Differentiation of osteoid-producing cells in vitro: possible evidence for the requirement of a microenvironment

Comparison of osteoblast spreading on microstructured dental implant surfaces and cell behaviour in an explant model of osseointegration

OBJECTIVES

To compare interactions between rat calvarial osteoblasts and titanium dental implants with different microstructured surfaces.

MATERIAL AND METHODS

Seven commercially available implants were used. Surfaces included plasma-sprayed, grit-blasted and/or acid-etched, smooth-machined and anodised titanium. Two methods were used to compare cell behaviour: (1) A cell-spreading assay in which percentages of cells at four different stages of attachment were identified by scanning electron microscopy and quantified within a 30 min attachment period. (2) Implants were placed in 'pocket culture' within nylon mesh sacs in contact with explanted calvarial bone fragments for 2 and 4 weeks.

RESULTS

Surfaces combining grit blasting and acid etching, of microporous topography, showed significantly enhanced rates of cell spreading in comparison with the others. Differential cell morphology was observed in both suspension assays and pocket cultures. In the latter, cells migrated onto all surfaces. Multicellular layers with extracellular matrix (ECM) were present between the layers and on the material surfaces after 2 weeks. After 4 weeks, cell layers were more consolidated, and microstructures were obscured by layers of cells and ECM. Mineralised tissue was seen in association with ECM on grit-blasted surfaces of rough and smooth microtopography.

CONCLUSIONS

The two methods provided complementary information: a rough surface of porous microstructure may enhance the rate of cell spreading. Differentiation and calcification occurred on surfaces of both rough and smooth microstructure.

Guidelines for using in vitro methods to study the effects of phyto-oestrogens on bone

These guidelines review the relevant literature on the way plant phyto-oestrogens act on bone and the responsiveness of different bone cell systems to phyto-oestrogenic compounds. The primary emphasis is on the experimental conditions used, the markers available for assessing osteoblast and osteoclast function, and their expected sensitivity. Finally, we assess the published results to derive some general recommendations for in vitro experiments in this area of research.

Effects of lipopolysaccharide extracted from Prevotella intermedia on bone formation and on the release of osteolytic mediators by fetal mouse osteoblasts in vitro

Prevotella intermedia, a Gram-negative obligate anaerobic black-pigmented oral bacterium, belongs to a small group of microorganisms that is closely associated with the initiation of periodontal diseases. Lipopolysaccharide (LPS), an outer membrane component, is one of the main virulence factors of this bacterium. The aim of this study was to examine the effects of Prev. intermedia lipopolysaccharide, extracted by the hot-phenol-water method, on differentiation (alkaline phosphatase activity) and mineralisation (calcium incorporation) of fetal mouse calvarial cells in vitro and to determine the release of the important osteolytic factors nitric oxide, interleukin-6 (IL-6) and matrix metalloproteinases by these cells after treatment with different concentrations of Prev. intermedia lipopolysaccharide (0.2-25 microg/ml). By gelatin zymography, we also characterized the matrix metalloproteinases released by these osteoblasts. Treatment with Prev. intermedia lipopolysaccharide dose-dependently inhibited bone formation by reducing alkaline phosphatase activity and calcium incorporation and induced the release of nitric oxide, IL-6 and the latent proforms of MMP-2 and MMP-9 by fetal mouse osteoblasts in organoid culture. These results indicate that the lipopolysaccharide from Prev. intermedia not only participates in periodontal tissue destruction and alveolar bone resorption, but also inhibits bone formation.

