Electron optical and analytical observations of rat growth plate cartilage prepared by ultracryomicrotomy: the failure to detect a mineral phase in matrix vesicles and the identification of heterodispersed particles as the initial solid phase of calcium phosphate deposited in the extracellular matrix

Oxidized Low-Density Lipoprotein Promotes In Vitro Calcification

Calcification plays an important role in the human body in maintaining homeostasis. In the human body, the presence of a high amount of oxidized low-density lipoprotein (ox-LDL) is a consistent feature of the local areas that are common sites of ectopic calcification, namely dental calculus, renal calculus, and the areas affected by arteriosclerosis. Hence, ox-LDL may have some effect on calcification. Scanning electron microscopy (SEM) observation revealed a high amount of amorphous calcium phosphate (ACP) when ox-LDL was included in the solution. In the in vitro experiment, the highest amount of precipitation of calcium phosphate was observed in the solution containing ox-LDL compared to the inclusion of other biomaterials and was 4.2 times higher than that of deionized water for 4.86 mM calcium and 2.71 mM phosphate. The morphology of calcium phosphate precipitates in the solution containing ox-LDL differed from that of the precipitates in solutions containing other biomaterials, as determined by transmission electron microscopy (TEM). Through the time course observation of the sediments using TEM, it was observed that the sediments changed from spherical or oval shape to a thin film shape. These results indicate that sediments acquired a long-range order array, and the phase transitioned from non-crystalline to crystalline with an increased time and density of ACP. Thus, it is concluded that ox-LDL promoted ACP precipitation and it plays an important role in ectopic calcification.

Precipitation of Inorganic Salts in Mitochondrial Matrix

In the mitochondrial matrix, there are insoluble, osmotically inactive complexes that maintain a constant pH and calcium concentration. In the present paper, we examine the properties of insoluble calcium and magnesium salts, such as phosphates, carbonates and polyphosphates, which might play this role. We find that non-stoichiometric, magnesium-rich carbonated apatite, with very low crystallinity, precipitates in the matrix under physiological conditions. Precipitated salt acts as pH buffer, and, hence, can contribute in maintaining ATP production in ischemic conditions, which delays irreversible damage to heart and brain cells after stroke.

The structure of cortical bone as revealed by the application of methods for the calcified matrix study

Calcination and decalcification are basic procedures useful to a morphological approach of a biological, composite material like cortical bone. The study was carried out on a whole human femur conserved in liquid (from an educational collection). Cortical fracturing and SEM observation of vascular canals surface collagen texture was used to study bone deproteination at scalar temperatures (400-1,200°C) and acid bone decalcification at crescent time intervals. Heating burned and vaporized the organic matrix with shrinkage of the bone specimens as documented by the weight loss and transverse surface morphometry. SEM showed a pattern of aligned spherulites at 400°C which maintained the collagen fibrils layout (like a mineral cast), followed by a spherulites fusion progression with the temperature increments. At 1200°C a crystalline-like structure of tightly-packed trapezohendron units. XRD analysis supported the SEM morphology displaying the complete Debey rings of hydroxyapatite and spotted Debey rings of withlockite. Surface Ca and P elution was documented after 12 hr of exposition to the acid solution by dissolution of spherulites and the whole canal surface decalcified in depth after 15 days by SEM-EDAX analysis. The periodic pattern of collagen fibrils was still evident up to 15 days of decalcification together with fine granular deposits of a not-collagenic proteic material, while after 30 days no period was observed in the decalcified fibrils. Collagen mineral cast at 400°C calcination. Complete crystalline transformation at 1200°C. Up to 15 days of decalcification fibrils period maintained.

Mineralization pathways in the active murine epiphyseal growth plate

Endochondral ossification in the growth plate of long bones involves cartilage mineralization, bone formation and the budding vasculature. Many of these processes take place in a complex and dynamic zone, the provisional ossification zone, of the growth plate. Here we investigate aspects of mineralization in 2D and 3D in the provisional ossification zone at different length scales using samples preserved under cryogenic or fully hydrated conditions. We use confocal light microscopy, cryo-SEM and micro-CT in the phase contrast mode. We show in 9 week old BALB/c mice the presence of vesicles containing mineral particles in the blood serum, as well as mineral particles without membranes integrated with the blood vessel walls. We also observe labeled mineral particles within cells associated with bone formation, but not in the hypertrophic cartilage cells that are involved with cartilage mineralization. High resolution micro-CT images of fresh hydrated tibiae, show that there are open continuous pathways between the blood vessel extremities and the hypertrophic chondrocyte zone. As the blood vessel extremities, the mineralizing cartilage and the forming bone are all closely associated within this narrow zone, we raise the possibility that in addition to ion transport, mineral necessary for both cartilage and bone formation is also transported through the vasculature.

A novel method for the collection of nanoscopic vesicles from an organotypic culture model

Nanovesicles, exosomes and other membrane bound particles excreted by cells are currently gaining research attention since they have been shown to play a significant role in many biologically related processes. Vesicles are now thought to mediate cellular communication, transmission of some diseases and pathologically mediated calcification. Matrix vesicles have long been proposed to be central to the controlled mineralisation of bone. They remain relatively poorly studied, however, since they are challenging to extract from biological media. One difficulty is the presence of a mineral content in comparison to pure lipid vesicles, meaning that standard separation process such as ultracentrifugation are unable to precisely separate on the basis of size or weight. In this paper we report the separation of matrix vesicles from an organotypic bone culture system using a process of immunoprecipitation. Matrix vesicles were extracted using polymeric beads that were modified with an antibody for tissue non-specific alkaline phosphatase (TNALP), a surface marker abundant in bone-derived vesicles. The vesicles isolated were positive for adenosine triphosphate (ATP), the substrate for TNALP and were demonstrated to have a high-binding affinity to type I collagen, the principal collagen type found in bone. This protocol enables more detailed study of the process and regulation of mineralisation.

Cellular nanovesicles have been shown to play a significant role in many biological processes. Organotypic bone culture systems are a source of physiologically-relevant mineralisation vesicles, which can be immuno-selected for investigation.

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.

Evaluation of Distraction Osteogenesis by Scanning Electron Microscopy

A model of bifocal distraction osteogenesis in the canine model was used to assess and quantitate the mineral content of the newly forming bone within the canine mandible. A 2-cm defect was created in the body of the mandible, and after a posterior osteotomy, the transport disk was advanced at 0.25 mm per 8 hours for 21 days and then held in rigid fixation for an additional week. As a control for this study, three additional dogs underwent the same procedure with the exception that the transport disk was not advanced. Electron dispersive spectroscopy analysis was performed on the newly formed regenerate bone and compared with areas of existing cortical bone of both the transport disk and the mandible. In the control model, special note was made of the pericortical callus at the osteotomy site as well as of the regenerative bone that filled the 2-cm defect in the body of the mandible. Calcium/phosphorous ratios were used to assess the composition of the mineralized regions of the mandible. The regenerate bone that filled the defect and the mineralized callus surrounding the site of osteoclasis in the control mandible were significantly different in composition when compared with the regenerate bone that formed during distraction osteogenesis. This suggests that distraction osteogenesis may effect an initial matrix production that is more similar in composition to the mature cortical bone from which it was derived than does periosteal regeneration and filling of an osseous defect.

