Formation of matrix vesicles by cultured chick embryo chondrocytes

Role of matrix vesicles and crystal ghosts in bio-mineralization

Matrix vesicles (MVs) are extracellular membrane-bound vesicles of about ~ 50-200 nm in diameter that play a role in the bio-mineralization process of hard tissue formation. The present review is based on the empirical phenomenon of primary mineralization process via matrix vesicle-mediated mechanism with special reference to crystal ghosts as well as the mechanism on the organic-inorganic relationship between matrix vesicles and crystal ghosts, and the transformation that these structures undergo during bio-mineralization.

MicroRNA Contents in Matrix Vesicles Produced by Growth Plate Chondrocytes are Cell Maturation Dependent

Chondrocytes at different maturation states in the growth plate produce matrix vesicles (MVs), membrane organelles found in the extracellular matrix, with a wide range of contents, such as matrix processing enzymes and receptors for hormones. We have shown that MVs harvested from growth zone (GC) chondrocyte cultures contain abundant small RNAs, including miRNAs. Here, we determined whether RNA also exists in MVs produced by less mature resting zone (RC) chondrocytes and, if so, whether it differs from the RNA in MVs produced by GC cells. Our results showed that RNA, small RNA specifically, was present in RC-MVs, and it was well-protected from RNase by the phospholipid membrane. A group of miRNAs was enriched in RC-MVs compared RC-cells, suggesting that miRNAs are selectively packaged into MVs. High throughput array and RNA sequencing showed that ~39% miRNAs were differentially expressed between RC-MVs and GC-MVs. Individual RT-qPCR also confirmed that miR-122-5p and miR-150-5p were expressed at significantly higher levels in RC-MVs compared to GC-MVs. This study showed that growth plate chondrocytes at different differentiation stages produce different MVs with different miRNA contents, further supporting extracellular vesicle miRNAs play a role as "matrisomes" that mediate the cell-cell communication in cartilage and bone development.

Lipid determination in bone marrow and mineralized bone tissue: From sample preparation to improved high-performance thin-layer and liquid chromatographic approaches

In view of their key roles in the bone physiology (e.g., in the biomineralization process) and their potential implication in bone pathologies, an approach to study lipids in situ is needed. The aim of the present paper is to propose an original procedure to characterize lipids in both bone marrow (BM) and mineralized tissue (MT) compartments, taking into consideration sample preparation, lipid extraction and analytical issues, when using small sample size (≤ 0.5g of rat femurs). The potential contamination of the MT by marrow lipids and the poor accessibility of certain lipids from the MT - two major issues in bone handling - were taking care, respectively by performing two cleaning steps after BM removal and by adding a demineralization step to the overall lipid extraction protocol. For lipid analyses, a multi-one-dimensional HP-TLC method was developed to analyze the major neutral and polar lipids at once and showed an excellent resolution (for 15 standards) and a good precision (inter-day RSD<13%). When subjected to the entire "lipid extraction-HP-TLC" protocol, spike recoveries of the standards ranged between 76 and 122%. This HP-TLC method was suitable for lipid determination in both BM and MT [e.g., the MT had 5-times lesser lipids and a lower TG/phospholipid ratio than the BM (P <0.05)], and was quite reliable in term of lipid quantification. The demineralization step allowed to extract additional phosphatidylserine and esterified cholesterol from the MT, suggesting that these two species were associated to the mineralized matrix possibly in relation to their physiological role in the bone. Moreover, a reverse phase HPLC method for fatty acid determination as naphthacyl esters was set up to study fatty acids in bone samples and was used to validate the HP-TLC data. The fatty acid profile of the MT exhibited lower linoleic acid (18:2 n-6) and linolenic acid (18:3 n-3+n-6) levels and higher arachidonic acid (20:4 n-6) and docosahexaenoic acid (22:6 n-3) levels (P<0.05, compared to BM), suggesting that the MT is more metabolically active than the BM in term of long chain fatty acid production. In sum, the present work should contribute to facilitate future studies in the bone lipid field in view to understand better their implication in the marrow fat expansion-associated bone pathologies, such as osteoporosis.

Role of matrix vesicles in biomineralization

BACKGROUND

Matrix vesicles have been implicated in the mineralization of calcified cartilage, bone and dentin for more than 40 years. During this period, their exact role, if any in the nucleation of hydroxyapatite mineral, and its subsequent association with the collagen fibrils in the organic matrix has been debated and remains controversial.

SCOPE OF REVIEW

This review summarizes studies spanning the whole history of matrix vesicles, but emphasizes recent findings and several hypotheses which have been recently introduced to explain in greater detail how matrix vesicles function in biomineralization.

MAJOR CONCLUSIONS

It is now generally accepted that matrix vesicles have some role(s) in mineralization; that they are the initial site of mineral formation; that MV bud from the plasma membrane of mineral forming cells, but that they take with them only a subset of the materials found in the parent membrane; that the three proteins, alkaline phosphatase, nucleotide pyrophosphatase phosphodiesterase and annexin V have important roles in the process and that matrix vesicles participate in regulating the concentration of PPi in the matrix. In contrast, many open questions remain to be answered.

GENERAL SIGNIFICANCE

Understanding the role of matrix vesicles in biomineralization will increase our knowledge of this important process.

Matrix vesicles originate from apical membrane microvilli of mineralizing osteoblast-like Saos-2 cells

In bone, mineralization is tightly regulated by osteoblasts and hypertrophic chondrocytes which release matrix vesicles (MVs) and control extracellular ionic conditions and matrix composition. MVs are the initial sites of hydroxyapatite (HA) mineral formation. Despite growing knowledge about their morphology and function, their biogenesis is not well understood. The purpose of this work was to determine the source of MVs in osteoblast lineage, Saos-2 cells, and to check whether MVs originated from microvilli. Microvilli were isolated from the apical plasma membrane of Saos-2 cells. Their morphology, structure, and function were compared with those of MVs. The role of actin network in MV release was investigated by using microfilament perturbing drugs. When examined by electron microscopy MVs and microvillar vesicles were found to exhibit similar morphology with trilaminar membranes and diameters in the same range. Both types of vesicles were able to induce HA formation. Their electrophoretic profiles displayed analogous enrichment in alkaline phosphatase, Na(+)/K(+) ATPase, and annexins A2 and A6. MVs and microvillar vesicles exhibited almost the same lipid composition with a higher content of cholesterol, sphingomyelin, and phosphatidylserine as compared to plasma membrane. Finally, cytochalasin D, which inhibits actin polymerization, was found to stimulate release of MVs. Our findings were consistent with the hypothesis that MVs originated from cell microvilli and that actin filament disassembly was involved in their biogenesis.

