The regenerative powers of many metazoans have intrigued biologists for generations and yet, the mechanisms involved in regenerative processes remain largely unknown. My laboratory uses the planarian flatworm, a classic subject of early regeneration experiments, to understand the molecular mechanisms underlying regeneration. The choice of planarians as a system to address the problem of regeneration is based upon: their remarkable developmental plasticity; the rapidity of their regenerative response; the ease with which they can be cultured in the laboratory; and the stem cell population that gives rise to their regenerative abilities.

After a planarian has been transected, the wounded area is rapidly covered by a thin layer of epidermal cells. Undifferentiated cells called neoblasts are then signaled to proliferate beneath the wound epithelium, giving rise to an unpigmented structure referred to as the regeneration blastema. As regeneration proceeds, neoblasts continue to accumulate within the blastema, causing it to grow exponentially. Within one week of the transection, differentiation of the missing structures occurs. In uninjured planaria, neoblasts are distributed throughout the parenchyma (mesenchyme) and, as the only mitotic cells in the animal, serve as the source of replacement cells during tissue renewal.