Crystalcollagen relationships in bone as observed in the electron microscope. III. Crystal and collagen morphology as a function of age

Journey of Mineral Precursors in Bone Mineralization: Evolution and Inspiration for Biomimetic Design

Bone mineralization is a ubiquitous process among vertebrates that involves a dynamic physical/chemical interplay between the organic and inorganic components of bone tissues. It is now well documented that carbonated apatite, an inorganic component of bone, is proceeded through transient amorphous mineral precursors that transforms into the crystalline mineral phase. Here, the evolution on mineral precursors from their sources to the terminus in the bone mineralization process is reviewed. How organisms tightly control each step of mineralization to drive the formation, stabilization, and phase transformation of amorphous mineral precursors in the right place, at the right time, and rate are highlighted. The paradigm shifts in biomineralization and biomaterial design strategies are intertwined, which promotes breakthroughs in biomineralization-inspired material. The design principles and implementation methods of mineral precursor-based biomaterials in bone graft materials such as implant coatings, bone cements, hydrogels, and nanoparticles are detailed in the present manuscript. The biologically controlled mineralization mechanisms will hold promise for overcoming the barriers to the application of biomineralization-inspired biomaterials.

Role of carboxylic organic molecules in interfibrillar collagen mineralization

Bone is a composite material made up of inorganic and organic counterparts. Most of the inorganic counterpart accounts for calcium phosphate (CaP) whereas the major organic part is composed of collagen. The interfibrillar mineralization of collagen is an important step in the biomineralization of bone and tooth. Studies have shown that synthetic CaP undergoes auto-transformation to apatite nanocrystals before entering the gap zone of collagen. Also, the synthetic amorphous calcium phosphate/collagen combination alone is not capable of initiating apatite nucleation rapidly. Therefore, it was understood that there is the presence of a nucleation catalyst obstructing the auto-transformation of CaP before entering the collagen gap zone and initiating rapid nucleation after entering the collagen gap zone. Therefore, studies were focused on finding the nucleation catalyst responsible for the regulation of interfibrillar collagen mineralization. Organic macromolecules and low-molecular-weight carboxylic compounds are predominantly present in the bone and tooth. These organic compounds can interact with both apatite and collagen. Adsorption of the organic compounds on the apatite nanocrystal governs the nucleation, crystal growth, lattice orientation, particle size, and distribution. Additionally, they prevent the auto-transformation of CaP into apatite before entering the interfibrillar compartment of the collagen fibril. Therefore, many carboxylic organic compounds have been utilized in developing CaP. In this review, we have covered different carboxylate organic compounds governing collagen interfibrillar mineralization.

A Brief History of Biomineralization Studies

Biomineralization is the process by which living organisms produce minerals. Although the term is recent (∼1970), the study of internal and external skeleton mineralization is older. This article describes the history of biomineralization studies. This story is strongly dependent on, but not only on, the history of analytical technique development. Events are chronologically described to easily track progress and connections between people. The background of the people who contributed to the progress is also briefly described.

Synthetic amorphous calcium phosphates (ACPs): preparation, structure, properties, and biomedical applications

Amorphous calcium phosphates (ACPs) represent a metastable amorphous state of other calcium orthophosphates (abbreviated as CaPO₄) possessing variable compositional but rather identical glass-like physical properties, in which there are neither translational nor orientational long-range orders of the atomic positions. In nature, ACPs of a biological origin are found in the calcified tissues of mammals, some parts of primitive organisms, as well as in the mammalian milk. Manmade ACPs can be synthesized in a laboratory by various methods including wet-chemical precipitation, in which they are the first solid phases, precipitated after a rapid mixing of aqueous solutions containing dissolved ions of Ca²⁺ and PO₄^3- in sufficient amounts. Due to the amorphous nature, all types of synthetic ACPs appear to be thermodynamically unstable and, unless stored in dry conditions or doped by stabilizers, they tend to transform spontaneously to crystalline CaPO₄, mainly to ones with an apatitic structure. This intrinsic metastability of the ACPs is of a great biological relevance. In particular, the initiating role that metastable ACPs play in matrix vesicle biomineralization raises their importance from a mere laboratory curiosity to that of a reasonable key intermediate in skeletal calcifications. In addition, synthetic ACPs appear to be very promising biomaterials both for manufacturing artificial bone grafts and for dental applications. In this review, the current knowledge on the occurrence, structural design, chemical composition, preparation, properties, and biomedical applications of the synthetic ACPs have been summarized.

