Abstract
BackgroundIn 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.ResultsSyllis 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.ConclusionsOur 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.
Highlights
One of the early stages of body regeneration in annelids is the development of the blastema, a tissue composed by relatively undifferentiated cells that are able to proliferate [1,2,3]
Migration of stem cells to the blastema was first documented for the sedentarian Lumbriculus by Randolph [8, 9] who named them neoblasts, a term widely used to refer to the flatworm pluripotent stem cells [10], despite there being no evidence of homology of those cell types between annelids and flatworms
The posterior body showed a prominent accumulation of S-phase cells in the growing segments and segment addition zone (SAZ, where new segments are generated [67]), with an overall average of 2.5x more S-phase cells than in the anterior body (n = 6 specimens)
Summary
One of the early stages of body regeneration in annelids is the development of the blastema, a tissue composed by relatively undifferentiated cells that are able to proliferate [1,2,3]. The annelid blastema is considered to develop through a process of dedifferentiation [1, 4,5,6]. Migration of stem cells to the blastema was first documented for the sedentarian Lumbriculus by Randolph [8, 9] who named them neoblasts, a term widely used to refer to the flatworm pluripotent stem cells [10], despite there being no evidence of homology of those cell types between annelids and flatworms. 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). 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
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