Induction of alkaline phosphatase activity by L-ascorbic acid in human osteoblastic cells: a potential role for CK2 and Ikaros

OBJECTIVE

To investigate the effect of L-ascorbic acid (AsA) on osteoblast differentiation, we examined the effects of AsA on in vitro osteoblastic differentiation markers such as collagen synthesis, alkaline phosphatase (ALP) activity, and receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) expression. The role of Ikaros and casein kinase 2 (CK2) in regulating osteoblast differentiation was also determined.

METHODS

This study examined the expression of RANKL and OPG, collagen synthesis, and ALP activity in AsA-treated osteoblast-like cells (MG63) using reverse transcription-polymerase chain reaction and biochemical assays. In addition, Ikaros activity and CK2 expression were assessed by electrophoretic mobility shift assays and western blot assays, respectively.

RESULTS

The results showed that AsA treatment slightly downregulated OPG mRNA expression, whereas the mRNA expression of RANKL and collagen was unaffected. AsA significantly increased ALP activity after 4 d, and this activation was inhibited by the CK2 inhibitors, 2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazimidazole. Small interfering RNA-mediated depletion of CK2-alpha also decreased ALP activity in AsA-stimulated cells. Moreover, western blot analysis showed that AsA induced the activation of CK2. AsA dose-dependently decreased the DNA binding affinity of the transcription factor Ikaros, which is a bifunctional differentiation factor. Moreover, cells treated with AsA and CK2 inhibitor exhibited increased Ikaros activity compared with those treated with AsA alone.

CONCLUSION

These results suggest that AsA stimulates osteoblastic differentiation by enhancing ALP activity and suppressing Ikaros activity. Moreover, this process might be related to CK2 regulation.

Analysis of the extracellular matrix vesicle proteome in mineralizing osteoblasts

Many key processes central to bone formation and homeostasis require the involvement of osteoblasts, cells responsible for accumulation and mineralization of the extracellular matrix (ECM). During this complex and only partially understood process, osteoblasts generate and secrete matrix vesicles (MVs) into the ECM to initiate mineralization. Although they are considered an important component of mineralization process, MVs still remain a mystery. To better understand their function and biogenesis, a proteomic analysis of MVs has been conducted. MVs were harvested by two sample preparation approaches and mass spectrometry was utilized for protein identification. A total of 133 proteins were identified in common from the two MV preparations, among which were previously known proteins, such as annexins and peptidases, along with many novel proteins including a variety of enzymes, osteoblast-specific factors, ion channels, and signal transduction molecules, such as 14-3-3 family members and Rab-related proteins. To compare the proteome of MV with that of the ECM we conducted a large-scale proteomic analysis of collagenase digested mineralizing osteoblast matrix. This analysis resulted in the identification of 1,327 unique proteins. A comparison of the proteins identified from the two MV preparations with the ECM analysis revealed 83 unique, non-redundant proteins identified in all three samples. This investigation represents the first systematic proteomic analysis of MVs and provides insights into both the function and origin of these important mineralization-regulating vesicles.

A simple and non-radioactive technique to study the effect of monophosphoesters on matrix vesicle-mediated calcification

A simple and non-radioactive technique based on O-cresolpthalein complexone assay was developed to study in vitro non-radioactive calcium (⁴⁰Ca) deposition by isolated matrix vesicles. Using this technique, the effect of various phosphoester substrates including ATP, AMP and β-GP on in vitro MV-calcification was studied. O-cresolpthalein complexone assay with non-radioactive calcium demonstrated that AMP or β-GP were more effective in promoting calcium deposition by isolated MVs than ATP. The application of this non-radioactive technique, which is highly sensitive and simple, would offer a useful alternative approach to the routinely used radiometric biomineralization assay which employs radioactive ⁴⁵Ca.

Primary culture of rat growth plate chondrocytes: an in vitro model of growth plate histotype, matrix vesicle biogenesis and mineralization

During endochondral ossification (EO), cartilage is replaced by bone. Chondrocytes of growth plate undergo proliferation, maturation, hypertrophy, matrix vesicle (MV) biogenesis and programmed cell death (PCD, apoptosis). The in vitro system presented here provides a potential experimental model for studying in vitro differentiation and MV biogenesis in chondrocyte cultures. Chondrocytes were obtained from collagenase-digested tibial and femoral growth plate cartilage of 7-week-old rachitic rats. The isolated chondrocytes were plated as monolayers at a density of 0.5 x 10(6) cells per 35-mm plate and grown for 17 days in BGJ(b) medium supplemented with 10% fetal bovine serum, 50 microg/ml ascorbic acid. Light microscopy revealed Sirius red-positive, apparent bone matrix in layers at the surfaces of cartilaginous nodules that developed in the cultures. The central matrix was largely alcian blue staining thus resembling cartilage matrix. Electron microscopy revealed superficial areas of bone like matrix with large banded collagen fibrils, consistent with type I collagen. Most of the central matrix was cartilaginous, with small fibrils, randomly arranged consistent with type II collagen. The presence of peripheral type I and central type II and type X collagen was confirmed by immunohistochemical staining. Immunohistochemistry with anti-Bone morphogenetic proteins 2, 4 and 6 showed that BMP expression is associated with maturing hypertrophic central chondrocytes, many of which were TUNEL positive and undergoing cell death with plasma membrane breaks, hydropic swelling and cell fragmentation. During early mineralization, small radial clusters of hydroxyapatite-like mineral were associated with matrix vesicles. Collagenase digestion-released MVs from the cultures showed a high specific activity for alkaline phosphatase and demonstrated a pattern of AMP-stimulated nonradioactive (40)Calcium deposition comparable to that observed with native MVs. These studies confirm that primary cultures of rat growth plate chondrocytes are a reasonable in vitro model of growth plate histotype, MV biogenesis and programmed cell death.

