Immunocytochemical localization of gamma-carboxyglutamic acid-containing proteins (osteocalcin) in rat bone and dentin

Correlations between gene expression and mineralization in the avian leg tendon

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

Anatomically-specific intratubular and interstitial biominerals in the human renal medullo-papillary complex

Limited information exists on the anatomically-specific early stage events leading to clinically detectable mineral aggregates in the renal papilla. In this study, quantitative multiscale correlative maps of structural, elemental and biochemical properties of whole medullo-papillary complexes from human kidneys were developed. Correlative maps of properties specific to the uriniferous and vascular tubules using high-resolution X-ray computed tomography, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, and immunolocalization of noncollagenous proteins (NCPs) along with their association with anatomy specific biominerals were obtained. Results illustrated that intratubular spherical aggregates primarily form at the proximal regions distant from the papillary tip while interstitial spherical and fibrillar aggregates are distally located near the papillary tip. Biominerals at the papillary tip were closely localized with 10 to 50 μm diameter vasa recta immunolocalized for CD31 inside the medullo-papillary complex. Abundant NCPs known to regulate bone mineralization were localized within nanoparticles, forming early pathologic mineralized regions of the complex. Based on the physical association between vascular and urothelial tubules, results from light and electron microscopy techniques suggested that these NCPs could be delivered from vasculature to prompt calcification of the interstitial regions or they might be synthesized from local vascular smooth muscle cells after transdifferentiation into osteoblast-like phenotypes. In addition, results provided insights into the plausible temporal events that link the anatomically specific intratubular mineral aggregates with the interstitial biomineralization processes within the functional unit of the kidney.

Enhancement of in Vitro Osteoblastic Potential after Selective Sorting of Osteoblasts with High Alkaline Phosphatase Activity from Human Osteoblast-Like Cells

In this article we describe the expansion of in vitro osteogenic capability of human osteoblasts (HOS cells) after sorting by fluorescence-activated cell sorting (FACS) with the osteoblastic marker of human bone alkaline phosphatase (AP) monoclonal antibody. After culturing for 7 days, the HOS cells were incubated with fluorescein isothiocyanate (FITC)-labeled AP monoclonal antibody. The antibody recognized the cells with high AP activity (high AP cells), which were about 76% of the total cells. After the HOS cells were sorted, the high AP cells could be recovered, and almost all of them reacted strongly with the AP antibody. Therefore, we were able to condense the high AP cells about 1.3 times. We further cultured the sorted cells as well as the unsorted control cells. After the initial seeding, the culturing periods for both groups of cells were 20 days. At the end of this period, we measured AP activity per DNA and osteocalcin contents. In contrast to the low condensation ratio of the high AP cells in the sorted fraction, the AP activity and osteocalcin contents were about nine times and four times greater than those of the unsorted cells, respectively. These results demonstrated that using the sorting technique to isolate the high AP cells might be a useful method for applications in bone tissue engineering.

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

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

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

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

Effects of radial shock waves therapy on osteoblasts activities

Radial shock waves therapy (RSWT) differs from extracorporeal shock waves therapy (ESWT) in that it produces a non-focused wave that is dissipated radially at the skin. Few studies have yet explored the effects of RSWT on bone tissue. Osteoblasts in culture flasks were studied by polymerase chain reaction after treatment with RSW (500 impulses, 0.05 mJ/mm(2)). An inhibited osteoblastogenesis was observed, with a statistically significant reduction in type 1 collagen, osterix, bone sialoprotein and receptor activator NF kappa ligand expression at 24 and 48 h, of osteocalcin at 24, 48 and 72 h, and osteopontin at 48 and 72 h. These findings show that RSWT is not indicated for treatment of delayed fracture union, pseudoarthrosis, and complex regional pain syndrome. The observed reduction in the receptor activator of nuclear factor-kB ligand/osteoprotegerin ratio suggests that it has an inhibiting effect on osteoclastogenesis, which could make it a useful tool for applications in proliferative diseases.

Regenerative capability of dental pulp cells after crown fracture

The purpose of this study was to evaluate the characteristics of dental pulp cells for tissue engineering derived from the fractured incisal portion of tooth crowns. Thirty Sprague-Dawley rats were used for histological and immunohistochemical analysis of nestin protein expression and to measure levels of mRNAs encoding osteocalcin, osteopontin, bone sialoprotein (BSP), dentin sialoprotein (DSP), heat shock protein (HSP) 27, vascular endothelial growth factor (VEGF), ATP-binding cassette transporter G2 (ABCG2), nestin, and p57(Kip2) . Odontoblasts at the incisal portion in the control group were oriented in a regular pattern, but those in the experimental group were randomly stratified. Immunohistochemically, only a few odontoblasts were positive for nestin at the incisal portion in the experimental group at 2 days. Some cells in the inner area in the control group were positive for nestin, but nestin-positive cells in the experimental group at the incisal portion were not observed. The mRNA expression for osteogenic or odontogenic markers in the experimental group was higher than in the control group. HSP27 mRNA expression in the experimental group at 2 days was higher than in the control group and in the experimental group at 7 days. mRNA expression of stem cell markers, such as ABCG2 and nestin, in the experimental group tended to decrease compared with the control. In conclusion, this study demonstrates that dental pulp stem cells derived from fractured teeth differentiate to osteogenic or odontogenic cells.

