Mineralization in osteoblast cultures: a light and electron microscopic study

Bone Formation in 2D Culture of Primary Cells

Relevance of mineralized nodules in two-dimensional (2D) osteoblast/osteocyte cultures to bone biology, pathology, and engineering is a decades old question, but a comprehensive answer appears to be still wanting. Bone-like cells, extracellular matrix (ECM), and mineral were all reported but so were non-bone-like ones. Many studies described seemingly bone-like cell-ECM structures based on similarity to few select bone features in vivo, yet no studies examined multiple bone features simultaneously and none systematically studied all types of structures coexisting in the same culture. Here, we report such comprehensive analysis of 2D cultures based on light and electron microscopies, Raman microspectroscopy, gene expression, and in situ messenger RNA (mRNA) hybridization. We demonstrate that 2D cultures of primary cells from mouse calvaria do form bona fide bone. Cells, ECM, and mineral within it exhibit morphology, structure, ultrastructure, composition, spatial-temporal gene expression pattern, and growth consistent with intramembranous ossification. However, this bone is just one of at least five different types of cell-ECM structures coexisting in the same 2D culture, which vary widely in their resemblance to bone and ability to mineralize. We show that the other two mineralizing structures may represent abnormal (disrupted) bone and cartilage-like structure with chondrocyte-to-osteoblast transdifferentiation. The two nonmineralizing cell-ECM structures may mimic periosteal cambium and pathological, nonmineralizing osteoid. Importantly, the most commonly used culture conditions (10mM β-glycerophosphate) induce artificial mineralization of all cell-ECM structures, which then become barely distinguishable. We therefore discuss conditions and approaches promoting formation of bona fide bone and simple means for distinguishing it from the other cell-ECM structures. Our findings may improve osteoblast differentiation and function analyses based on 2D cultures and extend applications of these cultures to general bone biology and tissue engineering research. Published 2022. This article is a U.S. Government work and is in the public domain in the USA. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Human Primary Odontoblast-like Cell Cultures—A Focused Review Regarding Cell Characterization

Cell cultures can provide useful in vitro models. Since odontoblasts are postmitotic cells, they cannot be expanded in cell cultures. Due to their extension into the dentin, injuries are inevitable during isolation. Therefore, “odontoblast-like” cell culture models have been established. Nowadays, there is no accepted definition of odontoblast-like cell cultures, i.e., isolation, induction, and characterization of cells are not standardized. Furthermore, no quality-control procedures are defined yet. Thus, the aim of this review was to evaluate both the methods used for establishment of cell cultures and the validity of molecular methods used for their characterization. An electronic search was performed in February 2022 using the Medline, Scopus, and Web of Science database identifying publications that used human primary odontoblast-like cell cultures as models and were published between 2016 and 2022. Data related to (I) cell culture conditions, (II) stem cell screening, (III) induction media, (IV) mineralization, and (V) cell characterization were analyzed. The included publications were not able to confirm an odontoblast-like nature of their cell cultures. For their characterization, not only a similarity to dentin but also a distinction from bone must be demonstrated. This is challenging, due to the developmental and evolutionary proximity of these two tissue types.

Prostate Osteoblast-Like Cells: A Reliable Prognostic Marker of Bone Metastasis in Prostate Cancer Patients

The main aim of this study was to investigate the putative association among the presence of prostate cancer cells, defined as prostate osteoblast-like cells (POLCs), and showing the expression of typical morphological and molecular characteristics of osteoblasts, the development of bone metastasis within 5 years of diagnosis, and the uptake of 18F-choline evaluated by PET/CT analysis. To this end, prostate biopsies (n = 110) were collected comprising 44 benign lesions and 66 malignant lesions. Malignant lesions were further subdivided into two groups: biopsies from patients that had clinical evidence of bone metastasis (BM+, n = 23) and biopsies from patients that did not have clinical evidence of bone metastasis within 5 years (BM-, n = 43). Paraffin serial sections were obtained from each specimen to perform histological classifications and immunohistochemical (IHC) analysis. Small fragments of tissue were used to perform ultrastructural and microanalytical investigations. IHC demonstrated the expression of markers of epithelial-to-mesenchymal transition (VIM), bone mineralization, and osteoblastic differentiation (BMP-2, PTX-3, RUNX2, RANKL, and VDR) in prostate lesions characterized by the presence of calcium-phosphate microcalcifications and high metastatic potential. Ultrastructural studies revealed the presence of prostate cancer cells with osteoblast phenotype close to microcalcifications. Noteworthy, PET/CT analysis showed higher uptake of 18F-choline in BM+ lesions with high positivity (≥300/500 cells) for RUNX2 and/or RANKL immunostaining. Although these data require further investigations about the molecular mechanisms of POLCs generation and role in bone metastasis, our study can open new and interesting prospective in the management of prostate cancer patients. The presence of POLCs along with prostate microcalcifications may become negative prognostic markers of the occurrence of bone metastases.

Non-sutural basicranium-derived cells undergo a unique mineralization pathway via a cartilage intermediate in vitro

The basicranium serves as a key interface in the mammalian skull, interacting with the calvarium, facial skeleton and vertebral column. Despite its critical function, little is known about basicranial bone formation, particularly on a cellular level. The goal of this study was therefore to cultivate a better understanding of basicranial development by isolating and characterizing the osteogenic potential of cells from the neonatal murine cranial base. Osteoblast-like basicranial cells were isolated, seeded in multicellular aggregates (designated micromasses), and cultured in osteogenic medium in the presence or absence of bone morphogenetic protein-6 (BMP6). A minimal osteogenic response was observed in control osteogenic medium, while BMP6 treatment induced a chondrogenic response followed by up-regulation of osteogenic markers and extensive mineralization. This response appears to be distinct from prior analyses of the calvarium and long bones, as basicranial cells did not mineralize under standard osteogenic conditions, but rather required BMP6 to stimulate mineralization, which occurred via an endochondral-like process. These findings suggest that this site may be unique compared to other cranial elements as well as the limb skeleton, and we propose that the distinct characteristics of these cells may be a function of the distinct properties of the basicranium: endochondral ossification, dual embryology, and complex loading environment.

Concise Review: In Vitro Formation of Bone-Like Nodules Sheds Light on the Application of Stem Cells for Bone Regeneration

: Harnessing the differentiation of stem cells into bone-forming cells represents an intriguing avenue for the creation of functional skeletal tissues. Therefore, a profound understanding of bone development and morphogenesis sheds light on the regenerative application of stem cells in orthopedics and dentistry. In this concise review, we summarize the studies deciphering the mechanisms that govern osteoblast differentiation in the context of in vitro formation of bone-like nodules, including morphologic and molecular events as well as cellular contributions to mineral nucleation, occurring during osteogenic differentiation of stem cells. This article also highlights the limitations of current translational applications of stem cells and opportunities to use the bone-like nodule model for bone regenerative therapies.

SIGNIFICANCE

Harnessing the differentiation of stem cells into bone-forming cells represents an intriguing avenue for the creation of functional skeletal tissues. Therefore, a profound understanding of bone development and morphogenesis sheds light on the regenerative application of stem cells in orthopedics and dentistry. In this concise review, studies deciphering the mechanisms that govern osteoblast commitment and differentiation are summarized. This article highlights the limitations of current translational applications of stem cells and the opportunities to use the bone-like nodule model for bone regenerative therapies.

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.

Nanohydroxyapatite shape and its potential role in bone formation: an analytical study

Bone cells (osteoblasts) produce a collagen-rich matrix called osteoid, which is mineralized extracellularly by nanosized calcium phosphate (CaP). Synthetically produced CaP nanoparticles (NPs) have great potential for clinical application. However few studies have compared the effect of CaP NPs with different properties, such as shape and aspect ratio, on the survival and behaviour of active bone-producing cells, such as primary human osteoblasts (HOBs). This study aimed to investigate the biocompatibility and ultrastructural effects of two differently shaped hydroxyapatite [Ca10(PO4)6(OH)2] nanoparticles (HA NPs), round- (aspect ratio 2.12, AR2) and rice-shaped (aspect ratio 3.79, AR4). The ultrastructural response and initial extracellular matrix (ECM) formation of HOBs to HA NPs were observed, as well as matrix vesicle release. A transmission electron microscopy (TEM)-based X-ray microanalytical technique was used to measure cytoplasmic ion levels, including calcium (Ca), phosphorus (P), sodium (Na) and potassium (K). K/Na ratios were used as a measure of cell viability. Following HA NP stimulation, all measured cytoplasmic ion levels increased. AR2 NPs had a greater osteogenic effect on osteoblasts compared with AR4 NPs, including alkaline phosphatase activity and matrix vesicle release. However, they produced only a moderate increase in intracellular Ca and P levels compared with AR4. This suggests that particular Ca and P concentrations may be required for, or indicative of, optimal osteoblast activity. Cell viability, as measured by Na and K microanalysis, was best maintained in AR2. Initial formation of osteoblast ECM was altered in the presence of either HA NP, and immuno-TEM identified fibronectin and matrilin-3 as two ECM proteins affected. Matrilin-3 is here described for the first time as being expressed by cultured osteoblasts. In summary, this novel and in-depth study has demonstrated that HA NP shape can influence a range of different parameters related to osteoblast viability and activity.