Alterations in the sensing and transport of phosphate and calcium by differentiating chondrocytes

During endochondral bone formation and fracture healing, cells committed to chondrogenesis undergo a temporally restricted program of differentiation that is characterized by sequential changes in their phenotype and gene expression. This results in the manufacture, remodeling, and mineralization of a cartilage template on which bone is laid down. Articular chondrocytes undergo a similar but restricted differentiation program that does not proceed to mineralization, except in pathologic conditions such as osteoarthritis. The pathogenesis of disorders of cartilage development and metabolism, including osteochondrodysplasia, fracture non-union, and osteoarthritis remain poorly defined. We used the CFK2 model to examine the potential roles of phosphate and calcium ions in the regulatory pathways that mediate chondrogenesis and cartilage maturation. Differentiation was monitored over a 4-week period using a combination of morphological, biochemical, and molecular markers that have been characterized in vivo and in vitro. CFK2 cells expressed the type III sodium-dependent phosphate transporters Glvr-1 and Ram-1, as well as a calcium-sensing mechanism. Regulated expression and activity of Glvr-1 by extracellular phosphate and parathyroid hormone-related protein was restricted to an early stage of CFK2 differentiation, as evidenced by expression of type II collagen, proteoglycan, and Ihh. On the other hand, regulated expression and activity of a calcium-sensing receptor by extracellular calcium was most evident after 2 weeks of differentiation, concomitant with an increase in type X collagen expression, alkaline phosphatase activity and parathyroid hormone/parathyroid hormone-related protein receptor expression. On the basis of these temporally restricted changes in the sensing and transport of phosphate and calcium, we predict that extracellular phosphate plays a role in the commitment of chondrogenic cells to differentiation, whereas extracellular calcium plays a role at a later stage in their differentiation program.

Establishment of a novel chondrocyte-like cell line derived from transgenic mice harboring the temperature-sensitive simian virus 40 large T-antigen gene

We established a clonal chondrocyte-like cell line (TC6, TC stands for large T immortalized chondrocyte-like cell line) derived from articular cartilage of transgenic mice harboring a temperature-sensitive simian virus 40 large T-antigen gene. TC6 cells exhibited spindle-like or polygonal morphology and grew well at 33 degrees C in alpha-minimal essential medium supplemented with 0.5% fetal bovine serum. After confluence, these cells formed nodules that were positive for staining with alcian blue. Northern blot analysis demonstrated that these cells expressed messenger RNAs (mRNA) of the genes encoding cartilage-specific proteins such as type II procollagen, link protein, and aggrecan. Furthermore, the expression of type II procollagen and link protein genes in TC6 cells was regulated by parathyroid hormone and basic fibroblast growth factor, suggesting the presence of the receptors for the hormone and cytokine. The expression of link protein mRNA in TC6 cells was regulated in a time-dependent manner and was enhanced in culture within a week and increased continuously up to 10-fold by the end of 4 weeks. Expression of mRNAs encoding type II procollagen and versican/PG-M also increased moderately during the culture period. TC6 cells expressed type I procollagen mRNA, however, its level declined along with time in culture in contrast to the enhancement of the genes encoding cartilage-specific molecules in these cells. Interestingly, alkaline phosphatase mRNA expression was barely detectable in the TC6 cells in their growing phase while it was enhanced dramatically more than 7-fold by day 14 in culture. These results indicate that the TC6 cells could serve as an excellent model for the studies on chondrocyte physiology.

Immunocytochemical localization of choline-phospholipids in postnatal mouse molars

This localization was studied using anti-choline phospholipid monoclonal IgM antibody (MC22-33F). By immunofluorescence, the apical portions of enamel-secreting ameloblasts and dentine-secreting odontoblasts were positively stained with MC22-33F. By immunoelectron microscopy, lysosomes in preodontoblasts, odontoblasts, preameloblasts and ameloblasts as well as matrix vesicles in dentine were strongly stained with MC22-33F. On the other hand, plasma membrane of these cells did not stain with MC22-33F. The frequency of positively reacting lysosomes in ameloblasts and odontoblasts and of matrix vesicles increased with cell maturation and mineral deposition.

Intron-exon structure, alternative use of promoter and expression of the mouse collagen X gene, Col10a-1

The entire mouse collagen X gene (Col10a-1) has been isolated. The gene is composed of three exons and two introns spanning 7.0 kb of the DNA sequence. Exons 2 and 3 together encode 15-bp of 5' untranslated sequence, a 2040-bp open reading frame and an 895-nucleotide 3' non-coding region. In the 5' flanking region of the gene, two consensus TATA-box sequences were found. Identification of the first exon by ribonuclease-protection assays and the determination of the 5' end of Col10a-1 mRNA transcripts by primer-extension analyses show that the more 3' TATA box is probably predominantly used and that there are at least three transcription start sites in the exon 1 sequence 3' to this, resulting in 5' untranslated regions of 78, 77 and 55 nucleotides. By means of rapid amplification of cDNA ends by polymerase chain reaction, an additional mRNA species was detected which overlapped the other Col10a-1 transcripts, including the 3' TATA box sequence, giving a 5' untranslated sequence of approximately 235 bases. This latter transcript starts approximately 20 bp 3' to the more 5' TATA box. The data suggest alternative use of promoters and transcription starts for the Col10a-1 gene. Comparison of the combined nucleotide and deduced amino acid sequences of exons 2 and 3 with chicken, bovine and human collagen X genes, showed a high degree of similarity indicating conservation of this gene throughout evolution. Mouse Col10a-1 mRNA was shown to be approximately 3.0 kb and the pepsinized protein, as detected by SDS/PAGE, was approximately 45 kDa. The mRNA and protein sizes correlate with that predicted by the open reading frame. Reverse-transcription polymerase chain reaction assays indicate that the mouse collagen X gene is first expressed at 13.5 days post coitum, temporally preceding the onset of endochondral ossification. In agreement with the generally accepted association of type-X collagen with endochondral ossification, in situ hybridization analyses indicate that Col10a-1 mRNA are restricted to the hypertrophic regions of growth cartilage.