Effect of shock wave treatment on platelet-rich plasma added to osteoblast cultures

The aim of this study was to verify the effects on osteoblast cultures of adding a platelet-rich plasma (PRP) concentrate pretreated with 500 shock wave (SW) at an energy flow density of 0.17 mJ/mm(2), emitted by an electromagnetic generator Minilith SL1 (STORZ, Germany), reproducing the conditions of our previous study in which we apply SW directly on osteoblasts. Real-time PCR showed that in osteoblast cultures with added PRP pretreated with SW, there was an increased expression at 48 h of insulin-like growth factor binding protein 3 (IGFBP-3) and runt-related transcription factor 2 (RUNX2) and at 72 h, of collagen type I, osteocalcin, insulin-like growth factor 1 (IGF-1) as well as IGFBP-3. Western blotting confirmed the increased protein synthesis of IGFBP-3. This experience suggests that extracorporeal shock wave treatment (ESWT) should stimulate osteogenesis also by indirect platelets-mediated network. It therefore seems possible that combining the two methods, ESWT and bioengineering procedures to infiltrate PRP and growth factors, could be a successful approach.

A novel spectrofluorometric technique for specific biocompatibility testing of implantable materials by cell culture. Report on use for multiparameter analysis of human osteoblasts cultured on commercially pure titanium and hydroxyapatite

The authors describe a novel spectrofluorometric technique based on double-labelled fluorescence imaging using immunoconjugates labelled with fluorochromes. Following isolation and characterization, cells are seeded on the surface of disks of the material(s) to be tested. After application of a primary antibody and an antibody bearing a fluorochrome, the signal emitted by the molecules in the extracellular matrix on the surface of the test disks is measured by spectrofluorimetry. Measurement is thus independent of the surface characteristics of the test material. Measured values are compared with pre-established standard curves. This technique facilitates determination of the characteristic molecules expressed by a given cell type,thus allowing accurate evaluation of the response of pertinent biological samples to implantable biomaterials.

Immunohistochemical identification of type I and type III collagen and chondroitin sulphate in human pre-dentine: a correlative FEI-SEM/TEM study

AIM

To identify type I- (I-CF) and type III-collagen fibrils (III-CF) and chondroitin 4/6 sulphate (CS) within human pre-dentine by means of a correlative analysis under field emission in-lens-scanning electron microscopy (FEI-SEM) and transmission electron microscopy (TEM).

METHODOLOGY

Human-extracted teeth were obtained and submitted to either a pre-embedding or a post-embedding immunolabelling procedure using monoclonal primary antibodies anti-I-CF, anti-III-CF and anti-CS. Gold-conjugated secondary antibodies were coupled to primary antibodies to visualize labelling under the electron beam. Correlative labelling patterns were obtained for I-CF and CS under both FEI-SEM and TEM.

RESULTS

Field emission in lens-SEM analysis revealed an intricate three-dimensional network of I-CF and CS clarifying the intimate relationship between the two main components of the pre-dentine organic matrix. TEM analysis revealed odontoblasts exhibiting intracellular labelling for CS, which became more intense and diffuse over the pre-dentine organic matrix. The same diffuse immunoreaction was revealed for I-CF, whereas a weak immunolocalization of III-CF was found scattered throughout the pre-dentine layer and over the collagen fibrils.

CONCLUSIONS

Both the pre- and post-embedding immunohistochemical approaches have led to the visualization of CF- and CS-labelling distribution within the pre-dentine layer, adding further knowledge on the elucidation of collagen-proteoglycans interaction in the organic matrix of human dental roots.

Advances in defining regulators of cementum development and periodontal regeneration

Substantial advancements have been made in defining the cells and molecular signals that guide tooth crown morphogenesis and development. As a result, very encouraging progress has been made in regenerating crown tissues by using dental stem cells and recombining epithelial and mesenchymal tissues of specific developmental ages. To date, attempts to regenerate a complete tooth, including the critical periodontal tissues of the tooth root, have not been successful. This may be in part due to a lesser degree of understanding of the events leading to the initiation and development of root and periodontal tissues. Controversies still exist regarding the formation of periodontal tissues, including the origins and contributions of cells, the cues that direct root development, and the potential of these factors to direct regeneration of periodontal tissues when they are lost to disease. In recent years, great strides have been made in beginning to identify and characterize factors contributing to formation of the root and surrounding tissues, that is, cementum, periodontal ligament, and alveolar bone. This review focuses on the most exciting and important developments over the last 5 years toward defining the regulators of tooth root and periodontal tissue development, with special focus on cementogenesis and the potential for applying this knowledge toward developing regenerative therapies. Cells, genes, and proteins regulating root development are reviewed in a question-answer format in order to highlight areas of progress as well as areas of remaining uncertainty that warrant further study.

Reduction of osteocalcin expression in aged human dental pulp

AIM

To investigate the expression of osteocalcin mRNA in young and in aged human dental pulp tissue to determine the characteristics of osteocalcin expression.

METHODOLOGY

Human dental pulp tissues of the third molars were obtained from healthy young (17-23 years) and aged (>50 years) subjects, and total RNA was extracted. Osteocalcin mRNA expression was determined by RT-PCR and by quantitative real-time RT-PCR (QRT-PCR). The threshold cycle (Ct) value, which reflects the amount of PCR, was calculated and the difference between the value in young and aged pulp was statistically analysed.

RESULTS

Osteocalcin mRNA was detected in all samples of human dental pulp tissue homogenates by RT-PCR analysis. Osteocalcin mRNA was expressed in young adult dental pulp but was decreased in aged human dental pulp. QRT-PCR analysis also showed a reduced expression of osteocalcin mRNA in aged human pulp. Expression of osteocalcin in young human pulp was significantly higher (about sixfold) than in aged pulp (P<0.01, Mann-Whitney U-test).

CONCLUSION

Reduction of osteocalcin expression may be associated with the loss of viability in human dental pulp tissue, and may be a characteristic of aged human dental pulps.

