Cellular proliferation dynamics during regeneration in Syllis malaquini (Syllidae, Annelida)

Abstract

Background In syllids (Annelida, Syllidae), the regenerative blastema was subject of many studies in the mid and late XXth century. This work on syllid regeneration showed that the blastema is developed by a process of dedifferentiation of cells near the wound, followed by their proliferation and redifferentiation (cells differentiate to the original cell type) or, in some specific cases, transdifferentiation (cells differentiate to a cell type different from the original). Up to date, participation of stem cells or pre-existing proliferative cells in the blastema development has never been observed in syllids. This study provides the first comprehensive description of Syllis malaquini’s regenerative capacity, including data on the cellular proliferation dynamics by using an EdU/BrdU labelling approach, in order to trace proliferative cells (S-phase cells) present before and after operation. Results Syllis malaquini can restore the anterior and posterior body from different cutting levels under experimental conditions, even from midbody fragments. Our results on cellular proliferation showed that S-phase cells present in the body before bisection do not significantly contribute to blastema development. However, in some specimens cut at the level of the proventricle, cells in S-phase located in the digestive tube before bisection participated in regeneration. Also, our results showed that nucleus shape allows to distinguish different types of blastemal cells as forming specific tissues. Additionally, simultaneous and sequential addition of segments seem to occur in anterior regeneration, while only sequential addition was observed in posterior regeneration. Remarkably, in contrast with previous studies in syllids, sexual reproduction was not induced during anterior regeneration of amputees lacking the proventricle, a foregut organ widely known to be involved in the stolonization control. Conclusions Our findings led us to consider that although dedifferentiation and redifferentiation might be more common, proliferative cells present before injury can be involved in regenerative processes in syllids, at least in some cases. Also, we provide data for comparative studies on resegmentation as a process that differs between anterior and posterior regeneration; and on the controversial role of the proventricle in the reproduction of different syllid lineages.

Comparative Aspects of Annelid Regeneration: Towards Understanding the Mechanisms of Regeneration

“…Posterior regrowth of the gut tissues also appears to be associated with the proliferation of endodermal epithelial cells. Experiments on P. dumerilii and Syllis malaquini with incubation with EdU prior to amputation support the idea that the intestinal epithelium contains stem cells that are normally required for physiological regeneration of the intestinal epithelium [39,45]. Active cell divisions of gut cells following amputation have been documented by autoradiography and histology for sabellid worms [131].…”

“…In Syllis malaquini, as recently shown with double EdU-chase/BrdU-pulse treatments, the blastema consists exclusively of cells that enter the S-phase after injury. These results suggest that the blastema is formed by cell dedifferentiation and redifferentiation, although it cannot be completely ruled out that stem cells are also involved in regeneration [45]. Once formed, the blastema shows a high proliferative activity of its cells, which has often been confirmed by using the DNA precursors [38][39][40][41][42].…”

“…In Autolitus pictus, Procerastea halleziana, and Syllis sp., regenerating fragments containing the head segments with only the ectodermal foregut (pharynx) cannot regenerate the lost posterior segments [62]. On the other hand, a regrowth of the intestine (endodermal origin) posteriorly from the foregut region (ectodermal origin) has been recently reported for syllid S. malaquini [45]. This result indicates that cell transdifferentiation might take place when the fragment contains no endodermal tissues from which the gut can be restored, as it was previously suggested [154,155].…”

“…Interestingly, the severing of the gut at the site of the wound is critically important for initiating blastemal outgrowth in Tubifex [10]. Moreover, endodermal cells were found in the inner blastema during the anterior regeneration of several annelid species [6,45].…”

Distinct stem-like cell populations facilitate functional regeneration of theCladonemamedusa tentacle

“…In order to test this hypothesis, we performed chasing experiments by labeling the two proliferative cell populations with distinct nucleoside analogs, EdU and BrdU (Figure 4A). This approach has been utilized in different animal systems as a way to track proliferative cell populations and their lineages (Harris et al, 2018; Ribeiro et al, 2021; Vega and Peterson, 2005). We labeled homeostatic proliferative cells in the bulb by EdU before amputation and repair-specific proliferative cells in the blastema by BrdU at 24 hpa (Figure 4A).…”