Dark field transmission electron microscopy as a tool for identifying inorganic nanoparticles in biological matrices

Dark field transmission electron microscopy has been applied herein to visualize the interactions of inorganic nanomaterials with biological systems. This new application of a known technique addresses a deficiency in status quo visualization techniques. High resolution and low noise images can be acquired to locate and identify crystalline nanoparticles in complex biological matrices. Moreover, through the composition of multiple images taken at different angular beam tilts, it is possible to image a majority of nanoparticles present at a site in dark field mode. This facilitates clarity regarding the internalization of nanomaterials in cellular systems. In addition, comparing dark field images recorded at different angular tilts yields insight into the character of nanoparticle faceting.

Colocation and role of polyphosphates and alkaline phosphatase in apatite biomineralization of elasmobranch tesserae

Elasmobranchs (e.g. sharks and rays), like all fishes, grow continuously throughout life. Unlike other vertebrates, their skeletons are primarily cartilaginous, comprising a hyaline cartilage-like core, stiffened by a thin outer array of mineralized, abutting and interconnected tiles called tesserae. Tesserae bear active mineralization fronts at all margins and the tesseral layer is thin enough to section without decalcifying, making this a tractable but largely unexamined system for investigating controlled apatite mineralization, while also offering a potential analog for endochondral ossification. The chemical mechanism for tesserae mineralization has not been described, but has been previously attributed to spherical precursors, and alkaline phosphatase (ALP) activity. Here, we use a variety of techniques to elucidate the involvement of phosphorus-containing precursors in the formation of tesserae at their mineralization fronts. Using Raman spectroscopy, fluorescence microscopy and histological methods, we demonstrate that ALP activity is located with inorganic phosphate polymers (polyP) at the tessera-uncalcified cartilage interface, suggesting a potential mechanism for regulated mineralization: inorganic phosphate (Pi) can be cleaved from polyP by ALP, thus making Pi locally available for apatite biomineralization. The application of exogenous ALP to tissue cross-sections resulted in the disappearance of polyP and the appearance of Pi in uncalcified cartilage adjacent to mineralization fronts. We propose that elasmobranch skeletal cells control apatite biomineralization by biochemically controlling polyP and ALP production, placement and activity. Previous identification of polyP and ALP shown previously in mammalian calcifying cartilage supports the hypothesis that this mechanism may be a general regulating feature in the mineralization of vertebrate skeletons.

Microcalcifications in breast cancer: an active phenomenon mediated by epithelial cells with mesenchymal characteristics

Background

Mammary microcalcifications have a crucial role in breast cancer detection, but the processes that induce their formation are unknown. Moreover, recent studies have described the occurrence of the epithelial–mesenchymal transition (EMT) in breast cancer, but its role is not defined. In this study, we hypothesized that epithelial cells acquire mesenchymal characteristics and become capable of producing breast microcalcifications.

Methods

Breast sample biopsies with microcalcifications underwent energy dispersive X-ray microanalysis to better define the elemental composition of the microcalcifications. Breast sample biopsies without microcalcifications were used as controls. The ultrastructural phenotype of breast cells near to calcium deposits was also investigated to verify EMT in relation to breast microcalcifications. The mesenchymal phenotype and tissue mineralization were studied by immunostaining for vimentin, BMP-2, β2-microglobulin, β-catenin and osteopontin (OPN).

Results

The complex formation of calcium hydroxyapatite was strictly associated with malignant lesions whereas calcium-oxalate is mainly reported in benign lesions. Notably, for the first time, we observed the presence of magnesium-substituted hydroxyapatite, which was frequently noted in breast cancer but never found in benign lesions. Morphological studies demonstrated that epithelial cells with mesenchymal characteristics were significantly increased in infiltrating carcinomas with microcalcifications and in cells with ultrastructural features typical of osteoblasts close to microcalcifications. These data were strengthened by the rate of cells expressing molecules typically involved during physiological mineralization (i.e. BMP-2, OPN) that discriminated infiltrating carcinomas with microcalcifications from those without microcalcifications.

Conclusions

We found significant differences in the elemental composition of calcifications between benign and malignant lesions. Observations of cell phenotype led us to hypothesize that under specific stimuli, mammary cells, which despite retaining a minimal epithelial phenotype (confirmed by cytokeratin expression), may acquire some mesenchymal characteristics transforming themselves into cells with an osteoblast-like phenotype, and are able to contribute to the production of breast microcalcifications.

A review of phosphate mineral nucleation in biology and geobiology

Relationships between geological phosphorite deposition and biological apatite nucleation have often been overlooked. However, similarities in biological apatite and phosphorite mineralogy suggest that their chemical formation mechanisms may be similar. This review serves to draw parallels between two newly described phosphorite mineralization processes, and proposes a similar novel mechanism for biologically controlled apatite mineral nucleation. This mechanism integrates polyphosphate biochemistry with crystal nucleation theory. Recently, the roles of polyphosphates in the nucleation of marine phosphorites were discovered. Marine bacteria and diatoms have been shown to store and concentrate inorganic phosphate (Pi) as amorphous, polyphosphate granules. Subsequent release of these P reserves into the local marine environment as Pi results in biologically induced phosphorite nucleation. Pi storage and release through an intracellular polyphosphate intermediate may also occur in mineralizing oral bacteria. Polyphosphates may be associated with biologically controlled apatite nucleation within vertebrates and invertebrates. Historically, biological apatite nucleation has been attributed to either a biochemical increase in local Pi concentration or matrix-mediated apatite nucleation control. This review proposes a mechanism that integrates both theories. Intracellular and extracellular amorphous granules, rich in both calcium and phosphorus, have been observed in apatite-biomineralizing vertebrates, protists, and atremate brachiopods. These granules may represent stores of calcium-polyphosphate. Not unlike phosphorite nucleation by bacteria and diatoms, polyphosphate depolymerization to Pi would be controlled by phosphatase activity. Enzymatic polyphosphate depolymerization would increase apatite saturation to the level required for mineral nucleation, while matrix proteins would simultaneously control the progression of new biological apatite formation.

Minerals and aligned collagen fibrils in tilapia fish scales: structural analysis using dark-field and energy-filtered transmission electron microscopy and electron tomography

The mineralized structure of aligned collagen fibrils in a tilapia fish scale was investigated using transmission electron microscopy (TEM) techniques after a thin sample was prepared using aqueous techniques. Electron diffraction and electron energy loss spectroscopy data indicated that a mineralized internal layer consisting of aligned collagen fibrils contains hydroxyapatite crystals. Bright-field imaging, dark-field imaging, and energy-filtered TEM showed that the hydroxyapatite was mainly distributed in the hole zones of the aligned collagen fibrils structure, while needle-like materials composed of calcium compounds including hydroxyapatite existed in the mineralized internal layer. Dark-field imaging and three-dimensional observation using electron tomography revealed that hydroxyapatite and needle-like materials were mainly found in the matrix between the collagen fibrils. It was observed that hydroxyapatite and needle-like materials were preferentially distributed on the surface of the hole zones in the aligned collagen fibrils structure and in the matrix between the collagen fibrils in the mineralized internal layer of the scale.