The role of periosteum in cartilage repair

Periosteum, which can be grown in cell and whole tissue cultures, may meet one or more of the three prerequisites for tissue engineered cartilage repair. Periosteum contains pluripotential mesenchymal stem cells with the potential to form either cartilage or bone. Because it can be transplanted as a whole tissue, it can serve as its own scaffold or a matrix onto which other cells and/or growth factors can be adhered. Finally, it produces bioactive factors that are known to be chondrogenic. The chondrocyte precursor cells reside in the cambium layer. These vary in total density and volume with age and in different donor sites. The advantages of whole tissue periosteal transplants for cartilage repair include the fact that this tissue meets the three primary requirements for tissue engineering: a source of cells, a scaffold for delivering and retaining them, and a source of local growth factors. Many growth factors that regulate chondrocytes and cartilage development are synthesized by periosteum in conditions conducive to chondrogenesis. These include transforming growth factor-beta 1, insulinlike growth factor-1, growth and differentiation factor-5, bone morphogenetic protein-2, integrins, and the receptors for these molecules. By additional study of the molecular events in periosteal chondrogenesis, it may be possible to optimize its capacity for articular cartilage repair.

The enhancement of periosteal chondrogenesis in organ culture by dynamic fluid pressure

Cartilage repair by autologous periosteal arthroplasty is enhanced by continuous passive motion (CPM) of the joint after transplantation of the periosteal graft. However, the mechanisms by which CPM stimulate chondrogenesis are unknown. Based on the observation that an oscillating intra-synovial pressure fluctuation has been reported to occur during CPM (0.6-10 kPa), it was hypothesized that the oscillating pressure experienced by the periosteal graft as a result of CPM has a beneficial effect on the chondrogenic response of the graft. We have developed an in vitro model with which dynamic fluid pressures (DFP) that mimic those during CPM can be applied to periosteal explants while they are cultured in agarose gel suspension. In this study periosteal explants were treated with or without DFP during suspension culture in agarose, which is conducive to chondrogenesis. Different DFP application times (30 min, 4 h, 24 h/day) and pressure magnitudes (13, 103 kPa or stepwise 13 to 54 to 103 kPa) were compared for their effects on periosteal chondrogenesis. Low levels of DFP (13 kPa at 0.3 Hz) significantly enhanced chondrogenesis over controls (34 +/- 7% vs 14 +/- 5%; P < 0.05), while higher pressures (103 kPa at 0.3 Hz) completely inhibited chondrogenesis, as determined from the percentage of tissue that was determined to be cartilage by histomorphometry. Application of low levels of DFP to periosteal explants also resulted in significantly increased concentrations of Collagen Type II protein (43 +/- 8% vs 10 +/- 5%; P < 0.05). New proteoglycan synthesis, as measured by 35S-sulphate uptake was increased by 30% in periosteal explants stimulated with DFP (350 +/- 50 DPM vs 250 +/- 75 DPM of 35S-sulphate uptake/microg total protein), when compared to controls though this difference was not statistically significant. The DFP effect at low levels was dose-dependant for time of application as well, with 4 h/day stimulation causing significantly higher chondrogenesis than just 30 min/day (34 +/- 7 vs 12 +/- 4% cartilage; P < 0.05) and not significantly less than that obtained with 24 h/day of DFP (48 +/- 9% cartilage, P > 0.05). These observations may partially explain the beneficial effect on cartilage repair by CPM. They also validate an in vitro model permitting studies aimed at elucidating the mechanisms of action of mechanical factors regulating chondrogenesis. The fact that these tissues were successfully cultured in a mechanical environment for six weeks makes it possible to study the actions of mechanical factors on the entire chondrogenic pathway, from induction to maturation. Finally, these data support the theoretical predictions regarding the role of hydrostatic compression in fracture healing.

Osteogenesis from cultured chick periostea has a specific requirement for chloride

Bone development, like embryonic development in general, depends on a particular internal electrical milieu. Ions are the carriers of currents that maintain this internal environment. In embryonic bone, chloride is a major carrier of such current. To explore the role chloride plays in embryonic bone development we performed several ion-removal experiments, using the chick periosteal osteogenesis (CPO) system as our model. We found that if chloride is reduced in the medium and replaced with a nontoxic anion, alkaline phosphatase (ALP) activity does not rise, nor does osteogenic development occur. However, acid phosphatase (AP) activity is not affected by level of chloride. Experiments using metabolic inhibitors showed that explants cultured in low chloride medium remain viable. Dose-response studies revealed that the response of ALP activity to chloride concentration is sigmoidal, with a [Cl-]0.5 of 45.9 mM. