Cell Reorganization and Blastema Formation

Once the challenge of an open wound has been overcome, the regeneration process requires assembling the resources necessary to rebuild the missing structures. The most common strategy in this phase is the buildup of a regeneration blastema, a mass of morphologically undifferentiated cells, from w'hich the new structures will develop. Blastema formation is arguably the process that gathers the most interest and debate among regeneration researchers, as this is where regeneration and embryogenesis differ the most. During embryonic development, structure formation goes through a trajectory in which cells move from pluripotent to differentiated states, while during regeneration, structures must be derived from cells found in adult tissue. Since most adult tissues are composed of differentiated cells, regenerating organisms need to either have a reserve supply of pluripotent, undifferentiated stem cells, or else have the ability to dedifferentiate or transdifferentiate already differentiated cells. Evidence for sourcing from reserve stem cells, differentiated cells, or both are found in many lineages showing extensive regenerative abilities. Conversely, lack of stem cells or irreversibility of cell differentiation is often hypothesized to be a cause for diminished regenerative abilities.

A related question is where do cells contributing materials for regeneration come from. In general, most cells contributing to regenerated tissues originate from regions close to the w'ound site, as in newt limb regeneration (Kragl et al. 2009). However, there are several cases where cells migrate from more distant regions, sometimes being the main source of regenerating materials, as seen in planarians and hydrozoans (Aboobaker 2011; Bosch 2007). Irrespective of their origin, cells contributing to a regeneration blastema usually undergo extensive proliferation; however, some species are known to be able to complete regeneration in the absence of cell proliferation. This latter observation led early regeneration researchers to classify regeneration processes on two general categories: epimorphic processes characterized by active cell proliferation and generation of a visible blastema, and morphallactic processes driven by transdifferentiation of existing cells without cell proliferation (Morgan 1901). Although Morgan explicitly stated that these process categories “are not sharply separated, and may even appear combined in the same form,” later authors tried (often forcefully) to fit regeneration of different structures and organisms exclusively into either of these categories, a habit that persists to the present. While some cases do exist where regeneration proceeds exclusively by epimorphosis or morphallaxis, in most situations it can be shown that both cell proliferation and tissue remodeling play fundamental and complementary roles. Thus, if these terms are to remain useful, they should be used to refer to specific processes occurring during regeneration, rather than labels applied to the whole developmental trajectory (Agata, Saito, and Nakajima 2007).

In summary, during the phase of cell reorganization, regenerating organisms deploy epimorphic and/or morphallactic processes to gather cellular resources that will be used for reconstructing the lost structures at the wound site.

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