Regeneration in Planaria

Inmetazoans, regeneration of lost body parts requiring the formation of a blastema is known as epimorphic regeneration. A blastema is composed primarily of two cell populations: an outer cell layer or ectoderm, derived from the epidermis that covers the wound surface after amputation/fission; and mesenchymal cells that proliferate and accumulate beneath this wound epidermis, eventually differentiating into the lost body parts (Figure 1). Even though the architecture of both planarian and vertebrate blastemas is the same, the mechanism of blastema formation in planarians differs from that of vertebrates in two basic aspects. First, the wound epithelium forms by epidermal cell shape modifications rather than by cell proliferation. Second, the mesenchymal cells are derived frompre-existing undifferentiated cells (neoblasts), instead of the cellular dedifferentation observed in vertebrates. Such relative simplicity of blastema formation, combined with the planarian’s basic body plan, developmental plasticity and evolutionary position,make these free-living members of the phylum Platyhelminthes a very attractive model system in which to study and understand the molecular principles governing metazoan regeneration.