The importance of being amorphous: calcium and magnesium phosphates in the human body

This article focuses on the relevance of amorphous calcium (and magnesium) phosphates in living organisms. Although crystalline calcium phosphate (CaP)-based materials are known to constitute the major inorganic constituents of human hard tissues, amorphous CaP-based structures, often in combination with magnesium, are frequently employed by Nature to build up components of our body and guarantee their proper functioning. After a brief description of amorphous calcium phosphate (ACP) formation mechanism and structure, this paper is focused on the stabilization strategies that can be used to enhance the lifetime of the poorly stable amorphous phase. The various locations of our body in which ACP (pure or in combination with Mg²⁺) can be found (i.e. bone, enamel, small intestine, calciprotein particles and casein micelles) are highlighted, showing how the amorphous nature of ACP is often of paramount importance for the achievement of a specific physiological function. The last section is devoted to ACP-based biomaterials, focusing on how these materials differ from their crystalline counterparts in terms of biological response.

First evidence of octacalcium phosphate@osteocalcin nanocomplex as skeletal bone component directing collagen triple-helix nanofibril mineralization

Tibia trabeculae and vertebrae of rats as well as human femur were investigated by high-resolution TEM at the atomic scale in order to reveal snapshots of the morphogenetic processes of local bone ultrastructure formation. By taking into account reflections of hydroxyapatite for Fourier filtering the appearance of individual alpha-chains within the triple-helix clearly shows that bone bears the feature of an intergrowth composite structure extending from the atomic to the nanoscale, thus representing a molecular composite of collagen and apatite. Careful Fourier analysis reveals that the non-collagenous protein osteocalcin is present directly combined with octacalcium phosphate. Besides single spherical specimen of about 2 nm in diameter, osteocalcin is spread between and over collagen fibrils and is often observed as pearl necklace strings. In high-resolution TEM, the three binding sites of the γ-carboxylated glutamic acid groups of the mineralized osteocalcin were successfully imaged, which provide the chemical binding to octacalcium phosphate. Osteocalcin is attached to the collagen structure and interacts with the Ca-sites on the (100) dominated hydroxyapatite platelets with Ca-Ca distances of about 9.5 Å. Thus, osteocalcin takes on the functions of Ca-ion transport and suppression of hydroxyapatite expansion.

Amorphous surface layer versus transient amorphous precursor phase in bone - A case study investigated by solid-state NMR spectroscopy

The presence of an amorphous surface layer that coats a crystalline core has been proposed for many biominerals, including bone mineral. In parallel, transient amorphous precursor phases have been proposed in various biomineralization processes, including bone biomineralization. Here we propose a methodology to investigate the origin of these amorphous environments taking the bone tissue as a key example. This study relies on the investigation of a bone tissue sample and its comparison with synthetic calcium phosphate samples, including a stoichiometric apatite, an amorphous calcium phosphate sample, and two different biomimetic apatites. To reveal if the amorphous environments in bone originate from an amorphous surface layer or a transient amorphous precursor phase, a combined solid-state nuclear magnetic resonance (NMR) experiment has been used. The latter consists of a double cross polarization ¹H→³¹P→¹H pulse sequence followed by a ¹H magnetization exchange pulse sequence. The presence of an amorphous surface layer has been investigated through the study of the biomimetic apatites; while the presence of a transient amorphous precursor phase in the form of amorphous calcium phosphate particles has been mimicked with the help of a physical mixture of stoichiometric apatite and amorphous calcium phosphate. The NMR results show that the amorphous and the crystalline environments detected in our bone tissue sample belong to the same particle. The presence of an amorphous surface layer that coats the apatitic core of bone apatite particles has been unambiguously confirmed, and it is certain that this amorphous surface layer has strong implication on bone tissue biogenesis and regeneration.

STATEMENT OF SIGNIFICANCE

Questions still persist on the structural organization of bone and biomimetic apatites. The existing model proposes a core/shell structure, with an amorphous surface layer coating a crystalline bulk. The accuracy of this model is still debated because amorphous calcium phosphate (ACP) environments could also arise from a transient amorphous precursor phase of apatite. Here, we provide an NMR spectroscopy methodology to reveal the origin of these ACP environments in bone mineral or in biomimetic apatite. The ¹H magnetization exchange between protons arising from amorphous and crystalline domains shows unambiguously that an ACP layer coats the apatitic crystalline core of bone et biomimetic apatite platelets.