Correlative spectroscopy of silicates in mineralised nodules formed from osteoblasts

Silicon supplementation has been shown to play an important role in skeleton development, however, the potential role that silicon plays in mediating bone formation, and an understanding of where it might localise in the resulting bone tissue remain elusive. An improved understanding of these processes could have important implications for treating pathological mineralisation. A key aspect of defining the role of silicon in bone is to characterise its distribution and coordination environment, however, there is currently almost no information available on either. We have combined a sample-preparation method that simultaneously preserved mineral, ions, and the extracellular matrix (ECM) with secondary ion mass spectroscopy (SIMS) and electron energy-loss spectroscopy (EELS) to examine the distribution and coordination environment of silicon in murine osteoblasts (OBs) in an in vitro model of bone formation. SIMS analysis showed a high level of surface contamination from polydimethysiloxane (PDMS) resulting from sample preparation. When the PDMS was removed, silicon compounds could not be detected within the nodules either by SIMS or by energy dispersive X-ray spectroscopy (EDX) analysis. In comparison, electron energy-loss spectroscopy (EELS) provided a powerful and potentially widely applicable means to define the coordination environment and localisation of silicon in mineralising tissues. We show that trace levels of silicon were only detectable from the mineral deposits located on the collagen and in the peripheral region of mineralised matrix, possibly the newly mineralised regions of the OB nodules. Taken together our results suggest that silicon plays a biological role in bone formation, however, the precise mechanism by which silicon exerts its physicochemical effects remains uncertain. Our analytical results open the door for compelling new sets of EELS experiments that can provide detailed and specific information about the role that silicates play in bone formation and disease.

Ultrastructural and mineral phase characterization of the bone-like matrix assembled in F-OST osteoblast cultures

Cell cultures are often used to study bone mineralization; however, not all systems achieve a bone-like matrix formation. In this study, the mineralized matrix assembled in F-OST osteoblast cultures was analyzed, with the aim of establishing a novel model for bone mineralization. The ultrastructure of the cultures was investigated using scanning electron microscopy, atomic force microscopy, and transmission electron microscopy (TEM). The mineral phase was characterized using conventional and high-resolution TEM, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and solid-state (31)P and (1)H nuclear magnetic resonance. F-OST osteoblast cultures presented a clear nodular mineralization pattern. The chief features of the mineralizing nodules were globular accretions ranging from about 100 nm to 1.5 μm in diameter, loaded with needle-shaped crystallites. Accretions seemed to bud from the cell membrane, increase in size, and coalesce into larger ones. Arrays of loosely packed, randomly oriented collagen fibrils were seen along with the accretions. Mineralized fibrils were often observed, sometimes in close association with accretions. The mineral phase was characterized as a poorly crystalline hydroxyapatite. The Ca/P atomic ratio was 1.49 ± 0.06. The presence of OH was evident. The lattice parameters were a = 9.435 Å and c = 6.860 Å. The average crystallite size was 20 nm long and 10 nm wide. Carbonate substitutions were seen in phosphate and OH sites. Water was also found within the apatitic core. In conclusion, F-OST osteoblast cultures produce a bone-like matrix and may provide a good model for bone mineralization studies.

The response of mineralizing culture systems to microtextured and polished titanium surfaces

The surface texture of titanium has a predictable effect on peri-implant tissue formation in vivo. When implanted in an osseous environment, smooth surfaces (R(a) < 0.5 mm) are generally apposed by fibrous tissue and textured surfaces (R(a) > 1.0 mm) are generally apposed by osseous tissue. Thus in vitro study assessed the mineralization and proliferation response of TF274, MC3T3-E1, murine femoral stromal cells and canine stromal cells to tissue culture plastic (R(a) = 0.001 mm), polished (R(a) = 0.01 mm) and irregularly textured (R(a) = 3.26 mm) titanium surfaces. Amongst all culture systems, proliferation was significantly decreased on textured vs. smooth surfaces. Midway through the culture of the canine marrow cells, the cell layer detached from the tissue culture plastic and polished titanium surfaces. The TF274, MC3T3-E1, murine femoral stromal cell systems formed a mineralized matrix on the tissue culture plastic and polished titanium surfaces which was not observed with the canine stromal cell system. Compared to the tissue culture plastic and polished titanium surfaces, matrix mineralization was significantly reduced on the textured titanium surfaces for the TF274, MC3T3-E1, murine femoral stromal systems, a result which was differed significantly in comparison to the canine stromal system. These results were surprising given the large number of reports concerning the in vivo response to titanium in clinical and pre-clinical studies. Further work is required to determine if the TF274, MC3T3-E1 and murine femoral stromal systems are suitable for the in vitro investigation of the effects of titanium surface texture on osteoblast activity.

Isolation and culture of rodent osteoprogenitor cells

Osteoblasts are the cells responsible for formation of new bone throughout life. Rats are one of the most widely studied mammalian species in skeletal biology and serve as useful models for many aspects of human skeletal physiology. The availability of genetically modified mice as research tools has greatly enabled our understanding of how specific genes contribute to the process of skeletogenesis. In order to explore the impact of biochemical, genetic, or pharmacological manipulation on bone formation, various osteogenic cell culture systems have been developed. Two of the most widely accepted rodent osteogenic culture models, using osteoprogenitor cells isolated from calvaria or bone marrow, are described in this chapter.

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.

Transforming growth factor-beta 1 (TGF-β1) prevents the age-dependent decrease in bone formation in human osteoblast/implant cultures

Titanium implants have been extensively used in orthopedic surgery and dentistry. Most of the patients who receive such implants are elderly with a compromised ability to heal and form new bone. By using an in vitro osteoblast/implant culture system, the potency of TGF-beta1 in enhancing mineralization of human osteoblast cultures from elderly subjects was investigated in this study. Primary human osteoblast (HOB) cells obtained from different age group human subjects [Young (Y), Middle (M), and Old (O)] were cultured on Ti alloy (Ti-6Al-4V) disks with or without continuous administration of 0.2 ng/mL TGF-beta1 in the medium for 2 or 4 weeks. TGF-beta1 significantly (p < 0.05) increased calcium content and the size of calcified nodules on implant disks in the O group, but had no effect on the Y or M groups. The number of calcified nodules was not different with or without TGF-beta1 in all age groups. As measured by Northern blot analysis and RT-PCR, TGF-beta1 significantly increased the expression of bone-specific extracellular matrix proteins, including alkaline phosphatase, Type I collagen, bone sialoprotein and osteocalcin, after both 2 and 4 weeks in the O group but not in the Y group. In conclusion, TGF-beta1 enhances mineralization on implant materials of osteoblast cultures from elderly human subjects.

Effect of strain on bone nodule formation by rat osteogenic cells in vitro

The purpose of this study was to assess in vitro bone nodule formation by cells exposed to a range of microstrain, at a sub-optimal oscillation frequency for bone formation. Fetal rat calvarial cells experienced a Flexercell regimen within either FLEX I (deformable) or FLEX II (non-deformable) substrates. Cells in FLEX I plates were exposed to growth medium only; those in FLEX II plates were exposed to either growth medium only, or growth medium + 10(-7) M IGF-1. Cell numbers were assessed from 1 to 6 days. Other cells were exposed to the Flexercell regimen (-2 kPa, 0.05 Hz) for 1-3 (Group 1), 3-6 (Group 2), 1-9 (Group 3) or 10-15 (Group 4) days and were maintained, at other times, under standard conditions. After 21 days, nodules were counted within each well and within the compression, <999, 1000-4900, 5000-9999, 10,000-14,999 and 15,000-25,000 microstrain regions of the FLEX I membrane. Cyclic deformation inhibited cell numbers from 1 to 6 days, compared to control or IGF-1 groups (P<0.001). The number of nodules in Groups 2 and 4 were greater than Groups 1 or 3 (P<0.001), but not different from control or IGF-1 groups. Compression or tensile microstrain significantly affected nodule formation in all groups, with Group 4 producing more nodules than other groups in most microstrain regions. Thus, the number of bone nodules produced by osteogenic cell cultures exposed to cyclic deformation was significantly affected by the timing of initiation and the characteristics and magnitude of the deformation regimen.

