differentiation pathways. The concept of targeted differentiation was further advanced with the discovery of progenitor cells having seemingly unlimited potential for differentiation. Having cells with unrestricted capacity raises the possibility for totally directed differentiation that could extend to humans a capacity for regenerative healing only experienced by certain amphibians.