Expression of osteocalcin and Jun D in the early period during reactionary dentin formation after tooth preparation in rat molars

We examined, in rats, the expression of osteocalcin and Jun D in the early stage of reactionary dentin formation after tooth preparation and the accompanying morphological changes. Reverse transcription/polymerase chain reaction analysis revealed strong expression of osteocalcin mRNA in pulp tissue at 2 and 3 days post-preparation compared with that in control teeth. Light microscopy demonstrated that, at the dentin-pulp interface, damaged odontoblasts were detached from the dentin matrix immediately after preparation, with neutrophils lining the dental surface after 1 day. After 2-3 days, differentiated odontoblasts appeared at the interface. Reactionary dentin with tubular structures was formed under the cavity after 10 days. Immunoelectron microscopy showed that trace amounts of osteocalcin were expressed in odontoblasts at 2 days post-preparation, and abundant osteocalcin was found in the highly developed Golgi apparatus and granules at 3 days post-preparation. Osteocalcin was also found on type I collagen fibrils in newly formed predentin. The existing dentinal tubules were filled with osteocalcin-coated type I collagen fibrils. We observed, by immunohistochemistry, that Jun D was temporally expressed in the nuclei of the odontoblasts at 1 and 2 days post-preparation. However, no Jun D was found in the dental pulp cells at any other time or in control teeth. Thus, osteocalcin expression is correlated with reactionary dentin formation, and Jun D is associated with osteocalcin expression in odontoblasts. Osteocalcin may also serve as an obturator of the dentinal tubules to protect dental pulp vitality against external irritants after preparation.

Surface modification of poly(ethylene-co-vinyl alcohol): hydroxyapatite immobilization and control of periodontal ligament cells differentiation

To reveal and control the differentiation and proliferation of the periodontal ligament (PDL) cells and to develop a highly organized hybrid implant possessing periodontium, hydroxyapatite (HAP) was immobilized on the poly(ethylene-co-vinyl alcohol) (EVA) by alternate soaking method following with carboxyl groups' introduction through ozone exposure. Human PDL cells were cultured on the ozone-exposed EVA, collagen-immobilized EVA, HAP-immobilized EVA, HAP plate, tricalcium phosphate plate, and conventional tissue culture dish. Cell proliferation was highest on the collagen-immobilized EVA and lowest on the HAP-immobilized EVA. Alkaline phosphatase activity and osteocalcin secretion were highest on the HAP-immobilized EVA. These results suggest that PDL cells were differentiated toward bone-like cells on the HAP-immobilized EVA.

In-situ visualization and quantification of mineralization of cultured osteogenetic cells

An osteoblastic cell line (HOS cells) produces a prominent osteoid matrix with mineralization. Fibroblasts, on the other hand, do not exhibit this mineralization. To evaluate the degree of mineralization, we added calcein to the culture medium and then observed the culture wells by using an image analyzer. The calcein uptake into the cell/matrix layer was detected in the HOS cells but not in the fibroblasts. The calcein uptake was also quantified in situ by using an image analyzer, which revealed high levels in the HOS cells, which correlated well with the calcium content of the mineralized matrix. Rat marrow cells were also cultured in media containing calcein, fetal bovine serum, beta-glycerophosphate, L-ascorbic acid 2-phosphate, and with or without dexamethasone. With the dexamethasone, the cells exhibited osteogenic differentiation that resulted in mineralized matrix formation after about 10 days. The matrix formation coincided with the appearance of calcein uptake into the cell/matrix layer, with the amount of calcein uptake increasing with time. By contrast, the culture without the dexamethasone did not exhibit matrix formation and the calcein uptake was negligible. In the case of both HOS cell and rat marrow cell cultures in vitro, calcein did not affect expressions of their alkaline phosphatase activity or osteocalcin production. Furthermore, histologic observation revealed that rat marrow cells subcultured with calcein could show osteogenic ability after in vivo implantation. These results suggest that the current method of detecting calcein uptake in a culture allows the monitoring of the osteogenic capacity of cultured cells, as well as the measurement of the amount of mineralization produced by the osteogenic cells. Given that osteogenic cultured cells/mineralized matrices are used in bone reconstruction surgery, the in situ monitoring method is invaluable in that it allows us to evaluate the osteogenic capacity of in vitro constructs.

Quantitative immunogold labeling of bone sialoprotein and osteopontin in methylmethacrylate-embedded rat bone

Methylmethacrylate (MMA) embedding of undecalcified bone is routinely employed for histomorphometric analyses. Although MMA-embedded bone has been used for immunolabeling at the light microscopic level after removal of the resin, there are no such reports for electron microscopy. The aim of the present study was to determine whether MMA embedding can be used for ultrastructural immunolabeling and how it compares to LR White (LRW), an acrylic resin frequently used for immunocytochemistry of bone. Rat tibiae were fixed by vascular perfusion with aldehyde and embedded either in MMA or LRW resin. Thin sections were processed for postembedding protein A-gold immunolabeling with antibodies to rat bone sialoprotein (BSP) and osteopontin (OPN). The density of gold particles over bone was quantified. The density and distribution of immunolabeling for BSP and OPN respectively, were comparable between MMA and LRW. These results indicate that MMA performs as well as LRW for the ultrastructural immunolabeling of noncollagenous bone matrix proteins.

Development and validation of a specific radioimmunoassay for equine osteocalcin

This study describes for the first time the development and validation of a sensitive and specific radioimmunoassay (RIA) for equine osteocalcin (OC) quantification using purified equine OC as standard, tracer, and immunogen for antibody formation in rabbits. The assay allowed to measure equine serum OC levels with a sensitivity of 0.2 ng/mL. Immunoreactive serum OC values of clinically normal, different-aged horses ranged from 3.68 to 127.31 ng/mL. Intra- and inter-assay coefficients of variation (CV) were 6.2 and 8.2%, respectively. Serial equine serum sample dilutions were linear. The recovery of equine OC from equine serum samples ranged from 93.88 to 107.9%. There was a tight correlation between OC values measured with the equine-specific OC RIA and two commercially available bovine-specific OC RIA kits. However, highest serum OC values were obtained with the equine-specific OC RIA. In conclusion, our equine-specific OC RIA is sensitive, linear, accurate, precise, and reproducible. The assay allowed to quantify OC in equine serum samples and might, therefore, be used to monitor equine osteoblast activity associated with bone diseases, exercise, therapy forms or diet.

