Regeneration in sponges 

The ability to regenerate is widespread in the animal kingdom, but the regenerative capacities and mechanisms vary widely. Sponges are known to possess remarkable reconstitutive and regenerative abilities ranging from wound healing or body part regeneration to the impressive re-building of a functional body from dissociated cells. Sponges, as an ancient animal lineage, are important models in studies aimed at understanding of the evolutionary history of animal regeneration mechanisms, including relationships between regeneration and development. Because of diversity of developmental mechanisms found among sponges, we have combined several microscopy techniques to study cellular mechanisms of reparative regeneration in four sponge species from different clades: Oscarella lobularis (Homoscleromorpha), Halisarca dujardini (Demospongiae), Sycon ciliatum and Leucosolenia complicata (Calcarea). Wound healing followed by complete restoration of lost body parts have been observed in all four species, although the cellular mechanisms varied. Epithelial morphogenesis, mainly spreading (flattening) and fusion of epithelial sheets was found to be the key morphogenetic process during regeneration for O. lobularis, S. ciliatum and L. complicata ; while mesenchymal morphogenesis by mesenchymal-epithelial transformations is the main mechanism active during H. dujardini regeneration. In addition, formation of blastema (accumulation of pluripotent cells and the dedifferentiated cells - choanocytes, pinacocytes beneath the wound surface) was observed in this species. Notably, transdifferentiation of choanocytes to pinacocytes accompanied the regeneration in all investigated sponges. Altogether, we demonstrate that regeneration in sponges involves a variety of processes utilized during regeneration in other animals (e.g., cell migration, dedifferentiation, blastema formation) and points to the particular importance of transdifferentiation in this process. While further studies will be needed to uncover the molecular mechanisms governing regeneration in sponges, Wnt and Tgf-beta pathways appear to play important roles in this process as they do in other metazoans. Financial support by RFBR n° 16-04-00084 and the Russian Science Foundation n° 17-14-01089 is gratefully acknowledged.