Biominerals--hierarchical nanocomposites: the example of bone

Many organisms incorporate inorganic solids in their tissues to enhance their functional, primarily mechanical, properties. These mineralized tissues, also called biominerals, are unique organo-mineral nanocomposites, organized at several hierarchical levels, from nano- to macroscale. Unlike man-made composite materials, which often are simple physical blends of their components, the organic and inorganic phases in biominerals interface at the molecular level. Although these tissues are made of relatively weak components under ambient conditions, their hierarchical structural organization and intimate interactions between different elements lead to superior mechanical properties. Understanding basic principles of formation, structure, and functional properties of these tissues might lead to novel bioinspired strategies for material design and better treatments for diseases of the mineralized tissues. This review focuses on general principles of structural organization, formation, and functional properties of biominerals on the example the bone tissues.

Control of vertebrate skeletal mineralization by polyphosphates

BACKGROUND

Skeletons are formed in a wide variety of shapes, sizes, and compositions of organic and mineral components. Many invertebrate skeletons are constructed from carbonate or silicate minerals, whereas vertebrate skeletons are instead composed of a calcium phosphate mineral known as apatite. No one yet knows why the dynamic vertebrate skeleton, which is continually rebuilt, repaired, and resorbed during growth and normal remodeling, is composed of apatite. Nor is the control of bone and calcifying cartilage mineralization well understood, though it is thought to be associated with phosphate-cleaving proteins. Researchers have assumed that skeletal mineralization is also associated with non-crystalline, calcium- and phosphate-containing electron-dense granules that have been detected in vertebrate skeletal tissue prepared under non-aqueous conditions. Again, however, the role of these granules remains poorly understood. Here, we review bone and growth plate mineralization before showing that polymers of phosphate ions (polyphosphates: (PO(3)(-))(n)) are co-located with mineralizing cartilage and resorbing bone. We propose that the electron-dense granules contain polyphosphates, and explain how these polyphosphates may play an important role in apatite biomineralization.

PRINCIPAL FINDINGS/METHODOLOGY

The enzymatic formation (condensation) and destruction (hydrolytic degradation) of polyphosphates offers a simple mechanism for enzymatic control of phosphate accumulation and the relative saturation of apatite. Under circumstances in which apatite mineral formation is undesirable, such as within cartilage tissue or during bone resorption, the production of polyphosphates reduces the free orthophosphate (PO(4)(3-)) concentration while permitting the accumulation of a high total PO(4)(3-) concentration. Sequestering calcium into amorphous calcium polyphosphate complexes can reduce the concentration of free calcium. The resulting reduction of both free PO(4)(3-) and free calcium lowers the relative apatite saturation, preventing formation of apatite crystals. Identified in situ within resorbing bone and mineralizing cartilage by the fluorescent reporter DAPI (4',6-diamidino-2-phenylindole), polyphosphate formation prevents apatite crystal precipitation while accumulating high local concentrations of total calcium and phosphate. When mineralization is required, tissue non-specific alkaline phosphatase, an enzyme associated with skeletal and cartilage mineralization, cleaves orthophosphates from polyphosphates. The hydrolytic degradation of polyphosphates in the calcium-polyphosphate complex increases orthophosphate and calcium concentrations and thereby favors apatite mineral formation. The correlation of alkaline phosphatase with this process may be explained by the destruction of polyphosphates in calcifying cartilage and areas of bone formation.

CONCLUSIONS/SIGNIFICANCE

We hypothesize that polyphosphate formation and hydrolytic degradation constitute a simple mechanism for phosphate accumulation and enzymatic control of biological apatite saturation. This enzymatic control of calcified tissue mineralization may have permitted the development of a phosphate-based, mineralized endoskeleton that can be continually remodeled.

Exocytotic process as a novel model for mineralization by osteoblasts in vitro and in vivo determined by electron microscopic analysis

The process of biomineralization has been examined during osteoblastic differentiation of bone marrow stroma cells (BMSCs) from embryonic chick in culture and in periosteum itself by a number of different techniques including transmission and scanning electron microscopy. In cell culture of BMSCs at days 20-25, crystals were accumulated extracellularly in the collagen matrix, resulting in large plate-like crystallites and noncollagen associated on the culture disk surface. In contrast, up to days 10-18, mainly intracellular mineralization was visible by numerous needle-like crystal structures in the cell cytoplasm and in vacuoles. After 20-30 days, the crystal content of these vacuoles is released, most probably by membrane fusion to the outside of the cells. Energy-dispersive X-ray analysis (EDX), electron spectroscopic imaging, and electron energy loss spectroscopy demonstrated that Ca, O, and P are located in the intra- and extracellular needle-like crystals. From EDX spectra a Ca/P ratio of 1.3 was estimated for the intracellular structures and a Ca/P ratio of 1.5, for the extracellular material (for comparison, the Ca/P ratio in tibiae is 1.6). X-ray diffraction and quantitative infrared spectral analyses also demonstrated an increase of crystalline bone apatite along the mineralization process. In addition to the finding in vitro, the presence of intracellular needle-like crystals in vacuoles could be demonstrated in vivo in osteoblastic cells of the periosteum in tibia of day 11. The results are in favor of a novel model for mineralization by osteoblasts, in which amorphous Ca/P material is directly secreted via an exocytotic process from vacuoles of the osteoblast, deposited extracellularly, propagated into the collagen fibril matrix, and matured to hydroxyapatite.

Two forms of apatite deposited during mineralization of the hen tendon

Old hen tendon provides a model suitable for the study of calcification in an extracellular matrix. In the present study, we observed the mineralizing substances of hen tendon by scanning electron microscopy of plasma-osmium-coated specimens and by transmission electron microscopy of those processed by a plasma-polymerization film replica method. The mineralizing front area revealed a number of elliptical particles fused to each other and forming rod-like structures oriented parallel to collagen fibrils. The area of advanced mineralization possessed non-mineralizing cavities, in which tendon cells were likely to exist. At this site, we recognized a second form of mineral structure, one in which the crystals had a scale-like morphology and were deposited onto the major first-form mineral component. This crystal form was similar to hydroxyapatite synthesized under wet reaction conditions. These findings strongly suggest that the second form of mineral formed independent of collagen fibrils existed together with the predominant, collagen-dependent form of mineral. We speculate that cell membranes and an extremely slow mineralization process may contribute to the formation of this form of mineral during the mineralization process in the hen tendon.

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.