Investigation of osteocalcin, osteonectin, and dentin sialophosphoprotein in developing human teeth

Biochemical investigations in rodents have shown that numerous mineralized matrix proteins share expression in bone, dentin, and cementum. Little information is available regarding the expression pattern of these proteins in human tissues, particularly during tooth formation. The aim of this study was to identify the expression pattern of the two major noncollagenous proteins of bone and dentin, osteocalcin (OC) and osteonectin (ON), in comparison to the dentin-specific protein, dentin sialophosphoprotein (DSPP). Mandibles from fetuses (5-26 weeks), neonate autopsies, forming teeth from 10-12-year-old patients, third molars extracted for orthodontic reasons, and bone tumors were collected with approval from the National Ethics Committee. Human OC, ON, and DSPP mRNAs were detected by reverse transcription-polymerase chain reaction (RT-PCR) in fetal mandibles (5-11 weeks) and in primary cell cultures of dental pulp. In addition, OC, ON, and DSPP proteins were localized in forming human mineralized tissues using immunohistochemistry. In vivo, DSPP expression was associated with tooth terminal epithelial-mesenchymal interaction events, amelogenesis and dentinogenesis. Transient DSPP expression was seen in the presecretory ameloblasts with continuous expression in the odontoblasts. In contrast, both osteoblasts and odontoblasts showed a temporal gap between OC and ON expression in early development. ON was expressed in the initial stages of cytodifferentiation, whereas OC was expressed only during the later stages, especially in the teeth. At the maturation stage of enamel formation, both proteins were detected in odontoblasts and their processes within the extracellular matrix. In contrast to bone, OC was not localized extracellularly within the collagen-rich dentin matrix (predentin or intertubular dentin), but was found in the mature enamel. ON was present mostly in the nonmineralized predentin. These results demonstrate for the first time that both OC and ON are produced by human odontoblasts and determine the expression pattern of DSPP in human teeth, and suggest that OC and ON move inside the canalicule via odontoblast cell processes becoming localized to specific extracellular compartments during dentin and enamel formation. These distinct extracellular patterns may be related to the nature of DSPP, OC, and ON interactions with other matrix-specific macromolecules (i.e., amelogenin, dentin matrix protein-1) and/or to the polarized organization of odontoblast secretion as compared with osteoblasts.

Gene expression and immunohistochemical localization of osteonectin in association with early bone formation in the developing mandible

We have compared the expression of osteonectin with that of osteocalcin and bone sialoprotein during bone formation in the rat mandible, using in situ hybridization and immunohistochemistry. Expression of osteonectin, osteocalcin and bone sialoprotein mRNAs were first observed in newly differentiated osteoblasts of the developing mandible at embryonic day 15 (E15) and subsequently increased with the number of osteoblasts through E20. Definitive osteonectin immunostaining was observed in newly differentiated osteoblasts, but not in the intercellular unmineralized matrix. Immunostaining for osteocalcin and bone sialoprotein was visible in osteoblasts and unmineralized matrix. Concomitant with the initiation of matrix mineralization at E16, mineralized bone matrix showed osteocalcin and bone sialoprotein immunostaining, but lacked osteonectin immunostaining. The same staining profile was observed during subsequent phases of bone formation at E17-20. However, sequential demineralization with ethanolic trimethylammonium EDTA and protease digestion of tissue sections demonstrated prominent osteonectin immunostaining of the mineralized bone matrix. Western blot analysis of osteonectin in extracts of fresh specimens at E18 and 20 revealed that an EDTA extract contains osteonectin having M, approximately 50 kDa. These results indicate that newly differentiated osteoblasts synthesize and secrete osteonectin, which is mainly incorporated into the mineralized bone matrix and becomes a specific component of developing manibula of foetal rats.

Non-invasive assessment of equine bone: an update

This paper summarizes traditional and current methods of non-invasive assessment of bone in the horse. The description and potential clinical utility of two non-invasive technologies with major development in the last decade are presented, namely, (1) serum biochemical markers for bone turnover and (2) quantitative ultrasound. Serum biochemical markers of bone formation valid in horses are osteocalcin, carboxy-terminal peptide of type I procollagen and bone-specific alkaline phosphatase. The cross-linked carboxy-terminal telopeptide of type 1 collagen c-telopeptides of type I collagen and total deoxypyridinoline are the serum markers for bone degradation. These markers respond more rapidly to skeletal changes than other bone assessment techniques, but ideally each horse needs to be compared with itself. Quantitative ultrasound is radiation free and is a well-tolerated technique for measuring bone properties in horses. This device allows bone speed of sound measurements at various sites using the axial transmission mode along the cortex and gives information about stiffness, architecture, porosity and bone mass.A combination of different non-invasive assessment techniques is recommended for the evaluation of bone biphasic modelling-remodelling activity and the mineral phase with its architecture. The potential clinical and research use of these techniques is considered.

