During mollusk shell formation, the mineral phase forms within an organic matrix composed of β-chitin, silk-like proteins, and acidic glycoproteins rich in aspartic acid. The matrix is widely assumed ...to play an important role in controlling mineralization. Thus, understanding its structure is of prime importance. Cryo-transmission electron microscopy (Cryo-TEM) studies of the matrix of the bivalve Atrina embedded in vitrified ice show that the interlamellar sheets are composed mainly of highly ordered and aligned β-chitin fibrils. The silk, which is quantitatively an important component of the matrix, could not be imaged within the sheets. Organic material was, however, observed between sheets. We infer that this is the location of the silk. As this material reveals no regular structure, we suggest that at least prior to mineralization the silk is in the form of a hydrated gel. This is supported by cryo-TEM structural observations of an artificial assembly of β-chitin with and without silk. This view of the nacreous organic matrix significantly changes previous models of the matrix structure and hence hypotheses pertaining to the mechanisms by which mineral formation occurs.
Biological photonic systems composed of anhydrous guanine crystals evolved separately in several taxonomic groups. Here, two such systems found in fish and spiders, both of which make use of ...anhydrous guanine crystal plates to produce structural colors, are examined. Measurements of the photonic‐crystal structures using cryo‐SEM show that the crystal plates in both fish skin and spider integument are ∼20‐nm thick. The reflective unit in the fish comprises stacks of single plates alternating with ∼230‐nm‐thick cytoplasm layers. In the spiders the plates are formed as doublet crystals, cemented by 30‐nm layers of amorphous guanine, and are stacked with ∼200 nm of cytoplasm between crystal doublets. They achieve light reflective properties through the control of crystal morphology and stack dimensions, reaching similar efficiencies of light reflectivity in both fish skin and spider integument. The structure of guanine plates in spiders are compared with the more common situation in which guanine occurs in the form of relatively unorganized prismatic crystals, yielding a matt white coloration.
Structural colors evolved by fish and spiders may be iridescent, as in the fish scale (left of image), and silver spider (center of image) or matt white spider (right of image). The color depends on whether the guanine crystals are plate‐like and regularly spaced, forming photonic crystal arrays, or prismatic and irregular, giving rise to light scattering. Here these different effects are examined, and some explanation of their properties is given.
Vertebrate mineralized tissues, present in bones, teeth and scales, have complex 3D hierarchical structures. As more of these tissues are characterized in 3D using mainly FIB SEM at a resolution that ...reveals the mineralized collagen fibrils and their organization into collagen fibril bundles, highly complex and diverse structures are being revealed. In this perspective we propose an approach to analyzing these tissues based on the presence of modular structures: material textures, pore shapes and sizes, as well as extents of mineralization. This modular approach is complimentary to the widely used hierarchical approach for describing these mineralized tissues. We present a series of case studies that show how some of the same structural modules can be found in different mineralized tissues, including in bone, dentin and scales. The organizations in 3D of the various structural modules in different tissues may differ. This approach facilitates the framing of basic questions such as: are the spatial relations between modular structures the same or similar in different mineralized tissues? Do tissues with similar sets of modules carry out similar functions or can similar functions be carried out using a different set of modular structures? Do mineralized tissues with similar sets of modules have a common developmental or evolutionary pathway?
3D organization studies of diverse vertebrate mineralized tissues are revealing detailed, but often confusing details about the material textures, the arrangements of pores and differences in the extent of mineralization within a tissue. The widely used hierarchical scheme for describing such organizations does not adequately provide a basis for comparing these tissues, or addressing issues such as structural components thought to be characteristic of bone, being present in dermal tissues and so on. The classification scheme we present is based on identifying structural components within a tissue that can then be systematically compared to other vertebrate mineralized tissues. We anticipate that this classification approach will provide insights into structure-function relations, as well as the evolution of these tissues.
