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

Investigating the interfaces of the epiphyseal plate: An integrated approach of histochemistry, microtomography and SEM

We investigated the interfaces of the epiphyseal plate with over- and underlying bone segments using an integrated approach of histochemistry, microtomography and scanning electron microscopy (SEM) to overcome the inherent limitations of sections-based techniques. Microtomography was able to provide an unobstructed, frontal view of large expanses of the two bone surfaces facing the growth plate, while SEM observation after removal of the soft matrix granted an equally unhindered access with a higher resolution. The two interfaces appeared widely dissimilar. On the diaphyseal side the hypertrophic chondrocytes were arranged in tall columns packed in a sort of compact palisade; the interposed extracellular matrix was actively calcifying into a thick mineralized crust growing towards the epiphysis. Behind the mineralization front, histochemical data revealed a number of surviving cartilage islets which were being slowly remodelled into bone. In contrast, the epiphyseal side of the cartilage consisted of a relatively quiescent reserve zone whose mineralization was marginal in amount and discontinuous in extension; the epiphyseal bone consisted of a loose trabecular meshwork, with ample vascular spaces opening directly into the non-mineralized cartilage. On both sides the calcification process took place through the formation of spheroidal bodies 1-2 μm wide which gradually grew by apposition and coalesced into a solid mass, in a way distinctly different from that of bone and other calcified tissues.

The chondro-osseous junction of articular cartilage

In the synovial joints the transition between the soft articular cartilage and the subchondral bone is mediated by a layer of calcified cartilage of structural and mechanical characteristics closer to those of bone. This layer, buried in the depth of articular cartilage, is not directly accessible and is mostly visualized in histological sections of decalcified tissue, where it appears as a darker strip in contact with the subchondral bone. In this study conventional histology and scanning electron microscopy (SEM) with secondary electron imaging (SE) or backscattered electron imaging (BSE) were used to discriminate the calcified and the uncalcified cartilage in high resolution on native, untreated tissue as well as in deproteinated or demineralized tissue. This approach evidenced a high heterogeneity of the calcified layer of articular cartilage. High resolution pictures revealed that the mineralization process originates by progressive accretion and confluence of individual, small mineral clusters, in a very different way from other hard tissues such as bone, dentin and mineralized tendons. Finally, selective removal of the soft matrix by thermal treatment allowed for the first time a face-on, unrestricted 3D view of the mineralization front.

In Vitro Mineralization of Collagen

Collagen mineralization is a biological process in many skeletal elements in the animal kingdom. Examples of these collagen-based skeletons are the siliceous spicules of glass sponges or the intrafibrillar hydroxyapatite platelets in vertebrates. The mineralization of collagen in vitro has gained interest for two reasons: understanding the processes behind bone formation and the synthesis of scaffolds for tissue engineering. In this paper, the efforts toward collagen mineralization in vitro are reviewed. First, general introduction toward collagen type I, the main component of the extracellular matrix in animals, is provided, followed by a brief overview of collagenous skeletons. Then, the in vitro mineralization of collagen is critically reviewed. Due to their biological abundance, hydroxyapatite and silica are the focus of this review. To a much lesser extent, also some efforts with other minerals are outlined. Combining all minerals and the suggested mechanisms for each mineral, a general mechanism for the intrafibrillar mineralization of collagen is proposed. This review concludes with an outlook for further improvement of collagen-based tissue engineering scaffolds.

Intermolecular channels direct crystal orientation in mineralized collagen

The mineralized collagen fibril is the basic building block of bone, and is commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ~2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO₃ and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril.

Biological stenciling of mineralization in the skeleton: Local enzymatic removal of inhibitors in the extracellular matrix

Biomineralization is remarkably diverse and provides myriad functions across many organismal systems. Biomineralization processes typically produce hardened, hierarchically organized structures usually having nanostructured mineral assemblies that are formed through inorganic-organic (usually protein) interactions. Calcium‑carbonate biomineral predominates in structures of small invertebrate organisms abundant in marine environments, particularly in shells (remarkably it is also found in the inner ear otoconia of vertebrates), whereas calcium-phosphate biomineral predominates in the skeletons and dentitions of both marine and terrestrial vertebrates, including humans. Reconciliation of the interplay between organic moieties and inorganic crystals in bones and teeth is a cornerstone of biomineralization research. Key molecular determinants of skeletal and dental mineralization have been identified in health and disease, and in pathologic ectopic calcification, ranging from small molecules such as pyrophosphate, to small membrane-bounded matrix vesicles shed from cells, and to noncollagenous extracellular matrix proteins such as osteopontin and their derived bioactive peptides. Beyond partly knowing the regulatory role of the direct actions of inhibitors on vertebrate mineralization, more recently the importance of their enzymatic removal from the extracellular matrix has become increasingly understood. Great progress has been made in deciphering the relationship between mineralization inhibitors and the enzymes that degrade them, and how adverse changes in this physiologic pathway (such as gene mutations causing disease) result in mineralization defects. Two examples of this are rare skeletal diseases having osteomalacia/odontomalacia (soft bones and teeth) - namely hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH) - where inactivating mutations occur in the gene for the enzymes tissue-nonspecific alkaline phosphatase (TNAP, TNSALP, ALPL) and phosphate-regulating endopeptidase homolog X-linked (PHEX), respectively. Here, we review and provide a concept for how existing and new information now comes together to describe the dual nature of regulation of mineralization - through systemic mineral ion homeostasis involving circulating factors, coupled with molecular determinants operating at the local level in the extracellular matrix. For the local mineralization events in the extracellular matrix, we present a focused concept in skeletal mineralization biology called the Stenciling Principle - a principle (building upon seminal work by Neuman and Fleisch) describing how the action of enzymes to remove tissue-resident inhibitors defines with precision the location and progression of mineralization.

Correlations between gene expression and mineralization in the avian leg tendon

Certain avian tendons have been studied previously as a model system for normal mineralization of vertebrates in general. In this regard, the gastrocnemius tendon in the legs of turkeys mineralizes in a well defined temporal and spatial manner such that changes in the initial and subsequent events of mineral formation can be associated with time and specific locations in the tissue. In the present investigation, these parameters and mineral deposition have been correlated with the expression of several genes and the synthesis and secretion of their related extracellular matrix proteins by the composite tenocytes of the tendon. Quantitative polymerase chain reaction analysis demonstrates that mRNA expression of the non-collagenous genes of bone sialoprotein, osteopontin, and osteocalcin corresponds well with the temporal and spatial onset and progression of mineralization. Immunolocalization separately confirms the synthesis and secretion of these matrix molecules. The expression of other non-collagenous genes such as decorin does not show strong correlation with turkey leg tendon mineralization, and expression of vimentin, a cytoskeletal component which may be regulated by biomechanical factors in the tendon, may lead to inhibition of osteocalcin expression during the development and mineralization of the tissue. The overall results of this work provide insight into direct temporal and spatial relations between the genes and proteins of interest as well as the formation and deposition of mineral in the avian tendon model.