Enhancement of the in vivo osteogenic potential of marrow/hydroxyapatite composites by bovine bone morphogenetic protein

A composite of marrow mesenchymal stem cells and porous hydroxyapatite (HA) has in vivo osteogenic potential. To investigate factors enhancing the osteogenic potential of marrow/HA composites, we prepared a bone morphogenetic protein (BMP) fraction from the 4M guanidine extract of bovine bone by heparin-sepharose affinity chromatography. Marrow/HA composites or composites containing marrow mesenchymal stem cells, BMP, and HA (marrow/BMP/HA composites) were implanted subcutaneously in 7-week-old male Fischer rats. BMP/HA composites and HA alone were also implanted. The implants were harvested after 2, 4, or 8 weeks and were prepared for histological and biochemical studies. Histological examination showed obvious de novo bone formation together with active osteoblasts at 2 weeks, as well as more extensive bone formation at 4 and 8 weeks in many pores of the marrow/BMP/HA composites. The marrow/HA composites did not induce bone formation at 2 weeks, but there was moderate bone formation at 4 weeks. At 2 weeks, only marrow/BMP/HA composites resulted in intensive osteogenic activity, judging from alkaline phosphatase and osteocalcin expression at both the protein and gene levels. These results indicate that the combination of marrow mesenchymal stem cells, porous HA, and BMP synergistically enhances osteogenic potential, and may provide a rational basis for their clinical application, although further in vivo experiment is needed.

Content and distribution of noncollagenous matrix proteins in bone and cementum: relationship to speed of formation and collagen packing density

The organic matrix of collagen-based calcified tissues consists of a supporting collagen meshwork and various noncollagenous matrix proteins (NCPs). Together, they contribute to determining the structure and biomechanical properties of the tissue. Their respective organization and interrelation can advantageously be examined by immunocytochemistry, an approach which allows correlation of composition with structure. The aim of this article is to review postembedding immuno- and lectin-gold-labeling data on the characterization of the noncollagenous compartment in rat and human bone and cementum, and on its relationship to collagen. The two major NCPs, bone sialoprotein and osteopontin, generally codistribute and accumulate in cement lines and in the spaces among the mineralized collagen fibrils. However, there are variations in their distribution and density of labeling throughout the tissue. Indeed, bone and cementum can form in environments that are either poor or enriched in NCPs. The amount of NCPs generally correlates with bone and cementum types and with speed of formation of the tissue and packing density of collagen fibrils. Taken together, the data suggest that production of both collagenous and noncollagenous constituents can be "modulated" during formation of collagen-based calcified tissues. It is concluded that, in addition to structural and compositional parameters, tissue dynamics must be taken into consideration in order to understand the significance of the apparent accumulation of NCPs at some sites and to determine the mechanisms of normal and pathological calcified tissue formation.

Morphological and immunocytochemical characterization of primary osteogenic cell cultures derived from fetal rat cranial tissue

Enzymatic digestion of bone tissue potentially releases a mixture of precursor, differentiating, and mature cells. Conceptually, early fetal osteogenic tissue should provide a more uniform population of cells than late embryonic or newborn bone in which cells have already differentiated. In this context, we have applied sequential enzymatic digestion to obtain and culture cells from 15-16-day fetal rat cranial tissue, a developmental age where deposition of bone matrix has not yet started at this site. These cultures were compared with those of osteogenic cells isolated from newborn rat calvariae and grown under similar conditions. Matrix production and composition were examined by colloidal gold immunocytochemistry using antibodies to bone sialoprotein (BSP), osteocalcin (OC), and osteopontin (OPN). The plated cells formed mineralized nodules by day 14. The presence of mineral was determined by von Kossa staining and backscattered electron imaging (BEI), and the accumulation of calcium and phosphorus within the nodules was demonstrated by X-ray microanalysis and elemental mapping. At early time intervals, cells were generally cuboidal in shape and showed a well-developed Golgi apparatus, which occasionally was immunoreactive for OPN. Labeling for BSP and OPN was found over mineralization foci and electron-dense material within, and at the periphery, of larger mineralized masses and over accumulations of afibrillar matrix at the dish surface. Osteocalcin immunoreactivity was also associated with electron-dense portions of the bone-like matrix. These data demonstrate the potential of presumptive fetal rat calvarial cells to form a bone-like matrix in vitro and suggest that the assembly and mineralization pattern show similarities to the process of intramembranous ossification. Such a culture system is of interest not only for studying cellular and matrix events of bone formation, but also factors which influence mesenchymal cells in committing themselves to the osteogenic pathway.

Biochemical markers of bone metabolism in draught and warmblood horses

Concentrations of the cross-linked carboxyterminal telopeptide of type I collagen (ICTP) and osteocalcin (OC) have been determined in the serum of one hundred clinically healthy adult Draught or Warmblood horses. The correlation between these two markers has been evaluated and the influence of gender, age and type of horse described. No significant variations were observed between animals of different sex, but a significant inverse correlation (P < 0.0001) with age was observed for both measured parameters. After correction for age, serum levels of OC were found to be lower in Draught [adjusted least square mean (LSM) = 6.612 micrograms.L-1] than in Warmblood horses (adjusted LSM = 8.596 micrograms.L-1), whereas levels of ICTP were higher in Draughts (adjusted LSM = 8.035 micrograms.L-1) than in Warmbloods (adjusted LSM = 6.643 micrograms.L-1). A significant correlation (P < 0.0001) was observed between OC and ICTP. This correlation was stronger if the type of horse was taken into account in the statistical model. The ratio OC:ICTP was independent of gender and age. A higher OC:ICTP ratio in Warmbloods compared to the Draught horses might reflect a higher bone remodelling level of horses submitted to regular daily work. It was concluded that ICTP and OC are influenced by the type of horse, and probably reflect a physiological difference in bone remodelling between these animals.

