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Sponges (phylum Porifera) exhibit surprisingly complex tissue dynamics, maintaining constant cell turnover and migration, rearranging internal structures, and regenerating after severe injuries. Such tissue plasticity relies on the activity of proliferating cells represented primarily by the food-entrapping cells, choanocytes. Although there are plenty of studies regarding the dynamics of regeneration and tissue rearrangement in sponges, cell cycle kinetics of choanocytes in intact tissues remains a controversial issue. This study is devoted to the comparative description of choanocyte cell cycle dynamics in intact tissues of two sponges, Halisarca dujardinii (class Demospongiae) and Leucosolenia corallorrhiza (class Calcarea). We have identified populations of proliferating cells and synchronized them in the S-phase to estimate the growth fraction of cycling cells. Using continuous exposure to labeled thymidine analog ethynyl deoxyuridine (EdU), we calculated choanocyte cell cycle duration and the length of the S phase. We also applied double labeling with EdU and antibodies against phosphorylated histone 3 to estimate the lengths of choanocyte M and G2 phases. Finally, flow-cytometry-based quantitative analysis of DNA content provided us with the lengths of G2 and G1 phases. We found that tissue growth and renewal in the studied sponges are generally maintained by a relatively large population of slowly cycling choanocytes with a total cell cycle duration of 40 h in H. dujardinii and 60 h in L. corallorrhiza. In both species, choanocytes are characterized by an extremely short M-phase and heterogeneity in the duration of the G2 phase., (© 2024 Federation of European Biochemical Societies.)

Microtubules are an indispensable component of all eukaryotic cells due to their role in mitotic spindle formation, yet their organization and number can vary greatly in the interphase. The last common ancestor of all eukaryotes already had microtubules and microtubule motor proteins moving along them. Sponges are traditionally regarded as the oldest animal phylum. Their body does not have a clear differentiation into tissues, but it contains several distinguishable cell types. The choanocytes stand out among them and are responsible for creating a flow of water with their flagella and increasing the filtering and feeding efficiency of the sponge. Choanocyte flagella contain microtubules, but thus far, observing a developed system of cytoplasmic microtubules in non-flagellated interphase sponge cells has been mostly unsuccessful. In this work, we combine transcriptomic analysis, immunofluorescence, and electron microscopy with time-lapse recording to demonstrate that microtubules appear in the cytoplasm of sponge cells only when transdifferentiation processes are activated. We conclude that dynamic cytoplasmic microtubules in the cells of sponges are not a persistent but rather a transient structure, associated with cellular plasticity.