Study of Endochondral Ossification in Human Fetalcartilage Anlagen of Metacarpals: Comparative Morphology of Mineral Deposition in Cartilage and in the Periosteal Bone Matrix

The progression of mineral phase deposition in hypertrophic cartilage and periosteal bone matrix was studied in human metacarpals primary ossification centers before vascular invasion began. This study aimed to provide a morphologic/morphometric comparative analysis of the calcification process in cartilage and periosteal osteoid used as models of endochondral ossification. Thin, sequential sections from the same paraffin inclusions of metacarpal anlagen (gestational age between the 20th and 22nd weeks) were examined with light microscopy and scanning electron microscopy, either stained or heat-deproteinated. This process enabled the analysis of corresponding fields using the different methods. From the initial CaPO₄ nucleation in cartilage matrix, calcification progressed increasing the size of focal, globular, randomly distributed deposits (size range 0.5-5 µm), followed by aggregation into polycyclic clusters and finally forming a dense, compact mass of calcified cartilage. At the same time, the early osteoid calcification was characterized by a fine granular pattern (size range 0.1-0.5 µm), which was soon compacted in the layer of the first periosteal lamella. Scanning electron microscopy of heat-deproteinated sections revealed a rod-like hydroxyapatite crystallite pattern, with only size differences between the early globular deposits of the two calcifying matrices. The morphology of the early calcium deposits was similar in both cartilage and osteoid, with variations in size and density only. However, integration of the reported data with the actual hypotheses of the mechanisms of Ca concentration suggested that ion transport was linked to the progression of the chondrocyte maturation cycle (with recall of H₂ O from the matrix) in cartilage, while ions transport was an active process through the cell membrane in osteoid. Other considered factors were the collagen type specificity and the matrix fibrillar texture. Anat Rec, 301:571-580, 2018. © 2017 Wiley Periodicals, Inc.

In situ examination of osteoblast biomineralization on sulfonated polystyrene-modified substrates using Fourier transform infrared microspectroscopy

Osteoporosis is a skeletal disorder that is characterized by the loss of bone mineral density (BMD) resulting in increased risk of fracture. However, it has been shown that BMD is not the only indicator of fracture risk, as the strength of bone depends on a number of factors, including bone mass, architecture and material properties. Physiological mineral deposition requires the formation of a properly developed extracellular matrix (ECM), which recruits calcium and phosphate ions into the synthesis of apatite crystals. Temporal and spatial compositional and structural changes of biological apatite greatly depend on the properties of the crystals initially formed. As such, Fourier-transform infrared microspectroscopy (FTIRM) is capable of examining adaptive remodeling by providing compositional information such as the level of mineralization and carbonate substitution, as well as quality and perfection of the mineral phase. The objective of this study was to evaluate the in vitro mineralization development of MC3T3-E1 murine calvarial preosteoblasts cultured on different substrata by comparing FTIRM measurements from two subclones (mineralizing subclone 4 and nonmineralizing subclone 24) maintained in culture for up to 21 days. The results showed that modulation of the substrate surface using a thin coating of sulfonated polystyrene (SPS) provided favorable conditions for the development of a mineralizable ECM and that the mineral formed by the osteoblasts was similar to that of fully mineralized bone tissue. Specifically, the mineralizing subclone produced significantly more mineral phosphate when cultured on SPS-coated substrates for 21 days, compared to the same culture on bare substrates. In contrast, the level of mineralization in nonmineralizing subclone was low on both SPS-coated and uncoated substrates. The mineralizing subclone also produced comparable amounts of collagen on both substrates; however, mineralization was significantly higher in the SPS culture. The nonmineralizing subclone produced comparable amounts of collagen on day 1 but much less on day 21. Collagen maturity ratio increased in the mineralizing subclone from day 1 to day 21, but remained unchanged in the nonmineralizing subclone. These results suggest that SPS-treatment of the substrate surface may alter collagen remodeling; however, other factors may also influence osteoblast mineralization in the long term.

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

Experimental studies have shown a great potential for periodontal regeneration. The limitations of periodontal regeneration largely depend on the regenerative potential at the root surface. Cellular intrinsic fiber cementum (CIFC), so-called bone-like tissue, may form instead of the desired acellular extrinsic fiber cementum (AEFC), and the interfacial tissue bonding may be weak. The periodontal ligament harbors progenitor cells that can differentiate into periodontal ligament fibroblasts, osteoblasts, and cementoblasts, but their precise location is unknown. It is also not known whether osteoblasts and cementoblasts arise from a common precursor cell line, or whether distinct precursor cell lines exist. Thus, there is limited knowledge about how cell diversity evolves in the space between the developing root and the alveolar bone. This review supports the hypothesis that AEFC is a unique tissue, while CIFC and bone share some similarities. Morphologically, functionally, and biochemically, however, CIFC is distinctly different from any bone type. There are several lines of evidence to propose that cementoblasts that produce both AEFC and CIFC are unique phenotypes that are unrelated to osteoblasts. Cementum attachment protein appears to be cementum-specific, and the expression of two proteoglycans, fibromodulin and lumican, appears to be stronger in CIFC than in bone. A theory is presented that may help explain how cell diversity evolves in the periodontal ligament. It proposes that Hertwig’s epithelial root sheath and cells derived from it play an essential role in the development and maintenance of the periodontium. The role of enamel matrix proteins in cementoblast and osteoblast differentiation and their potential use for tissue engineering are discussed.

Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells

Bone tissue is continuously remodeled through the concerted actions of bone cells, which include bone resorption by osteoclasts and bone formation by osteoblasts, whereas osteocytes act as mechanosensors and orchestrators of the bone remodeling process. This process is under the control of local (e.g., growth factors and cytokines) and systemic (e.g., calcitonin and estrogens) factors that all together contribute for bone homeostasis. An imbalance between bone resorption and formation can result in bone diseases including osteoporosis. Recently, it has been recognized that, during bone remodeling, there are an intricate communication among bone cells. For instance, the coupling from bone resorption to bone formation is achieved by interaction between osteoclasts and osteoblasts. Moreover, osteocytes produce factors that influence osteoblast and osteoclast activities, whereas osteocyte apoptosis is followed by osteoclastic bone resorption. The increasing knowledge about the structure and functions of bone cells contributed to a better understanding of bone biology. It has been suggested that there is a complex communication between bone cells and other organs, indicating the dynamic nature of bone tissue. In this review, we discuss the current data about the structure and functions of bone cells and the factors that influence bone remodeling.