Analysis of gene expression in osteogenic cultured marrow/hydroxyapatite construct implanted at ectopic sites: a comparison with the osteogenic ability of cancellous bone

We investigated the in vivo osteogenic ability of cultured marrow cells subcultured in porous hydroxyapatite. This osteogenic ability was compared with that of cancellous bone grafts. Fresh marrow cells were obtained from young adult rat femora and cultured in a standard medium for 10 days, then trypsinized and used to make constructs of porous hydroxyapatite (HA) and cultured marrow cells. An additional 2-week culture (subculture) was performed for the construct in standard medium with and without the addition of dexamethasone (Dex). The 2-week subcultured constructs then were implanted into subcutaneous sites of syngeneic rats. These implants and the rat cancellous bone were harvested and prepared for gene expression analysis of alkaline phosphatase (ALP) and osteocalcin (OC) as well as for histological analysis. ALP and OC mRNAs could be detected in Dex-treated subcultured constructs 1 week after implantation with an increase at 2 weeks, comparable to the observation in cancellous bone. Histological analysis showed active bone formation even 1 week postimplantation. In contrast, the subcultured constructs without the addition of Dex did not show bone formation, and only small levels of ALP and OC mRNAs were found. These results indicate that the bone tissue formed by grafting the Dex-treated construct of cultured marrow cells and hydroxyapatite possesses a high osteoblastic activity comparable to that of viable cancellous bone. Thus the prefabricated osteogenic subcultured marrow/HA construct may be applicable in bone reconstructive surgery.

Spaceflight has compartment- and gene-specific effects on mRNA levels for bone matrix proteins in rat femur

In the present study, we evaluated the possibility that the abnormal bone matrix produced during spaceflight may be associated with reduced expression of bone matrix protein genes. To test this possibility, we investigated the effects of a 14-day spaceflight (SLS-2 experiment) on steady-state mRNA levels for glyceraldehyde-3-phosphate dehydrogenase (GAPDH), osteocalcin, osteonectin, and prepro-alpha(1) subunit of type I collagen in the major bone compartments of rat femur. There were pronounced site-specific differences in the steady-state levels of expression of the mRNAs for the three bone matrix proteins and GAPDH in normal weight-bearing rats, and these relationships were altered after spaceflight. Specifically, spaceflight resulted in decreases in mRNA levels for GAPDH (decreased in proximal metaphysis), osteocalcin (decreased in proximal metaphysis), osteonectin (decreased in proximal and distal metaphysis), and collagen (decreased in proximal and distal metaphysis) compared with ground controls. There were no changes in mRNA levels for matrix proteins or GAPDH in the shaft and distal epiphysis. These results demonstrate that spaceflight leads to site- and gene-specific decreases in mRNA levels for bone matrix proteins. These findings are consistent with the hypothesis that spaceflight-induced decreases in bone formation are caused by concomitant decreases in expression of genes for bone matrix proteins.

The effect of altered functional forces on the expression of bone-matrix proteins in developing mouse mandibular condyle

Mechanical forces are known to have an effect on bone formation, maintenance and remodelling, and there is evidence that the development of the mandibular condyle in the rat is influenced by the consistency of the diet. Here a mouse model was used to investigate the relation between food, condylar development and the expression of bone sialoprotein (BSP), osteopontin (OPN), osteocalcin (OC) and type 1 collagen (COL I). Twenty-four 19-day-old male mice were randomly divided into three groups. Groups 1 and 2 were fed hard pellets and soft powdered food, respectively, for 2 weeks. Group 3 mice were fed soft food for 1 week followed by a week of hard pellets. Incisors of mice in groups 2 and 3 were trimmed twice a week to reduce occlusal forces. After killing the animals, mandibular condyles were collected for RNA extraction, in situ hybridization and immunohistochemical analyses. Histological sections showed that the condyles of mice in group 2 were underdeveloped, with a thinner layer of cartilage and fewer bone trabeculae. Northern hybridization of total RNA of the condyle from mice in this soft-food group also exhibited a significant decrease in the amounts of BSP, OPN, OC and COL I, representing 79%, 75%, 77% and 79% respectively, of that from mice fed hard food. In situ hybridization of these bone-matrix proteins demonstrated signals in bone-forming cells and BSP mRNA was also seen in the hypertrophic cartilage cells in the developing condyle. Immunohistochemical study demonstrated an obvious difference in the intensity of staining, especially for BSP. Results from group 3 were similar to those from group 1. The observed decrease in bone matrix-protein expression confirms that the consistency of the diet affects the development of the mouse mandibular condyle and that a soft diet diminishes the rate of bone formation.

RGTA11, a new healing agent, triggers developmental events during healing of craniotomy defects in adult rats

RGTA are chemically defined compounds which proved to be very potent healing agents in various tissue repair models including skin, muscle and nerve. These chemicals are believed to protect endogenously released heparin-binding growth factors and enhance their bioavailability during healing. In craniotomy defects that do not heal spontaneously in adults, RGTA promoted dose-dependent skull closure. The aim of this work was to characterize, in the same model, the events associated with wound closure by studying the expression of the osteoblastic phenotype and the distribution of some matrix proteins during RGTA11-induced bone healing. Craniotomy defects in rats were implanted with collagen plasters soaked in a solution of RGTA11 (1.5 micrograms per piece). The skulls were removed 30 days after wounding, a stage of almost complete bone filling in treated samples. Bone formed only at the edges of the defect in controls, while it formed also at the center in the form of nodules in the treated samples. RGTA11 modified the amount and distribution of the tissues including bone in the wounds. In some RGTA11-treated samples, skull closure by bone occurred and the median suture was restored. In the treated defects, alkaline phosphatase-positive (osteoprogenitor) cells were far more numerous and were distributed differently. Type I and III collagen and fibronectin deposition was markedly enhanced in the bone compartment of the wounds. Secretory osteoblasts released type III collagen. Osteocalcin expression was enhanced by RGTA11. RGTA11 thus modified the healing pattern by increasing both the cellularity and the synthesis of a bone-competent extracellular matrix, thereby restoring the original anatomy of the skull. Flat bone regeneration can be triggered in adults through developmental events (i.e. nodule formation, secretion of type III collagen by osteoblasts, suture restoration...) that are no longer operative in the wounds of mature individuals.