The use of physiological solutions or media in calcium phosphate synthesis and processing

This review examined the literature to spot uses, if any, of physiological solutions/media for the in situ synthesis of calcium phosphates (CaP) under processing conditions (i.e. temperature, pH, concentration of inorganic ions present in media) mimicking those prevalent in the human hard tissue environments. There happens to be a variety of aqueous solutions or media developed for different purposes; sometimes they have been named as physiological saline, isotonic solution, cell culture solution, metastable CaP solution, supersaturated calcification solution, simulated body fluid or even dialysate solution (for dialysis patients). Most of the time such solutions were not used as the aqueous medium to perform the biomimetic synthesis of calcium phosphates, and their use was usually limited to the in vitro testing of synthetic biomaterials. This review illustrates that only a limited number of research studies used physiological solutions or media such as Earle's balanced salt solution, Bachra et al. solutions or Tris-buffered simulated body fluid solution containing 27mM HCO3(-) for synthesizing CaP, and these studies have consistently reported the formation of X-ray-amorphous CaP nanopowders instead of Ap-CaP or stoichiometric hydroxyapatite (HA, Ca10(PO4)6(OH)2) at 37°C and pH 7.4. By relying on the published articles, this review highlights the significance of the use of aqueous solutions containing 0.8-1.5 mMMg(2+), 22-27mM HCO3(-), 142-145mM Na(+), 5-5.8mM K(+), 103-133mM Cl(-), 1.8-3.75mM Ca(2+), and 0.8-1.67mM HPO4(2-), which essentially mimic the composition and the overall ionic strength of the human extracellular fluid (ECF), in forming the nanospheres of X-ray-amorphous CaP.

Calcium orthophosphates and human beings: a historical perspective from the 1770s until 1940

The historical development of a scientific knowledge on calcium orthophosphates from the 1770s until 1940 is described. Many forgotten and poorly known historical facts and approaches have been extracted from old publications and then they have been analyzed, systematized and reconsidered from the modern point of view. The chosen time scale starts with the earliest available studies of 1770s (to the best of my findings, calcium orthophosphates had been unknown before), passes through the entire 19th century and finishes in 1940, because since then the amount of publications on calcium orthophosphates rapidly increases and the subject becomes too broad. Furthermore, since publications of the second half of the 20th century are easily accessible, a substantial amount of them have already been reviewed by other researchers. The reported historical findings clearly demonstrate that the substantial amount of the scientific facts and experimental approaches have been known for very many decades and, in fact, the considerable quantity of relatively recent investigations on calcium orthophosphates is just either a further development of the earlier studies or a rediscovery of the already forgotten knowledge.

Amorphous Calcium Phosphates

Amorphous calcium phosphates (ACPs) represent a unique class of biomedically relevant calcium orthophosphate salts, in which there are neither translational nor orientational long-range orders of the atomic positions. Nevertheless, the constancy in their chemical composition over a relatively wide range of preparation conditions suggests the presence of a well-defined local structural unit, presumably, with the structure of Ca9(PO4)6 – so-called Posner’s cluster. ACPs have variable chemical but rather identical glass-like physicochemical properties. Furthermore, all ACPs are thermodynamically unstable compounds and, unless stored in dry conditions or doped by stabilizers, spontaneously they tend to transform to crystalline calcium orthophosphates. Although some order within general disorder is the most distinguishing feature of ACPs, the solution instability of ACPs and their easy transformation to crystalline phases might be of a great biological relevance. Namely, the initiating role ACPs play in matrix vesicle biomineralization raises the importance of this phase from a mere laboratory curiosity to that of a key intermediate in skeletal calcification. Furthermore, ACPs are very promising candidates to manufacture artificial bone grafts.

