Detection of markers for proliferation, stem cell, and EMT in the gills of Pacific abalone Haliotis discus hannai

The hemocytes of molluscs take various roles including immunity and wound healing. Oyster gill was found to undergo hematopoiesis and produce adult somatic progenitor cells. In gastropod molluscs, however, such special gill structure has not been reported. In the present study, we used Pacific abalone, a gastropod mollusc, and explored unique gill structures undergoing active proliferation. By immunofluorescence staining, two proliferation markers, proliferating cell nuclear antigen (PCNA) and phospho-histone H3 (H3P), were detected in marginal regions of the gill filaments, which were characterized by thick epithelium with cilia. In situ hybridization for Sox genes, the stem cell markers, also showed signals in the corresponding regions. BrdU, a DNA replication-detecting reagent, and nuclear-localized β-catenin, a putative epithelial–mesenchymal transition (EMT) marker, were also detected in the same regions. These results suggest that the marginal regions of gill filaments are highly proliferative, possess stemness, and likely undergoing EMT. Further immunofluorescence staining for hemocytes with PCNA, H3P, and β-catenin revealed that these cells also exhibited these markers, implying a possible link between the hemocytes and the gill cells. These results showed a structure containing actively proliferating cells in abalone gills, and points to the importance of classification of cell population in this tissue.