Abstract
Biomineralization refers to the dynamic physiological processes whereby living organisms elaborate mineralized tissues. The existence of extremely abundant molluscan species implies the diversity of mineralized tissues, since the majority of them (Conchifera) produce shells that vary in size and shape. Over the past decades, great progress has been made on the study of the cellular biology of shell biomineralization. The construction of the molluscan shell is the archetype of a biologically controlled mineralization which requires specialized cellular machinery. It has been so far demonstrated that the cells involved in shell formation come from two different tissues: 1) outer mantle epithelial cells (OME) secrete the organic matrix, among which shell matrix proteins (SMPs) determine mineralogical and crystallographic properties of shell; and 2) circulating hemocytes which take part in the deposition of intracellular biominerals and deliver them to the mineralization sites. Mounts of novel SMPs have been identified by using molecular biology techniques (gene cloning, in situ hybridization, immunohistochemistry et al.) coupled with high-throughput sequencing data (genome, proteome, secretome and transcriptome) , and their corresponding functions during shell formation have also been confirmed. The cellular activity of OME and hemocytes during shell formation are significantly increased during shell regeneration process. A potential cellular basis model for molluscan shell formation is proposed. 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 firstly transported to the hemolymph, and then engulfed by hemocytes (mainly granulocytes), which will disintegrate and release shell proteins and CaCO3 crystals at mineralization front. Besides, OME and hemocytes may be involved in the nucleation and remodeling process of CaCO3 mineralization. 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.
Highlights
Biomineralization refers to an extraordinary dynamic biological process whereby a living organism produces biomineral structures 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)
Scientists have traditionally recognized the matrix-mediated hypothesis, which states that the organic matrix exclusively control the molluscan shell formation by providing the framework, inducing crystal nucleation, and regulating crystal growth extracellularly, thereby forming the crystal morphologies that are unique to the various layers of molluscan shell (Addadi et al, 1987, 2006; Lowenstam and Weiner, 1989)
The results clearly demonstrate that Pf Y2 is a critical macromolecule and performs a variety of biological functions during shell formation
Summary
Biomineralization 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). Scientists have traditionally recognized the matrix-mediated hypothesis, which states that the organic matrix exclusively control the molluscan shell formation by providing the framework, inducing crystal nucleation, and regulating crystal growth extracellularly, thereby forming the crystal morphologies that are unique to the various layers of molluscan shell (Addadi et al, 1987, 2006; Lowenstam and Weiner, 1989) These results were mostly revealed from in vitro experiments through mimicking internal microenvironment, the effect of matrix proteins on shell mineralization is questionable (Sikes et al, 2000; Mount et al, 2004). This section summarizes the SMPs identified so far from the shells of molluscs, and emphasizes the physiological function of several critical SMPs through biochemical and micromorphological studies during shell biosynthesis
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