Effects of metallic ions and diphosphonates on inhibition of pericardial bioprosthetic tissue calcification and associated alkaline phosphatase activity

This study focused on the association of extrinsic alkaline phosphatase (AP) activity with both early and advanced calcification of glutaraldehyde-pretreated bovine pericardial bioprosthetic (GPBP) tissue, and the inhibition of both calcification and AP activity by pre-incubation in diphosphonates (sodium-ethanehydroxydiphosphonate [NaEHDP], aminopropanehydroxydiphosphonate [APD]) and metallic salts (FeCl3, Ga(NO3)3, AICI3). GPBP specimens were implanted subcutaneously in 3 wk old male rats after pre-incubation. Following explantation of the tissue at 72 h and 21 d, calcification was assessed morphologically by light microscopy and chemically by atomic adsorption spectroscopy for calcium content and by molybdate complexation for phosphorus. AP activity was characterized by enzymatic hydrolysis of paranitrophenyl phosphate and by histochemical studies. In both control and pretreated groups, AP levels were greater in 72 h explants than 21 d retrievals, which demonstrated extensive calcification in control explants. All pre-incubations that resulted in inhibition of calcification after 21 d, except for APD, were associated with 72 h AP content which was lower than control specimens. The typical time of initiation of calcification was 72 h, as determined by previous studies with this model system. Covalently bound APD inhibited calcification. Increased AP activity in the APD group may be due to the toxicity of this agent with resultant acute inflammation, or other incompletely understood effects of diphosphonates on calcification and AP. Furthermore, EHDP and Ga3+ incubations were also associated with decreased GPBP AP at 72 h compared to control, but were not effective for inhibiting calcification after 21 d. We concluded that inhibition of peak GPBP AP activity is not necessarily associated with the prevention of GPBP mineralization.

A phorbol ester induces secretion of alkaline phosphatase activity in human osteosarcoma cells

The phorbol ester 12-O-tetradecanoyl-13-acetate (TPA) blocked the growth of, and induced the appearance of processes in the human osteosarcoma cell line U-2 OS. The phorbol ester decreased the intracellular level of alkaline phosphatase (APase) activity (as measured per mg cell protein) and caused a marked increase in the APase activity secreted from the cells into the culture medium. The secretion of APase appeared after a lag period of 4-6 hours of TPA treatment, and it could also be visualized with histological staining. Differential ultracentrifugation of the culture media showed that the APase was released to the media in the form of vesicles. The vesicles were studied by electron microscopy and appeared similar to matrix vesicles isolated from cartilage and chondrocytes. It is thus concluded that TPA is able to induce the primary steps of mineralization in these cells.

Coordinate inhibition of alkaline phosphatase and type X collagen syntheses by 1,25-dihydroxyvitamin D3 in primary cultured hypertrophic chondrocytes

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (2.3 x 10(-12) - 1.4 x 10(-6) [M]) on alkaline phosphatase, collagen, and cell proliferation were examined in primary cultured hypertrophic chondrocytes prepared from the distal epiphyseal growth plate of the tibias of 12-day chick embryos. 1,25(OH)2D3 showed time- and dose-dependent inhibitory effects on the alkaline phosphatase and collagen levels. The inhibition of alkaline phosphatase activity became detectable at 2 x 10(-11) [M] and reached 10% of control at 10(-7) [M]. The concentration of 1,25(OH)2D3 giving a 50% inhibition of the enzyme level was approximately 3 x 10(-10) [M]. Of the two extracellular collagen pools, a cell-associated matrix pool showed a more dramatic decrease (to 10% of control) than a culture medium pool (to 50% of control) at increased 1,25(OH)2D3 concentrations. The degree of inhibition was different for each type of chondrocyte-specific collagen (types II, IX, X, and XI). Types II and IX were inhibited in a parallel manner to only 60-80% of control. On the other hand, types X and XI were more greatly reduced up to 10% of control, and their dose-dependent inhibitory curves were similar to that of alkaline phosphatase. On cell proliferation, 1,25(OH)2D3 had a biphasic effect: stimulation at 10(-10)-10(-8) [M] and inhibition at higher levels. The results revealed the significant involvement of 1,25(OH)2D3 in the metabolism of two probable calcification-related products, alkaline phosphatase and type X collagen.

Post natal development of the canine elbow joint: a light and electron microscopical study

The elbows of 13 puppy cadavers were dissected, samples were taken for light and electron microscopy, and the thickness of the articular cartilage of the distal humerus and proximal ulna was measured. Throughout post natal development differences were found in the arrangement of the growth plate and articular chondrocytes. At birth, the articular surface had remnants of a fibrous limiting membrane that was continuous with the perichondrium, a finding not previously recorded in dogs. Orientation of the collagen fibrils within the matrix of the articular cartilage was initially lacking but became established by three weeks. In the humerus cartilage canals were present up to 12 weeks old. The articular cartilage of the humeral condyle varied in thickness across the joint surface, being thicker on the medial than on the lateral side; it was also thicker at the apex of the medial coronoid process. These regions of thick cartilage correspond with the sites where cartilage defects arise in elbow osteochondrosis. No histological evidence was found that the medial cornoid process of the ulna is a separate centre of ossification.

In vitro studies on the regulation of endochondral ossification by vitamin D

The research described in this article has focused on the complex autocrine, paracrine, and endocrine regulation of endochondral ossification using vitamin D metabolites and TGF-beta as models. By comparing results from a number of laboratories utilizing a diverse array of in vivo and in vitro systems, a coherent picture is beginning to emerge. Vitamin D metabolites influence cell differentiation and maturation and have direct effects on cell function. Differentiation of the mesenchymal cells into chondroblasts is regulated by both 1,25-(OH)2D3 and 24,25-(OH)2D3, as well as by TGF-beta. The resting zone chondrocytes respond primarily to 24,25-(OH)2D3 in terms of matrix synthesis and matrix vesicle biochemistry. They synthesize both metabolites and other factors that stabilize matrix vesicle enzymes like AHSG. In addition to the paracrine role these factors may play in regulating the matrix, it is possible that they may influence the cells in the growth plate itself. Growth zone chondrocytes also synthesize both metabolites, but respond primarily to 1,25-(OH)2D3 for the parameters measured in the studies described. These cells also synthesize TGF-beta which further increases alkaline phosphatase activity, perhaps via an autocrine stimulation of the cell. While cells from the calcified zone have not yet been studied directly in culture, it is likely that they respond to paracrine signals from the avascular cartilage as well as to serum-derived factors. How the signals are transferred among the cells is unknown. Certainly one can postulate information flow in both upward and downward directions. The signal transduction mechanisms for the factors at the cellular level are complex. While it is known that 1,25-(OH)2D3 stimulates gene transcription and stabilization of mRNA for proteins like alkaline phosphatase, its nongenomic effects are only beginning to emerge. Membrane effects of this metabolite have been shown in intestine and kidney in conjunction with studies on Ca flux. It is becoming increasingly evident that other steroid hormones may operate in similar ways. Studies with the rat costochondral chondrocytes are the first to show that there are specific membrane effects for at least two vitamin D metabolites and that membrane enzymes, including those involved in phospholipid metabolism, can be differentially regulated by them. Furthermore, these experiments have provided for the first time a clear hypothesis for how cells can regulate events in the extracellular matrix after the matrix vesicles are produced and incorporated into the matrix.