Altered Cbfa1 expression and biomineralization in an osteosarcoma cell line

Osteoblast differentiation and expression are regulated by Cbfa1 transcription factors. Recent evidence suggests that Cbfa1 may also regulate bone mineralization. The purpose of this study was to characterize Cbfa1 expression in relation to mineralization in rat UMR106-01 osteoblastic cell cultures. UMR106-01 BSP cultures consistently form bone-like mineral, whereas the UI subclone mineralize gradually. BSP and UI cultures were grown for 48 h and then treated with beta-glycerophosphate. BSP cultures had alizarin red stained calcifications and mineral-like deposits within 24 h of phosphate. Atomic absorption spectroscopy measured significantly (P<0.0001) more calcium in the phosphate-treated BSP cultures than in the UI. Cbfa1 message was detected in the BSP and UI cultures, but the Cbfa1 N-terminal isoform was deficient in UI and appeared to be up-regulated in the phosphate-treated BSP cultures. Cbfa1 protein levels were also reduced in the UI. DNA sequence from the RT-PCR products was utilized to design Taqman Real-time RT-PCR reagents. Quantitative Real-time RT-PCR analysis showed that Cbfa1 mRNA levels relative to endogenous 18 s rRNA were lower in the slower mineralizing UI cultures. Furthermore, the Cbfa1 N-terminal isoform mRNA levels were significantly (P<0.001) lower in the slower mineralizing cultures. Transfection with Cbfa1 or isoform antisense caused a significant (P<0.001) reduction in mineralization. Therefore, Cbfa1 expression may be associated with bone-like mineral formation in rat UMR106-01 osteoblastic cell cultures.

The effects of patient age on human osteoblasts' response to Ti-6Al-4V implants in vitro

Osseointegrated implants are a common therapy for the elderly population as lifespan increases. Understanding the effects of age and sex on osseointegration is important for successful implant therapy. Therefore, the response of primary human osteoblasts (HOB) to implant materials was studied. HOBs were obtained by outgrowth of cells from bone from orthopaedic procedures and categorized as Young (Y), <15; Middle (M), 30-50; and Old (O), >60 years old. Initially the HOB phenotype was determined on tissue culture plastic. Alkaline phosphatase (ALP) staining and activity were significantly increased in HOBs from older patients. Message levels of type I collagen (COL), bone sialoprotein (BSP) and ALP were significantly higher (from 2.3- to 3.8-fold) in Y subjects compared to M and O patients at 2 weeks. Studies of the response of HOBs to implant materials were undertaken using Ti-6Al-4V disks prepared in a manner similar to orthopaedic implants. A 1.4-fold (p<0.05) increase in cell attachment was found in HOBs from Y compared with O in female subjects but not in male subjects. Cell proliferation at 24 h was not significantly different by age or sex, nor was DNA content different at 2 and 4 weeks. Mineralization in HOB-implant cultures was 2.3-fold higher in Y than in O, and 1.7-fold higher in Y compared to M HOBs from female but not male subjects at 4 weeks. Northern blot and RT-PCR analysis at 2 weeks of culture showed significantly higher levels (1.6-2.3-fold) of COL, BSP, and osteocalcin (OC) mRNAs in Y HOBs compared to M and O HOBs from female subjects. We conclude that human osteoblasts from older female patients have a decreased ability to form bone on implants.

Bone-like nodules formed by human bone marrow stromal cells: comparative study and characterization

Autologous bone marrow stromal cells have been proposed as an adjuvant in the treatment of bone nonunion. This cell therapy would require the establishment of culture conditions that permit the rapid expansion of these cells ex vivo while retaining their potential for further differentiation. Our aim was to achieve a full differentiation process using human bone marrow aspirates. We first analyzed the effects of mineralization medium (with ascorbic acid and phosphate) and dexamethasone (dex) during the primary culture of human bone marrow stromal (HBMS) cells on the proliferation/differentiation behavior of first-passage cells. The most appropriate schedule was then selected to further characterize this differentiation model. We showed that primary culture of HBMS cells in proliferation medium (DMEM supplemented with 10% fetal calf serum), with a 48-h treatment by mineralization medium and dex resulted in a better osteoblastic differentiation of first-passage cells than primary culture carried out in mineralization medium with or without dex. We showed that culture of HBMS cells under these conditions (primary culture in proliferation medium, followed by subculture in mineralization medium) led to the formation of specifically mineralized bone-like nodules similar to the ones observed with rat bone marrow stromal cells. Our nodules exhibited three distinct cell types, reproducing in vitro a tissue-like structure. This treatment demonstrated an optimal proliferation and expression of osteoblastic markers such as alkaline phosphatase, osteocalcin, and type I collagen. The primary culture allowed the multiplication of the number of adherent progenitor cells at the initial time of plating by a mean factor of 44,000, which was found to be negatively correlated with age. Thus, this differentiation model could provide a new tool to elaborate an autologous cell therapy designed to enhance osteogenesis.

Role of abnormal neutral endopeptidase-like activities in Hyp mouse bone cells in renal phosphate transport

We investigated whether the absence of Phex (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) in the Hyp mouse affects the expression and activity of neprilysin (NEP) and of endothelin-converting enzyme-like endopeptidase (ECEL1/DINE) in bone marrow stromal cells (BMSC) and osteoblasts (Ob). Total NEP-like activity was higher in Ob than in BMSC regardless of genotype, and Hyp cells showed higher activities than normal. Conditioned media (CM) from Hyp BMSC and Ob inhibited inorganic phosphate (P(i)) uptake by mouse proximal tubule cells, and incubating Hyp Ob with phosphoramidon prevented the production of the inhibitor of renal P(i) uptake. A linear relationship was observed between the NEP-like activity of Hyp and normal cells and the inhibition of P(i) uptake. NEP and ECEL1/DINE mRNA levels were higher in Hyp cells than in normal cells, and in situ hybridization of ECEL1/DINE confirmed higher levels of expression in the Hyp mouse than in normal cells. In conclusion, we observed a correlation between the inhibition of P(i) uptake by CM from Hyp cells and elevated NEP-like activities.

A modified method to culture human osteoblasts from bone tissue specimens using fibrin glue

INTRODUCTION

To establish primary osteoblast cultures is a challenge. The methods for isolation mostly comprise digestion with extracellular matrix degrading enzymes after mincing the bone samples. These methods are labour intensive and lead to an inefficient recovery of cells. Therefore, the aim of this study was to develop a more reliable method for culturing human osteoblasts.

MATERIALS AND METHODS

Bone tissue specimens were obtained from 20 patients undergoing reconstructive operations. Bone specimens were dissected and put into petri dishes with the bottom covered with fibrin glue. To identify the nature of the outgrowing cells, cytological staining was performed, i.e. Von Kossa, Azan, Dahl's, alkaline phosphatase, and collagen type I.

RESULTS

Mean time interval of cellular outgrowth was 12 days after preparing the bone tissue specimens. Confluence of the cell cultures was reached after four to five weeks on average. All cells were positively stained using Von Kossa, alkaline Phosphatase and collagen type I. The matrix consisted of lime, calcium and collagens.

CONCLUSION

A simplified method to culture osteoblasts from all kinds of bone tissue specimens is presented. The fibrin glue allows firm adhesion of the specimens to the petri dish. This allows the cells to grow out without disturbance. Normally, due to movements during medium exchange the adhesive bonds are disrupted. The fibrin glue retains the adhesive bonds. This method allows studying human osteoblasts in different clinical settings.

17Beta-estradiol stimulates mineralized bone nodule formation when added intermittently to SaOS-2 cells

It is now well established that estrogen inhibits bone resorption. However, its effect on bone formation remains controversial. We studied the effect of 17beta-estradiol (E2) on mineralized bone nodule formation in long-term cultures of osteosarcoma SaOS-2 cells. We showed that SaOS-2 cells formed mineralized nodules which under electron microscopy revealed a bone structure with active osteoblasts, entrapped osteocytes, extracellular collagen fibrils and hydroxyapatite deposits, making this system a valid model to study bone formation in vitro. Intermittent addition of E2 for 6 hours during a 48-hour cycle of changes of medium, starting from day 3, resulted in a dose-dependent stimulation of mineralized bone nodule number and area, as well as alkaline phosphatase activity. In conclusion, we report for the first time a stimulatory effect of E2 on mineralized bone nodule formation in human osteoblasts in culture.

