The migration of cells from the water-vascular system to the coelom provides evidence supporting the origin of coelomocytes in sea cucumber Apostichopus japonicus

Abstract

Apostichopus japonicus, a member of Holothuroidea (phylum Echinodermata), is known for its strong ability to regenerate its coelomocytes. The constitutive recovery of coelomocytes occurs at 6 h post-evisceration. Rapid coelomocyte recovery, combined with limited proliferation of residual coelomocytes at the early recovery stage, suggests that migration from other sites is the likely mechanism. In this study, we investigated migration of coelomocytes and several other components, including cell-free coelomic fluid, protein, and bacteria from the water-vascular system to the coelom by transplantation of these components through the papillae. The coelomic fluid had migrated substantially at 6 h post-transplantation (hpt). Glutathione S-transferase was detected migrating into the coelom at 6 and 12 hpt with an increasing trend. Vibrio splendidus, a pathogenic bacterium, and Escherichia coli, a non-pathogenic bacterium of A. japonicus, were both found to migrate at 6–72 hpt. Fluorescence labeling and allogeneic transplantation of live coelomocytes were used to investigate coelomocyte migration. Coelomocytes migrated from the water-vascular system to the coelom at 2 and 12 hpt, with a migration rate of 1.2 % and 3.7 %, respectively. However, migration rate increased significantly at 6 and 12 hpt under evisceration, which were 2.5- and 1.3-fold than that in healthy A. japonicus. Pathogenic infection significantly increased coelomocyte migration rate at each point post-transplantation, with a peak of 7.60-fold at 48 hpt. The findings have shown that different components can migrate from the water-vascular system to the coelom in A. japonicus. This suggests that the water-vascular system may act as a storage pool for coelomocytes to supplement coelomocyte physiological loss. This study enhances our understanding of the immune defense mechanisms and regenerative processes in echinoderms.