Intraspecific variability of cell reaggregation during reproduction cycle in sponges.

The ability of sponge cells to reaggregate and reconstruct intact functional organism is known for more than 100 years. This process was studied in numerous species of sponges, and its interspecific variability is well described. However, some data also indicate the existence of a certain intraspecific variability of the cell reaggregation. The present study deals with the cell reaggregation in two demosponges, Halichondria panicea and Halisarca dujardinii, during different periods of their sexual reproduction. In both species, cell reaggregation shows a common pattern at all studied periods of reproduction. However, the course of the reaggregation process significantly depends on the reproduction period of an individual used in the experiment, thus demonstrating pronounced intraspecific variability, which concerns the rate of the cell reaggregation and the final stage of the process. This variability occurs due to tissue rearrangements that accompany reproduction and changes cell composition and amount of available somatic stem cells in sponge tissues, and consequently alters morphogenetic potencies of a cell suspension and multicellular aggregates. In both Halichondria panicea and Halisarca dujardinii, the growth period is the most favorable for the reaggregation process, while the cell reaggregation is depressed during periods of embryogenesis and restoration of somatic tissues after the reproduction. At the same time, the structure of a particular stage of reaggregation and morphogenetic processes underlying the development of multicellular aggregates are always identical, independently from the period of the reproductive cycle.

Transdifferentiation and mesenchymal-to-epithelial transition during regeneration in Demospongiae (Porifera).

Origin and early evolution of regeneration mechanisms remain among the most pressing questions in animal regeneration biology. Porifera have exceptional regenerative capacities and, as early Metazoan lineage, are a promising model for studying evolutionary aspects of regeneration. Here, we focus on reparative regeneration of the body wall in the Mediterranean demosponge Aplysina cavernicola. The epithelialization of the wound surface is completed within 2 days, and the wound is completely healed within 2 weeks. The regeneration is accompanied with the formation of a mass of undifferentiated cells (blastema), which consists of archaeocytes, dedifferentiated choanocytes, anucleated amoebocytes, and differentiated spherulous cells. The main mechanisms of A. cavernicola regeneration are cell dedifferentiation with active migration and subsequent redifferentiation or transdifferentiation of polypotent cells through the mesenchymal-to-epithelial transformation. The main cell sources of the regeneration are archaeocytes and choanocytes. At early stages of the regeneration, the blastema almost devoid of cell proliferation, but after 24 hr postoperation (hpo) and up to 72 hpo numerous DNA-synthesizing cells appear there. In contrast to intact tissues, where vast majority of DNA-synthesizing cells are choanocytes, all 5-ethynyl-2'-deoxyuridine-labeled cells in the blastema are mesohyl cells. Intact tissues, distant from the wound, retains intact level of cell proliferation during whole regeneration process. For the first time, the apoptosis was studied during the regeneration of sponges. Two waves of apoptosis were detected during A. cavernicola regeneration: The first wave at 6-12 hpo and the second wave at 48-72 hpo.