Temporal and local appearance of alkaline phosphatase activity in early stages of guided bone regeneration. A descriptive histochemical study in humans

Alkaline phosphatase (ALP) catalyzes the hydrolysis of phosphate esters and it seems to be a prerequisite for normal skeletal mineralization. Also, ALP is the most widely recognized marker of osteoblast phenotypes. By a tissue regenerative technique called Guided Bone Regeneration (GBR), it is possible nowadays to regenerate small bony defects. The aim of the present study was to investigate early events in bone healing and neogenesis by studying histochemically the temporal and local appearance of the marker Alkaline Phosphatase (ALP) in a GBR model system. Nine healthy volunteers (5 males, 4 females, mean age 31.7 years) participated in the experiment. After raising a mucoperiosteal flap from the mandibular second molar to the retromolar area in each volunteer, a hollow titanium test cylinder was placed into a congruent bony bed and the coronal end of the cylinder was closed with an ePTFE-membrane. Then the flap was adapted and sutured to obtain primary wound closure. After 2, 7 and 12 weeks, the regenerated tissue within the cylinders was harvested. Histologically, ALP activity was observed associated with the osteoid seams in the very basal part of the regenerate where new bone trabeculae were in the process of being formed. More coronally, large round cells seemed to secrete an ALP-positive substance since in the center of such cell clusters strong ALP activity located extracellularly was detected. In the present experiment, ALP seemed to have been an early sign of osteoblast secretion of a matrix which subsequently was determined to become osteoid. ALP activity was never seen isolated within connective tissue and away from bone. This is an indication that its source is linked to existing bone. The present study has documented for the first time the appearance of ALP activity in guided bone regenerations in humans. It has revealed that: 1) Osteogenesis in guided bone regeneration is preceded by localized, marked expression of ALP in an organized connective tissue environment. 2) Bone neogenesis is an early event in this experimental setup and may be detected already 2 weeks after wounding. 3) Expression of ALP and subsequent bone neogenesis is originating from and topographically linked to pre-existing bone structures.

Primary mineralization at the surfaces of implants

Osteogenesis around implants is affected by the physical and chemical characteristics of the biomaterials used. The osteoprogenitor cells must migrate to the implant site and synthesize and secrete a mineralizable extracellular matrix. Because this is neo-bone formation, the mechanism by which the cells calcify their matrix involves extracellular organelles called matrix vesicles in a process termed "primary mineralization". Two different methods for assessing the effects of implant materials on primary mineralization are presented in this report. In the first approach, different implant materials used in dentistry and orthopedic surgery were placed in rat tibial bones after marrow ablation. Two groups of implants were used, bone-bonding and non-bonding materials. We examined the effects of the materials on calcification morphometrically by quantitating changes in matrix vesicle morphology and distribution in endosteal tissue around implants as compared with normal endosteal bone healing. In addition, matrix vesicles were isolated from the endosteal tissue around the implant as well as from the contralateral limb and were examined biochemically. The results demonstrated that bone-bonding materials induced a greater increase in matrix vesicle enzyme activity than did non-bonding materials. However, all materials caused changes in matrix vesicles that were different from those seen in normal endosteal bone formation following injury. The effects of implant materials on biochemical markers of mineralization, including specific activities of matrix vesicle alkaline phosphatase and phospholipase A2 and phosphatidylserine content, demonstrated a high correlation with the morphometric observations with regard to enhancement and/or delay of primary mineralization. In the other approach, we used a radioisotopic method to evaluate the effects of implant materials on primary mineralization. This analysis revealed that implants alter bone healing, as shown by the differential uptake of 99mTc and 32P in different bone compartments. Decreased 32P uptake by the organic phase in the presence of bone-bonding implants suggests that cleavage of 99mTcMD32P into its technetium and methylene diphosphonate moieties was inhibited by the presence of the implants. In summary, these approaches to evaluating the effects of materials on primary mineralization demonstrate that the marrow ablation model can easily distinguish between bone-bonding and non-bonding materials. The use of this model can be valuable in the development of new materials.

Structure, composition, and maturation of newly deposited calcium-phosphate crystals in chicken osteoblast cell cultures

Characterization of the very early calcium phosphate (CaP) crystals deposited in bone or in osteoblast cell cultures has been hampered by the overwhelming presence of organic matrix components and cells that obscure spectral analyses. We have overcome this problem using isolated protein-free crystals and have obtained new data including 31P nuclear magnetic resonance (NMR) spectra for the first time from mineral crystals deposited during osteoblast calcification in culture. Crystals were isolated from cultures at two time points: (a) at first calcium accumulation (day 8-10) and (b) after 60 days of culture, to assess maturational changes. The analyses show that the chemical composition overall and short range order of the early and mature crystals are characteristic of the apatite crystals found in young embryonic chick bone in vivo. No mineral phase other than apatite was detected by any of the methods used. 31P NMR spectroscopy identified the HPO4 groups as those present in bone apatite. Similar to bone apatites, no OH groups were detected by Fourier transform infrared (FTIR) spectroscopy. The temporal maturational changes in composition and structure of the mineral phase were difficult to assess because of the continuous deposition of crystals throughout culturing. The pathway of the maturational changes observed were similar to those occurring in chick bone in vivo and synthetic apatite crystals in vitro although to a much smaller extent.

Enhanced production of mineralized nodules and collagenous proteins in vitro by calcium ascorbate supplemented with vitamin C metabolites

BACKGROUND

Vitamin C or ascorbate is important in wound healing due to its essential role in collagen synthesis. To study wound healing in the periodontium, cells adherent to expanded polytetrafluoroethylene (ePTFE) augmentation membranes, recovered from edentulous ridge augmentation procedures, have been established in culture in our laboratories. The objective of this study was to determine whether treatment of these cells with a calcium ascorbate, which contains vitamin C metabolites (metabolite-supplemented ascorbate), would increase the production of collagenous protein and mineralized tissue in vitro, as compared to unsupplemented calcium ascorbate (ascorbate).

METHODS

Cells derived from ePTFE membranes were cultured with beta-glycerophosphate and the test agents for 2 to 5 weeks, and the surface areas of the cell cultures occupied by mineralized nodules were measured using computerized image analysis. One experiment tested the effects of calcium threonate, one of the vitamin C metabolites in metabolite-supplemented ascorbate. Incorporation of radioactive proline and glycine was used as a measure of total protein (radioactivity precipitated by trichloracetic acid) and collagenase-digestible protein (radioactivity released by collagenase digestion.) Co-localization of collagen and fibronectin was examined by immunofluorescence.

RESULTS

In vitro treatment of these cells with metabolite-supplemented ascorbate increased the area of the cell cultures occupied by mineralized nodules after 5 weeks. Cell cultures treated with metabolite-supplemented ascorbate also exhibited significant increases in total protein. The increase in collagenous proteins in these cultures accounted for 85% of the increase in total protein. The greatest difference between treatment groups was observed in the cell-associated fraction containing the extracellular matrix. The additional collagen exhibited normal co-distribution with fibronectin. In cultures treated with ascorbate spiked with calcium threonate, the area of mineralized tissue was significantly greater than in ascorbate-treated cultures, but was less than that observed in cultures treated with metabolite-supplemented ascorbate.

CONCLUSIONS

In vitro treatment with ascorbate containing vitamin C metabolites enhanced the formation of mineralized nodules and collagenous proteins. Calcium threonate may be one of the metabolites influencing the mineralization process. Identifying factors which facilitate the formation of mineralized tissue has significant clinical ramifications in terms of wound healing and bone regeneration.

Transmission and scanning electron microscopic analysis of mineralized loci formed by human periodontal ligament cells in vitro

Fibroblasts isolated from human periodontal ligament (PDL) were cultured under a medium supplemented with ascorbic acid, beta-glycerophosphate and dexamethasone. The cultures were assessed for their ability to elaborate a mineralized matrix. Cell cultures stained positive when analyzed for alkaline phosphatase activity throughout the culture period. After about 3 weeks in culture, the cells produced a calcified matrix. Light microscopy showed formation of clusters of different shapes and sizes. Von Kossa staining revealed mineral deposits as amorphous brown-black precipitates. Transmission electron microscopy showed cells in multilayers and mineralized formations in close association with a dense network of collagen fibers. Scanning electron microscopy revealed smooth formations rising over the cultures with an abundant fiber matrix. We conclude that human PDL fibroblasts can be induced to form a mineralized matrix which shares features with bone mineralized matrix but most likely represents a more immature type of in vitro mineralization. Moreover, the present study further supports the osteoblastic potential of these cells.

