Abstract
An important goal for understanding regeneration is determining how polarity is conferred to the regenerate. Here we review findings in two groups of polychaete annelids that implicate the ventral nerve cord in assigning dorso‐ventral polarity, and specifically ventral identity, to the regenerate. In nereids, surgical manipulations indicate that parapodia develop where dorsal and ventral body wall territories contact. Without a nerve cord at the wound site, the regenerate differentiates no evident polarity (with no parapodia) and only dorsal identity, while with two nerve cords the regenerate develops a twinned dorso‐ventral axis (with four parapodia per segment instead of the normal two). In sabellids, a striking natural dorso‐ventral inversion in parapodial morphology occurs along the body axis and this inversion is morphologically correlated with the position of the nerve cord. Parapodial inversion also occurs in segments in which the nerve cord has been removed, even without any segment amputation. Together, these data strongly support a role for the nerve cord in annelid dorso‐ventral pattern regulation, with the nerve cord conferring ventral identity.
Highlights
The establishment of body axes is a key process in animal development
A series of grafting experiments have demonstrated that contact between dorsal and ventral body wall induces the formation of parapodia (Boilly-Marer, 1969, 1971a,1971b), a process sometimes referred to as parapodial “regeneration.” In these experiments, parapodia were grafted onto different regions of the body or, alternatively, sections of body wall from different regions of the body were grafted to the dorsal or ventral side of the body, with grafts extending over several consecutive segments
Given that the DV polarity of parapodia appears to be correlated with the proximity of the nerve cord to the body wall, it is tempting to propose that the nerve cord could be an important regulatory factor in DV patterning during morphallactic regeneration, and tissue reorganization more broadly, in sabellids
Summary
The establishment of body axes is a key process in animal development. Body polarity cues provide developing tissue with positional information required for the development of normal phenotypes and the establishment of polarity typically occurs very early during development. Recent molecular studies in both planarians and acoels have implicated the BMP pathway in the establishment of DV polarity during regeneration (Gavino & Reddien, 2011; Srivastava et al, 2014), supporting the idea that at least some aspects of axis polarity development during regeneration redeploy processes occurring during embryogenesis. Surgical manipulations in both amphibians and planarians have implicated tissue interactions, and contact between dorsal and ventral body wall tissues, as being critical in blastema initiation during regeneration (Campbell & Crews, 2008; Carlson, 1974, 1975; Kato, Orii, Watanabe, & Agata, 1999, 2001) Such experimental studies of regeneration can be challenging to execute but can provide information about tissue-level processes, even in the absence of molecular knowledge of the processes. We hope that this review can help to fuel further research on DV polarity establishment during regeneration, both in annelids and in other animals, leading to a better understanding of the fundamental mechanisms that confer polarity to regenerated tissues
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