Secretion of chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase from cultured chick embryo chondrocytes

We found that chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase were released into the culture medium from the cultured chick embryo chondrocytes. Since the release of the sulfotransferases was observed not only in serum-supplemented medium but also in serum-free medium, the released sulfotransferases were unlikely to be derived from serum. Addition of ascorbate to the serum-free medium supported the continuous release of the sulfotransferases. Monensin, which is known to cause dilatation of the Golgi apparatus and to inhibit sulfation of proteoglycan, was found to affect the release of the sulfotransferases. In the presence of 10(-6) M monensin, chondroitin 6-sulfotransferase activity in the cell layer was decreased to less than one tenth of the control, and the rate of the release of the activity became much smaller than the control after the initial rapid release. The activity of chondroitin 4-sulfotransferase was also affected by monensin, but the reduction of the chondroitin 4-sulfotransferase activity in the cell layer was not so great as the reduction of chondroitin 6-sulfotransferase activity. Unlike to the microsomal sulfotransferases, both chondroitin 6-sulfotransferase and chondroitin 4-sulfotransferase released into the culture medium were retained in the soluble fraction after centrifugation at 100,000 x g for 60 min, and were not activated by detergent. pH optimum and requirements for sulfhydryl compounds of the released sulfotransferases were similar to those observed previously in the chondroitin sulfotransferases from chick embryo cartilage and from cultured chick embryo chondrocytes. These results suggest that chondroitin sulfotransferases, which are localized in the Golgi apparatus, may be secreted to the extracellular space in a soluble form under the culture conditions.

Increased matrix vesicle protein in rachitic rat epiphyseal growth plates

Extracellular, membrane-bound vesicles are widely regarded to be the initial site of calcification in a variety of tissues under normal and pathological conditions. Alkaline phosphatase is believed to play a vital role in this process by hydrolysing ester phosphates or mineral inhibitors, e.g. inorganic phosphates. In the present study, matrix vesicles from normal and rachitic rat growth plates were compared with regard to specific activity of alkaline phosphatase, total vesicle protein and ultrastructural distribution of alkaline phosphatase activity. Matrix vesicles were released from normal or rachitic growth plates by collagenase digestion and isolated by differential centrifugation. Enzyme cytochemical localization involving a cerium capture method was performed on vesicles collected by vacuum filtration on Millipore filters. SDS gels and Western blots on fractions of both normal and rachitic matrix vesicles showed major proteins to be almost identical and confirmed the presence of alkaline phosphatase in both. Total matrix vesicle protein ((mg total matrix vesicle protein/rat) x 10(2)) per rat was significantly greater for the rachitic animals (9.0 +/- 2.0 vs. 4.0 +/- 1.0), P less than 0.0001. Alkaline phosphatase specific activity (units alkaline phosphatase/mg vesicle protein) in the rachitic and normal matrix vesicles was 25.29 +/- 9.36 and 18.78 +/- 3.37, respectively (0.05 less than P less than 0.1). Electron dense cerium phosphate deposits were localized to the outer membrane surface of matrix vesicles derived from both types of rats. This data, the first to quantify the relationship between rickets, matrix vesicle protein and alkaline phosphatase specific activity, suggests that matrix vesicles from rachitic and normal rats have biochemical and morphological similarity.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the growth plate in longitudinal bone growth

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

Retinoic acid treatment induces type X collagen gene expression in cultured chick chondrocytes

The vitamin A derivative retinoic acid (RA) is widely thought to be involved in cartilage development, but its precise roles and mechanisms of action in this complex process remain unclear. We have tested the hypothesis that RA is involved in chondrocyte maturation during endochondral ossification and, in particular, is an inducer of maturation-associated traits such as type X collagen and alkaline phosphatase. Immature chondrocytes isolated from the caudal region of Day 19 chick embryo sterna were seeded in secondary monolayer cultures and treated either with a high dose (100 nM) or with physiological doses (10-35 nM) of RA for up to 3 days. We found that after an initial lag of about 24 h, physiological doses of RA indeed induced type X collagen gene expression in the immature cells. This induction was not accompanied by obvious changes in expression of the type II collagen and large aggregating proteoglycan core protein genes. As revealed by immunocytochemistry, 30-35% of the cells in cultures treated with RA for 3 days were engaged in type X collagen production. Interestingly, these cells were relatively similar in size to chondrocytes in which no type X collagen was detected, suggesting that chondrocytes can initiate type X collagen production independent of cell hypertrophy. RA treatment also led to increased alkaline phosphatase activity occurring as early as 24 h after the start of treatment. The data in this study indicate that RA may have a role in endochondral ossification as an inducer/promoter of maturation-associated traits during chondrocyte maturation.

