The Functions Of The Mantle Tissue And Hemocytes In The Shell Growth Of Pinctada Fucata

The pearl oyster Pinctada fucata is an economically important species for the aquacultured seawater pearls.Its shell structure is similar to that of pearls and exhibits outstanding mechanical properties.Therefore,understanding the formation mechanism of the shells will improve the yield and quality of the cultured pearls and provide guidance for the manufacture of biomimetic materials.In this study,we studied the regulation mechanism of the mantle tissue and hemocytes in the process of shell growth.In the process of shell repair,the spatial position and morphology of the mantle directly determined the microstructure of the regenerated shell.Simultaneously,the mantle tissue could promote calcium carbonate nucleation by upregulating the organic matrix secretion,thereby accelerating the shell regeneration.During the formation of flat pearl,it was found that the composition of the epidermal cells and their gene expressions were dramatically changed;when a curved glass was inserted into the extrapallial space(EPS)as an explant,calcium carbonate deposits were only found on the outer surface of the explant which was could directly contact with the mantle cells,further supporting that the mantle directly controls the shell deposition process.Further investigation of the process of secreting shell components from the mantle revealed that the mantle directly deposited the organic matrix onto the growing interface of the shell via secreting vesicles.However,proteomic analysis of the extrapallial fluids showed that most of the matrix proteins secreted by the mantle were not present as a free form in the EPS.Therefore,we speculate that the mantle cells directly secrete matrix proteins to the mineralization interface,thus regulating the shell growth in a close manner.As the main immunocomponent,hemocytes are supposed to be involved in shell mineralization.Our results suggest that hemocytes can migrate in and out of the EPS,and their composition is similar to hemocytes in the circulatory system.When a large amount of microorganisms was present in the EPS,the deposition of nacre was severely affected,and it even led to shell disease.Hemocytes agglutinates and phagocytoses the pathogenic microorganisms,and maintained the microenvironment of the EPS by exerting cellular immunity and humoral immunity,therefore ensuring the successful deposition of the shell.After immune stimulation and shell notch damage,hemocytes significantly upregulate genes involved in calcium metabolism.At the same time,we found that some hemocytes might be involved in the transport of calcium ions,and these cells were rich in calcium ions and moved quickly,which were beneficial for calcium transportion.This suggests that hemocytes plays a role in immune protection and serves as a potential calcium pool in shell formation.In addition,we explored the evolution and formation mechanism of the polygonal arrangement of prismatic layers.We found two different growth modes in the prismatic layer,namely the Voronoi distribution during horizontal expansion and the two-dimensional foam evolution during vertical growth.These two processes are governed by thermodynamic rules.The heterogeneity distribution of the organic matrix determines the boundary conditions for the prism layer to grow under mathematics and physics laws.Our results indicate that the mantle tissue plays a leading role in shell growth and directly control the shell deposition,while hemocytes play an auxiliary role by ensuring and promoting the shell growth,which deepens our understanding of the upstream cellular regulation process of shell growth and will provide a theoretical support for the aquaculture of the pearls.