Regeneration in Annelids
The ability to make new segments during axial regeneration varies across the truly segmented phyla. Among vertebrates, examples of axial regeneration involving formation of new segments are rather rare (Mochii, Taniguchi, and Shikata 2007; Hutchins et al. 2014), and the regenerated segments are often simpler and more disorganized than those formed during embryonic development. Consequently, studies comparing segmentation mechanisms between embryogenesis and regeneration are lacking. Among arthropods, axial regeneration is exceedingly infrequent, and only involves reconstruction of the terminal region, with no known cases of segment reconstruction. In contrast, most annelids show excellent axial regeneration and can reconstruct segments similar to those made during embryonic development and post-embryonic posterior growth (Bely 2006). Since segments produced by regeneration are not only indistinguishable from those made during embryogenesis and growth, but also develop faster, regeneration (in particular, posterior regeneration) offers a convenient way to study the developmental genetic mechanisms of annelid segmentation.
Annelida is one of the few phyla where phylogenetic distribution of axial regenerative abilities has been formally surveyed and analyzed (Zattara and Bely 2016). This comparative work has shown that the stem group annelids likely had very good axial regenerative abilities, being capable of restoring both posterior and anterior ends. The basic annelid architecture is relatively well conserved across the phylum, minimizing the problem of homology and facilitating comparisons of the regenerative process. Furthermore, annelid regeneration has been a focus of research for a long time, resulting in a rich literature that is now being actively updated and expanded thanks to a recent surge in interest that is applying powerful new tools to dissect the developmental and evolutionary mechanisms underlying this process.
Stages of Axial Regeneration
In most annelids that have been studied, axial regeneration proceeds through a similar series of steps that begin with an injury response and end in either a complete anterior or posterior end (in species capable of regeneration) or an incomplete stump stalling at some point in the process (in species with diminished or absent regeneration). The non-segmental, terminal structures—the prostomium anteriorly and the pygidium posteriorly—differentiate first, followed by a variable number of segments. The complete regeneration process is usually divided in five stages; (1) wound healing; (2) blastema formation; (3) blastema differentiation; (4) resegmentation; and (5) growth (Figure 10.2).
Stage 1, wound healing, begins immediately after amputation. After closure of the wound by muscle contraction, cells migrate into the wounded segment to provide an immune response and generate a wound plug by clotting. This stage is characterized
FIGURE 10.2 Generic stages of annelid posterior and anterior regeneration, along with their main events. The wound/blastemal epithelium is shown by a dotted red line; brown anterior regions represent non-segmental prostomium and peristomium; ocher posterior region represents the pygidium; dark red posterior subterminal band and graded dark red region represent the segment addition zone and remaining posterior growth zone, respectively; green regions represent undifferentiated blastemal tissues.
by local to body-wide downregulation of cell proliferation. The adjacent epidermis then extends over the wound forming a wound epithelium; transition to the next stage is marked by a sharp upregulation of several genes known to be key developmental regulators in other systems.
Stage 2, blastema formation, begins with invasion of the wound site by neurites originating in nerves from the ventral nerve cord and peripheral nerves, and the upregulation of local cell proliferation, which results in the formation of a blastema composed of undifferentiated cells. Blastema formation is characterized by expression of a large suite of genes and intense mitotic activity. This stage eventually grades into the next stage.
Stage 3, blastema differentiation, is characterized by morphological differentiation of the non-segmental terminal caps (prostomium and peristomium at the anterior end, pygidium at the posterior end) and the intercalation of additional tissues that form segment addition zones and segmental tissues. Muscular fibers originating from the longitudinal muscle bands of the stump extend until they reach the developing terminal caps, while the brain starts differentiating at the anterior end.
Stage 4, resegmentation, is when segmental units start to be distinguishable at the segmental region of the blastema, and primordia of segmental organs and structures appear. At this stage, the brain completes differentiating, and new ventral nerve cord ganglia form. Fibers of circular muscle form fine rings between the epidermis and the longitudinal muscle.
Stage 5, growth, involves finishing differentiation of all regenerated structures, and growth of the regenerated tissues to reach the adult proportions relative to the stump.
Key processes occurring along these five stages are a wound-healing response, cell migration, cell proliferation, formation of a regeneration blastema, neural regeneration, muscle regeneration, and segmentation. These processes are common to most species able to regenerate. However, details and relative timing vary across species. Furthermore, the aforementioned staging system is based on convenient median landmarks rather than clear developmental boundaries. As a result, many of these processes span two or more stages. Since processes are more comparable across species than timing, further discussion focuses on describing processes rather than stages.