General principle of ordered apatitic crystal formation in enamel and collagen rich hard tissues

The biomineralization processes in different hard tissues like enamel, circumpulpal dentine, epiphyseal growth plates were analyzed morphologically and ultrastructurally by an energy filtering transmission electron microscope. In the primary stage of crystal formation Ca- and phosphate-ions accumulate at charged sites, "active sites", along the fiber matrix-molecules of the extracellular matrix. After exceeding the critical radius for nucleation, crystal nuclei appear that develop to "chains" of stable nanometer-sized paracrystalline particles. In the latest studies of small area electron diffraction it was found that in the earliest stage of crystal formation these mineral chains show a parallel orientation in the direction of the c-axis of apatite. This was supported by a texture of the 002 reflection in the corresponding diffraction patterns. Since apatite is bipolar in this direction crystal growth would be in like manner in both directions. Thus the center-to-center distances between nucleating sites along the matrix macromolecules show with the chains of nanometer islands the same process of biomineralization in the different mineralizing hard tissue systems. This way of crystal formation might be a general principle of apatitic biomineralization.

Chondrogenic potential of skeletal cell populations: selective growth of chondrocytes and their morphogenesis and development in vitro

Most vertebrate embryonic and post-embryonic skeletal tissue formation occurs through the endochondral process in which cartilage serves a transitory role as the anlage for the bone structure. The differentiation of chondrocytes during this process in vivo is characterized by progressive morphological changes associated with the hypertrophy of these cells and is defined by biochemical changes that result in the mineralization of the extracellular matrix. The mechanisms, which, like those in vivo, promote both chondrogenesis in presumptive skeletal cell populations and endochondral progression of chondrogenic cells, may be examined in vitro. The work presented here describes mechanisms by which cells within presumptive skeletal cell populations become restricted to a chondrogenic lineage as studied within cell populations derived from 12-day-old chicken embryo calvarial tissue. It is found that a major factor associated with selection of chondrogenic cells is the elimination of growth within serum-containing medium. Chondrogenesis within these cell populations appears to be the result of permissive conditions which select for chondrogenic proliferation over osteogenic cell proliferation. Data suggest that chondrocyte cultures produce autocrine factors that promote their own survival or proliferation. The conditions for promoting cell growth, hypertrophy, and extracellular matrix mineralization of embryonic chicken chondrocytes in vitro include ascorbic acid supplementation and the presence of an organic phosphate source. The differentiation of hypertrophic chondrocytes in vitro is associated with a 10-15-fold increase in alkaline phosphatase enzyme activity and deposition of mineral within the extracellular matrix. Temporal studies of the biochemical changes coincident with development of hypertrophy in vitro demonstrate that proteoglycan synthesis decreases 4-fold whereas type X collagen synthesis increases 10-fold within the same period. Ultrastructural examination reveals cellular and extracellular morphology similar to that of hypertrophic cells in vivo with chondrocytes embedded in a well formed extracellular matrix of randomly distributed collagen fibrils and proteoglycan. Mineral deposition is seen in the interterritorial regions of the matrix between the cells and is apatitic in nature. These characteristics of chondrogenic growth and development are very similar in vivo and in vitro and they suggest that studies of chondrogenesis in vitro may provide a valuable model for the process in vivo.

Expression of type X collagen and matrix calcification in three-dimensional cultures of immortalized temperature-sensitive chondrocytes derived from adult human articular cartilage

Chondrocytes isolated from normal adult human articular cartilage were infected with a retroviral vector encoding a temperature-sensitive mutant of the simian virus 40 large tumor antigen and a linked geneticin (G418)-resistance marker. G418-resistant colonies were then isolated, ring-cloned, and expanded in serum-containing media. Several immortalized chondrocyte cell lines were established from the clones that survived, some of which have been maintained in continuous culture for over 2 years. Despite serial subcultures and maintenance as monolayers, these cells retain expression of markers specific for cells of the lineage, namely type II collagen and aggrecan, detected immunocytochemically. We also examined the phenotype of three of these immortalized cell lines (designated HAC [human articular chondrocyte]) using a pellet culture system, and in this report, we present evidence that a prototype of these lines (HAC-F cells) expresses markers normally associated with hypertrophic chondrocytes. When HAC-F cells were cultivated in centrifuge tubes, for periods of up to 63 days, at 39 degrees C with mild and intermittent centrifugation they continued to express both lineage markers; total type II collagen/pellet remained stable, whereas there was a temporal decrease in cartilage-specific glycosaminoglycans content. In addition, in the presence of ascorbate but in the absence of a phosphate donor or inorganic phosphate supplement, the cells also begin to express a hypertrophic phenotype characterized by type X collagen synthesis and extensive mineralization of the extracellular matrix in late stage cultures. The mRNA encoding type X collagen was detected in the cell pellets by reverse transcriptase polymerase chain reaction as early as day 2, and anti-type X collagen immunoreactivity was subsequently localized in the matrix. The mineral was characterized by energy-dispersive X-ray microanalysis as containing calcium (Ca) and phosphorus (P) with a Ca:P peak height ratio close to that of mineralized bone tissue. The unexpected phenotype of this human chondrocyte cell line provides an interesting opportunity for studying chondrocyte maturation in vitro.

Space-related bone mineral redistribution and lack of bone mass recovery after reambulation in young rats

This study reports the effects of a 14-day spaceflight followed by a 14-day reambulation period on bones of 56-day-old male rats compared with synchronous (S) and vivarium (V) control animals. Femur, tibia, and humerus bone mineral densities (BMD); bone calcium and phosphorus concentrations ([Ca2+] and [P]), measured by X-ray microanalysis (XRM), on tibia, vertebra, and calvaria; and histomorphometric data on proximal primary and secondary spongiosae (I and II SP, respectively) of the tibia and humerus were measured. After the flight in flown rats (compared with S), BMD was lower in the distal femur and remained similar to S in humerus and tibia, [Ca2+] and [P] were lower in tibia II SP and higher in calvaria, tibia I SP width and II SP bone volume were lower, resorption was markedly higher in tibia II SP, and no difference in formation parameters was observed. After reambulation, BMD was lower in long bones of both flight and S groups compared with V. Bone loss appeared in humeral II SP and worsened in tibial II SP in flown rats. Tibial formation parameters were higher in flown rats compared with V and S, indicating the onset of an active recovery. Tibial XRM [Ca2+] and [P] in flown rats remained below control levels.

Analysis of a general principle of crystal nucleation, formation in the different hard tissues

We have found, at high EM magnification, on ultrathin sections of shock-frozen, freeze-dried, embedded pieces of the developing hard tissues, that the primary crystallites consist of strands composed of nanometer-sized apatitic islands, which rapidly coalesce to needles and afterward to platelets. By small-area electron diffraction, with energy-filtered electrons, it was clarified that these strands are already crystallographically oriented along the bipolar c-axis so that the center-to-center distances between the islands would reflect the distances between crystal-nucleating sites along the matrix. The EM analysis of the cross-cut stained unmineralized and of the unstained mineralized collagen fibers of turkey tibia tendon shows that the staining "nuclei" and the early crystallites, appearing as dark dots, surround "light" round structures, which we interpret as the collagen microfibrils, surrounded by the apatitic crystallites.