THE FINE STRUCTURE OF BONE CELLS

An electron microscopic study of Araldite-embedded, undecalcified human woven and chick lamellar bone is presented. The fine structure of the cells of bone in their normal milieu is described. Active osteoblasts possess abundant granular endoplasmic reticulum, numerous small vesicles, and a few secretion droplets. Their long cytoplasmic processes penetrate the osteoid. The transition of osteoblasts into osteoid osteocytes and then into osteocytes is traced and found to involve a progressive reduction of cytoplasmic organelles. Adjoining the osteocytes and their processes is a layer of amorphous material which is interposed between the cell surfaces and the bone walls of their respective cavities. Osteoclasts contain numerous non-membrane-associated ribosomes, abundant mitochondria, and little granular endoplasmic reticulum, thus differing markedly from other bone cells. The brush border is a complex of cytoplasmic processes adjacent to a resorption zone in bone. No unmineralized collagen is seen at resorption sites and it appears that collagen is removed before or at the time of mineral solution. All bone surfaces are covered by cells, some of which lack distinctive qualities and are designated endosteal lining cells. The structure of osteoid, bone, and early mineralization sites is illustrated and discussed.

A new rhenanite (β-NaCaPO4) and hydroxyapatite biphasic biomaterial for skeletal repair

Biphasic beta-rhenanite (beta-NaCaPO(4))-hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2)) biomaterials were prepared by using a one-pot, solution-based synthesis procedure at the physiological pH of 7.4, followed by low-temperature (300-600 degrees C) calcination in air for 6 h. Calcination was for the sole purpose of crystallization. An aqueous solution of Ca(NO(3))(2). 4H(2)O was rapidly added to a solution of Na(2)HPO(4) and NaHCO(3), followed by immediate removal of gel-like, poorly-crystallized precursor precipitates from the mother liquors of pH 7.4. Freeze-dried precursors were found to be nanosize with an average particle size of 45 nm and a surface area of 128 m(2)/g. Upon calcination in air, precursor powders crystallized into biphasic (60% HA-40% rhenanite) biomaterials, while retaining their submicron particle sizes and high surface areas. beta-rhenanite is a high solubility sodium calcium phosphate phase. Samples were characterized by XRD, FTIR, SEM, TEM, ICP-AES, TG, DTA, DSC, and surface area measurements.

Electron microscopic observations on the three-dimensional morphology of apatite crystallites of human dentine and bone

In sections of human dentine (carious and sound) and bone examined with the electron microscope, apatite crystallites were seen to present long thin profiles somewhat suggestive of a cylindrical shape, broad profiles indicative of a plate-like shape, and profiles intermediate between these two extremes. With a special stereoscopic specimen holder allowing the specimen to be tilted through an angle of 30 degrees it was possible to record images of two profiles of the same crystallite from different angles and thus gain information concerning the 3-dimensional morphology of crystallites showing a thin profile. In all fields so examined, the thin-profile crystallites that were properly oriented with respect to the axis of tilt exhibited a different width dimension in each of the two micrographs. From this it is concluded that the thin profiles actually represented edge views of plate-like crystallites.

Age and temperature related changes to the ultrastructure and composition of human bone mineral

This X-ray diffraction (XRD) investigation of heat-treated human femoral bone showed that the main mineral phase of both unheated bone and bone heated to 600 degrees C resembled that of a poorly crystalline form of hydroxyapatite. The rod-shaped apatite crystals in unheated bone persisted in bone heated up to 400 degrees C. Recrystallization at approximately 600 degrees C, produced larger crystals, which either retained their original morphology or changed to tabular or equidimensional shapes. The size of the apatite crystals in unheated and heated bone specimens was dependent on both temperature and age. When heated above 600 degrees C the crystallinity of the bone mineral increased, and the XRD pattern more closely resembled that of hydroxyapatite. Partial decomposition of the hydroxyapatite phase to calcium oxide above 1000 degrees C, and beta-tricalcium phosphate, alpha-tricalcium phosphate, and calcium oxide phosphate between 1200 degrees C and 1400 degrees C, indicated that the original apatite phase was both calcium deficient and contained carbonate. The relative peak intensities of the thermal decomposition products were related to some extent to the age of the deceased person and reflected the compositional changes that occur during bone aging. Because the thermally induced changes to the composition and ultrastructure of bone mineral were influenced by the age of the individual, this investigation proposed that the heat treatment of bone tissue may offer an alternative way of studying bone aging.