Refinement of collagen-mineral interaction: a possible role for osteocalcin in apatite crystal nucleation, growth and development

Mineralization of vertebrate tissues such as bone, dentin, cementum, and calcifying tendon involves type I collagen, which has been proposed as a template for calcium and phosphate ion binding and subsequent nucleation of apatite crystals. Type I collagen thereby has been suggested to be responsible for the deposition of apatite mineral without the need for non-collagenous proteins or other extracellular matrix molecules. Based on studies in vitro, non-collagenous proteins, including osteocalcin and bone sialoprotein, are thought to mediate vertebrate mineralization associated with type I collagen. These proteins, as possibly related to mineral deposition, have not been definitively localized in vivo. The present study has reexamined their localization in the leg tendons of avian turkeys, a representative model of vertebrate mineralization. Immunocytochemistry of osteocalcin demonstrates its presence at the surface of, outside and within type I collagen while that of bone sialoprotein appears to be localized at the surface of or outside type I collagen. The association between osteocalcin and type I collagen structure is revealed optimally when calcium ions are added to the antibody solution in the methodology. In this manner, osteocalcin is found specifically located along the a4-1, b1, c2 and d bands defining in part the hole and overlap zones within type I collagen. From these data, while type I collagen itself may be considered a stereochemical guide for intrafibrillar mineral nucleation and subsequent deposition, osteocalcin bound to type I collagen may also possibly mediate nucleation, growth and development of platelet-shaped apatite crystals. Bone sialoprotein and osteocalcin as well, each immunolocalized at the surface of or outside type I collagen, may affect mineral deposition in these portions of the avian tendon.

Annulus fibrosus cells can induce mineralization: an in vitro study

BACKGROUND CONTEXT

There is still no consensus as to whether the calcification observed in degenerate intervertebral discs (IVDs) is a cause or a consequence of disc degeneration.

PURPOSE

To investigate the mineralization potential of healthy (independent of other associated changes) annulus fibrosus (AF) cells under controlled in vitro conditions.

STUDY DESIGN/SETTING

In vitro study to investigate the mineralization potential of the AF cells.

METHODS

Annulus fibrosus cells, isolated from bovine IVDs, were grown in monolayer. The effect of cell density, culture time, age of cell source, and passage on the percentage of AF cells with alkaline phosphatase activity (ALPa) was evaluated. Gene expression of mineralization-associated markers was determined. Cells were immunostained for Type I, II, and X collagens. To study mineralization potential, AF cells and AF cells that were sorted into two populations, high (top 5% ± 1%) or low (bottom 5% ± 1%) ALPa expressors, were grown in the presence of β-glycerophosphate for 2 weeks.

RESULTS

The percentage of AF cells that express ALPa changes with time in culture and seeding density for primary immature and mature cell sources but not for passaged cells. Gene expression of ALP, matrix metallopeptidase-13 (MMP-13), osteopontin, and runt-related transcription factor 2 was upregulated by Day 7. Under mineralization-inducing conditions, high ALPa expressors and unsorted AF cells formed von Kossa-positive nodules, composed of hydroxyapatite as determined by electron diffraction analysis. Low ALPa expressors had significantly fewer von Kossa-positive nodules (p<.01) compared with high ALPa expressors. Cells showed colocalization of Type I collagen and ALPa. No Type II collagen was detected suggesting that these were AF cells and not chondrocytes.

CONCLUSIONS

Annulus fibrosus cells have mineralizing capability and form hydroxyapatite crystalline deposits when cultured under appropriate conditions. This system could be used to investigate mineralization mechanisms in the AF during pathological calcification and at the AF-bone interface in disc degeneration.

Effect of acetaminophen and nonsteroidal anti-inflammatory drugs on gene expression of mesenchymal stem cells

We have previously shown that mesenchymal stem cells (MSCs) from patients with osteoarthritis (OA) constitutively express type X collagen, a marker of late-stage chondrocyte hypertrophy, osteogenic marker genes, including alkaline phosphatase (ALP), bone sialoprotein (BSP), and osteocalcin (OC), and chondrogenesis marker gene aggrecan (ACAN). As patients with arthritis often take nonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen (Acet), the purpose of the study was to assess whether these drugs can affect the gene expression of human MSCs. MSCs isolated from the bone marrow of patients with OA or normal donors were cultured without (control) or with Acet or NSAIDs, which include ibuprofen, diclofenac (Dic), naproxen, and celebrex. After 3 days of culture, the expression of type X collagen alpha 1 (COL10A1), ACAN, COL1A1, as well as ALP, BSP, OC, and Runt-related transcription factor 2 was analyzed by real-time reverse transcription (RT)-polymerase chain reaction. The results showed that COL10A1 and the osteogenic and chondrogenic marker genes can be regulated by NSAIDs and Acet in normal MSCs. In contrast, Acet did not significantly affect COL10A1 expression in OA MSCs, while Dic is the only drug that had no significant effect on all markers in normal MSCs. The upregulation of COL10A1 in normal MCSs by Acet and Npx may explain why stem cells from patients with OA express COL10A1 constitutively. This knowledge may help in designing better strategies for stem cell differentiation into chondrocyte-like cells, from this source, with Dic being a viable option for treating OA pain, with an eye toward preventing the potential to enhance calcification in the repair of cartilage and degenerated intervertebral discs.

A model for the ultrastructure of bone based on electron microscopy of ion-milled sections

The relationship between the mineral component of bone and associated collagen has been a matter of continued dispute. We use transmission electron microscopy (TEM) of cryogenically ion milled sections of fully-mineralized cortical bone to study the spatial and topological relationship between mineral and collagen. We observe that hydroxyapatite (HA) occurs largely as elongated plate-like structures which are external to and oriented parallel to the collagen fibrils. Dark field images suggest that the structures (“mineral structures”) are polycrystalline. They are approximately 5 nm thick, 70 nm wide and several hundred nm long. Using energy-dispersive X-ray analysis we show that approximately 70% of the HA occurs as mineral structures external to the fibrils. The remainder is found constrained to the gap zones. Comparative studies of other species suggest that this structural motif is ubiquitous in all vertebrates.

Calcification in human intervertebral disc degeneration and scoliosis

Calcification is a pathological process that may lead to impairment of nutrient supply and disc metabolism in degenerative and scoliotic intervertebral discs (IVDs). The purpose of this study was to assess the calcification potential of IVDs in degenerative disc disease (DDD) and adolescent idiopathic scoliosis (AIS). For this purpose, 34 IVDs from 16 adult patients with DDD and 25 IVDs from 9 adolescent patients with AIS were obtained at surgery. The concave and convex parts of the scoliotic discs were analyzed separately. Von Kossa staining was performed to visualize calcium deposits, while type X collagen (COL X) expression associated with endochondral ossification was measured by immunohistochemistry. Alkaline phosphatase activity and calcium and inorganic phosphate concentrations were used as indicators of calcification potential. Results showed the presence of calcium deposits and COL X in degenerative and scoliotic IVDs, but not in control discs, and the level of the indicators of calcification potential was consistently higher in degenerative and scoliotic discs than in control discs. The results suggest that disc degeneration in adults is associated with ongoing mineral deposition and that mineralization in AIS discs might reflect a premature degenerative process.

Intrafibrillar Mineralization of Collagen using a Liquid-Phase Mineral Precursor

Intrafibrillar mineralization of type-I collagen with hydroxyapatite (HA) is the basis of the complex biological composite known as bone, which from a material science perspective is a fascinating example of an interpenetrating bioceramic composite. Using a polymer-induced liquid-precursor (PILP) process, collagen substrates were highly infiltrated with a liquid-phase mineral precursor to calcium carbonate (CaCO3). At sections of partially mineralized collagen, banded mineral patterns were observed perpendicular to the collagen fibrils, while other fibrils were completely mineralized. An acid etch, used to preferentially remove superficial mineral, further revealed such banded patterns in fully mineralized samples. Removal of the collagen matrix with a dilute hypochlorite solution showed an interpenetrating mineral phase, with mineral disks that spanned the diameter of the pre-existing collagen fibrils, supporting our hypothesis that intrafibrillar mineralization can be achieved via capillary action applied to a liquid-phase mineral precursor.

