Several times in this blog I have pointed out the importance to extend our regenerative studies to more different species in order to have a broader view of this fascinating process as well as for a more comparative approach. Platyhelminthes are amazing animals not only because many of them show very high regenerative capabilities but also because these are based upon the presence of adult pluripotent stem cells. Among Platyhelminthes, model species include the freshwater planarians Schmidtea mediterranea and Dugesia japonica as well as the marine flatworm Macrostomum lignano.
A report from Ulrich Dirks and Jörg A. Ott has started to characterize the proliferative response associated to the regenerative process in the Catenulida, Paratenula galateia (http://www.ncbi.nlm.nih.gov/pubmed/22729484). Catenulida represent the most basally branching group of the Platyhelminthes. P. galateia is an interesting free-living animal with no mouth and gut, that reproduces asexually by paratomy and that harbors intracellular bacterial symbionts. These animals can be divided into an anterior rostrum and the trophosome region where the symbionts are found. In the rostrum these animals have a brain from which rostral nerves extend up to the most anterior tip. Two main longitudinal nerves extend from the rostrum towards the posterior regions of the animal. S-phase cells localize mostly throughout the trophosome up to behind the brain, so not dividing cells are seen in the most anterior part of the rostrum. When amputated behind the brain, the trophosome is able to regenerate a new rostrum; however, the rostrum is not able to regenerate the trophosome.
The authors analyse the dynamics of neoblast proliferation by combining EdU and BrdU pulses during rostrum regeneration. By 48h of regeneration the wound is closed by the constriction of circular muscles and the flattening of epidermal cells. At this stage EdU and BrdU positive cells are evenly distributed in the whole trophosome fragment. On the other side, stainings with an anti-serotonin antibody show the truncated longitudinal cords in the wound area. After 5 days of regeneration there is strong accumulation of proliferative cells within the forming blastema. Interestingly, an accumulation of proliferating cells is also found along the longitudinal nerve cords. At this stage, however, the amputated nerve cords appear still truncated and not extending into the blastema. The rest of the body shows an even distribution of low density of proliferating cells. Then, around 7 days of regeneration the new tip of the rostrum starts growing and a strong accumulation of proliferating cells is still observed in the blastema. Also, a prominent commissure appears at the anterior end of the nerve cords and some neuronal processes extend anteriorly, possibly corresponding to the regenerating rostral nerves. Finally, regeneration appears to be practically completed by day 11 after amputation. On the other side, all the rostrum fragments die after a couple of weeks without any sign of posterior regeneration.
Thus, this study represents a first step in trying to determine the proliferative dynamics of the neoblasts during regeneration in this Catenulida. However, and as the authors state, a more detailed time course of the regenerating process in terms of the proliferative response of the neoblasts is needed. Also, it will be interesting to further investigate how these neoblasts within the blastema exit the cell cycle and differentiate into the different cell types. Finally, it would be also interesting to study whether the failure to regenerate posterior regions from a rostrum fragment is caused by the low number of neoblasts present in the rostrum or the fact that the rostrum lacks enough symbionts for its nutrition. Maybe it would be interesting to analyze, in case the authors have not done it yet, what happens when the amputation is done not behind the brain but through the middle region of the trophosome. Would then the anterior fragment bearing the rostrum and part of the trophosome be able to regenerate the posterior end?