Tissue homeostasis in sponges: Quantitative analysis of cell proliferation and apoptosis

Abstract

Background: Tissues of multicellular animals are maintained due to a tight balance between cell proliferation and programmed cell death. Phylum Porifera is an early branching group of metazoans essential to understanding the key mechanisms of tissue homeostasis. This paper is dedicated to the comparative analysis of proliferation and apoptosis in intact tissues of two sponges belonging to distinct Porifera lineages, Halisarca dujardinii (class Demospongiae) and Leucosolenia variabilis (class Calcarea). Results: Labeled nucleotides EdU and anti-phosphorylated histone 3 antibodies reveal a considerable number of cycling cells in intact tissues of both species. The main type of cycling cells are choanocytes -flagellated cells of the aquiferous system. The rate of proliferation remains constant in areas containing choanoderm. Cell cycle distribution assessed by the quantitative DNA stain reveals the classic cell cycle distribution curve. During EdU pulse-chase experiments conducted in H. dujardinii, the contribution of the choanocytes to the total amount of EdU-positive cells decreases, while contribution of the mesohyl cells increases. These findings could indicate that the proliferation of the choanocytes is not solely limited to the renewal of the choanoderm, and that choanocytes may participate in the general cell turnover through migration. The number of apoptotic cells in intact tissues of both species is insignificant. In vivo studies in both species with TMRE and CellEvent Caspase-3/7 indicate that apoptosis might be independent of mitochondrial outer membrane permeabilization. Conclusions: A combination of confocal laser scanning microscopy and flow cytometry provides a quantitative description of cell turnover in intact sponge tissues. Intact tissues of H. dujardinii (Demospongiae) and L. variabilis (Calcarea) are highly proliferative, indicating either high rates of growth or cell turnover. Although the number of apoptotic cells is low, apoptosis could still be involved in the regular cell turnover..

Cell cycle dynamics of food‐entrapping cells of sponges: an experimental approach

“…The labeling fraction of choanocytes (the proportion of EdU-positive choanocytes to the number of choanocytes) was 10.5 AE 1.8% and 13.4 AE 1.0%, respectively (Table 1). As we reported earlier [21], the absolute majority (> 90%) of labeled cells in both species are represented by choanocytes, flagellated food-entrapping cells of the aquiferous system. Minor populations of cycling cells, however, differ between species, probably reflecting the difference in the anatomical and histological structure of studied sponges.…”

“…Still, choanocytes are not the only cycling cells in sponges. As expected, we found archaeocytes (amoeboid mesohyl cells) to incorporate labeled nucleotides in H. dujardinii; in demosponges, both choanocytes and archaeocytes are thought to maintain complex tissue dynamics in intact animals through vigorous transdifferentiation and cell migration [14,21,33]. In calcareous sponge L. corallorrhiza, however, proliferating amoebocytes are few (if any), and cycling cells outside the choanoderm are represented by epithelial-like exopinacocytes, endopinacocytes, and ostia-forming porocytes.…”

“…There has long been a debate about which cell type acts as the primary stem cells in sponges. While certain amoeboid cells of mesohyl were traditionally regarded as the main type of stem cells in sponges [2,4,14,32], recent studies [6,20,21,33] showed that the majority of proliferating cells in intact tissues is actually comprised food-entrapping cells of the aquiferous system, choanocytes. Here, we also show that choanocytes are the main proliferating cell type both in intact sponges and after the S-phase synchronization.…”

“…In sponges, certain amoeboid cells of mesohyl were traditionally thought to be pluripotent stem cells [4,[14][15][16]. However, recent studies have shown choanocytes to play a leading role in regeneration [11,[17][18][19] as well as in regular cell turnover [5,6,20,21]. If so, the dynamics of choanocyte proliferation should determine the general rate of cell turnover and regeneration speed.…”

“…1). While there are several papers devoted to regular cell turnover and regeneration dynamics in these species [11,19,21], the kinetics of choanocyte cell cycle has long been an unsettled issue. The intention of this study is to characterize proliferating cell populations and choanocyte cell cycle dynamics in intact tissues of H. dujardinii and L. corallorrhiza.…”