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Many animals are able to regenerate different types of appendages (understood as body wall outgrowths). Among vertebrates, limbs and tails from salamanders and lizards are well-known examples. On the other side, some invertebrates also regenerate appendages such us parapodia, tentacles, opercula, palps and gills. Molluscs comprise a very diverse phylum with an extreme morphological diversity. Up to date, no known representative has been shown to be able to regenerate the whole body. However, several mollusc species can regenerate a variety of structures such as: foot, tentacles, siphon, shell and mantle, and even the head. Cephalopod molluscs are well known for their capacity to regenerate their arms. However, very little quantitative data is available about this process and even less about the cellular and molecular mechanisms involved. In a paper from the laboratory of Jedediah Tressler Nathan J. Tublitz the authors provide a detailed description and quantification of the regeneration process in two species of cuttlefish (http://www.ncbi.nlm.nih.gov/pubmed/23982859). Studying arm regeneration in cephalopods is also important as these structures are as complex as many vertebrates appendages.
It is also obvious that an initial detailed description and quantitative data on the regeneration process in terms of growth rate, behavioural changes, timing of regeneration and functional recovery is necessary prior to tackle the cellular and molecular mechanisms that regulate it, especially in those non-model animals.
First, the authors present the data on Sepia officinalis in which the right 3rd arm was amputated from nine juveniles. In all cases, the amputated arms were regenerated by 39 days, after which the newly formed arms were indistinguishable from the contralateral control arms. They divided the regeneration process in 5 stages: at stage I (days 0-3) the leading edge of the arm appeared smooth with little bleeding. Two days after the amputation the regenerating arm appeared frayed and covered with a mucus-like substance. At this stage only a few new suction cups and no new chromatophores were observed. As a consequence of the amputation the behaviour of the animals was significantly altered as they showed: unbalanced swimming, impaired prey manipulation for ingestion, altered normal body posturing behaviours and lack of any colour change behaviour. Stage II (days 4-15) was characterized by the smooth, slightly hemispherical appearance displayed by the leading edge of the regenerating arm. Growth across the entire width of the arm was symmetrical. New suction cups and chromatophores were seen at this stage. Also, a normal balanced swimming was seen and from day 9 normal food manipulation reappeared. Stage III (days 16-20) began with the appearance of a growth bud on the lateral side of the leading edge, resulting in an asymmetric shape of the regenerating arm. The new arm kept growing at a higher rate that the contralateral control and new suction cups and chromatophores were added at the distal tip. By the end of this stage the regenerating arm was also used for normal body postures. Stage IV (days 21-24) was defined by the emergence of an elongated tip from the growth bud. Suction cup regeneration appeared to be completed by the end of this stage. In the final stage V (days 25-39) the elongated tip took on a tapered appearance. New chromatophores were added until their density in the tip resembled that from control arms. Here, all the checked behaviours, including the brown tip behaviour, were recovered.
In the other species, Sepia pharaonis, arms were also regenerated in 39 days and followed the same 5 stages as in S. officinalis. Based on the location of the new suction cups, chromatophores and the presence of the growth bud and elongated tip, it seemed that new tissue was added directly to the tip of the regenerating arm. Whereas the growth rate of the regenerating arm was quite constant throughout regeneration in S. pharaonis, it varied depending on the stage in S. officinalis.
In summary, in this paper the authors report on the fine description of arm regeneration in two species of cuttlefish at the morphological and behavioural recovery levels. Next step should be to get insights into the cellular and molecular processes governing this regenerative process.