Matrix mineralization in MC3T3-E1 cell cultures initiated by beta-glycerophosphate pulse

MC3T3-E1 cells, grown in the presence of serum and ascorbate, express alkaline phosphatase and produce an extensive collagenous extracellular matrix that can be mineralized by the addition of beta-glycerophosphate (beta-GP). In the present work, we study the influence of concentration and duration of beta-GP treatment on the mineralization pattern in 4-week-old cell cultures. Amount and structure of mineral deposition were monitored by von Kossa staining, light, and electron microscopy, as well as small-angle X-ray scattering (SAXS) of unstained specimens. SAXS measures the total surface of the mineral phase and is therefore preferentially sensitive to very small crystals (typically <50 nm). It was used to determine the ratio (M) of small crystals to collagen matrix. A variety of mineralization patterns was observed to occur simultaneously, some associated with collagen within nodules or in deeper layers of the cultures and some independent of it. At a beta-GP concentration of 10 mmol, mineralization was initiated after about 24 h and continued to increase, irrespective of whether the high level of beta-GP was maintained or reduced to 2 mmol. With shorter pulses (<24 h), no significant mineralization was observed in the week following beta-GP pulse. With continuous treatment at 5 mmol beta-GP, the first signs of mineralization were detected 14 days after the beginning of treatment in the 4-week-old cultures, but no mineralization at all occurred at lower beta-GP concentrations. When cells were grown without ascorbic acid for 4 weeks, only two cell layers without collagen matrix were found. In these cultures, no mineralization detectable by SAXS could be induced with beta-GP. These data indicate that, in viable cells, high doses of beta-GP are essential for the nucleation of mineral crystals, but not for the progression of mineralization once crystals had been nucleated. In contrast, when 4-week-old cell cultures were devitalized, M was found to increase immediately, even at 2 mmol beta-GP. These results suggest that, in MC3T3-E1 cell cultures, cell viability is essential for prevention of spontaneous mineralization of the extracellular matrix.

The effects of cold storage and endotoxin challenge on osteoblast viability and interleukin-6 production

Autogenous hip marrow is an excellent source of pluripotential cells for regenerative procedures. However, before this treatment modality can be employed a method to attenuate osteoclast activity must be developed. The shock of cold storage (4 degrees C) is thought to abate osteoclast activity through the downregulation of osteolytic cytokines produced by osteoblasts. The objective of this study was to evaluate the effects of cold storage (4 degrees C) and endotoxin challenge on bone cell culture viability and interleukin-6 (IL-6) production. These cells (osteoblasts) were primarily harvested from murine calvaria utilizing sequential digestions, separated by density gradient and combined. Twelve-well cell culture plates were inoculated with 2 x 10(4) cells/ml and placed in cold storage for 1-14 d. After cold storage the cultures were then incubated at 37 degrees C for 1-20 d. A set of replicate plates was also challenged with 10 ng/ml endotoxin upon incubation at 37 degrees C for 4 consecutive days. Cells were evaluated daily for alkaline phosphatase activity. Cell culture supernatants were also collected daily and batch assayed for IL-6 production. Cell cultures did not survive more than 48 h of cold storage. There was a decrease in IL-6 secretion in all refrigerated cultures and a significant decrease in those cells refrigerated for 48 h versus control cultures (p < 0.05). Replicate cultures treated with endotoxin secreted significantly increased amounts of IL-6 in both the control cultures and the cultures exposed to 24 h of cold storage versus non-endotoxin-treated control cultures (p < 0.05). These observations suggest that after 48 h of cold storage autogenous marrow may be safe to use because of the dramatic decrease in IL-6 production by osteoblasts.

Direct demonstration of a humorally-mediated inhibition of renal phosphate transport in the Hyp mouse

The murine Hyp model reproduces the characteristics of human X-linked hypophosphatemia (XLH), an inherited disease causing renal loss of phosphate (Pi), severe rickets and osteomalacia. A current hypothesis considers that a humoral factor may be responsible for the renal Pi loss, although in vitro experiments with renal cell models have failed to demonstrate the presence of such a factor in XLH or in the Hyp mouse model. To test this hypothesis directly, we prepared primary mouse proximal tubule cell cultures (MPTC), expressing normal features of proximal tubule cells. These cells possess high alkaline phosphatase activity, and respond to human parathyroid hormone fragment 1-34 (PTH) with a four- to sixfold increase in cAMP production but do not respond to either arginine vasopressin (AVP) or to salmon calcitonin (sCT). They also show sodium-dependent phosphate, glucose and amino acid uptake. The presence of 10% Hyp mouse serum in HAMF12/DMEM media (1 mM Pi) for the last 48 hours of culture of MPTC reduced Pi uptake (0.1 mM 32P-Pi in the presence of 140 mM NaCl) by 45.7 +/- 3.9% (P < 0.01) as compared to normal mouse serum. This effect of Hyp mouse serum was dose-dependent between 5 to 20% (final concentration) in culture media for the last 48 hours of culture (P < 0.01 by analysis of variance). This effect of Hyp mouse serum was also time-dependent, with a lag time of at least 12 hours. Indeed, no significant inhibition of Pi uptake could be detected with incubations less than 12 hours in the presence of 10% Hyp mouse serum, whereas a maximal effect was obtained after 24 hours of incubation and remained unchanged after 36 and 48 hours. The inhibition of phosphate uptake by Hyp mouse serum was specific, since neither sodium-dependent glucose nor alpha-aminobutyric acid uptake was modified under these conditions. MPTC cells showed a very nice adaptation to Pi concentration in the media; low Pi (0.4 mM final concentration in the presence of 10% serum) stimulated Pi uptake, whereas high Pi concentration (3 mM) reduced Pi uptake by these cells as compared to regular HAMF12/DMEM media containing 1 mM Pi. Normal and Hyp mouse serum both inhibited Pi uptake by MPTC following adaptation in low or normal Pi media, however, Hyp mouse serum always showed a stronger inhibition than normal serum. In contrast, adaptation of MPTC in high Pi media resulted in no inhibition of phosphate uptake either in the presence of normal or Hyp mouse serum. We next questioned whether conditioned media from confluent Hyp mouse primary osteoblast-like cell cultures could affect Pi uptake by MPTC. These osteoblast-like cells expressed high alkaline phosphatase and produced the bone specific protein, osteocalcin. When MPTC were treated for 48 hours with Hyp mouse bone cell media conditioned for the last 48 hours of cultures, Pi uptake was specifically inhibited by 30.5 +/- 4.1% (P < 0.025) as compared to normal mouse bone cell-conditioned media. This effect of primary Hyp mouse bone cell-conditioned media is specific for these cells since it was not observed with CHO cell-conditioned media, nor with either mouse fibroblast (NCTC), normal mouse Kupffer cell- or Hyp mouse Kupffer cell-conditioned media. This effect also persisted through a number of passages of Hyp mouse bone cells, since conditioned-media from cells at their third passage still resulted in a 32 +/- 9.4% inhibition (P < 0.02). These results are the first to show an effect of Hyp mouse serum on Pi uptake by primary renal cell cultures in vitro. This effect is dose- and time-dependent, requiring 24 hours for maximum response, and is blocked in Pi rich media. These results also suggest that a specific intrinsic cellular defect, present in Hyp mouse osteoblasts, is responsible for the release of and/or the modification of a factor that can reach the circulation and which inhibits renal phosphate reabsorption. The molecular nature of this factor and its mode of action remains to be determined.

Nongenomic regulation of protein kinase C isoforms by the vitamin D metabolites 1 alpha,25-(OH)2D3 and 24R,25-(OH)2D3

Prior studies have shown that vitamin D regulation of protein kinase C activity (PKC) in the cell layer of chondrocyte cultures is cell maturation-dependent. In the present study, we examined the membrane distribution of PKC and whether 1 alpha,25-(OH)2D3 and 24R,25-(OH)2D3 can directly regulate enzyme activity in isolated plasma membranes and extracellular matrix vesicles. Matrix vesicle PKC was activated by bryostatin-1 and inhibited by a PKC-specific pseudosubstrate inhibitor peptide. Depletion of membrane PKC activity using isoform-specific anti-PKC antibodies suggested that PKC alpha is the major isoform in cell layer lysates as well as in plasma membranes isolated from both cell types; PKC zeta is the predominant form in matrix vesicles. This was confirmed in Western blots of immunoprecipitates as well as in studies using control peptides to block binding of the isoform specific antibody to the enzyme and using a PKC zeta-specific pseudosubstrate inhibitor peptide. The presence of PKC zeta in matrix vesicles was further verified by immunoelectron microscopy. Enzyme activity in the matrix vesicle was insensitive to exogenous lipid, whereas that in the plasma membrane required lipid for full activity. 1,25-(OH)2D3 and 24,25-(OH)2D3 inhibited matrix vesicle PKC, but stimulated plasma membrane PKC when added directly to the isolated membrane fractions. PKC activity in the matrix vesicle was calcium-independent, whereas that in the plasma membrane required calcium. Moreover, the vitamin D-sensitive PKC in matrix vesicles was not dependent on calcium, whereas the vitamin D-sensitive enzyme in plasma membranes was calcium-dependent. It is concluded that PKC isoforms are differentially distributed between matrix vesicles and plasma membranes and that enzyme activity is regulated in a membrane-specific manner. This suggests the existence of a nongenomic mechanism whereby the effects of 1,25-(OH)2D3 and 24,25-(OH)2D3 may be mediated via PKC. Further, PKC zeta may be important in nongenomic, autocrine signal transduction at sites distal from the cell.

