Among those structures that can be regenerated by different animals we can find several types of appendages: from amphibian and insect legs to Hydra’s tentacles or arms and siphons from molluscs. Now a recent paper from the laboratory of Dave Ferrier introduces us to the regeneration of the operculum in the polychaete Pomatoceros lamarckii (http://www.ncbi.nlm.nih.gov/pubmed/24799350). As the authors point out there are not many models to study appendage regeneration within the Lophotrocozoans, as compared to other invertebrates (Ecdysozoans) and vertebrates.
P. lamarckii are serpulid polychaetes (annelids) with two types of head appendages capable of regeneration: the radioles (tentacles) for feeding and respiration and the operculum that can close the tube as a defensive strategy. In this study, the authors analyzed how the operculum regenerates at the morphological level as well as taking in account the dynamics of cell proliferation during regeneration. The opercular filament can be divided into two main parts separated by a prominent groove: a basal peduncle and a cup (the operculum). The cup is closed distally by the so-called opercular plate. This plate bears a spine and several prongs.
Upon amputation the first signs of regeneration are the elongation of the stump and the emergence of the new prongs of the spine. Then, by day one a small swelling is observed in the middle of the stump. This swelling enlarges and becomes cup-shaped with an expanding distal plate. Next, the groove that connects the cup with the peduncle is formed. Finally, wing buds develop on the peduncle below this groove. Therefore, it seems that during regeneration the new regions of the opercular filament differentiate following a disto-proximal sequence. In terms of the timing of all these regeneration steps, no significant differences were observed between different sex and size animals.
Next, the authors wanted to investigate the dynamics of cell proliferation during this process. They used two approaches: BrdU labeling and anti-phospho histone H3 immunostaining to detect mitotic cells. One first observation made by the authors was that operculum regeneration proceeds without the formation of a blastema (understood as the formation of an undifferentiated mass of cells at the stump). In fact, during the early stages of regeneration there is very little if any proliferation in the distal plate and spine. Moreover, the lack of BrdU labeled cells in these regions suggests that they do not come from proliferating cells elsewhere. Therefore, it seems that these new distal parts of the opercular filament, including the connective tissue of the cup, regenerate by morphallaxis (remodeling of the pre-existing tissues without cell proliferation). At later stages, the number of proliferative labeled epithelial cells increases in the wall of the new cup and the peduncle and remains high during this stage. At final stages of regeneration, BrdU concentrates in the regenerated wings in the distal peduncle. Remarkably, during the whole process, proliferative cells are uniformly found from the base of the peduncle to the distal cup, with no distinct or preferred proliferation domain.
In summary, it appears that operculum regeneration takes place without blastema formation and through an initial “morphallactic” phase in which the remodeling of the distal stump would give rise to the new distal structures: the spine, plate and the connective tissue of the cup. Later, the regeneration of more proximal regions would depend on cell proliferation, but not at the cut surface but all along the more proximal regenerating regions (from the new cup to peduncle). Also, it is interesting to notice that, morphologically, the new regions appear in a disto-proximal sequence of events.
Future experiments with molecular markers of specific cell types and regions should help to better understand the whole process of operculum regeneration as well as determine the origin of the regenerative cells.