Post-translational control of collagen fibrillogenesis in mineralizing cultures of chick osteoblasts

Cultured osteoblasts from chick embryo calvaria were used as a model system to investigate the post-translational extracellular mechanisms controlling the macroassembly of collagen fibrils. The results of these studies demonstrated that cultured osteoblasts secreted a collagenous extracellular matrix that assembled and mineralized in a defined temporal and spatial sequence. The assembly of collagen occurred in a polarized fashion, such that successive orthogonal arrays of fibrils formed between successive cell layers proceeding from the culture surface toward the media. Mineralization followed in the same manner, being observed first in the deepest and oldest fibril layers. Collagen fibrillogenesis, the kinetics of cross-link formation, and collagen stability in the extracellular matrix of the cultures were examined over a 30 day culture period. Between days 8 and 12 in culture, collagen fibril diameters increased from < 30 nm to an average of 30-45 nm. Thereafter, diameters ranged in size from 20 to 200 nm. Quantitation of the collagen cross-linking residues, hydroxylysyl pyridinoline (HP) and lysyl pyridinoline (LP), showed that these mature cross-links increased from undetectable levels to concentrations found in normal chick bone. Analysis of the kinetics of their formation by pulse-chase labeling the cultures with [3H]lysine showed a doubling time of approximately 5 days. The relationships between cross-link formation, fibrillogenesis, and collagen stability were examined in cultures treated with beta-aminopropionitrile (beta-APN), a potent inhibitor of lysyl oxidase and cross-link formation. In beta-APN-treated cultures, total collagen synthesis was increased twofold, with no change in mRNA levels for type I collagen, whereas the amount of collagen accumulated in the cell layer was decreased by 50% and mineral deposition was reduced. The rate of collagen retention in the matrix was assessed by pulse-chase analysis of [3H]proline over a 16 day period in control and beta-APN-treated cultures. In control cultures, about 20% of the labeled collagen was lost from the cell layers over a 16 day period compared with > 80% in the presence of beta-APN. The beta-APN-treated cultures also showed a wider diversity of fibril diameters with a median in the > 45-60 nm range. In summary, these data suggest that cross-linking and assembly of collagen fibrils secreted by osteoblasts in vitro occur in a fashion similar to that found in vivo. The rate of cross-link formation is relatively constant and may be correlated with increasing collagen mass.(ABSTRACT TRUNCATED AT 400 WORDS)

Cross-linking connectivity in bone collagen fibrils: the COOH-terminal locus of free aldehyde

Quantitative analyses of the chemical state of the 16c residue of the alpha 1 chain of bone collagen were performed on samples from fetal (4-6-month embryo) and mature (2-3 year old) bovine animals. All of this residue could be accounted for in terms of three chemical states, in relative amounts which depended upon the age of the animal. Most of the residue was incorporated into either bifunctional or trifunctional cross-links. Some of it, however, was present as free aldehyde, and the content increased with maturation. This was established by isolating and characterizing the aldehyde-containing peptides generated by tryptic digestion of NaB3H4-reduced mature bone collagen. We have concluded that the connectivity of COOH-terminal cross-linking in bone collagen fibrils changes with maturation in the following way: at first, each 16c residue in each of the two alpha 1 chains of the collagen molecule is incorporated into a sheet-like pattern of intermolecular iminium cross-links, which stabilizes the young, nonmineralized fibril as a whole. In time, some of these labile cross-links maturate into pyridinoline while others dissociate back to their precursor form. The latter is likely due to changes in the molecular packing brought about by the mineralization of the collagen fibrils. The resultant reduction in cross-linking connectivity may provide a mechanism for enhancing certain mechanical characteristics of the skeleton of a mature animal.

A contribution with review to the description of mineralization of bone and other calcified tissues in vivo

This manuscript considers certain aspects of mineral deposition in bone and other vertebrate calcifying tissues in order to examine physical, chemical, and biological factors important in the mineralization process. The paper in a discussion format principally presents a new data and the formulation of concepts based on such data as well as a summary of background material as necessary review. Mineralization is found to occur at spatially independent sites throughout the organic extracellular tissue matrices. Matrix vesicles and collagen fibrils each may serve as independent nucleation centers for mineral with vesicle mineralization being local and collagen mineralization dominating the tissues as a whole. Collagen fibril organization is suggested to be such that hole zones are aligned in three dimensions, creating extensive channels for mineral accommodation. Nucleation occurs initially in hole zones and crystal growth leads to the development of plate-like mineral particles whose orientation, disposition, and sizes within fibrils are detailed. Effects of diffusion, crystallinity, and critical nucleation and growth events are described with respect to their influence on mineral deposition in bulk and local regions of tissue matrices.