X-ray diffraction, electron microscopy, and Fourier transform infrared spectroscopy of apatite crystals isolated from chicken and bovine calcified cartilage

Apatite crystals of the calcified zone of the subarticular cartilaginous growth plates of the long bones of young growing chickens and calves were isolated by low temperature reaction with hydrazine and plasma ashing and examined by electron microscopy, electron diffraction and microprobe analysis, and computer-generated deconvolution of the spectra obtained by Fourier transform infrared spectroscopy. The crystal habit was that of wide, very thin, relatively long rectangular plates, which tended to form small clusters of crystals, possibly because reaction with hydrazine alone did not remove all of the organic matrix constituents. Further reaction with low power plasma ashing released more of the isolated crystals although to a lesser extent than was possible with bone. Stereograms of the small clusters showed that many of the crystals in the small isolated aggregates of crystals were bent and/or curved. Together with the resultant overlap of individual adjacent crystals, they also produced images of sharp, very dense lines, reminiscent of the electron-dense needle or rod-like appearances frequently observed by transmission electron microscopy of thin sections of calcified cartilage and thought to represent the habit of the apatite crystals. No true rod or needle-like crystals were observed in the isolated crystals. Although the overall general apatitic structure of the apatite crystals was similar to that of the apatitic crystals of bone, the individual crystals were significantly larger than those of bone from the same specimen, and there were small but significant differences in the concentrations of acid phosphate and carbonate groups and in their short range order.

Matrix vesicles and focal proteoglycan aggregates are the nucleation sites revealed by the lanthanum incubation method: a correlated study on the hypertrophic zone of the rat epiphyseal cartilage

Correlated studies were performed with light and electron microscopy, and backscattered electron image in conjunction with X-ray microanalysis, of lanthanum-incubated epiphyseal cartilage of the young rat. The hall-mark of this procedure is the appearance of LaP electron-dense deposits (not present in control sections) in precise sites of the hypertrophic zone. The ultrastructural study revealed a dual nature of these sites: "dense matrix vesicles" and "focal filament aggregates". The dense matrix vesicles are a specific type of matrix vesicle with the intrinsic capacity of precipitating LaP mineral, as soon as they originate from the hypertrophic chondrocytes. Furthermore, the matrix vesicles were found to be heterogeneous because lanthanum-devoid, "light matrix vesicles" were also present. The focal filament aggregates, which were not recognized in unstained sections and in controls, are apparently focal concentrations of proteoglycans with high lanthanum binding capacity, although the presence in them of other components (e.g., type X collagen, C-propeptide of type II collagen) cannot be excluded. The were in close connection with the light matrix vesicles in the upper hypertrophic zone, and were loaded with a variable quantity of LaP irregular electron-dense deposits in the lower hypertrophic zone. These irregular deposits are similar to, but distinct from, calcification nodules. The lanthanum incubation method indirectly detects the matrix Ca-binding components (which bind La ions), and the calcification initiation sites (which precipitate a LaP-mineral phase). A sequence is proposed of successive steps of LaP nucleation within the focal filament aggregates, which possibly mimics calcium phosphate deposition. Such a sequence seems to require the participation not only of dense matrix vesicles, but also of the filamentous components of the focal aggregates, possibly together with the activity of alkaline phosphatase.

The isolation and characterization of magnesium whitlockite crystals from human articular cartilage

A number of basic calcium phosphate crystals have been demonstrated in human articular tissues. The exact relationship between crystal deposition and disease remains obscure, although there is evidence supporting a rapid degenerative arthropathy within a specific set of patients. Limited reports of 'cuboid' calcium phosphate microcrystals in articular cartilage have been made over the last 10 years. In this study the occurrence of such crystals, not apparent by light microscopy, in human articular cartilage has been confirmed by transmission electron microscopy and X-ray microanalysis of tissue prepared by aqueous and anhydrous processing techniques. A crystal isolation technique involving collagenase digestion, centrifugation and sodium hypochlorite treatment was developed enabling crystal characterization by electron and X-ray diffraction. Crystals were identified as magnesium whitlockite; the first report of this mineral in articular cartilage. The presence of this mineral phase in normal and osteoarthritic articular cartilage is discussed with consideration given to physical conditions known to favor whitlockite formation and those extant in articular cartilage.

Initial enamel crystals are not spatially associated with mineralized dentine

During epithelial-mesenchymal interactions associated with mammalian tooth development, epithelially-derived and mesenchymally-derived extracellular matrix molecules form a discrete dentine-enamel junction. The developmental and molecular processes required to form this junction are not known. To address this problem we designed studies to test the hypothesis that ectodermally-derived epithelial cells synthesize and secrete enamel proteins which function to nucleate and regulate the growth of enamel calcium phosphate crystals. Initial enamel crystals were detected separate from the adjacent dentine. Electron-microprobe analyses revealed that early enamel crystals were octacalciumphosphate or tricalciumphosphate rather than hydroxyapatite. Thereafter, enamel crystals became confluent with the adjacent, albeit significantly smaller hydroxyapatite crystals associated with mineralized dentine. Therefore, we interpret our data to indicate that de novo enamel crystal nucleation and growth are independent from the mineralization processes characterized for dentine. We further argue that gene expression of enamel protein appears to have a constitutive function during early enamel formation and that supramolecular aggregates of amelogenin and enamelin provide the microenvironment for the nucleation and crystal growth of the initial enamel matrix.

Investigation of the early mineralisation on collagen in dentine of rat incisors by quantitative electron spectroscopic diffraction (ESD)

The earliest crystallites in dentine appear as chains of "dots" in ultra-thin sections viewed by transmission electron microscopy. These dots rapidly coalesce along the longitudinal directions of the collagen microfibrils to form needle-like structures that coalesce preferentially in lateral directions to form ribbon-like or plate-like crystallites. This morphological interpretation is supported by line-scans of the corresponding zero-loss filtered electron spectroscopic diffraction patterns, which demonstrate the crystalline structure of the dentine mineral (apatite). The intensity ratio of the Debye-Scherrer rings of the characteristic Bragg-reflections (002 to 300, together with 1 or 2 unresolved reflections) shows a maximum in the region of early chain-like and needle-like crystallites, decreasing with maturation of the dentine mineral to the ribbon-plate-like crystallites. Detailed investigations using line-scans of the zero-loss filtered electron spectroscopic diffraction patterns through the dentine zone show that the intensity ratio found near the mineralisation front is repeated 3-5 times at distances of about 10-20 microns. This may represent a circadian pattern of mineralisation corresponding to light microscopically visible incremental lines in dentine.

Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Growth plate chondrocytes play a pivotal role in promoting longitudinal bone growth. The current review represents a brief survey of the phenomena involved in this process at the cellular level; it delineates the contributions made by various activities during the course of the chondrocyte life cycle, notably proliferation and hypertrophy, and illustrates how the relative contributions may be modulated according to the particular needs of an organism at critical phases of growth. The cellular mechanisms by which a few well characterized growth-promoting substances exert their influences are discussed in the light of recent findings pertaining to epiphyseal plate chondrocytes in vivo.