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Bone is a complex hierarchically structured family of materials that includes a network of cells and their interconnected cell processes. New insights into the 3-D structure of ...various bone materials (mainly rat and human lamellar bone and minipig fibrolamellar bone) were obtained using a focused ion beam electron microscope and the serial surface view method. These studies revealed the presence of two different materials, the major material being the well-known ordered arrays of mineralized collagen fibrils and associated macromolecules, and the minor component being a relatively disordered material composed of individual collagen fibrils with no preferred orientation, with crystals inside and possibly between fibrils, and extensive ground mass. Significantly, the canaliculi and their cell processes are confined within the disordered material. Here we present a new hierarchical scheme for several bone tissue types that incorporates these two materials. The new scheme updates the hierarchical scheme presented by Weiner and Wagner (1998). We discuss the structures at different hierarchical levels with the aim of obtaining further insights into structure–function-related questions, as well as defining some remaining unanswered questions.
The most abundant mineral produced in the wood and leaves of trees is calcium oxalate monohydrate (whewellite), and after burning the wood the ash obtained is calcite. In the case of the Tamarix sp. ...tree, the freshly prepared ash is calcium sulfate (anhydrite). The aim of this study is to determine the calcium sulfate mineral phase in the fresh wood of Tamarix aphylla prior to burning. SEM images of the crystals show that they express smooth faces, are about 5–15 microns in their longest dimensions and are located in the ray cells. Fourier transform infrared spectroscopy (FTIR) and Raman microspectroscopy of the crystals in the wood and after extraction, both showed that the crystals are composed of calcium sulfate hemihydrate (bassanite). As elemental analyses of the crystals showed that in addition to calcium and sulfur, around 20 atom percent of the cations are sodium and potassium, we also obtained an X-ray powder diffraction pattern. This pattern excluded the possibility that the crystals are composed of another related mineral, and confirmed that, indeed, the crystals in the T. aphylla wood are composed of calcium sulfate hemihydrate (bassanite).
Bones exposed on tropical savannah grasslands of Amboseli National Park, Kenya undergo extensive post-mortem alteration within 40 years. A combined analytical approach involving TEM microscopy, trace ...metal analysis, FTIR spectroscopy, and petrographic analysis has revealed a complex, dynamic diagenetic environment operating within exposed bones, driven by evaporative transport of soil water from the bone/soil interface to the upper exposed surface of the bone. This process results in extensive bone/soil-water interaction, and is responsible for increases in the concentrations of trace elements such as Ba and La of 100 – >1000% within 15 years. The maximum and mean size of bone crystallites increases with continued exposure. This change in mean crystallite length is correlated positively with increases in bone crystallinity, which in turn is associated with degradation of the bone protein. Microbial decomposition is rarely observed in the Amboseli bones, but where present resulted in severe dissolution–reprecipitation of bone mineral. Many bones showed extensive permineralization of the larger vascular spaces with calcite and barite and, to a lesser extent, crandallite. Permineralization of unburied bones may account for 95% reduction in macro (micron–millimeter scale) porosity in the bone within 2 years of death.
We produce a model for pre-burial diagenesis of bone in arid tropical environments that highlights extensive alteration of bone chemistry within 1–40 years post-mortem.
Abstract The components of the tooth–periodontal ligament (PDL)–alveolar bone complex act in a synergistic manner to dissipate the loads incurred during mastication. The complex incorporates a ...diverse array of structural features for this purpose. These include the non-mineralized and hence soft PDL that absorbs much of the initial loads. The internal structure of the tooth also includes soft interphases that essentially surround the dentine core. These interphases, although stiffer than the PDL, still are more compliant than the dentine core, and are thus key components that allow the tooth itself to deform and hence help dissipate the compressive loads. There is also direct evidence that even under moderate compressive loads, when the tooth moves in the alveolar bone socket, this movement is guided by specific locations where the tooth comes into contact with the bone surface. The combination of all these responses to load is that each tooth type appears to move and deform in a specific manner when loaded. Much, however, still remains to be learned about these three-dimensional responses to load and the factors that control them. Such an understanding will have major implications for dentistry, that include a better understanding of phenomena such as abfraction, the manner in which tooth implants function even in the absence of a PDL-like tissue and the implications to bone remodelling of the movements imposed during orthodontic interventions.