Immunocytochemical detection of dentine matrix protein 1 in experimentally induced reactionary and reparative dentine in rat incisors

OBJECTIVE

Although the general mechanisms of dentinogenesis are understood, several aspects regarding tertiary dentine formation still deserve investigation, especially regarding the presence and distribution of some noncollagenous matrix proteins. As dentine matrix protein 1 (DMP 1) is present in primary dentine, it is possible that this protein may also be present in the dentine matrix secreted after injury, but there are no immunocytochemical studies attempting its detection in tertiary dentine. The aim of this study was to examine the ultrastructural immunolocalization of DMP 1 in the tertiary dentine after extrusion of the rat incisor.

STUDY DESIGN

Upper incisors were extruded 3mm and then repositioned into their sockets. After several periods, the incisors were fixed and processed for transmission electron microscopy and for immunocytochemistry for DMP 1.

RESULTS

Extrusion yielded both types of tertiary dentine, which varied in aspect and related cells. DMP 1 was found in the mineralized matrix of all types of dentine, presenting high affinity for collagen, but rare colloidal gold particles over predentine. DMP 1 was evident in the supranuclear region and inside the nucleus of some odontoblast-like cells.

CONCLUSION

The observed association between DMP 1 and collagen seem to be essential for reactionary and reparative dentine formation.

Calcospherulites isolated from the mineralization front of bone induce the mineralization of type I collagen

Previous work has suggested that "calcospherulites" actively participate in the mineralization of developing and healing bone. This study sought to directly test this hypothesis by developing a method to isolate calcospherulites and analyzing their capacity to seed mineralization of fibrillar collagen. The periosteal surface of juvenile rat tibial diaphysis was enriched in spherulites of approximately 0.5-mum diameter exhibiting a Ca/P ratio of 1.3. Their identity as calcospherulites was confirmed by their uptake of calcein at the tibial mineralization front 24 h following in vivo injection. Periosteum was dissected and unmineralized osteoid removed by collagenase in order to expose calcospherulites. Calcein-labeled calcospherulites were then released from the mineralization front by dispase digestion and isolated via fluorescence flow sorting. X-ray diffraction analysis revealed they contained apatite crystals (c-axis length of 17.5+/-0.2 nm), though their Ca/P ratio of 1.3 is lower than that of hydroxyapatite. Much of their non-mineral phosphorous content was removed by ice-cold ethanol, elevating their Ca/P ratio to 1.6, suggesting the presence of phospholipids. Western blot analyses showed the presence of bone matrix proteins and type I collagen in these preparations. Incubating isolated calcospherulites in collagen hydrogels demonstrated that they could seed a mineralization reaction on type I collagen fibers in vitro. Ultrastructural analyses revealed crystals on the collagen fibers that were distributed rather uniformly along the fiber lengths. Furthermore, crystals were observed at distances well away from the observed calcospherulites. Our results directly support an active role for calcospherulites in inducing the mineralization of type I collagen fibers at the mineralization front of bone.

Altered collagen in tartrate-resistant acid phosphatase (TRAP)-deficient mice: a role for TRAP in bone collagen metabolism

Tartrate-resistant acid phosphatase (TRAP) is an iron-containing protein that is highly expressed by osteoclasts, macrophages, and dendritic cells. The enzyme is secreted by osteoclasts during bone resorption, and serum TRAP activity correlates with resorptive activity in disorders of bone metabolism. TRAP is essential for normal skeletal development. In knockout mice lacking TRAP, bone shape and modeling is altered with increased mineral density. Here, we report the effect of TRAP on the biochemical and biomechanical properties of collagen, the major protein constituting the bone matrix, using these mice. Femurs from TRAP-/- and wild-type mice were used in these studies. The biomechanical properties were investigated using a three-point bending technique. Collagen synthesis was determined by measuring cross-link content using high-performance liquid chromatography and amino acid analysis. Collagen degradation was determined by measuring matrix metalloproteinase-2 (MMP-2) activity. The rates of collagen synthesis and degradation were significantly greater in bones from TRAP-/- mice compared with wild type. At 8 weeks, there was an increase in the intermediate cross-links but no significant difference in animals aged 6 months. There was a significant increase in mature cross-links at both ages. A significant increase in MMP-2 production both pro and active was observed. A significant increase in ultimate stress and Young's modulus of elasticity was needed to fracture the bones from mice deficient in TRAP. We conclude that both synthesis as well as degradation of collagen are increased when TRAP is absent in mice at 8 weeks and 6 months of age, showing that TRAP has an important role in the metabolism of collagen.

Gene targeting reveals the role of Oc90 as the essential organizer of the otoconial organic matrix

A critical part of the functional development of our peripheral balance system is the embryonic formation of otoconia, composite crystals that overlie and provide optimal stimulus input to the sensory epithelium of the gravity receptor in the inner ear. To date neither the functions of otoconial proteins nor the processes of crystal formation are clearly defined. Using gene targeting and protein analysis strategies, we demonstrate that the predominant mammalian otoconin, otoconin-90/95 (Oc90), is essential for formation of the organic matrix of otoconia by specifically recruiting other matrix components, which includes otolin, a novel mammalian otoconin that we identified to be in wildtype murine otoconia. We show that this matrix controls otoconia growth and morphology by embedding the crystallites during seeding and growth. During otoconia development, the organic matrix forms prior to CaCO3 deposition and provides optimal calcification efficiency. Histological and ultrastructural examinations show normal inner ear epithelial morphology but reduced acellular matrices, including otoconial, cupular and tectorial membranes, in Oc90 null mice, likely due to an absence of Oc90 and a profound reduction of otolin. Our data demonstrate the critical roles of otoconins in otoconia seeding, growth and anchoring and suggest mechanistic similarities and differences between otoconia and bone calcification.

