Regenerative ability in annelids is highly variable, but many groups are capable of reconstructing extensive regions of the body lost to injury, and mapping these traits to molecular phylogenies indicate that such regenerative powers are ancestral to the phylum (Zattara and Bely 2016). The process of axial regeneration in annelids presents many similarities to regeneration in other phyla: after wounding and wound healing, the wound epithelium initiates a reconstructive process that begins by reestablishing the terminal regions of the body, and then intercalates the remaining missing structures. This order of events is also seen during regeneration of flatworms, nemerteans, echinoderms, vertebrates, and many other groups, and combines epimorphic and morphallactic processes to restore the original morphology of the injured individual (Agata, Saito, and Nakajima 2007).
Regenerative trajectories (and related fission trajectories) have many fundamental differences with embryogenesis. On the one hand, while embryogenesis always begins from a highly stereotyped and predictable point, the fertilized egg, the starting point for regeneration is variable and unpredictable. Such unpredictability is likely an important challenge, as evidenced by the fact that many groups, including vertebrates, arthropods, and annelids, have evolved specific breakpoints, and might undergo corrective autotomy (self-amputation) at those points if amputated elsewhere (Bely and Nyberg 2010; Bely 2014). On the other hand, even similar morphogenetic processes often occur with different timing between embryogenesis and regeneration. For example, muscles and neurons develop simultaneously in annelid embryogenesis, while neurons develop earlier than muscle cells during regeneration (Kozin, Filippova, and Kostyuchenko 2017). Thus, regeneration cannot be assumed to represent a simple recapitulation of embryonic development.
Prior to morphogenetic events, the regenerative process needs to amass adequate resources: cells capable of moving, proliferating, reorganizing, and differentiating into new tissues. In annelids, such cells are sourced from existing tissues. What fraction of those cells are reserve stem cells and what fraction represent dedifferentiated cells is not yet fully understood. Furthermore, such fraction is likely to vary from group to group. However, current evidence does not support a fully stem cell-based regeneration, as seen in turbellarian flatworms. Evidence for transdifferentiation of cells across germ layers is also scant: most observational and experimental evidence supports that each germ layer furnishes material for its own derivatives.
However, after the stages of wound healing and blastema formation, the morphogenetic processes of regeneration converge with those of embryogenesis, both develop- mentally and at the gene expression level. This makes perfect sense: millions of years of evolution of embryonic developmental pathways and processes have already generated complex and robust developmental genetic networks that can be readily deployed by activation of relatively few hub genes. After forming a blastema, the regenerative process reaches a more stereotypical, predictable point and is ready to reconstruct the missing structures by launching the developmental pathways that are also used during embryo- genesis. Thus, the developmental trajectory of regeneration can be seen as a process that starts as a wound-healing response and leads to the reboot of embryonic development.
The ability of annelid regeneration to rebuild the segment formation zone and reboot the segment formation process is not only amazing on its own, but also highly fortunate, as it offers a unique window to study both conservation and evolution of the segmentation process. Breeding most species of annelids is not an easy task, and thus only a handful of species currently make good systems to study embryonic development. In contrast, many annelids can regenerate at least their posterior end (Bely 2006; Zattara and Bely 2016), providing a widely diverse set of systems for comparative studies of the developmental and genetic mechanisms underlying the formation of repeated body segments.