The periodontal ligament (PDL), a soft tissue connecting the tooth and the bone, is essential for tooth movement, bone remodeling and force dissipation. A collagenous network that connects the tooth ...root surface to the alveolar jaw bone is one of the major components of the PDL. The organization of the collagenous component and how it changes under load is still poorly understood. Here using a state-of-the-art custom-made loading apparatus and a humidified environment inside a microCT, we visualize the PDL collagenous network of a fresh rat molar in 3D at 1μm voxel size without any fixation or contrasting agents. We demonstrate that the PDL collagen network is organized in sheets. The spaces between sheets vary thus creating dense and sparse networks. Upon vertical loading, the sheets in both networks are stretched into well aligned arrays. The sparse network is located mainly in areas which undergo compressive loading as the tooth moves towards the bone, whereas the dense network functions mostly in tension as the tooth moves further from the bone. This new visualization method can be used to study other non-mineralized or partially mineralized tissues, and in particular those that are subjected to mechanical loads. The method will also be valuable for characterizing diseased tissues, as well as better understanding the phenotypic expressions of genetic mutants.
Crystallization pathways in bone Mahamid, Julia; Addadi, Lia; Weiner, Steve
Cells, tissues, organs,
01/2011, Letnik:
194, Številka:
2-4
Journal Article
Recenzirano
Many biomineralization processes involve the sequestering of ions by cells and their translocation through the cells to the final deposition site. In many invertebrate crystallization pathways the ...cells deposit an initial highly disordered mineral phase with intracellular vesicles, and this mineral is subsequently transported into the final deposition site outside the cell. As this initial mineral phase is metastable, it can easily dissolve or crystallize during sample preparation and examination. A cryogenic electron microscopy study of the forming fin bone of a zebra fish strain with continuously growing fins shows that the cells responsible for bone tissue formation do have mineral-bearing intracellular vesicles and that the mineral phase is a highly disordered calcium phosphate. We also show that globules of disordered calcium phosphate are present in the extracellular collageneous matrix and that they are not membrane bound. Close to the mineralization front these globules appear to penetrate into the collagen fibrils where they crystallize to form mature bone. This crystallization pathway is similar to pathways observed in invertebrates, and it differs from the matrix vesicle pathway documented for a variety of vertebrate mineralizing tissues as the extracellular mineral globules are not membrane bound.
The mollusc shell prismatic layer of Atrina rigida is composed of an assemblage of large and relatively perfect single calcite crystals, embedded in an organic matrix. A key to elucidating basic ...mechanisms of mineralization is understanding the structures of the matrix, the mineral and the relations between them. The matrix that envelopes each prism (the inter-prismatic matrix) is composed mainly of glycine-rich proteins, while the matrix inside each prism (intra-crystalline matrix) is composed of a network of chitin fibers. Prisms grow by deposition of mineral particles on the chitin fibers. The mineral particles are associated with highly acidic proteins from the Asprich family, which presumably stabilize an amorphous mineral precursor. We infer that once in contact with the already formed crystalline prism, the particles crystallize by epitaxial nucleation. In nacre, sheets of beta-chitin are interspaced by silk-like proteins in a hydrated gel-like state. beta-Chitin forms a scaffold onto which the acidic proteins are adsorbed. Some of these are organized into a crystal nucleation site, where nucleation of aragonite, supposedly from colloidal amorphous calcium carbonate particles, is induced. Comparing the mechanisms of growth of the nacreous and prismatic layers can help to understand the underlying strategies of formation of mineralized structures.
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