Dissociation of mineral and collagen orientations may differentially adapt compact bone for regional loading environments: results from acoustic velocity measurements in deer calcanei

In limb bone diaphyses, it is hypothesized that collagen and extra-fibrillar mineral are aligned differently in relatively simple loading conditions (e.g., habitual longitudinal compression) when compared to complex or potentially deleterious strain environments (e.g., habitual shear or tension). These putative differences in collagen/mineral organization might be adaptations that enhance toughness and fatigue resistance by controlling the direction of microdamage propagation. This study examined relationships between the non-uniform strain distribution of wild deer calcanei and elastic anisotropy of cortical bone specimens in three preparations: (1) demineralized (collagen only), (2) deproteinized (mineral only), and (3) untreated. Using simulated functional loading, the following strain data were obtained from the dorsal "compression", plantar "tension", and medial and lateral ("neutral axis") cortices of one calcaneus of each of seven pairs: (1) peak strain magnitude, (2) prevalent/predominant strain mode (compression, tension, shear), and (3) principal strain orientation with respect to the bone's long axis. In the contralateral calcanei, elastic anisotropy ratios (ARs) were calculated using acoustic velocity (longitudinal and transverse) measurements from a pair of orthogonally sliced specimens (representing each of three preparation types) from each cortex. In a separate set of seven adult calcanei, predominant collagen fiber orientation (CFO) was measured using circularly polarized light (CPL) in the four cortical locations. Results showed that, in general, elastic anisotropy was significant in each region, with ARs being significantly different from isotropy (where AR=1.0). Compared to CFO, mineral orientation more strongly influenced this anisotropy, which was most notable in the plantar "tension" cortex. High correlations (r values from -0.675 to -0.734, P<0.05) were found between collagen anisotropy obtained from acoustic data when compared to the CPL data. Significant correlations of mineral and collagen anisotropy were also found between strain mode, magnitude, and orientation (all r values approximately -0.750). The habitual compression, tension, and shear (neutral axis) regions also had different collagen/mineral organizations, which may be important in accommodating the well-known disparity in the mechanical properties of bone in these loading modes.

A locus on chromosome 7 determines dramatic up-regulation of osteopontin in dystrophic cardiac calcification in mice

Calcification of necrotic tissue is frequently observed in chronic inflammation and atherosclerosis. A similar response of myocardium to injury, referred to as dystrophic cardiac calcinosis (DCC), occurs in certain inbred strains of mice. We now examined a putative inhibitor of calcification, osteopontin, in DCC after transdiaphragmal myocardial freeze-thaw injury. Strong osteopontin expression was found co-localizing with calcification in DCC-susceptible strain C3H/HeNCrlBr, which exhibited low osteopontin plasma concentrations otherwise. Osteopontin mRNA induction was 20-fold higher than in resistant strain C57BL/6NCrlBr, which exhibited fibrous lesions without calcification and little osteopontin expression. Sequence analysis identified several polymorphisms in calcium-binding and phosphorylation sites in osteopontin cDNA. Their potential relevance for DCC was tested in congenic mice, which shared the osteopontin locus with C57BL/6NCrlBr, but retained a chromosomal segment from C3H/HeNCrlBr on proximal chromosome 7. These mice exhibited strong osteopontin expression and DCC comparable to C3H/HeNCrlBr suggesting that a trans-activator of osteopontin transcription residing on chromosome 7 and not the osteopontin gene on chromosome 5 was responsible for the genetic differences in osteopontin expression. A known osteopontin activator encoded by a gene on chromosome 7 is the transforming growth factor-beta1, which was more induced (3.5x) in C3H/HeNCrlBr than in C57BL/6NCrlBr mice.

Nucleation of calcium phosphate by surface-bound extracellular matrix

The native extracellular matrix (ECM) laid down on silicon and titanium surfaces by osteoblast-like SAOS-2 cells was exposed by selective removal of cells. This type of material surface ECM-Si, ECM-Ti was shown to promote the nucleation of calcium phosphate from a simulated body fluid (SBF). Microscopic and spectroscopic results revealed the effect was associated with a collagen fiber-free extracellular matrix.

The attachment and growth behavior of osteoblast-like cells on microtextured surfaces

In previous studies, we showed that the application of microgrooves on a surface can direct cellular morphology and the deposition of mineralized matrix of osteoblast-like cells (Biomaterials 20 (1999) 1293; Clin. Oral Impl Res. 11 (2000) 325). In this study, we evaluated the attachment and growth behavior of these cells, using scanning- and transmission electron microscopy (SEM/TEM). Smooth and microgrooved polystyrene substrates were made (groove depth 0.5-1.5 microm, groove- and ridge width 1-10 microm). On these substrates, osteoblast-like cells were cultured for periods up to 16 days. SEM showed that the cells, and their extensions, closely followed the surface on smooth and wider grooved (>5 microm) substrates. In contrast, narrow grooves (<2 microm) were bridged. After 16 days of incubation, the matrix showed extensive deposition of collagen fibrils, and the formation of calcified nodules. With TEM it was shown that on the smooth and wider grooved substrates, focal adhesions were spread throughout the surface. However, on narrow grooves focal adhesions were always positioned on the edges of surface ridges only. Apparently, most extracellular matrix (ECM) was produced by the cells that directly adhered to the substrate. Deposition of ECM was seen in the surface grooves, as well as in between the cell layers. On basis of the current study and previous experiments, we conclude that microgrooves are able to influence bone cell behavior by (1) determining the alignment of cells and cellular extensions, (2) altering the formation and placement of cell focal adhesions, and (3) altering ECM production. Therefore, microgrooved surfaces seem interesting to be applied on bone-anchored implants.

Hierarchical structures in fibrillar collagens

The collagen family includes several large transcripts, usually exceeding 1000 amino acid residues per single chain. As a group, they make up 1/3 of all the protein of the body and are responsible for modelling the framework of connective tissues; individually, they show both a wide variety and a complex hierarchy of mutual interactions, and form a range of functional aggregates including a variety of fibrils, microfibrils and basal membranes. Of the collagens, the fibril-forming types (i.e. the types I, II III, V and XI) are the most abundant and the most extensively studied. At the primary structure level, the amino acid sequence of all collagens is now known in detail and it shows a distinctive domain organization, its composition being dominated by the amino acid glycine (roughly 1/3 of all residues) and by post-translational hydroxylation of proline and lysine residues. Collagen secondary and tertiary structure, which together give origin to a classic triple helix, were painstakingly determined in the 1950s and 1960s. In contrast with the primary, secondary and tertiary structure, the supramolecular arrangement within collagen fibres seems to be far more elusive, and none of the models so far advanced can be said to be universally accepted. Half a century of research and debate spawned numerous mutually incompatible models, most of them focussing either on a quasi-crystalline supramolecular array or on several forms of microfibrillar aggregates, while radial fibrils, epitaxial fibrils and other structural models have almost been ignored. In many cases, data gained with a single technique from a single tissue were arbitrarily given a general legitimacy, whilst other well-documented morphological evidence went virtually unnoticed by the scientific community.Moreover, in recent years there has been a growing interest in the multiple interactions of collagens with the other macromolecules of the extra-cellular matrix, as their structure and their functional role become known. It is now indisputable that collagen interacts and forms functional entities with several other macromolecules of the extracellual matrix. This paper will succinctly review some current concepts on the structural biology of collagen higher-order structures.

The structure and function of normally mineralizing avian tendons

The leg tendons of certain avian species normally calcify. The gastrocnemius, or Achilles, tendon of the domestic turkey, Meleagris gallopavo, is one such example. Its structure and biomechanical properties have been studied to model the adaptive nature of this tendon to external forces, including the means by which mineral deposition occurs and the functional role mineralization may play in this tissue. Structurally, the distal rounded, thick gastrocnemius bifurcates into two smaller proximal segments that mineralize with time. Mineral deposition occurs at or near the bifurcation, proceeding in a distal-to-proximal direction along the segments toward caudal and medial muscle insertions of the bird hip. Mineral formation appears mediated first by extracellular matrix vesicles and later by type I collagen fibrils. Biomechanical analyses indicate lower tensile strength and moduli for the thick distal gastrocnemius compared to narrow, fan-shaped proximal segments. Tendon mineralization here appears to be strain-induced, the muscle forces causing matrix deformation leading conceptually to calcium binding through the exposure of charged groups on collagen, release of sequestered calcium by proteoglycans, and increased diffusion. Functionally, the mineralized tendons limit further tendon deformation, reduce tendon strain at a given stress, and provide greater load-bearing capacity to the tissue. They also serve as important and efficient elastic energy storage reservoirs, increasing the amount of stored elastic energy by preventing flexible type I collagen regions from stretching and preserving muscle energy during locomotion of the animals.