Occurrence of osteoblast necroses during ossification of long bone cortices in mouse fetuses

Previous investigations concerned with in vitro osteogenesis and mineralization have revealed some indication of a participation of cell necroses in the course of calcification. These observations were confirmed by in vivo investigations on desmoid ossification in fetal mouse calvariae, where abundant necrotic osteoblasts were found at the mineralization border and in the osteoid. In the present study, ossification of long bone cortices from fetal mice was investigated by use of electron microscopy. Specimens obtained from the collection of the Institute of Anatomy, Free University of Berlin (mouse fetuses, forearm; rat fetuses, forearm) were reinvestigated for control purposes. In all cases, mineralization of osteoid was accompanied by cell necroses. Cell degeneration was characterized by swelling of the endoplasmic reticulum and loss of the plasma membrane resulting in freely distributed vesicular structures. Cell debris was incorporated within the mineral. Initially, cell necroses in the perichondrium occurred in the region surrounding the hypertrophic cartilage and the matrix of which showed spots of endochondral mineralization. Necrotic osteoblasts occurred simultaneously with mineralization of the osteoid. During further ossification of the long bone cortices, the number of necrotic cells increased markedly. In addition to necrotic cells, healthy osteoblasts, osteocytes and perichondral tissue were present, indicating that an artifact can be excluded. The importance of cell necroses in the process of mineralization is as yet unclear. Possibly, the cells act as calcium and/or phosphate stores, which are liberated by cell death to increase the amount of mineral constituents at sites of mineralization.

Electron microscopy of calcification during high-density suspension culture of chondrocytes

Chondrocyte cultures grown in centrifuge tubes with intermittent centrifugation differentiate into hypertrophic chondrocytes and form calcification. We examined chondrocytes cultured in this system electron microscopically. Rat growth-plate chondrocytes were seeded in a plastic centrifuge tube and cultured in the presence of Eagle's minimum essential medium supplemented with 10% fetal bovine serum and 50 micrograms of ascorbic acid per ml. Specimens were examined by using electron microscopy and selected-area electron-diffraction techniques. In the early stage of culture, a few chondrocytes were scattered and extracellular matrices were not observed. In the middle stage of the cultures, the chondrocytes resembled proliferative cells. Matrix vesicles appeared to be budding from the cell surfaces of chondrocytes and were observed sparsely in the extracellular matrices, which were well formed around the chondrocytes. Matrix vesicles increased substantially during the following cultures. In the mature stage of the cultures, crystal formation related to matrix vesicles was observed. In the 33-day cultures, several masses of calcified matrix were formed and it was confirmed to be apatite by selected-area electron diffraction analysis. The chondrocytes appeared hypertrophic during this same stage. The 56-day culture was similar to the 33-day culture. It was concluded that this culture system provides an extracellular-matrix mineralization which is produced by chondrocytes per se.

Tubule formation and elemental detection in developing opossum enamel

Most marsupials and some placental mammals possess enamel characterized by the presence of tubules, and the cellular origin of these structures has been the subject of a number of previous studies (See, for example, Lester, 1970; Azevedo and Goldberg, 1987). In the present report, tooth germs of the American opossum were examined to determine the structure and composition of enamel tubules during development and to analyze the enamel matrix relative to that of placental mammals with atubular enamel. For this purpose, tissues prepared by aqueous (decalcified and undecalcified) and anhydrous (undecalcified) methods were investigated by conventional transmission (TEM) and high voltage electron microscopy (HVEM), as well as by electron probe x-ray microanalysis (EPMA), selected-area electron diffraction (SAED), and electron spectroscopic imaging (ESI). Results indicate that most enamel tubules in the opossum begin as cytoplasmic remnants of Tomes' processes of ameloblasts. During development of the matrix, some of the tubules do not appear to be continuous throughout the prismatic layer. Sulfur is detectable around the lumen of the tubule in decalcified sections by EPMA and in and around the tubule by ESI. Calcium/phosphorus (Ca/P) molar ratios of the mineralizing matrix are generally higher than those found in enamel of other mammals and appear to decrease rather than increase with enamel maturation. The summary of data indicates the presence of sulfated glycoproteins or proteoglycans in this tissue, specifically around enamel tubules. Calcium and phosphorus are also present within the tubules, with the sulfated groups possibly binding calcium to prevent mineralization of the enamel tubules themselves.

Early mineral deposition in calcifying tendon characterized by high voltage electron microscopy and three-dimensional graphic imaging

Extracellular matrix organization and the spatial relationship between collagen fibrils, vesicular structures, and the first deposits of mineral in the calcifying leg tendon from the domestic turkey, Meleagris gallopavo, have been investigated by high voltage electron microscopy and three-dimensional computer graphic imaging of serial thick tissue sections. The work demonstrates that the tendon extracellular matrix is a complex assembly of somewhat flexible, highly aligned collagen fibrils with different diameters and occasionally opposite directionality. Smaller collagen fibrils appear to branch from larger fibrils or to aggregate to form those of greater size. While the matrices are dominated by fibrils, space exists between adjacent packed fibrils. The three-dimensional perspective indicates that approximately 60% of the total tendon volume is extrafibrillar over the regions examined. The first observable mineral in this tissue is extrafibrillar and appears to derive from vesicles. This view of three-dimensional matrix-mineral spatial relations supports earlier two-dimensional results that mineral is initially associated with membrane-invested vesicles and is deposited between collagen fibrils, but it is distinct in showing the mineral at different depths in the matrix rather than at a single depth as deduced from two-dimensional conventional electron microscopy. These results are important in the onset and development of tendon calcification in that they suggest, first, that collagen fibrils appear to be aligned three-dimensionally such that their hole zones are in contiguous arrangement. This situation may create channels or grooves within the collagen volume to accommodate extensive mineral deposition in association with the fibrils. Second, the results indicate that there are widely dispersed sites of vesicle-mediated mineralization in the tendon matrix, that the bulk of mineralization in this tissue is collagen-mediated, and that, while vesicles may possibly exert some local influence temporally on mineralization of neighboring collagen, vesicle- and collagen-mediated mineralization arise at spatially and structurally distinct sites by independent nucleation phenomena. Such concepts are fundamental in considerations of possible mechanisms of mineralization of tendon and potentially of other normally calcifying vertebrate tissues in general.

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.

