Rediscovering the Immortal Hydra : Stem Cells and the Question of Epigenesis

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Abstract
Configurations 11.1 (2003) 1-26 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: 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 restricted capacity for self-repair. These processes have been attributed to resident adult stem cells—cells that are able to turn into and replace specific differentiated cell types, as they age and die, while at the same time continuing to renew themselves in an undifferentiated state. The authors go on to note that the cellular dynamics involved in adult mammalian tissue repair are highly "reminiscent" of those occurring in early embryological development. Not only does wound healing, in its own limited way, recall the more plastic self-regenerative capabilities of the newt, the flatworm, and the Hydra, but it also seems to partially reenact the processes of embryogenesis. This observation points to a further chain of associations, one that has contributed to the experimental isolation of embryonic stem cells (ES cells)—cells that, when isolated from the inner cell mass of the early embryo, are able to renew themselves indefinitely in an undifferentiated state while also giving rise to many if not all of the differentiated cell types of the body. What interests me here is the conceptual movement that leads the writers of this introduction from the spectacular self-regenerative [End Page 2] acts of Hydra, amphibians, and flatworms, to the dynamics of tissue renewal in the adult human body, and back again to the process of mammalian embryogenesis. In what sense does this line of inquiry inform the technical ambitions of stem cell research? If such diverse phenomena can be attributed to the different actualizations of the "stem cell," as the authors seem to claim, might it be possible to treat them as more or less extreme cases of...

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