Cartilage calcification studied by proton nuclear magnetic resonance microscopy

A three-dimensional (3D) mineralizing culture system using hollow fiber bioreactors has been developed to study the early stages of endochondral ossification by proton nuclear magnetic resonance (NMR) microscopy. Chondrocytes harvested from the cephalic half of the sterna from 17-day-old chick embryos were terminally differentiated with 33 nM of retinoic acid for 1 week and mineralization was initiated by the addition of 1% beta-glycerophosphate to the culture medium. Histological sections taken after 6 weeks of development in culture confirmed calcification of the cartilage matrix formed in bioreactors. Calcium to phosphorus ratios (1.62-1.68) from X-ray microanalysis supported electron diffraction of thin tissue sections showing the presence of a poorly crystalline hydroxyapatite mineral phase in the cultures. After 4 weeks of culture, quantitative proton NMR images showed water proton magnetization transfer rate constants (km) were higher in premineralized cartilage compared with uncalcified cartilage, a result suggesting collagen enrichment of the matrix. Notably after 5 weeks mineral deposits formed in bioreactors principally in the collagen-enriched zones of the cartilage with increased km values. This caused marked reductions in water proton longitudinal (T1) and transverse (T2) relaxation times and water diffusion coefficients (D). These results support the hypothesis that mineralization proceeds in association with a collagen template. After 6 weeks of culture development, the water proton T2 values decreased by 13% and D increased by 7% in uncalcified areas, compared with the same regions of tissue examined 1 week earlier. These changes could be attributed to the formation of small mineral inclusions in the cartilage, possibly mediated by matrix vesicles, which may play an important role in cartilage calcification. In summary, NMR images acquired before and after the onset of mineralization of the same tissue provide unique insights into the matrix events leading to endochondral mineral formation.

Age- and genotype-dependence of bone material properties in the osteogenesis imperfecta murine model (oim)

Cortical mineralization of long bones was studied in collagen alpha2(I)-deficient mice (oim) used as a model for human osteogenesis imperfecta. Aspects of the age development of the mice were characterized by combining nanometer- to micrometer-scale structural analysis with microhardness measurements. Bone structure was determined from homozygous (oim/oim) and heterozygous (oim/+) mice and their normal (+/+) littermates as a function of animal age by small-angle X-ray scattering (SAXS) and quantitative backscattered electron imaging (qBEI) measurements. SAXS studies found anomalies in the size and arrangement of bone mineral crystals in both homozygous and heterozygous mice aged 1-14 months. Generally, the crystals were smaller in thickness and less well aligned in these mice compared with control animals. An increase in the mean crystal thickness of the bone was found within all three genotypes up to an age of 3 months. Vicker's hardness measurements were significantly enhanced for oim bone (homozygotes and heterozygotes) compared with controls. The microhardness values were correlated directly with increased mineral content of homozygous and heterozygous compared with control bone, as determined by qBEI analysis. There was also a significant increase of mineral content with age. Two possibilities for collagen-mineral association are discussed for explaining the increased hardness and mineral content of oim/oim bone, together with its decreased toughness and thinner mineral crystals. As a consequence of the present measurements, one model for oim bone could incorporate small and densely packed mineral crystals. A second model for possible collagen-mineral association in oim material would consist of two families of mineral crystals, one being smaller and the other being much larger than the crystals found in normal mouse long bones.

Mineralized dermal layer of the Brazilian tree-frog Corythomantis greeningi

Some species of anuran amphibians possess a calcified dermal layer (the Eberth-Kastschenko layer) located between the "stratum spongiosum" and the "stratum compactum." This layer consists of calcium phosphate deposits, proteoglycans, and glycosaminoglycans. Although regarded as a protective layer against desiccation, a calcium reservoir, or possibly a remnant of a dermal skeleton present in anuran ancestors, very little is known about its origin, structure, and function. Thus, we studied the structure and composition of the mineralized dermal layer of Corythomantis greeningi, a peculiar hylid from the Brazilian semiarid region (caatinga), using conventional and cryosubstitution methods combined with transmission, scanning, and analytical electron microscopy. Results show that the dermal layer consists of dense, closely juxtaposed, globular structures. Although the electron opacity of the globules was variable, depending on the type of preparation, crystal-like inclusions were present in all of them, as confirmed by dark field microscopy. Electron probe X-ray microanalysis showed calcium, phosphorus, and oxygen, and electron diffraction revealed a crystalline structure comparable to that of a hydroxyapatite.

Insulin-like growth factor-1 and -2 stimulate osteoprogenitor proliferation and differentiation and adipocyte formation in cell populations derived from adult rat bone

In the present study, we test the hypothesis that the stimulation of osteoprogenitor differentiation by the glucocorticoid dexamethasone (Dex) is mediated, at least in part, through components of the insulin-like growth factor (IGF) system. Because it has been suggested that osteoprogenitors and adipocyte progenitors originate from the same precursor cells, and that their differentiation in many systems is reciprocally regulated, the effects of Dex and IGF on adipocyte formation were also evaluated in the same cultures. In view of the presence of IGFs and their binding proteins in serum, we also evaluated to what degree the effects of IGF-1 and IGF-2 on differentiation of osteoblasts and adipocytes were affected by the serum concentration of the culture media. Bone cell populations were isolated from vertebrae of 3-month-old female Wistar rats using an explant culture technique. Osteoprogenitor differentiation was evaluated by a colony assay: Bone-forming osteoblastic colonies (bone nodules) derived from single osteoprogenitors were identified by alkaline phosphatase (ALP) staining and/or staining for mineralized matrix according to the von Kossa technique. Unmineralized nodules and osteoblastic colonies were subsequently identified by their distinctive color and morphology after methylene blue counterstaining. Differentiated adipocytes were identified by Sudan IV staining. IGF-1 and IGF-2 stimulated both osteoprogenitor and adipocyte progenitor differentiation in a dose-dependent pattern. The stimulation of osteoprogenitor differentiation by IGF was not dependent on Dex, but differentiation of adipocytes was. The stimulatory effects of IGF-1 and IGF-2 on osteoprogenitor differentiation were greater in media containing 2.5% fetal bovine serum (FBS) than in media containing 5% or 10% FBS, whereas stimulation of adipocyte formation was greater in media containing 10% FBS. Neutralizing antibody against the type 1 IGF receptor (IGF-1R) partially blocked IGF- and Dex-induced osteoprogenitor differentiation, but did not affect IGF-induced adipocyte formation. This suggests that IGF-stimulated osteoprogenitor differentiation is mediated through IGF-1R, that the stimulation of adipocyte formation by IGF is not, that the stimulatory effects of Dex on osteoprogenitor differentiation are partially mediated through IGF-1R, and that the effects on adipocyte differentiation appear to be mediated through signaling pathways other than the IGF-1R.