Osteocalcin expression in young and aged dental pulps as determined by RT-PCR

Dental pulps were obtained from third molars of young adults (17-25 yr) or from molar teeth of individuals > 50 yr of age and examined for the expression of osteocalcin (OC) mRNA by RT-PCR. OC was selected as a determinant of pulp vitality, because it has long been associated with the production of hard tissue matrix in teeth and bone. For comparative purposes, the expression of OC in each pulp was normalized relative to its housekeeping gene-product GAPDH by the establishment of a OC/GAPDH ratio. This study demonstrated that OC expression, presumably by cells of odontoblast lineage, does not diminish relative to the extant cell population. Our findings suggest, despite a reduction in volume and cell numbers, that the pulps of aging teeth retain a capacity for dentin deposition and a potential for caries and trauma resistance.

Sequential expression of bone matrix proteins during rat calvaria osteoblast differentiation and bone nodule formation in vitro

We investigated the expression of osteocalcin (OC), bone sialoprotein (BSP), osteonectin (ON), and alkaline phosphatase (ALP) during cell differentiation and bone nodule formation by fetal rat calvaria cells, using immunofluorescent and immunogold techniques at light and electron microscopic levels. Six hours after plating all proteins were expressed in calvaria cells. However, expression was not detected during the proliferation phase after plating. Cell morphological modifications were observed in osteoblastic cells expressing ALP, OC, and BSP, but not ON. During the matrix formation phase, all proteins were expressed with various intensities and OC was limited to differentiated osteoblastic cells. EM observations demonstrated that BSP was selectively associated with clusters of needle-like crystals, but not with collagen fibers, in mineralization foci and in the mineralized matrix. OC was localized intracellularly and in all the extracellular compartments, and was concentrated at the mineralization front. ON was distributed uniformly throughout the osteoid and mineralized matrix, which was intensely labeled. The results show that the expression of bone matrix proteins during differentiation of calvaria cells and nodule formation in vitro duplicate what is observed during osteogenesis in vivo.

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

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

Transient accumulation of proteins at interrod and rod enamel growth sites

Conceptually, there should be a brief interval in time when newly secreted proteins "pile up" at secretory sites just outside the membrane of ameloblasts. Indeed, previous cytochemical studies have suggested that glycosylated and/or sulfated glycoproteins accumulate at enamel growth sites. Colloidal gold lectin cytochemistry and immunocytochemistry with antibodies to enamel proteins and phosphoserine, combined with cycloheximide and brefeldin A to inhibit protein synthesis and secretion, were applied to characterize the distribution of newly formed proteins at enamel interrod and rod growth sites. Although enamel growth sites show a "rarefied" appearance, the results indicate that one or more subclasses of enamel proteins accumulate near the cell surface at sites where elongation of enamel crystallites contributes to thickening of the enamel layer. These proteins are glycosylated and/or phosphorylated and, at least in the case of the glycosylated ones, are rapidly processed after they are released extracellularly. In contrast, immunolabeling for amelogenins is generally weaker near the cell surface and more intense at a short distance away from the site where crystallites elongate. The data suggest that the enamel proteins accumulating at growth sites likely belong to the non-amelogenin category and play a transient role in promoting the lengthening of crystallites. It is concluded that areas near the ameloblast membrane where certain enamel proteins accumulate in fact constitute the equivalent of a mineralization front.

Immediate bone forming capability of prefabricated osteogenic hydroxyapatite

We employed culture technology, which provides bone tissue in vitro, to expand and promote the osteogenic ability of marrow cells in porous hydroxyapatite (HA). Marrow cells were obtained from rat femur and cultured in a standard medium for 10 days, then trypsinized to make composites of HA and the cells. An additional 2-week culture (subculture) was done for the composite in a standard medium with or without the addition of dexamethasone (Dex). The 2-week subcultured composites were implanted into subcutaneous sites of syngeneic rats. These implants were harvested and prepared for the biochemical analysis of alkaline phosphatase activity and bone Gla protein content, as well as histological analysis of decalcified and undecalcified sections. In Dex-treated composites, high alkaline phosphatase activity could be detected 1 week after implantation and was maintained until 8 weeks after implantation. The bone Gla protein content could also be detected 1 week after implantation, followed by a steady increase with the passage of time until 8 weeks after implantation. The histological analysis showed active bone formation even 1 week after implantation. The bone formation was evidenced by active osteoblast lining and the appearance of calcein labeling following calcein injection 1 week after implantation. Thus, Dex-treated subcultured marrow cells in pore regions of HA showed a high osteogenic response immediately after transplantation. In contrast, Dex-untreated composite did not show bone formation and contained traces of these biochemical parameters. These results indicate that the inherent osteogenic ability of marrow stromal stem cells in pore regions of HA can be stimulated using tissue culture technology; and thus, formed osteogenic HA can show immediate osteoblastic activity in in vivo situations, suggesting the applicability of the HA in clinical situations.

The effect of recombinant human basic fibroblast growth factor rhFGF-2 on human osteoblast in growth and phenotype expression

This paper describes the studies of human recombinant basic fibroblast growth factor (rhFGF-2) for its effects on human osteoblast growth and phenotype expression. During a 24-h period of treatment, rhFGF-2 highly stimulated DNA synthesis in a dose-related fashion with a maximum stimulation of 150% for 1 ng/ml. On the other hand, rhFGF-2 decreases alkaline phosphatase activity, synthesis of type I collagen, and cumulative amount of osteocalcin. Moreover, rhFGF-2 provoked a threefold increase in the amount of intracellular cAMP. Scatchard plots show the presence of two classes of [125I] rhFGF-2 receptors. This data suggests that rhFGF-2 which stimulate cell replication may act indirectly as an anabolic agent and stimulate some of the phenotypic expression markers.