The role of the growth plate in longitudinal bone growth

The epiphyseal growth plate is the main site of longitudinal growth of the long bones. At this site, cartilage is formed by the proliferation and hypertrophy of cells and synthesis of the typical extracellular matrix. The formed cartilage is then calcified, degraded, and replaced by osseous tissue. Proliferation and differentiation of cartilage cells (i.e., chondrocytes) as studied mostly in culture, is regulated by various endocrine, paracrine, and autocrine agents such as growth hormone, insulin-like growth factor-I (IGF-I), transforming growth factor (TGE-beta), and vitamin D metabolites (1,25-dihydroxycholecalciferol and 24,25-dihydroxycholecalciferol). Avian chondrocyte proliferation is enhanced by agents which use adenosine 3':5'-cyclic monophosphate as a second messenger, such as parathyroid hormone or prostaglandin-E2, and is depressed by guanosine 3':5'-cyclic monophosphate agonists, such as atrial natriuretic peptide. Several of the regulating agents also affect synthesis of the main extracellular components (i.e., collagen and proteoglycans) and their transfer to the extracellular space. Cartilage calcification involves matrix vesicles secreted by the chondrocytes at a specific stage. Calcification probably involves some initial nucleation agent and participation of phosphatases. During sexual maturation, the growth plate closes by an unknown mechanism and longitudinal bone growth ceases. Disorders in the metabolism of the controlling agents or the cellular responses in growth plate may lead to several deformities classified as dysplasias. In poultry, this class of disorders is represented by chondrodystrophy and dyschondroplasia.

Structural studies of the mineral phase of calcifying cartilage

The calcified cartilage of the epiphyseal growth plate of young calves has been studied by x-ray diffraction. Fourier transform infrared spectroscopy, magic angle 31P nuclear magnetic resonance spectroscopy, and chemical composition. The powdered tissue was separated by density centrifugation as a function of mineral content and thus qualitatively of the age of the calcium-phosphorus mineral phase. The individual density centrifugation fractions were examined separately. X-ray diffraction of the samples, especially of the lowest density fractions, revealed very poorly crystalline apatite. Fourier transform infrared spectroscopy and 31P nuclear magnetic resonance spectroscopy revealed the presence of significant amounts of nonapatitic phosphate ions. The concentration of such nonapatitic phosphates increases during the early stages of mineralization but then decreases as the mineral content steadily rises until full mineralization is achieved. The total concentration of carbonate ions was found to be much lower in calcified cartilage than in bone from the same organ (scapula). The carbonate ions are located in both A sites (OH-) and B sites (PO4(3-)), with a distribution similar to that found in bone mineral. However, discrepancies between infrared resolution factors of phosphate and carbonate bands are consistent with a heterogeneous distribution of carbonate ions in poorly organized domains of the solid phase of calcium phosphate. These initial studies permit one to characterize the calcium phosphate mineral phase as a very poorly crystalline, immature calcium phosphate apatite, rich in labile nonapatitic phosphate ions, with a low concentration of carbonate ions compared with bone mineral of the same animal, indeed from the bone of the same organ (scapula).

Patterns of mineralization in vitro

Various patterns of mineralization are found in the organism during fetal and postnatal development. Different findings and theories have been published in the literature with regard to the mechanisms of mineralization, many of which are controversely discussed. In the present study the different patterns of mineralization observed in the organoid culture system of fetal rat calvarial cells were investigated by electron microscopy. In organoid culture, calvarial cells grow and differentiate at high density, and deposition of osteoid and mineralization of the matrix occur to a very high extent. Different types of mineralization could be observed more or less simultaneously. It was found that hydroxyapatite crystals were formed at collagen fibrils as well as in the interfibrillar space. Mineralization was frequently seen in necrotic cells and cellular remnants as well as in extra- and intracellular vesicles. Addition of bone or dentin matrices or the artificial hydroxyapatite Interpore 200 to the cells caused an increased mineralization in the vicinity and on the surface of the matrices with and without participation of collagen. On previously formed mineralized nodules, an apposition of mineralizing material appeared due to matrix secretion by osteoblasts. It is concluded that initiation of mineralization occurs--at least in vitro--at every nucleation point under appropriate conditions. These mineralization foci enlarge by further apposition as well as by cellular secretion of a mineralizing matrix. Furthermore, cell necroses may liberate mineralizable vesicles. All these patterns of mineralization are the result of different activities of one cell type.

Gene expression and extracellular matrix ultrastructure of a mineralizing chondrocyte cell culture system

Conditions were defined for promoting cell growth, hypertrophy, and extracellular matrix mineralization of a culture system derived from embryonic chick vertebral chondrocytes. Ascorbic acid supplementation by itself led to the hypertrophic phenotype as assessed by respective 10- and 15-fold increases in alkaline phosphatase enzyme activity and type X synthesis. Maximal extracellular matrix mineralization was obtained, however, when cultures were grown in a nutrient-enriched medium supplemented with both ascorbic acid and 20 mM beta-glycerophosphate. Temporal studies over a 3-wk period showed a 3-4-fold increase in DNA accompanied by a nearly constant DNA to protein ratio. In this period, total collagen increased from 3 to 20% of the cell layer protein; total calcium and phosphorus contents increased 15-20-fold. Proteoglycan synthesis was maximal until day 12 but thereafter showed a fourfold decrease. In contrast, total collagen synthesis showed a greater than 10-fold increase until day 18, a result suggesting that collagen synthesis was replacing proteoglycan synthesis during cellular hypertrophy. Separate analysis of individual collagen types demonstrated a low level of type I collagen synthesis throughout the 21-d time course. Collagen types II and X synthesis increased during the first 2 wk of culture; thereafter, collagen type II synthesis decreased while collagen type X synthesis continued to rise. Type IX synthesis remained at undetectable levels throughout the time course. The levels of collagen types I, II, IX, and X mRNA and the large proteoglycan core protein mRNA paralleled their levels of synthesis, data indicating pretranslational control of synthesis. Ultrastructural examination revealed cellular and extracellular morphology similar to that for a developing hypertrophic phenotype in vivo. Chondrocytes in lacunae were surrounded by a well-formed extracellular matrix of randomly distributed collagen type II fibrils (approximately 20-nm diam) and extensive proteoglycan. Numerous vesicular structures could be detected. Cultures mineralized reproducibly and crystals were located in extracellular matrices, principally associated with collagen fibrils. There was no clear evidence of mineral association with extracellular vesicles. The mineral was composed of calcium and phosphorus on electron probe microanalysis and was identified as a very poorly crystalline hydroxyapatite on electron diffraction. In summary, these data suggest that this culture system consists of chondrocytes which undergo differentiation in vitro as assessed by their elevated levels of alkaline phosphatase and type X collagen and their ultrastructural appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Vectorial sequence of mineralization in the turkey leg tendon determined by electron microscopic imaging

Turkey leg tendons were used as a model tissue to study the spatial and temporal relationships of mineral deposition between matrix vesicles and collagen fibrils by various electron microscopic techniques--bright field, selected-area dark field (SADF), and electron spectroscopic imaging (ESI). These latter imaging techniques enabled the direct localization and spatial distributions of both apatite crystals and atomic elements (Ca, P) within matrix vesicles and collagen. In longitudinal planes of section, a consistent vectorial gradient of mineralization was observed which started with the first localization of apatite mineral in matrix vesicles; with further development, the mineral spread from the vesicle to the extravesicular interstices and then into the adjacent collagen fibrils. Once intrafibrillar, the mineral was observed to advance both laterally and axially. The association of vesicle/collagen mineral was examined by ESI analysis of Ca and P elemental maps and appeared as a continuum between the vesicles and the adjacent collagen fibrils. Similarly, an intimate spatial relationship was observed between the mineral of vesicles and collagen in transversely cut sections of tendon. The sequential development of this mineralized matrix is discussed in light of matrix vesicle/collagen interactions.

