We noted in Chapter 1 that small body segments of planaria can regenerate whole animals. Regeneration was known to require a population of proliferating stem cell–like cells, termed neoblasts, that are present throughout the adult body, but a key question was whether regeneration is accomplished, as in most animals with this capability, by the collective activity of multiple lineage-restricted stem or progenitor cells, or whether pluripotent stem cells are involved. Recent experiments showed that adult planaria contain lineage-restricted neoblasts as well as pluripotent stem cells, termed cNeoblasts.
The key studies used gamma-irradiation to inhibit most or all cell division in adult planaria; the treated animals could not regenerate and suffered massive tissue loss because of failed replacement of aged differentiated cells. The few functional proliferating neoblast cells remaining after irradiation could be identified by a marker gene termed smedwi-1. Several days after irradiation, individual neoblasts formed colonies of smedwi-1-positive cells that contained multiple types of differentiated body cells (see the chapter-opening figure), and it was hypothesized that this smedwi-1-positive subpopulation of neoblasts was pluripotent. To test this hypothesis, single neoblasts were transplanted into lethally irradiated planaria that lacked all of their own neoblasts. Remarkably, several transplant recipients lived past 7 weeks and regenerated, from the single transplanted cell, neuronal, intestinal, and other differentiated cell types that were distributed throughout the body. These animals eventually regained feeding behavior and had regenerated complex tissues, including photoreceptors. These experiments indicated that at least some of the neoblast stem cells in adult planaria are indeed pluripotent, providing a cellular basis for the remarkable regenerative abilities of planaria. Despite much effort, no pluripotent stem cells have ever been reliably identified in any adult vertebrate organism.