Three-dimensional spatial relationship between the collagen fibrils and the inorganic calcium phosphate crystals of pickerel (Americanus americanus) and herring (Clupea harengus) bone

High-voltage (1.0 MV) electron microscopy and stereomicroscopy, electron probe microanalysis, electron diffraction and three-dimensional computer reconstruction, have been used to examine the spatial relationship between the inorganic crystals of calcium phosphate and the collagen fibrils of pickerel and herring bone. High-voltage stereo electron-micrographs were obtained of cross-sections of the cylinder-shaped intramuscular bones in uncalcified regions, in regions where only one or only several crystals had been deposited in some of the fibrils, and in successive sections containing progressively more mineral crystals until the stage of full mineralization was reached. High-resolution electron probe microanalysis confirmed that the electron-dense particles contained calcium and phosphorus. In the earliest stages of mineralization and progressing throughout the mineralization process, the crystals are located only within the collagen fibrils; crystals are not observed free in the extracellular spaces between collagen fibrils. The progressive increase in the mass of mineral deposited in the bone tissue with time occurs, essentially, completely within the collagen fibrils including the stage of full mineralization. At this stage, cross-sectional profiles of collagen fibrils are completely obliterated by mineral. A small number of crystals that are located on or close to the surface of the fibrils appear to extend a very short distance into the spaces between the fibrils. These ultrastructural observations of the very onset of calcification in which nucleation of the calcium phosphate crystals is clearly shown to begin within specific volumes of collagen fibrils, and of the subsequent temporal and spatial sequences of this phenomenon, which shows that calcification continues wholly within the collagen fibrils until maximum calcification is achieved, add important information on the basic physical chemical mechanism of the calcification and the structural elements that are involved. The spatial and temporal independence of the sites where mineralization is initiated establishes that such ultrastructural locations within individual collagen fibrils represent independent, physical chemical nucleation loci. The findings are totally inconsistent with the proposal that crystals must first be deposited in matrix vesicles, or other components such as mitochondria, and subsequently released and propagated in the interfibrillar space, until they eventually reach and impregnate the hole zone regions of the collagen fibrils. Three-dimensional computer reconstruction of serial transverse and longitudinal sections demonstrates periodic swellings along the collagen fibrils, corresponding to the hole zone region of their axial period as mineralization proceeds.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic imaging of mineral and collagen in the calcifying turkey tendon

Topographic imaging, a method of providing a direct view of ultrastructure in three dimensions, has been newly applied to a study of mineral crystals and collagen from calcifying turkey leg tendon. Individual crystals obtained from intact tendon were observed as thin platelets of irregular shape having a relatively smooth surface. Mineralized collagen fibrils isolated singly or examined in thin tissue sections were found to exhibit the characteristic 64-70 nm period and were associated with platelets and needle-like mineral. The crystals were disposed in numerous fashions, notably as small groups of platelets within individual collagen hole zones, as a number of needle-like densities arranged parallel to one another, or as a combination of platelets and needles over entire stretches of single collagen fibrils. The topographic observations of crystals and crystal-collagen interaction clearly demonstrate the plate-like habit of the mineral in calcifying turkey tendon and suggest that these crystals are located both within and on the surface of collagen fibrils. In certain sites such as the collagen hole zones, the crystals appear organized in a specific manner, possibly with a preferred c-axial orientation. Crystals of hydroxyapatite prepared in vitro and examined topographically are similar in habit and texture to the crystals from tendon. When interpretation of this method is corroborated by other independent microscopic techniques, topographic imaging has widespread potential application in many fields of study in which structural surface features of biological tissues or non-biological materials are of interest at the electron microscope level.

Organization and development of the mineral phase during early ontogenesis of the bony fin rays of the trout Oncorhynchus mykiss

Characterization of mineral deposition has been studied by electron optical methods during early ontogenesis of lepidotrichia, the bony fin rays, of the trout Oncorhynchus mykiss (the former Salmo gairdneri). The fin rays consist of an extracellular granular ground substance containing in part a network of collagen fibrils within the basal lamella of the fin dermoepidermal interface. Growth of individual rays proceeds in a proximodistal direction. The mineral phase appears as electron-dense needle or plate-like particles and is associated with the collagenous matrix. On analysis of progressively maturing tissue, the mineral was characterized as a poorly crystalline hydroxyapatite with Ca/P molar ratios in the range of 1.0-1.4, corresponding to distal and proximal areas, respectively. With selected-area electron diffraction and dark field imaging of lepidotrichia, the mineral particles were found to be about 3-10 nm thick and 12-20 nm in length (along their crystallographic c-axes), possibly aggregated into larger crystals 35-40 nm long observed with bright field microscopy. No definitive relation was found between either the c- or a,b-axes images of the crystals and the periodic structure of collagen, which forms the framework for mineral deposition in this and in other vertebrate calcifying tissues.

