Cell Proliferation

Cell proliferation patterns after amputation have been characterized by observation of mitotic nuclei in histological sections and more recently by incorporation of thymidine analogs during S-phase and immunodetection of phosphorylated histones and proliferating cell nuclear antigen (PCNA) as markers for cell replication. Postamputation cell proliferation patterns vary throughout the process of regeneration, also showing differences across species. In most cases, however, cell proliferation is a key step in regeneration, and experimental blockage of proliferation usually abrogates or strongly delays the regenerative process (Bely 2014; Planques et al. 2018).

Studies on several annelid species show that axial amputation triggers a fast and sharp decrease in cell proliferation; this effect is more marked after anterior amputation, and can be weak or absent after posterior amputation (Paulus and Muller 2006; Zattara and Bely 2011, 2013; Jong and Seaver 2016; Planques et al. 2018). When present, this shutdown becomes evident as soon as 30 minutes after amputation and can involve exclusively segments close to the wound site or extend to the whole body. Proliferation remains low until the onset of blastema formation, usually within 1 day after amputation. This proliferation shutdown and delay might be part of a resource allocation strategy aimed to stall current resource sinks (i.e., growth) while early post-amputation preparatory steps, like wound-healing, immune response, and cell dedifferentiation, take place (Zattara and Bely 2013; Bely 2014).

After this shutdown, initiation of the next phase of regeneration, blastema formation, is evidenced by a marked increase in proliferation near the wound site. The earliest proliferative activity is often observed near the severed end of the ventral nerve cord, suggesting that this structure has an important role in initiating blastema formation (Bely 2014). Cell proliferation soon spreads to all tissues, including endodermal, mesodermal, and ectodermal derivatives, adjacent to the wound site. Proliferation usually extends away from the cut site to a variable number of proximal segments, but is more intense in the blastema and developing regenerated structures, supporting that most of the regenerate is derived from proliferation of proximal cells and not from cells that migrate from more distal regions.

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