Homozygous achondroplasia: morphologic and biochemical study of cartilage

We have performed histochemical, immunohistochemical, electron microscopic, and biochemical studies on the upper tibial cartilage from a case of homozygous achondroplasia. The growth zone was narrow and disorganized. Columnization was absent except for a few areas with short rows of cells. Hypertrophy was reduced to scattered clusters of cells. The provisional calcification was patchy and primary trabeculae were thick and irregularly arranged. Islands of fibrous or fibrocartilagineous tissue were found along the growth zone. The matrix did not stain with safranin O and lacked metachromasia, except for pericellular rims around the hypertrophic cell clusters. Staining with antibodies against the large proteoglycan monomers and chondroitin-4-sulfate was weakly positive. Electron microscopic examination showed that only a few cells had degenerative signs. In most areas of the matrix, proteoglycan granules were absent. Areas with dense collagen fibers were seen. In contrast to the growth zone, the cartilage of the remaining epiphyses had normal histochemical, immunohistochemical, and electron microscopic appearance. The large proteoglycan monomers had a normal composition and hydrodynamic size. Type II and XI collagen, pepsin fragments of type IX collagen, and several noncollagenous proteins extracted from cartilage had a normal electrophoretic migration. It is suggested that a mutation affecting a matrix component or a regulatory pathway present only or predominantly in the growth area of the chondroepiphysis might explain the findings.

Differential expression of biological effects in maturationally distinct subpopulations of growth plate chondrocytes

Countercurrent centrifugal elutriation is an accurate and reproducible technique which can separate cells on the basis of size. This technique was utilized for the separation of isolated chick growth plate chondrocytes. Mean cellular volume, alkaline phosphatase activity, and Type X collagen synthesis progressively increased in each of seven successive separated fractions. The chondrocytes from the different fractions expressed a differential response to the growth regulator, transforming growth factor beta (TGF beta). TGF beta receptor binding to these cell fractions suggests that the different biological effects may be due to differences in the number of receptors expressed per cell as well as to differences in the proportion of high and low affinity TGF beta receptors present.

Enzyme cytochemical localization of alkaline phosphatase in cultures of chondrocytes derived from normal and rachitic rats

Epiphyseal growth plate cartilages were removed from rats which had been maintained on normal laboratory chow or a rachitogenic diet. Chondrocytes were released from the growth plates by collagenase digestion and cultured in tissue chamber slides. After 7, 10 and 12 days of culture, the chondrocytes were removed as intact multilayers and processed for electron microscopical enzyme cytochemical studies. Alkaline phosphatase activity in the cultures was visualized by means of a cerium based capture method. Electron-dense cerium phosphate deposits were localized on the membrane of matrix vesicles and plasma membranes of chondrocytes derived from normal and rachitic animals. The appearance of first crystals within matrix vesicles was characterized by a concomitant decrease in alkaline phosphatase activity in the membrane of these structures. Calcification was initiated at approximately the same time in cultures of chondrocytes derived from normal or rachitic animals. The results suggest that rickets has no serious effects on the capacity of chondrocytes to support matrix calcification in vitro. Additionally, the evidence indicates that alkaline phosphatase-positive matrix vesicles play a significant role in the initiation of this process.

Matrix vesicles as a marker of endochondral ossification

Endochondral ossification in bone development and repair, and in induced bone formation in mesenchymal tissues, involves recruitment of mesenchymal cells, their differentiation into chondrocytes, and calcification of the cartilagenous matrix. Stimulation of proteoglycan synthesis is used as a biochemical marker of chondrogenesis, however it does not distinguish among chondrogenic phenotypes. Chondrocytes derived from the resting zone and adjacent growth zone cartilage of the costochondral junction of young rats, produce matrix vesicles in culture which are enriched in alkaline phosphatase specific activity with respect to the plasma membrane. Matrix vesicles isolated from cultures of neonatal rat muscle mesenchymal cells are not enriched in this enzyme activity. Alkaline phosphatase in matrix vesicles produced by growth zone chondrocytes is stimulated by 1,25(OH)2D3; enzyme in matrix vesicles produced by resting zone chondrocytes is stimulated by 24,25(OH)2D3; enzyme in matrix vesicles isolated from mesenchymal cell cultures is responsive to neither metabolite. Matrix vesicle phospholipase A2 is stimulated by 1,25(OH)2D3 in growth zone chondrocytes cultures; inhibited by 24,25(OH)2D3 in resting zone chondrocyte cultures; and is unaffected by either metabolite in mesenchymal cell cultures. These observations suggest that matrix vesicle production, as defined by alkaline phosphatase enrichment, and responsiveness of matrix vesicle enzymes to vitamin D metabolites, can be used as markers of phenotypic maturation during chondrogenesis in vivo and in vitro.

Culture and growth characteristics of chondrocytes encapsulated in alginate beads

Methodology is described for the culture of avian and mammalian chondrocytes in ionotrophically gelled "semi-solid" and "hollow" alginate beads. Chondrocytes grown in "semi-solid" gels exhibited a spherical shape as opposed to a fibroblastic morphology observed in monolayer culture. In the "semi-solid" beads, the cells grew as small clumps and as large aggregates. The aggregates were round or elliptical in appearance and surrounded by a dense Alcian Blue positive halo. Preliminary studies with collagen and chitosan matrixes encapsulated in "hollow" beads suggest that cell growth and morphology are profoundly influenced by the composition of the cellular environment. Chondrocyte structure and function in the "semi-solid" and "hollow" beads were partially characterized by light microscopy, histochemical and biochemical means. The encapsulation methodology is readily applicable for the culture of chondrocytes in single beads, in multiwell dishes, or mass culture.

Regulation of matrix vesicle metabolism by vitamin D metabolites

Matrix vesicles are membrane organelles found in the extracellular matrix of calcifying cells. Vitamin D-responsive alkaline phosphatase specific activity has been localized to matrix vesicles in chondrocyte and osteoblast cultures. The effect of hormone is both metabolite and cell specific. Alkaline phosphatase in matrix vesicles produced by resting zone chondrocytes is stimulated by 24,25(OH)2D3 whereas alkaline phosphatase in matrix vesicles produced by growth zone chondrocytes is responsive to 1,25(OH)2D3. However, mesenchymal cell cultures, which exhibit a chondrogenic phenotype when exposed to bone inductive proteins in vitro, produce vesicles with alkaline phosphatase activity that is unaffected by either 1,25(OH)2D3 or 24,25(OH)2D3. Incorporation and release of arachidonic acid into phosphatidylethanolamine is also differentially regulated by 1,25(OH)2D3 and 24,25(OH)2D3 in chondrocytes. These data suggest that vitamin D metabolites may regulate endochondral ossification by altering matrix vesicle enzyme activities, perhaps through changes in membrane phospholipid metabolism.