Ultrastructural analysis of mineralized matrix from human osteoblastic cells: effect of tumor necrosis factor-alpha

Recent work by a number of investigators has demonstrated that the process of bone matrix formation and mineralization is under the influence of growth factors and cytokines present in the local environment. Utilizing primary and established osteoblast cell culture systems, these studies have examined the regulation of bone matrix protein synthesis and deposition into the extracellular matrix (ECM) and subsequent mineralization. In previous studies, we have utilized the human osteoblastic cell line, HOS TE85, to study the effects of Tumor Necrosis Factor-alpha (TNF-alpha) on the regulation of matrix proteins and proteolytic function in monolayer cultures as well as during the development and calcification of ECM formed by HOS TE85 cells during extended culture. Our studies demonstrate that TNF-alpha inhibited formation and mineralization of nodules. In the study reported here, we evaluated the ultrastructural morphology of the cell-matrix complex formed by HOS TE85 cells in the presence and absence of TNF-alpha at selected time points during the matrix development process utilizing both transmission electron microscopy and light microscopy. In the presence of TNF-alpha, the cell-matrix complex does not develop normally, with a lack of organization and mineralization, when compared to untreated cells. The lack of mineralization appears to result from the lack of normal collagen fibril deposition and formation of an appropriate ECM essential for the mineralization process. These results support our previous observations that TNF-alpha inhibits HOS TE85 cells from forming a mineralizing ECM by inhibiting incorporation of collagen into the ECM and inducing the synthesis of proteolytic enzymes capable of degrading collagen in the ECM.

Analysis of osteoblast mineral deposits on orthopaedic/dental implant metals

Neonatal rat calvarial osteoblasts were cultured on Ti-6Al-4V, Co-Cr-Mo alloy, 316L stainless steel and polystyrene (reference substrate) in the presence of ascorbic acid and 10 mM beta-glycerophosphate for 16, 17, 18, 19, 20, 21, 24 and 28 d. Scanning electron microscopy examination revealed that osteoblasts cultured on these orthopaedic/dental implant metals synthesized and deposited an extracellular matrix containing collagenous and non-collagenous components, as well as mineral nodules of various morphologies. Energy dispersive spectrometry revealed that the mineral deposits consisted of three distinct chemical compositions: calcium phosphate, calcium-sulphur-phosphorus, and calcium only. Backscattered electron imaging demonstrated that both the calcium phosphate and calcium-only deposits were electron dense, while the calcium-sulphur-phosphorus deposits were electron translucent. X-ray diffraction analysis indicated that the bulk of the osteoblast mineral deposits was amorphous hydroxyapatite; in addition, electron diffraction analysis revealed small regions of crystalline hydroxyapatite.

Ecto-alkaline phosphatase considered as levamisole-sensitive phosphohydrolase at physiological pH range during mineralization in cultured fetal calvaria cells

Alkaline phosphatase (ALP) activity expressed on the external surface of cultured fetal rat calvaria cells and its relationship with mineral deposition were investigated under pH physiological conditions. After replacement of culture medium by assay buffer and addition of p-nitrophenyl phosphate (pNPP), the rate of substrate hydrolysis catalyzed by whole cells remained constant for up to seven successive incubations of 10 min and was optimal over the pH range 7.6-8.2. It was decreased by levamisole by a 90% inhibition at 1 mM which was reversible within 10 min, dexamisole having no effect. Values of apparent Km for pNPP were close to 0.1 mM, and inhibition of pNPP hydrolysis by levamisole was uncompetitive (Ki = 45 microM). Phosphatidylinositol-specific phospholipase C (PI-PLC) produced the release into the medium of a p-nitrophenyl phosphatase (pNPPase) sensitive to levamisole at pH 7.8. The released activity whose rate was constant up to 75 min represented after 15 min 60% of the value of ecto-pNPPase activity. After 75 min of PI-PLC treatment the ecto-pNPPase activity remained unchanged despite the 30% decrease in Nonidet P-40-extractable ALP activity. High levels of 45Ca incorporation into cell layers used as index of mineral deposition were decreased by levamisole in a stereospecific manner after 4 h, an effect which was reversed within 4 h after inhibitor removal, in accordance with ecto-pNPPase activity variations. These results evidenced the levamisole-sensitive activity of a glycosylphosphatidylinositol-anchored pNPPase consistent with ALP acting as an ecto-enzyme whose functioning under physiological conditions was correlated to 45Ca incorporation and permit the prediction of the physiological importance of the enzyme dynamic equilibrium at the cell surface in cultured fetal calvaria cells.

Ultrastructural characterization and immunolocalization of osteopontin in rat calvarial osteoblast primary cultures

As part of ongoing studies aimed at clarifying the early events of bone matrix deposition and mineralization, we have characterized primary osteoblast cultures using ultrastructural and immunocytochemical methods. Osteogenic cells were isolated by sequential enzymatic digestion of newborn rat (2-4-day-old) calvariae and grown for periods of 7 to 28 days on polystyrene, Thermanox plastic, or sputtered titanium. Bone-like nodules, showing a stratified organization of cells and collagen, were examined by scanning and transmission electron microscopy, and further characterized for mineral by backscattered electron imaging and X-ray microanalysis. Colloidal gold immunocytochemistry was used to examine the distribution of osteopontin in these nodules. Cells at the surface of the nodules were rounded, while those within the nodules generally appeared more flattened. Both cell types, particularly at early culture intervals, exhibited well-developed protein synthetic organelles. Collagen fibrils were present between the cell layers and some individual fibrils appeared mineralized. Aggregates of needle-shaped crystallites were sometimes apposed to the cell surface, frequently within invaginated regions of the cell membrane, while other mineralized masses of various sizes were present within the collagenous scaffolding. The periphery of the mineralized masses was often delimited by an electron-dense, lamina limitans-like layer. Focal accumulations and/or a more complete layer of afibrillar, mineralized organic matrix were sometimes observed at the interface between the cells and the surface of the culture dish. Osteopontin was immunodetected over the afibrillar and collagenous mineralized matrix throughout the cultures and, in some cases, labeling was concentrated over the peripheral, electron-dense material delimiting the mineralized masses. In conclusion, these data indicate that calvaria-derived osteoblasts produce an extracellular matrix with structural and compositional similarities to bone. Although not a regular observation, the accumulation of osteopontin on the surface of the culture substrate and at the periphery of masses of mineralized matrix may be analogous to what takes place in vivo at naturally occurring bone interfaces.

Monoclonal antibodies as tools for studying the osteoblast lineage

Knowledge of the number and kinds of differentiation steps characterizing cells of the osteoblast lineage is inadequate. To analyze further osteoblast differentiation, a number of labs have generated monoclonal antibodies to osteogenic cells, derived from both normal bone and osteosarcomas. A variety of immunolabelling patterns on primary cell cultures, cell lines, and tissue sections has been reported, including cell surface, cytoplasmic, and extracellular matrix-associated patterns. Most of the antibodies selected recognize predominantly the mature osteoblast and osteocyte; in addition, however, antibodies have been generated that recognize pre-osteoblasts. Some recognize cells of both the osteoblast and chondroblast lineages and may contribute to a better understanding of the lineage and phenotypic relationships between these two cell types. In addition to recognition in vivo of cell subpopulations of discrete maturational stages, changes in the immunolabelling patterns in vitro have also documented a differentiation sequence in cells undergoing osteogenesis in cell and tissue cultures. In at least two cases, the antibodies have been used to isolate subpopulations of cells from bone, including relatively pure populations of osteocytes. With the exception of several antibodies that are against alkaline phosphatase or known matrix proteins including osteocalcin, the nature of the macromolecular species recognized by most of the antibodies generated to date are unknown. Recently, however, one antibody was used to clone the cDNA for the beta-galactoside-binding lectin, galectin 3 or epsilon binding protein (epsilon BP; IgE-binding protein; Mac-2), from a lambda gt11 osteoblast expression library; another was used to clone from an ROS 17/2.8-COS cell expression library the cDNA for OTS-8, a putative target gene of early response genes stimulated in response to phorbol esters in MC3T3-E1 cells. Neither of these macromolecules had previously been identified in bone cells, but the recent molecular and cellular analyses have shown them to be developmentally and/or hormonally regulated in osteoblastic cells. These antibodies extend the available markers and support earlier observations that a variety of molecules are differentially expressed by cells at different stages of the osteoblast lineage. This chapter will not be an exhaustive survey of all immunocytochemical and immunohistochemical analyses of osteogenic cells and tissues but will focus on the approach of eliciting novel monoclonal antibodies by the injection of osteogenic cells or crude bone extracts and its potential for establishing new markers of the osteoblast lineage. We have not included a large number of studies documenting the use of antibodies raised against several known bone matrix proteins; while these have been crucial in developing our current understanding of osteogenic differentiation, we sought rather to highlight the potential of the "random" injection approach.