Postembedding colloidal-gold immunocytochemistry of noncollagenous extracellular matrix proteins in mineralized tissues

Immunocytochemistry is a powerful tool for investigating protein secretion, extracellular matrix assembly, and cell-matrix and matrix-matrix/mineral relationships. When applied to the tissues of bones (bone and calcified cartilage) and teeth (dentin, cementum, and enamel), where calcium phosphate-containing extracellular matrices are the predominant structural component related to their weight-bearing and masticatory roles, respectively, data from immunocytochemical studies have been prominent in advancing our understanding of mineralized tissue modeling and remodeling. The present review on the application of postembedding, colloidal-gold immunocytochemistry to mineralized tissues focuses on the advantages of this approach and relates them to conceptual, theoretical, and experimental data currently available discussing matrix-mineral interactions and extracellular matrix formation and turnover in these tissues. More specifically, data are summarized regarding the distribution and role of noncollagenous proteins in different mineralized tissues, particularly in the context of how they interface with mineral, and how this relationship might be affected by the various tissue-processing steps and immunocytochemical strategies commonly implemented to examine the distribution and function of tissue proteins. Furthermore, a technical discussion is presented that outlines several different possibilities for epitope exposure in mineralized tissues during preparation of thin sections for transmission electron microscopy. Cell biological concepts of protein secretion by cells of the mineralized tissues, and subsequent extracellular matrix assembly and organization, are illustrated by examples of high-resolution, colloidal-gold immunolabeling for osteopontin, bone sialoprotein, and osteocalcin in the collagen-based mineralized tissues and for enamel protein (amelogenin) in enamel.

Pulpo-dentinal complex revisited

OBJECTIVES AND METHODS

The value of the concept of a pulpo-dentinal complex was assessed on human teeth treated according to the ISO test on biological evaluation. The teeth were extracted after 1 or 3 months and examined histologically. Biochemical and biological data available from the dental literature were also re-examined.

RESULTS

During the early development of the tooth, pulp and dentine establish close links and form an undivided organ. However, examination of the tissues at later stages of development casts doubt on the validity of such a concept. Major differences are reviewed in this report between the cells (odontoblasts and heterogeneous pulpal cells) and extracellular matrix (collagens, non-collagenic proteins and phospholipids) located either in the odontoblast-dentine area or in the pulp. It seems also that clear-cut differences are detected during inflammatory and repair processes.

CONCLUSION

It is concluded that, although the existence of a dentino-pulpal reaction cannot be denied, the concept of a pulpo-dentinal complex is an oversimplification and should be revisited. This may have implications in the evaluation of restorative treatments and in the design of a tissue repair strategy.

Estimation of age at death based on aspartic acid racemization in noncollagenous bone proteins

Age at death determination based on the extent of aspartic acid racemization in dentin has been reported to be highly accurate and reproducible. To test the applicability of this method to human bone, aspartic acid racemization in noncollagenous proteins of bone was investigated. A close relationship was found between age at death and the extent of aspartic acid racemization in osteocalcin, the most abundant noncollagenous protein of the organic bone matrix. Our findings indicate that osteocalcin is a permanent, 'aging' constituent of the organic bone matrix whose D-aspartic acid content increases with age because of in vivo racemization. Thus, the extent of aspartic acid racemization in bone osteocalcin is a measure of the age of the peptide and hence of the entire organism. The relationship between age at death and the extent of aspartic acid racemization in purified bone osteocalcin appears to be close enough to serve as a basis for determination of age at death in forensic medicine.

Dentinogenesis

The formation of dentin, dentinogenesis, comprises a sophisticated interplay between several factors in the tissue, cellular as well as extracellular. Dentin may be regarded as a calcified connective tissue. In this respect, as well as in its mode of formation, it is closely related to bone. Using dentinogenesis as an experimental model to study biomineralization provides several practical advantages, and the results may be extrapolated to understand similar processes in other tissues, primarily bone. After describing dentin structure and composition, this review discusses items such as the morphology of dentinogenesis; the dentinogenically active odontoblast, transport, and concentrations of mineral ions; the constituents of the dentin organic matrix; and the presumed mechanisms involved in mineral formation.

Differentiation of heterogeneous phenotypes in human osteoblast cultures in response to 1,25-dihydroxyvitamin D3

1,25(OH)2D3 (1,25-dihydroxyvitamin D3) inhibits the cell proliferation of human osteoblast-like cell cultures, but stimulates the synthesis of two of the phenotypic markers of the osteoblast, alkaline phosphatase and osteocalcin. It is not known whether all cells which synthesize alkaline phosphatase also synthesize osteocalcin in response to 1,25(OH)2D3. In this study we addressed this question by examining the response of human osteoblast-like cell cultures to 1,25(OH)2D3, using concurrent histochemical and immunochemical staining for alkaline phosphatase and osteocalcin, respectively. The cells were grown in the presence or absence of 1,25(OH)2D3 (10(-9) M) for 48 h. Co-localisation of osteocalcin and alkaline phosphatase in osteoblast-like cell cultures showed that not all cells which synthesize osteocalcin (about 9%) in response to 1,25(OH)2D3 synthesize alkaline phosphatase (about 24%) and vice versa. There was also a proportion of osteoblast-like cells which produce both osteocalcin and alkaline phosphatase simultaneously (about 12%). These findings suggest that during differentiation of bone-derived cells in cultures, in response to 1,25(OH)2D3, heterogeneous phenotypes with respect to expression of alkaline phosphatase and osteocalcin appear.