Phylogenetic aspects of tissue regeneration: role of stem cells. A concise overview.

Many different forms of regeneration are known among the members of the animal kingdom. Invertebrates commonly generate new individuals by sprouting and splitting off of body parts, so that the processes of asexual reproduction and of regeneration as a response to injury can hardly be distinguished. Among the adult vertebrates, regeneration of lost body parts has become exceptional rather than the rule. However, the capacity for regeneration of tissues and for the remodeling of injured organs is much better preserved than is generally appreciated. The main reason for this misconception is the enormous variety of mechanisms for replenishment, repair, and remodeling that coexist in one and the same animal. In addition, there is a considerable variation in the response to damage inflicted by different forms of injury. Our conceptions of wound repair and tissue regeneration have been radically changed by the recent discoveries of stem cell plasticity and of dedifferentiation of supposed end cells in mice and men. Seemingly, these mechanisms should provide our bodies with near inexhaustible powers of regeneration. Yet, failures to achieve just that are among the most pressing problems of modern medicine. Apparently, counterforces have evolved that prevent the orderly replacement of lost cells. The present challenge for regenerative medicine is to unravel those barriers and find means to overcome them.