Cell biology of calcified tissues: experimental models of differentiation and mechanisms by which local and systemic factors exert their effects

Interpretation of the cell biology literature, as it relates to formation and mineralization of calcifying tissues, is complicated by the plethora of models available. Some culture models use heterogeneous populations of cells while others use relatively homogeneous populations. The issues are further confused by comparison of monolayer and three dimensional cultures. In addition, transformed and nontransformed cell lines are also used. As little clinical data about the age and sex of the original donor for many of these cell lines is lacking, it is impossible to know where in the cell lineage the cells were when they were isolated, yet this information can have a direct impact on the data obtained and their interpretation. Furthermore, many responses are attributed to the cell, while much of the effect observed may be targeted to the matrix. These issues are discussed and a potential mechanism explaining how cells can modulate events in the matrix nongenomically is presented.

Functional characterization of prostaglandin E2 inducible osteogenic colony forming units in cultures of cells isolated from the neonatal rat calvarium

Prostaglandin E2 (PGE2) increases the number of mineralized nodules that form in cultures of rat calvarial (RC) cells. The purpose of our study was to characterize PGE2-inducible osteogenic colony forming units (CFU-Os) by determining their number, the cell populations from which they were released, their specific responsive period to PGE2, and their proliferating and differentiating characteristics under the stimulation of PGE2. Limiting dilution analysis was used to determine the number of PGE2-inducible CFU-Os. Sequential digestion of intact rat parietal bones with collagenase isolated 5 subpopulations of RC cells that were used to estimate the cell populations where PGE2-inducible CFU-Os resided. The responsive period of PGE2-inducible CFU-Os to PGE2 was evaluated by treating cultures of mixed RC cells for all possible combinations of days 1-10, 11-20, and 21-30. PGE2 effects on proliferation and differentiation of CFU-Os were evaluated by comparing the DNA synthesis and AP activity in subpopulations I and IV on days 3, 6, and 9. Results showed: (1) PGE2-inducible CFU-Os represent 0.27% of cells in the mixed RC population, (2) the majority of determined and PGE2-inducible CFU-Os were found in the subpopulations released during the 60-100 min digestion periods, (3) the response of PGE2-inducible CFU-Os is limited to the first 10 days of culture, and (4) PGE2-stimulated nodule formation is associated with an early increase in DNA synthesis and a sustained increase in alkaline phosphatase activity. We conclude that, functionally, PGE2-inducible CFU-Os are slowly proliferating AP negative cells primarily found in the subpopulations III-V. PGE2 stimulates them to proliferate and become AP+, and function as determined CFU-Os to form mineralized nodules in vitro.

Osteosarcoma hybrids can preferentially target alkaline phosphatase activity to matrix vesicles: evidence for independent membrane biogenesis

Alkaline phosphatase is the marker enzyme for matrix vesicles, extracellular organelles that play a major role in primary bone formation and calcification. Recently, we developed osteosarcoma x fibrosarcoma hybrids in which alkaline phosphatase expression was greatly reduced, a phenomenon known as extinction. In the present study, we used to cell hybrids, LTA-1 and LTA-5, constructed from a human osteoblast-like osteosarcoma. TE85, and a mouse fibrosarcoma, La-t-, to examine the differential distribution of alkaline phosphatase between matrix vesicles and the plasma membrane, postulated to be the parent membrane from which matrix vesicles are derived. While alkaline phosphatase in plasma membranes was extinguished, enzyme activity in matrix vesicles from LTA-1 hybrid cells was 34.2% of that present in matrix vesicles from the TE85 parent cells and 200 times that found in La-t- matrix vesicles. Matrix vesicles from LTA-5 had alkaline phosphatase levels similar to La-t-. When other membrane enzymes (phospholipase A2, 5'-nucleotidase, and Na+/K+ ATPase) were examined, hybrid matrix vesicle and plasma membrane levels were similar to those of TE85 and significantly higher than in La-t- membrane fractions. Northern analysis detected mRNA for alkaline phosphatase in TE85 cells, but not in the hybrids or La-t- cells. In contrast, reverse transcription-polymerase chain reaction (RT-PCR) revealed alkaline phosphatase mRNA in the hybrid cells, but at very low levels. Taken together, the data indicate that regulation of plasma membrane and matrix vesicle alkaline phosphatase is independent and suggest that matrix vesicle biogenesis is independent and distinct from that of plasma membrane biogenesis. Analysis of 1B- and 1L-type alkaline phosphatase mRNA by RT-PCR showed that alternate promoter usage of the alkaline phosphatase gene was not responsible for the differential localization of this enzyme in matrix vesicle. Thus, it is likely that matrix vesicle and plasma membrane alkaline phosphatase are regulated differently at a post-transcriptional level.

Mineralization of bone-like extracellular matrix in the absence of functional osteoblasts

When grown in medium containing ascorbic acid and beta-glycerol phosphate, mouse MC3T3-E1 cells express an osteoblast phenotype and produce a highly mineralized extracellular matrix. The purpose of this study was to independently examine the role of the collagenous matrix and functional osteoblasts on the mineralization process. Cultures with and without an extensive collagenous matrix were prepared by growing MC3T3-E1 cells in the presence and absence of ascorbic acid. Matrix-rich cultures mineralized at much lower calcium phosphate ion products than nonmatrix cultures. At higher ion products, spontaneous precipitation in the medium and cell layers of nonmatrix cultures were observed. In contrast, mineral in matrix-rich cultures was still exclusively associated with collagen fibrils and not with ectopic sites in the cell layer or medium. To examine the effect of cell viability on matrix mineralization, cells were grown 8 or 16 days in the presence of ascorbic acid, then killed and incubated in a mineralizing medium. Significant mineralization was not observed in the collagenous matrix of 8-day killed cultures or age-matched controls. At 16 days mineral was associated with collagen fibrils at specific foci in the matrix of both viable and killed cultures. This observation is consistent with the concept that collagenous matrices must undergo a maturation process before they can support a mineral induction and growth. It further shows that osteoblast-like cells are not required for mineralization of mature matrices, but are required for matrix maturation.

Osteoblast and chondroblast differentiation

Recognition of discrete commitment and differentiation stages requires characterization of changes in proliferative capacity together with the temporal acquisition or loss of expression of molecular and morphological traits. Both cell lines and primary cultures have been useful for analysis of transitional steps in the chondroblast (CB) and osteoblast (OB) lineages. One striking feature is that OBs and CBs share expression of some molecules, including newer markers such as epsilon BP (galectin-3), while also having unique markers. The fact that hypertrophic chondrocytes appear able to downregulate cartilage markers and upregulate OB markers also points to an interesting lineage relationship that needs to be explored further. Recently, we have focused on the osteoprogenitors that divide and differentiate into mature OBs forming bone nodules in fetal rat calvaria cell cultures. We use cellular, immunocytochemical, and molecular approaches, including PCR on small numbers of cells, to discriminate stages. Nodule formation is characterized by loss of proliferative capacity and sequential increased marker expression, that is, alkaline phosphatase (AP), followed by bone sialoprotein (BSP), and osteocalcin. Upregulation of collagen type I and biphasic expression of osteopontin, with two peaks corresponding to proliferation and differentiation stages, also occurs. A variety of other molecules are also upregulated in the mature OB, including epsilon BP and CD44s. By replica plating and PCR, we have begun to study the expression of the messenger RNAs (mRNAs) for potential regulatory molecules (e.g., PTHrP) and their receptors (e.g., PTHR, FGFR-1, and PDGFR alpha) and have found all to be modulated during the progression from committed osteoprogenitor to mature OB.(ABSTRACT TRUNCATED AT 250 WORDS)

