Molluscan separate the biomineral formation from the ambient environment ( Rahman and Shinjo, 2012) and exhibit a huge diversity of biomineral morphologies, such as shells from most molluscs, epithelial spicules of the basal mollusk Wirenia argentea (Solenogastres) ( Todt and Wanninger, 2010), scales and plates in bivalves, operculum in gastropod Rapana venosa ( Hashimoto et al., 2012), intracellular detoxifying granules in the common garden snail Helix aspersa ( Howard et al., 1981), egg capsules of the Patagonian neogastropod Odontocymbiola magellanica ( Bigatti et al., 2010), love dart of land snails ( Lodi and Koene, 2016), pearls from pearl oyster, statoconia in Aplysia californica ( Kondrachuk and Wiederhold, 2004), and statoliths in Nassarius reticulatus (Caenogastropoda) ( Galante-Oliveira et al., 2014). The phylum Mollusca is the second largest invertebrate phylum, which benefits from the protection from their external biomineralized structure, the shell, a kind of the mastery of cellular-engineered microstructures ( Kocot et al., 2016). In the metazoan world, CaCO 3 skeletons are the most abundant and most commonly encountered biominerals ( Lowenstam and Weiner, 1989 Simkiss and Wilbur, 1989 Marin et al., 2007 Cusack and Freer, 2008 Marin et al., 2008). So far, at least 60 different biominerals have been identified to play versatile functions, including tissues support, embryonic and UV protection, shelter against predation, nutrition, reproduction, gravity, light or magnetic field perceptions, storage of mineral ions ( Cusack and Freer, 2008 Marin et al., 2008 Islam and Peng, 2018). Biomineral structures are of wide existence in nature with polymorphism and multiple functions. These cells and cell products work co-operatively to produce an organo-mineral shell, which is composed of various biomineral ultra-structures and macromolecular organic components.īiomineralization refers to an extraordinary dynamic biological process whereby a living organism produces biomineral structures (a rigid skeleton or a non-skeletal mineral) at ambient temperature in environments ranging from polar to tropical ( Simkiss and Wilbur, 1989 Cusack and Freer, 2008 Shi et al., 2013 Tang et al., 2018). OME and hemocytes may also be involved in the nucleation and remodeling process of CaCO 3 mineral. The shell matrix proteins, mostly secreted from OME, and a few secreted from hemocytes or other organs, are either directly delivered to the mineralization site via exosome or classical secretory pathway, or first transported to the hemolymph, and then engulfed by hemocytes (mainly granulocytes), which will disintegrate and release shell proteins and CaCO 3 crystals at the mineralization front. A potential cellular basis model for molluscan shell formation is proposed. The cellular activity of OME and hemocytes during shell formation are significantly increased during shell regeneration process. Many novel SMPs have been identified by using molecular biology techniques (i.e., gene cloning, in situ hybridization, immunohistochemistry) coupled with high-throughput sequencing data (genome, proteome, secretome and transcriptome), and their corresponding functions during the shell formation have also been confirmed. Circulating hemocytes take part in the deposition of intracellular biominerals and deliver them to the mineralization sites. OMEs secrete the organic matrix, among which shell matrix proteins (SMPs) determine mineralogical and crystallographic properties of shell. It has been so far demonstrated that the cells involved in shell formation come from two different sources: outer mantle epithelial cells (OME) and circulating hemocytes. The construction of the molluscan shell is the archetype of biologically controlled mineralization which requires specialized cellular machinery. Over the past decades, great progress has been made on the study of the cellular biology of shell biomineralization. The existence of extremely abundant molluscan species shows the diversity of mineralized tissues, since the majority of them (Conchifera) produce shells that vary in size and shape. 3Liaoning Key Laboratory of Marine Animal Immunology and Disease Control, Dalian Ocean University, Dalian, Chinaīiomineralization refers to the dynamic physiological processes whereby living organisms elaborate mineralized tissues.2Laboratory for Marine Fisheries Science and Food Production Processes, Qingdao National Laboratory for Marine Science and Technology, Qingdao, China.1Liaoning Key Laboratory of Marine Animal Immunology, Dalian Ocean University, Dalian, China.Xiaorui Song 1, Zhaoqun Liu 1, Lingling Wang 1 and Linsheng Song 1,2,3*
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