Osteoadherin, a cell-binding keratan sulfate proteoglycan in bone, belongs to the family of leucine-rich repeat proteins of the extracellular matrix

Osteoadherin is a recently described bone proteoglycan containing keratan sulfate. It promotes integrin (alphav beta3)-mediated cell binding (Wendel, M., Sommarin, Y., and Heinegârd, D. (1998) J. Cell Biol. 141, 839-847). The primary structure of bovine osteoadherin has now been determined by nucleotide sequencing of a cDNA clone from a primary bovine osteoblast expression library. The entire translated primary sequence corresponds to a 49,116-Da protein with a calculated isoelectric point for the mature protein of 5.2. The dominating feature is a central region consisting of 11 B-type, leucine-rich repeats ranging in length from 20 to 30 residues. The full, primary sequence contains four putative sites for tyrosine sulfation, three of which are at the N-terminal end of the molecule. There are six potential sites for N-linked glycosylation present. Osteoadherin shows highest sequence identity, 42%, to bovine keratocan and 37-38% identity to bovine fibromodulin, lumican, and human PRELP. Unique to osteoadherin is the presence of a large and very acidic C-terminal domain. The distribution of cysteine residues resembles that of other leucine-rich repeat proteins except for two centrally located cysteines. Northern blot analysis of RNA samples from various bovine tissues showed a 4.5-kilobase pair message for osteoadherin to be expressed in bone only. Osteoadherin mRNA was detected by in situ hybridization in mature osteoblasts located superficially on trabecular bone.

Developmental appearance and distribution of bone sialoprotein and osteopontin in human and rat cementum

BACKGROUND

Bone sialoprotein (BSP) and osteopontin (OPN), two major noncollagenous proteins (NCPs) in collagen-based mineralized tissues, have been implicated in mineral deposition and cell- and matrix-matrix interactions during root development. However, their role in cementogenesis is still a subject of debate. Since distribution of proteins is indicative of function, we have analyzed their temporo-spatial appearance in relation to that of cementum collagen.

METHODS

Human premolars and rat molars at various stages of root development characterized by differing rates of formation were fixed in aldehyde and embedded in epoxy and LR White resin. Sections were processed for ultrastructural analysis and postembedding colloidal gold (immuno)cytochemistry.

RESULTS

Incubations with antibodies against BSP and OPN and with lectins recognizing prominent sugars in these proteins generally revealed similar labeling patterns in both human and rat teeth, with gold particles accumulating mainly in the interfibrillar spaces. The lectin Helix pomatia, specific for N-acetyl-D-galactosamine, was distinctive in that it consistently reacted with human cementum, but only sporadically labeled rat cementum. Regardless of both the species and the stage of root development, mineralization initiated in mantle predentin in association with distinct foci immunoreactive for BSP and OPN. In human teeth, the deposition of cementum collagen began before the start of dentin mineralization and thus prior to any detectable labeling for BSP and OPN. However, at early stages of root formation in the rat, cementum collagen appeared after BSP and OPN accumulated on the root surface, whereas at advanced stages the deposition of cementum collagen, BSP and OPN coincided.

CONCLUSIONS

The temporo-spatial differences in the appearance of BSP and OPN relative to cementum collagen correlate well with known differences in the speed of root elongation and explain the variable appearance of the dentino-cemental junction. The data reveal no causal relationship between BSP and OPN and the differentiation of cementoprogenitor cells and indicate that the distribution of collagen fibrils ultimately determines the amount and pattern of accumulation of these NCPs. There also is no consistent planar accumulation of BSP and OPN between dentin and cementum such as the cement lines found between "old" and "new" bone. It is concluded that the interlacement of collagen fibrils at the dentino-cemental junction, across which mineralization spreads, represents the primary attachment mechanism between cementum and dentin.

Osteogenic cell cytotoxicity and biomechanical strength of the new ceramic Diopside

Diopside was prepared by sintering a powder compact composed of CaMgSi2O6 at 1573K for 2 h. In order to clarify the biocompatibility of Diopside, the cytotoxicity of Diopside against the osteogenic cell line MC3T3-E1 and the bone-Diopside interface strength were examined. On both the 14th and 21st days of incubation of MC3T3-E1 cells with Diopside, ALP activities were not significantly lower than those of the CTRL. TEM photographs of MC3T3-E1 on Diopside after 14 days of incubation showed active secretion of crystals from osteoblast-like cells. Scanning electron microscopic analysis showed that the cells on Diopside formed multiple cell layers similar to those on the CTRL both 14 and 21 days after incubation. These results showed that Diopside had no cytotoxic effect on MC3T3-E1. The pulling test showed that failure loads of Diopside were significantly lower than those of AWGC. Histologically, there was no fibrous tissue or foreign body reaction at the bone interface. SEM-EPMA showed that Diopside had attached to the bone via a calcium-phosphorus layer. SEM back-scattered electron imaging showed that the Diopside plate had degraded to a porous state 12 weeks after implantation. These findings indicate that Diopside is a biodegradable ceramic.

Crack growth resistance in cortical bone: concept of microcrack toughening

The role of microcracking in cortical bone as a toughening mechanism has been investigated in conjunction with the variation in fracture toughness with crack length. Fracture toughness tests were conducted on miniaturised compact tension specimens made from human and bovine cortical bone and the resultant microstructural damage, present in the form of microcracking on the surface, was analysed around the main propagating crack. It was found that the fracture toughness (Kc) and the cumulative number of microcracks increased linearly with crack extension in human and bovine cortical bone, although both Kc and number of microcracks were considerably higher in the latter case. Based on these results, a mechanism, derived from the resistance (R) curve concept developed for microcracking brittle solids, is proposed to explain the fracture of cortical bone, with microcracking distributed between a frontal process zone and a significant process zone wake. Evidence to support this mechanism is given from the existing bone literature, detailed scanning electron microscopical observations and the distribution of microcracks in the process zone wake.

Development of tight junctions between odontoblasts in early dentinogenesis as revealed by freeze-fracture

BACKGROUND

Mature odontoblasts possess junctional structures constituted by adherens, gap, and tight junctions. Although adherens and gap junctions appear early between odontoblasts, there is no information on the appearance and development of tight junctions between odontoblasts. In this study, we have examined freeze-fracture replicas of early dentinogenesis to study the development of tight junctions between odontoblasts and to determine whether these junctions are of zonular or macular type.

METHODS

Upper first molar tooth germs of Wistar rats between 1 and 3 days old were fixed in buffered 4% glutaraldehyde/4% formaldehyde and subsequently cryoprotected with cacodylate-buffered glycerol. Freeze-fracture replicas were obtained in a Balzers 301 apparatus, and early stages of dentinogenesis were examined in a Jeol 100 CX II electron microscope.

