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If there were no regeneration, there could be no life. If everything regenerated there would be no death.
-R. J. Goss, Principles of Regeneration (1969)
The authors of a recent textbook attribute the original impetus to experiment with human stem cells to the observation of extraordinary regenerative powers in certain animals:
Early evidence that stem cells exist in the somatic tissues of animals arose from observations of the regeneration of entire organisms, including the head, from small sections of the Hydra soma. Substantial somatic regeneration also occurs among other invertebrates, including members of relatively highly organized groups such as annelids. Limb regeneration can also be observed in insects and, among vertebrates, this property extends to the amphibians, which can regenerate the distal portions of limbs following their amputation.1
The concept of regeneration refers to the ability of the developed organism to replace lost tissues or organs through the growth or remodeling of somatic cells. As the authors of this introduction observe, some animals possess remarkable powers of self-regeneration. Flatworms and starfish can regenerate whole organisms from isolated fragments. Chop a flatworm (planarian) into a hundred pieces, feed it occasionally, and it will grow back as a hundred perfectly formed new worms. Insects can grow new legs. Among vertebrates, newts and other amphibians are capable of restoring limbs, tails, lower jaws, and even the eye lens. Perhaps most astonishing of all, the tentacle-waving freshwater coelenterate Hydra is not only able to survive amputations, it can also be dissolved into a single-cell solution, from which it will reconstitute itself over a period of weeks.
Humans and other mammals seem to be precluded from these extreme feats of self-regeneration, although the textbook authors point out that the formation of multiple tissues during wound healing is consistent with the possibility that mammals "have retained progenitor [stem] cells capable of repairing limited damage to organs."2 They also observe that certain tissues of the adult human body retain a limited potential for self-renewal throughout life: Skin is constantly shed and renewed. The entire adult human skeleton is regenerated every eight to ten years. Blood cells, gut epithelium, epidermis, and the cellular lining of the uterus are all replaced on a regular basis, while liver, muscle, and blood vessels have a more...