Evolutionary perspective

Limb and organ regeneration is common among organisms. Alvarado wrote an excellent review of the evolutionary aspects of regeneration (9). Stem cell activity and regeneration can be studied at the most basic level in simple organisms such as the planarian (10) (Fig. 1). In the planarian, the molecular mechanisms underlying asexual modes of reproduction are indistinguishable from mechanisms of regeneration following injury. In the hydra, similar molecular messages that stimulate asexual reproduction are triggered by injury. Primitive organisms capable of regenerating damaged body parts include hydra, planarian, mollusks, insects, crustaceans, and echi-noderms (starfish). Chordates that can regenerate include amphibians such as frogs and salamanders.

Almost every phylum has species that are able to regenerate lost body parts (9). Regeneration in these organisms requires the ability of cells in the injured tissue to "dedifferentiate," which requires the ability of these organisms to regulate pluripotentiality. Sites of injury in chordates that regenerate form an area of dedifferentiated cells called the "regeneration blastema," and the cells within this structure recapitulate molecular developmental processes that occur during embryogenesis (9).

In summary, the ability to regenerate is a common trait shared by many species. The reason some classes of animals have lost the ability to regenerate is unclear. Alvarado hypothesized that the ability to regenerate confers neither a positive nor negative evolutionary bias, allowing this trait to disappear in many classes of animals, including mammals (9). However, reversal of cell fates and stem cell plasticity may be vestiges of these evolutionarily ancient processes.

Fig. 1. Bromodeoxyuridine labeling of regenerative stem cells in planarians Phagocata sp. (upper left); Girardia dorotocephala (lower left); and Schmidtea mediterranea (right). Scale bars: A, 150 microns; B, 300; C, 450. (From ref. 10. Photo courtesy Dr. Alejandro Alvarado. Used with permission of Academic Press.) (See color plate 1 in the insert following p. 82.)

Fig. 1. Bromodeoxyuridine labeling of regenerative stem cells in planarians Phagocata sp. (upper left); Girardia dorotocephala (lower left); and Schmidtea mediterranea (right). Scale bars: A, 150 microns; B, 300; C, 450. (From ref. 10. Photo courtesy Dr. Alejandro Alvarado. Used with permission of Academic Press.) (See color plate 1 in the insert following p. 82.)

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