FT-IR microscopy of endochondral ossification at 20 mu spatial resolution

A Fourier transform infrared spectrometer has been coupled with an optical microscope to study the distribution and characteristics of the mineral phase in calcifying tissues at 20 mu spatial resolution. This represents the first biophysical application of this technique. High quality spectra were obtained in a relatively short scan time (1-2 minutes) from thin longitudinal sections of normal and rachitic rat femurs. Substantial spatial variations in the extent and structure of the mineral phase were observed as a function of spatial position both within and beyond the growth plates, as judged by the phosphate vibrations in the 900-1200 cm-1 spectral region. The current experiments reveal the utility of FT-IR microscopy in identification of sites where mineralization has occurred. In addition to vibrations from the inorganic components, the Amide I and Amide II motions of the protein constituents are readily observed and may be useful as a probe of protein/mineral interactions.

Transmission electron microscopy of lattice planes in human alveolar bone apatite crystals

Periodic fringes corresponding to six different lattice planes have been observed in apatite crystals of human normal alveolar bone by transmission electron microscopy. Three of these sets of fringes have spacings less than 3.5 A corresponding to the Scherzer resolution of the microscope used. The (0002) lattice plane of hydroxyapatite of 3.4 A d-spacings, the (2111) lattice plane with a d-spacing of 2.81 A, and the (3030) lattice plane with a d-spacing of 2.72 A have been identified. The (0002) and (2121) lattice planes have been observed for the first time in bone microcrystals. Some of the crystals studied were characterized by a mean width/thickness ratio of 6.91, typical of platelike habit, whereas observations of crystals aligned along the (1210) and (1211) directions showed a needlelike habit. The mean length of the bone apatite crystals was 470 A. A dark line similar to the one observed in enamel and dentine crystals was also seen. The bone microcrystals observed have shown a high sensitivity to beam damage.

Dolomite as a possible magnesium-containing phase in human tooth enamel

A solid-state chemical model is derived to estimate the solubility of dolomites having different degrees of cation ordering. It is shown that the solubility of dolomites formed by precipitation at ordinary temperatures is such that it allows for the formation of dolomite in tooth enamel during in vivo mineralization.

Determination of mineral-organic bonding effectiveness in bone--theoretical considerations

It is postulated that the effectiveness of bonding between the mineral and organic phases could be an important influence on the behavior of bone with respect to its mechanical properties, metabolic activity, and aging effects associated with these factors. Changes in bonding effectiveness might also be related to the etiology of osteoporosis. If this hypothesis is correct, it would be of interest to determine the amount of debonding present in bone. An analysis that employs both macromechanical and micromechanical composite theory is performed to show how this quantity could be calculated. The approach taken is first to determine the elastic moduli of a characteristic volume from bulk elastic properties of bone and the mineral crystallite orientation distribution. Voigt and Reuss type averages are used to obtain upper and lower bounds. Modifications of the Halpin-Tsai equations that apply to chopped fiber composites are then used to calculate the amount of debonding between the phases in the characteristic volume. All of the parameters employed in the theory are measurable using established techniques. To apply the theory quantitatively the following information must be known: 1) the density and elastic moduli of the bone (and its phases), and 2) the mineral orientation distribution.

Amorphous calcium phosphate in casein micelles of bovine milk

The calcium phosphate remaining after hydrazine deproteination of casein micelles isolated from bulk skim milk exhibits under the electron microscope a very fine and uniform granularity being formed by small subunits with a true diameter of approximately 2.5 nm. This material, which is about 10 percent by weight citrate, termed calcium phosphate citrate (CPC) complex, also contains Mg and Zn at molar ratios of 0.03 and 0.003 respectively. Radial distribution function (RDF) and infrared analyses show that CPC is a Mg-containing amorphous calcium phosphate (ACP) similar to synthetic and cytoplasmic ACP. presence of CPC in casein micelles as an amorphous colloid bonded with phosphoproteins provides the means for storing in milk large amounts of Ca (16 mM) and Pi (10 mM) in a readily utilizable form but at a higher ion concentration than found in biological solutions.