A transmission electron microscopy examination of the interface between osteoblasts and metal biomaterials

Transmission electron microscopy was used to examine the interface between metal implant materials and bone cells. Specifically, neonatal rat calvaria osteoblasts were cultured on CoCrMo alloy and on 316L stainless steel discs (mechanically polished to a 0.3 micron finish) in Dulbecco's Modified Eagle Medium (supplemented with 10% fetal bovine serum, 50 micrograms/mL ascorbic acid, and 10 mM beta-glycerophosphate) under standard, sterile, cell culture conditions for 14 to 28 days. At the end of the prescribed time periods, the cells were fixed and embedded in resin before removing the metal substrates using an electrolytic dissolution technique and a 7% NaCl solution. Transmission electron microscopic examination of stained, ultrathin sections of the biological samples revealed an intact interface with microscopic details characteristic to the cell line and similar to those reported in the literature for animal and explant studies. The osteoblasts exhibited continuous contact and intimate apposition to both the CoCrMo and stainless steel substrate surfaces and grew in multilayered structures; an electron dense layer (composed of mucopolysaccharides and proteins) was observed at the surface of both substrates; collagen fibrils and mineralized foci were observed in the extracellular matrix interspersed among the multilayered osteoblasts.

The effects of cytokines and growth factors on osteoblastic cells

In this short review, some regulatory mechanisms that are involved in the control of normal bone formation are proposed, based on several in vivo and in vitro models our group has utilized recently to study osteoblast differentiation and mineralized bone matrix formation. Of course, these proposals must be assessed in the light of the limitations of the models, which probably represent a simplification of the complex and different ways in which normal mammalian bone is formed at different sites. Nevertheless, it is likely that the same general types of control mechanisms are active in each of the different types of bone formation. In adult humans, bone formation predominantly occurs by remodeling, the process by which bone which has recently been resorbed by osteoclasts is replaced by teams of osteoblasts. Other types of bone formation such as endochondral bone formation and appositional bone formation are also important, particularly during growth and adolescence. The end results of each of these processes are the same, namely a complex mineralized proteinaceous bone matrix. These processes are modulated by systemic hormonal influences, which are particularly important with respect to pituitary hormones and sex steroids during growth and adolescence, and by local cellular microenvironmental differences. The former will not be discussed here. Rather, we will concentrate on the local events and factors which are likely involved in the bone formation process occurring during normal bone remodeling.

Establishment of two rat osteosarcoma cell lines (YROS-1 and YROS-2) induced by radioactive phosphorus

Two rat osteosarcoma cell lines, YROS-1 and YROS-2, were established from two experimental osteosarcomas and induced by internal irradiation with radioactive phosphorus. Both cell lines formed a monolayer cell sheet in vitro with focal piling. The YROS-1 cells were refractile and spindle or polygonal in shape, whereas the YROS-2 cells were flat, spread and polygonal in shape. Ultrastructurally, the YROS-1 cells had well-developed rough-surfaced endoplasmic reticulum with focal pericellular deposition of calcified matrix, whereas YROS-2 had abundant polysomes and intracytoplasmic filaments. Both cell lines grew stably with population doubling times of 23 and 39 h, respectively. Flow cytometry revealed that YROS-1 was rich in proliferating cells compared to YROS-2, with a higher colony-forming efficiency. YROS-1 showed high alkaline phosphatase activity, while YROS-2 possessed low activity. When subcutaneously transplanted into lumbodorsal area of athymic nude mice, only YROS-1 formed tumors with frequent lung metastasis.

Dexamethasone promotes von kossa-positive nodule formation and increased alkaline phosphatase activity in costochondral chondrocyte cultures

This study examined the effect of dexamethasone on von Kossa-positive nodule formation and alkaline phosphate specific activity of costochondral chondrocytes at two distinct stages of maturation. The nodules formed by the more mature growth zone chondrocyte cultures contained von Kossa-positive deposits in the extracellular matrix that had a punctate morphology. The nodules formed by the less mature resting zone cells also contained von Kossa-positive deposits, but differentiation was delayed by three-to-five days compared to the growth zone cell cultures. Dexamethasone stimulated the number of nodules formed and shortened the length of time required for von Kossa-positive nodule formation in both types of cultures. During the first 48 h of exposure to dexamethasone, alkaline phosphatase specific activity in the cell layer of both resting zone and growth zone cultures was increased in a dose-dependent manner. At 12 days post-confluence and thereafter, enzyme activity was inhibited in the dexamethasone-treated cultures. Changes in matrix vesicle alkaline phosphatase specific activity reflected those changes seen in the cell layer after dexamethasone treatment, but with higher magnitude, suggesting that one effect of dexamethasone might be to regulate matrix vesicle function. With the exception of one culture, the chondrocytes did not synthesize type X collagen under any of the experimental conditions used. Fourier transform infrared spectroscopy (FTIR) failed to detect the presence of calcium phosphates in any of the cultures exposed to dexamethasone except one. These results demonstrate that dexamethasone promotes early differentiation events, including nodule formation and increased alkaline phosphatase activity, in costochondral chondrocyte cultures. The failure to detect type X collagen synthesis and mineralization in both dexamamethasone-treated and control cultures suggests that these cultures lack the factors necessary for terminal differentiation and mineralization.

Rapidly forming apatitic mineral in an osteoblastic cell line (UMR 106-01 BSP)

This study evaluated a rapid biomineralization phenomenon exhibited by an osteoblastic cell line, UMR 106-01 BSP, when treated with either organic phosphates [beta-glycerophosphate (beta-GP), Ser-P, or Thr-P], inorganic phosphate (P(i)), or calcium. In a dose-dependent manner, these agents (2-10 mM) stimulated confluent cultures to deposit mineral in the cell layer (ED50 of approximately 4.6 mM for beta-GP (30 +/- 2 nmol Ca2+/microgram DNA) and approximately 3.8 mM (29 +/- 2 nmol Ca2+/microgram DNA) for P(i)) with a plateau in mineral formation by 20 h (ET50 approximately 12-15 h). beta-GP or P(i) treatment yielded mineral crystals having an x-ray diffraction pattern similar to normal human bone. Alizarin red-S histology demonstrated calcium mineral deposition in the extracellular matrix and what appeared to be intracellular paranuclear staining. Electron microscopy revealed small, needle-like crystals associated with fibrillar, extracellular matrix deposits and intracellular spherical structures. Mineral formation was inhibited by levamisole (ED50 approximately 250 microM), pyrophosphate (ED50 approximately 1-10 microM), actinomycin C1 (500 ng/ml), cycloheximide (50 micrograms/ml), or brefeldin A (1 microgram/ml). These results indicate that UMR 106-01 BSP cells form a bio-apatitic mineralized matrix upon addition of supplemental phosphate. This process involves alkaline phosphatase activity, ongoing RNA and protein synthesis, as well as Golgi-mediated processing and secretion.

Adenosine 5'-triphosphate promotes mineralization in differentiating chick limb-bud mesenchymal cell cultures

When chick limb-bud mesenchymal cells are plated in micromass culture, they differentiate to form a mineralizable cartilage matrix. Previous studies have demonstrated that, when the total inorganic phosphate concentration of the medium is adjusted to 3-4 mM by adding inorganic phosphate to the basal medium, the mineralized matrix formed resembles that of chick calcified cartilage in ovo. When the high-energy phosphates adenosine 5'-triphosphate (ATP) or creatine phosphate are used as supplements in place of inorganic phosphate, the mineralized matrix as analyzed by electron microscopy and Fourier transform infrared microscopy is also similar to that in ovo. This is in marked contrast to the mineralized matrix formed in the presence of 2.5-5 mM beta-glycerophosphate, where mineral deposition is random and mineral crystal sizes in general are larger. This is also in contrast to the known ability of ATP to inhibit mineral deposition in solution in the absence of cells. In the differentiating mesenchymal cell culture system, ATP does not alter the rate of cell proliferation (DNA content), the rate of matrix synthesis (3H-leucine uptake), the mean crystallite length, or the rate of mineral deposition (45Ca uptake) when contrasted with cultures supplemented with inorganic phosphate. However, ATP does increase the mineral to matrix ratio, especially around the edge of the culture, where a type I collagen matrix is presented. It is suggested that ATP promotes mineral deposition by providing a high-energy phosphate source, which may be used to phosphorylate extracellular matrix proteins and to regulate calcium flux through cell membranes.