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.

Electron microscopic evaluation of the occurrence of matrix vesicles in cartilage

Troubled by variations in the descriptions of shape, appearance, and content of matrix vesicles and the conflicting reports of increased numbers of vesicles in the mineralizing regions of the growth plate contrasted with larger numbers in the resting zone, we embarked on a review of matrix vesicles in the growth plate using a comparison of different fixation techniques. We found matrix vesicles resembling cell debris at all levels of the growth plate, with no particular association with mineral. Lipid bodies surrounded by a membrane of proteoglycan have also been seen in large numbers. The cell debris-like matrix vesicles have been the common finding in reports of digested centrifuged cartilage and may represent cytoplasmic processes. Lipid bodies surrounded by proteoglycan may be similar to the membrane-bound vesicle described by Ali (Fed. Proc., 35:135-142, 1987) and by Bonucci (Clin. Orthod., 78:108-133, 1971).

Endochondral mineralization in cartilage organoid culture

In the development of secondary bone, mineralization of the cartilage matrix is the first step in endochondral mineralization. The circumstances of cartilage mineralization are not known. Influences of the periosteal tissue have been mentioned. In order to investigate the role of osteoblastic cells in endochondral mineralization, cartilage organoid cultures were induced to mineralize by the addition of beta-glycerophosphate (beta-GP). In cartilage organoid culture, embryonic mouse limb bud mesenchymal cells were grown at high-density. The cells differentiated into mature chondrocytes and produced hyaline cartilage matrix. When cartilage had formed after 6 days in vitro, 10 mM beta-GP was added. The developed mineralized cartilage was investigated by morphological means. Seven days after the addition of beta-GP, the first mineralized spots were visible mainly in the internodular, noncartilage tissue. After 12 to 14 days, large areas of cartilage were mineralized, and after 21 days, nearly the whole culture had been mineralized. Electron microscopic investigations showed a dramatic alteration of the cartilage matrix followed by a homogeneous mineralization of the cartilage matrix. The chondrocytes in the mineralized area died and faded. Typical rod-like apatite crystals were visible at the border between the mineralized and the unmineralized matrix. This result closely resembles the in vivo situation of cartilage mineralization. Addition of osteoblastic calvarial cells enhanced the mineralization process, as did the addition of conditioned medium of calvarial cell monolayers. Under these treatments, mineralization started after 3 days and reached a maximum after 14 days. On the other hand, addition of mouse skin fibroblast-like cells without a direct contact to the cartilage inhibited cartilage mineralization. These results indicate that osteoblastic cells induce endochondral mineralization, whereas fibroblast-like cells inhibit this mineralization via soluble factors.

Visualization of sulfur-containing components associated with proliferating chondrocytes from rat epiphyseal growth plate cartilage: possible proteoglycan and collagen co-migration

Electron microscopy of epiphyseal growth plate cartilage from normal 4-5-week-old rats has revealed extensive fibrillar aggregates and globules in the pericellular spaces of proliferating chondrocytes. These cells contained small globules and diffusely coiled, fine filaments located within large, membrane-invested vacuoles. All such structures were observed after a variety of different tissue fixation regimes, including glutaraldehyde, osmium tetroxide, and potassium pyroantimonate. The fibrillar aggregates and globules were often overlapping and intermeshed and extended to 0.5 micron in length from their point of origin at cell membranes. Vacuoles were usually found at the periphery of cells, and some, by membrane fusion with the cell envelope, appeared contiguous with extracellular spaces wherein their contents could be discharged. Fine filaments and globules were occasionally observed in the Golgi complex and cisternae of endoplasmic reticulum of the chondrocytes. Further characterization of the cellular and pericellular components by electron microscopic radioautography, electron probe microanalysis, and electron spectroscopic imaging indicated the presence of sulfur, a result suggesting these aggregates, filaments, and globules in part represent proteoglycans in various stages of synthesis, secretion, and assembly. Additional radioautography utilizing 3H-proline implied that filament bundles are also composed of collagen, a result posing the possibility that this protein and the putative proteoglycans may co-migrate both intracellularly and within pericellular matrices. In extracellular matrices adjacent to cell lacunae, the fibrillar aggregates appeared in close association with typical collagen type II fibrils, an observation providing evidence for proteoglycan-collagen network formation in this region of the rat epiphysis. These microscopic and analytical data in situ would support certain studies in vitro of proteoglycan-collagen type II and IX association and are important in describing the interaction of such cartilage components ultimately involved in matrix formation.

Immunoelectron microscopic studies of type X collagen in endochondral ossification

Immunofluorescence and immunoelectron microscopy were used in conjunction with a monoclonal antibody to investigate the localization of type X collagen in the proximal tibial growth plate of 7-d-old chicks. This molecule was detected throughout the hypertrophic zone first appearing when chondrocytes exhibited hypertrophy: it was absent from the proliferative zone. Type X collagen was primarily associated with type II collagen fibrils as demonstrated by immunogold staining. Type X collagen was not concentrated in the focal calcification sites nor was it associated with matrix vesicles. These observations suggest that type X collagen may play a role other than that directly related to the nucleation of calcification.

Cartilage macromolecules and the calcification of cartilage matrix

The calcification of cartilage matrix in endochondral bone formation occurs in an extracellular matrix composed of fibrils of type II collagen with which type X collagen is closely associated. Also present within this matrix are the large proteoglycans containing chondroitin sulfate which aggregate with hyaluronic acid. In addition, the matrix contains matrix vesicles containing alkaline phosphatase. There is probably a concentration of calcium as a result of its binding to the many chondroitin sulfate chains. At the time of calcification, these proteoglycans become focally concentrated in sites where mineral is deposited. This would result in an even greater focal concentration of calcium. Release of inorganic phosphate, as a result of the activity of alkaline phosphatase, can lead to the displacement of proteoglycan bound calcium and its precipitation. The C-propeptide of type II collagen becomes concentrated in the mineralizing sites, prior to which it is mainly associated with type II collagen fibrils and is present in dilated cisternae of the enlarged hypertrophic chondrocytes. The synthesis of type II collagen and the C-propeptide, together with alkaline phosphatase, are regulated by the vitamin D metabolites 24,25(OH)2 cholecalciferol and 1,25 (OH)2 cholecalciferol. At the time of calcification, type X collagen remains associated with type II collagen fibrils. It may play a role in preventing the initial calcification of these fibrils focusing mineral formation in focal interfibrillar sites. This process of calcification is clearly very complex, and involves different interacting matrix molecules and is carefully regulated at the cellular level.