Mechanism of de novo mineral formation by matrix vesicles

Matrix vesicles (MV) induce mineralization by compartmentalization of ion accumulation and crystal nucleation within membrane-enclosed extracellular microstructures. MV derive from cell surface microvilli by processes that cause selective enrichment of specific proteins, enzymes, lipids, and electrolytes. Incubated in synthetic cartilage lymph (SCL), MV accumulate Ca2+ and Pi, inducing mineral formation in a sequence of stages that can be altered by specific affectors. Rapid uptake of mineral ions by MV precedes formation of the first crystalline phase, octacalcium phosphate (OCP), which later converts to apatite (HAP). Early uptake of Ca2+ and Pi by MV is pH and protease sensitive, and is stimulated by o-phenanthroline (OP), a Zn2+ chelator. Recent studies reveal that a quantitatively major group of MV proteins bind to Ca2+ with high affinity in a lipid-dependent manner. These MV proteins appear to be involved in transport and accumulation of Ca2+ and Pi by MV, and may catalyze nucleation of the first mineral phase.

Intrinsic and extrinsic controls of the hypertrophic program of chondrocytes in the avian columella

Immunohistochemical studies of the chick columella have shown that the extracellular matrix of this ossicular cartilage template is composed largely of type II collagen. As development proceeds, synthesis of type X collagen, a hypertrophic cartilage-specific molecule, is initiated by endochondral chondrocytes within the zone of cartilage cell hypertrophy. Subsequently, these cells and their surrounding extracellular matrix are removed, resulting in marrow cavity formation. We have examined which of these processes are programmed within the columella chondrocytes themselves, and which require involvement of exogenous factors. Prehypertrophic columella from 12-day chick embryos were grown either in organ culture on Nuclepore filters or as explants on the chorioallantoic membrane of host embryos. Chondrocytes from the same source were grown in monolayer cell cultures. In both organ culture and cell culture, chondrocytes developed to the stage at which some of them entered the hypertrophic program and initiated the production of type X collagen as determined by immunofluorescence histochemistry with a monoclonal antibody specific for that collagen type. The organ cultures, however, did not progress to the next stage, in which detectable removal of the type X collagen-containing matrix occurs. When identical columella were grown on the chorioallantoic membrane of host chicks, the type X collagen-containing matrix which formed was rapidly removed, resulting in the formation of a marrow cavity. Thus, progression of endochondral chondrocytes to the deposition of type X collagen-containing matrix seems to be programmed within the cells themselves. Subsequent removal of this matrix requires the involvement of exogenous factors.

Differential expression of phenotype by resting zone and growth region costochondral chondrocytes in vitro

This study establishes an in vitro model for examining endochondral cartilage cell metabolism. Chondrocytes derived from the resting cell zone and adjacent growth zone of rat costochondral cartilage were compared for retention of phenotype in culture. At third passage confluence, two cell populations differ morphologically and biochemically. Resting zone cells are fibroblast-like, with smooth cell membranes and little rough endoplasmic reticulum. Growth zone cells are more polygonal, smaller in diameter, with numerous cytoplasmic extensions of the plasma membranes and abundant rough endoplasmic reticulum. Both cell populations produce matrix vesicles that are comparable morphologically to matrix vesicles isolated enzymatically from epiphyseal cartilage. While membrane vesicles are released into the media by cells derived from the resting zone as well as from the growth cartilage, alkaline phosphatase activity is enriched in media vesicles produced by growth cartilage cells. Alkaline phosphatase enriched vesicles appear to be preferentially incorporated into the extracellular matrix. Both the plasma membrane marker enzyme activity and the membrane phospholipid composition are differentially expressed in matrix vesicles and plasma membranes and are cell specific. Matrix vesicles produced by resting zone cells are enriched in alkaline phosphatase, 5'-nucleotidase, ouabain sensitive Na+/K+ ATPase and cardiolipin when compared to the cell membrane. In addition, the plasma membranes of these cells contain more phosphatidylcholine plus sphingomyelin than do growth cartilage plasma membranes. Resting zone cell matrix vesicles have less phosphatidylethanolamine than do vesicles from growth cartilage cultures. Matrix vesicles produced by growth cartilage cells contain one proteolipid at 43,000 Mr which comigrates with plasma membrane proteolipid and an additional proteolipid at approximately 3,000 Mr. These data indicate that both cells retain differential expression of phenotype in culture and that one expression of this phenotype is production of specific extracellular matrix vesicles.

Inorganic pyrophosphate release by rabbit articular chondrocytes in vitro

Release of inorganic pyrophosphate (PPi) by rabbit articular chondrocytes in vitro was measured by a newly developed assay which utilizes radioactive orthophosphate (32Pi) labeling and anion exchange high performance liquid chromatography. Chondrocytes in monolayer and high density culture failed to release PPi. Explants (cartilage fragments), however, released newly formed PPi into the culture medium. Trypsin treatment of cartilage fragments almost completely blocked the PPi extrusion. Collagenase treatment had no effect on PPi extrusion. There was no clear correlation between proteoglycan synthesis, measured by 35SO4 incorporation, and PPi release. Suppression of proteoglycan synthesis with tunicamycin did not influence the PPi release of the explants.

Isolation and characterization of osteogenic cells derived from first bone of the embryonic tibia

Osteogenesis in the embryonic long bone rudiment occurs initially within an outer periosteal membrane and subsequently inside the cartilaginous core as a consequence of the endochondral ossification process. In order to investigate the development of these two different mechanisms of bone formation, embryonic chick tibial cell isolates were prepared from sites of first periosteal bone formation and from the immediately underlying hypertrophic cartilaginous core region. Mid-diaphyseal periosteal collars and the corresponding cartilage core were microdissected free from Hamburger-Hamilton stage 35 (Day 9) chick tibias and separately digested with a trypsin-collagenase enzyme mixture. The released cell populations were cultivated in vitro and characterized by morphological analysis, histochemical localization of alkaline phosphatase, alizarin red S staining for mineral deposition, growth rate [( 3H]thymidine uptake), and proteoglycan content. Results of these studies showed that periosteal collar cell cultures form nodule-like structures that stain positive with alkaline phosphatase and alizarin red S. Light and electron microscopic observation revealed cell and matrix morphologies similar to that of intact periosteum. The nodules were composed of plump cell types embedded within a mineralized matrix surrounded by a fibroblastic cell layer. Core cartilage cell cultures displayed typical characteristics of the hypertrophic state in their visual appearance and proteoglycan composition. The formation of osseous-like structures in periosteal collar cell cultures but not in core chondrocyte cell cultures demonstrates the relatively autonomous nature of intramembranous ossification while emphasizing the dependence of the endochondral ossification process upon an intact vascularized environment present in the developing tibia.