RESULTS

In the stage of early dentine matrix prior to mineralization, odontoblasts exhibit only gap junctions. With the progression of development, the distal plasma membranes of odontoblasts show numerous short tight junctions formed by fused particles and grooves. In the stage of advanced mineralization, branched and continuous rows of fused particles or grooves constitute tight junctions of the focal or macular type.

CONCLUSIONS

The present study shows that tight junctions of focal or macular type appear on distal plasma membrane of early odontoblasts during differentiation. Formation of tight junctions indicates the establishment of a distal membrane domain and maturation of odontoblasts. These events occur as mantle dentine formation ceases and circumpulpar dentine formation begins.

Evidence of ectokinase-mediated phosphorylation of osteopontin and bone sialoprotein by osteoblasts during bone formation in vitro

Osteopontin (OPN) and bone sialoprotein (BSP) are phosphorylated glycoproteins that, together with osteonectin/secreted protein, acidic, rich in cysteine (SPARC) and osteocalcin, comprise the major non-collagen proteins of bone. Although phosphorylation of OPN and BSP, which is known to influence the biological properties of these proteins, has been shown to occur intracellularly, recent studies have demonstrated ectokinase activity in bone cell populations [Mikuni-Takagaki, Kakai, Satoyoshi, Kawano, Suzuki, Kawase and Saito (1995) J. Bone Miner. Res. 10, 231-241]. To determine whether OPN and BSP are phosphorylated by ectokinase activity we have used [gamma-32P]ATP and [gamma-32P]GTP as cell-impenetrable phosphate donors to analyse for ectokinase activity in osteoblastic UMR106.06 cells and fetal rat calvarial cells (FRCCs). By pulse-labelling confluent cells with radiolabelled nucleotides, the phosphorylation of endogenous and exogenously added OPN and BSP was demonstrated together with the labelling of a number of cell surface proteins. These phosphorylation reactions were inhibited by a cell-impermeable ectokinase inhibitor, K252b, and cell surface phosphorylation was also inhibited by exogenously added OPN and BSP substrates, indicating competition for the ectokinase enzyme. However, phosphorylation of OPN and BSP, both of which can mediate cell attachment through Arg-Gly-Asp (RGD) motifs, was not inhibited by an RGD peptide, suggesting that binding of OPN and BSP to cell surface integrins is not required. In similar experiments, ectokinase-mediated phosphorylation of OPN and BSP was demonstrated during mineralized tissue formation by FRCCs in vitro. These studies demonstrate that OPN and BSP secreted by bone cells are phosphorylated by a casein kinase II-like ectokinase present on the surface of osteoblastic cells.

Detection of osteopontin in calcified human aortic valves

Cardiac valve calcification often results in obstruction of blood flow, which eventually leads to valve replacement. The molecular mechanisms resulting in valve calcification are unknown. Collagen and specific bone matrix proteins are thought to provide the framework for ectopic tissue calcification. This investigation was performed to determine whether the bone matrix protein osteopontin was present in calcified human aortic valves. Proteins extracted from human aortic valve tissue were subjected to polyacrylamide gel electrophoresis followed by Western blotting, using polyclonal antibodies directed against osteopontin. Fresh frozen tissue sections were also screened for osteopontin and macrophages using immunohistochemical techniques. Osteopontin was present in both heavily and minimally calcified aortic valves and absent in noncalcified purely regurgitant or normal aortic valves by both radioimmunoassay (n = 16) and immunohistochemical techniques (n = 8). Osteopontin colocalized with valvular calcific deposits, and macrophages were identified in the vicinity of osteopontin. These results, in addition to showing that osteopontin is present in calcified human aortic valves, suggest that osteopontin is a regulatory protein in pathological calcification. Identification of the cells producing osteopontin in abnormal cardiac valves and of proximate stimuli for its secretion may lead to novel therapeutic strategies to prevent and/or reverse calcific valve disease.

Immunodetection of enamel- and cementum-related (bone) proteins at the enamel-free area and cervical portion of the tooth in rat molars

Enamel and dentin at the cervical portion of the tooth are frequently covered by a collagen-free matrix referred to as acellular afibrillar cementum (AAC). It is believed that AAC deposition occurs when the enamel organ is displaced or disrupted, and mesenchymal cells from the dental follicle gain access to the tooth surface, differentiate into cementoblasts, and secrete noncollagenous proteins typically found in collagen-based mineralized tissues. A similar thin layer of mineralized matrix is found at the enamel-free area (EFA) of rodent molars, but in this case the matrix is covered by inner enamel epithelium (IEE) throughout development. We have, therefore, used this site as a paradigm to test the hypothesis that typical mesenchymal matrix proteins can also be found in association with epithelial cells. To this end, we have analyzed the presence and distribution of enamel- and cementum-related matrix proteins at the EFA and at the cervical portion of the tooth. Rat mandibular molars were processed for colloidal gold immunolabeling with antibodies to amelogenins, bone sialoprotein (BSP), osteopontin (OPN), osteocalcin (OC), and dentin sialoprotein (DSP), and the plasma proteins alpha 2 HS-glycoprotein and albumin. The EFA matrix was immunoreactive for amelogenins as well as for BSP, OPN, OC, and alpha 2 HS-glycoprotein, but not for albumin and DSP. The AAC was, similar to the EFA matrix, labeled for BSP, OPN, OC, and alpha 2 HS-glycoprotein. These data show for the first time that the EFA matrix is comprised of a mixture of enamel- and cementum-related proteins, a situation that parallels the distribution of matrix constituents at the cervical portion of the tooth. Since the EFA matrix is deposited on top of the mineralized dentin, and since the enamel organ seals off the forming matrix, it is concluded that EFA cells are responsible for the production of these proteins. Consistent with previous reports showing that epithelial cells can produce both BSP and OPN in some circumstances, the data also suggest that AAC may be deposited by cells of epithelial origin. Furthermore, they lend support to the possibility that cells derived from Hertwig's epithelial root sheath may likewise be capable of producing cementum matrix proteins.

Abnormal compartmentalization of cartilage matrix components in mice lacking collagen X: implications for function

There are conflicting views on whether collagen X is a purely structural molecule, or regulates bone mineralization during endochondral ossification. Mutations in the human collagen alpha1 (X) gene (COL10A1) in Schmid metaphyseal chondrodysplasia (SMCD) suggest a supportive role. But mouse collagen alpha1 (X) gene (Col10a1) null mutants were previously reported to show no obvious phenotypic change. We have generated collagen X deficient mice, which shows that deficiency does have phenotypic consequences which partly resemble SMCD, such as abnormal trabecular bone architecture. In particular, the mutant mice develop coxa vara, a phenotypic change common in human SMCD. Other consequences of the mutation are reduction in thickness of growth plate resting zone and articular cartilage, altered bone content, and atypical distribution of matrix components within growth plate cartilage. We propose that collagen X plays a role in the normal distribution of matrix vesicles and proteoglycans within the growth plate matrix. Collagen X deficiency impacts on the supporting properties of the growth plate and the mineralization process, resulting in abnormal trabecular bone. This hypothesis would accommodate the previously conflicting views of the function of collagen X and of the molecular pathogenesis of SMCD.