Developmental acquisition of type X collagen in the embryonic chick tibiotarsus

The temporal and spatial distribution of short chain skeletal (Type X) collagen was immunohistochemically examined in the chick tibiotarsus from 6 days of embryonic development to 1 day posthatching. The monoclonal antibody employed (AC9) was recently produced and characterized as being specific for an epitope located within the helical domain of the type X collagen molecule (T. M. Schmid and T. F. Linsenmayer, J. Cell Biol., in press). The earliest detectable appearance of type X collagen was at 7.5 days, at which time it was restricted to a middiaphyseal location (i.e., in the primary center of ossification). This was in marked contrast to type II collagen, which appears earlier and is distributed throughout the cartilaginous anlagen. With increasing embryonic age, the reactivity with the type X antibody progressively extended toward the epiphyses, lagging somewhat behind the progression of chondrocyte hypertrophy. The anti-type X collagen antibody also reacted with the bony matrix itself, but the immunofluorescent signal produced by this source was considerably less than that produced by cartilage. At 19 days of development, a new small site of type X deposition was initiated in an epiphyseal location, which subsequently enlarged in circumference. These results are consistent with our previous biochemical studies suggesting that, in cartilage, type X collagen is specifically a product of that population of chondrocytes which have undergone hypertrophy.

Ultrastructural characterization of extracellular matrix vesicles in the mineralizing fronts of apical cementum in cats

Fine structure and the organic-inorganic relationship in mineralized fronts of cementum was studied with post-embedding demineralization and staining method. Typical matrix vesicles were observed between apical cementum and cementoblasts. Some vesicles were enclosed by a unit membrane; others contained needle-like crystals. Cementum crystals were found on unstained sections as filament- and needle-like structures which disappeared completely on EDTA treatment; similarly-shaped structures re-appeared following uranyl and lead staining. These findings suggest that matrix vesicles might act as initiators of additional cementogenesis and that crystal ghosts seen in cementum consist of organic materials.

Isolation of a plasma membrane-enriched fraction from collagenase-suspended rachitic rat growth plate chondrocytes

An attempt was made to concentrate plasma membranes of homogenized chondrocytes isolated by collagenase digestion of rachitic rat epiphyseal growth plate cartilage. This study reports the characterization of enzymes in the plasma membrane of isolated chondrocytes and their comparison with extracellular matrix vesicle components. The plasma membrane-enriched fractions that were obtained showed a sevenfold increase in 5'-nucleotidase and a 15-fold increase in alkaline phosphatase, both of which are regarded as plasma membrane markers. SDS-polyacrylamide gel electrophoretic profiles of proteins extracted from membrane fractions contained several major protein bands also seen in isolated matrix vesicles. These studies indicate the usefulness of concentrating plasma membrane components from isolated chondrocytes, after the chondrocytes have been enzymatically freed from investing matrix and other stromal components by collagenase.

Correlation between distribution of cytoskeletal proteins and release of alkaline phosphatase-rich vesicles by epiphyseal chondrocytes in primary culture

Matrix vesicles, extracellular microstructures known to be involved in endochondral calcification, are rich in alkaline phosphatase and have been shown to contain actin. The mechanism of matrix vesicle formation in chondrocytes is not well understood. Chondrocytes from the epiphyseal growth plate, when grown in primary culture, elaborate alkaline phosphatase-rich vesicles. We examined the distribution of the cytoskeletal proteins actin, myosin, tubulin, and vinculin at various time-points during culture using indirect immunofluorescent labeling. Concomitantly, the production of alkaline phosphatase-containing matrix vesicles was also followed. Cell morphology changed noticeably at two distinct stages during the 22-day culture period: Immediately after release from the growth plate the cells were rounded, but after 4 days of culture they began to spread out and acquire irregular shapes with distinct filopodia. By 13 days, as the cells attained confluency, they reacquired a rounded, polygonal appearance. At all time-points, tubulin was seen as a dense network of microtubules radiating from the perinuclear region throughout the cytoplasm toward the cell periphery. Initially actin was seen in filamentous form, but displayed a punctate distribution focused at contact points during the cell-spreading stage of culture. After confluency, actin was concentrated at cell-cell junctions. Initially, vinculin was diffusely distributed, but became focused in multiple adhesion plaques and at the termini of filopodia during the cell-spreading stage of culture. Following confluency vinculin became concentrated at cell-cell junctions. Myosin was observed at all time-points in small, intensely localized focal points in the cytoplasmic region of the cells and was consistently absent from the nuclear and peripheral regions. The amount of myosin in the cells increased steadily with time in culture. Elaboration of alkaline phosphatase-rich vesicles, which corresponded closely with the rounded morphology of early and late stages of culture, may be correlated with contact inhibition.

Calcification of cartilage matrix in chondrocyte cultures derived from rachitic rat growth plate cartilage

Chondrocytes obtained from collagenase-digested epiphyseal growth plate cartilage of rachitic rats were grown in multilayer cultures. The cultured chondrocytes produced a metachromatic matrix and further electron microscopic examination revealed typical features of cartilage matrix collagen fibrils and matrix vesicles. The alkaline phosphatase activity in cultures was high during the entire 3-week culture period. Acid phosphatase showed a marked increase in activity during the first week of culture. The appearance of apatite crystals in the synthesized matrix was monitored by electron microscopy over a 3-week period. First crystals were consistently found to be associated with matrix vesicles, and in the older cultures calcification spread into the surrounding matrix. No collagen fibrils associated with mineralization were observed during the early culture period. This study clearly demonstrates that in chondrocyte cultures the first mineral crystals were found within or in close association with matrix vesicles. This gives further support to the hypothesis that matrix vesicles are the primary site of mineralization in cartilage. In addition to calcification studies it is suggested that this model is suitable for studying the effects of hormones or other agents on rachitic chondrocytes in vitro.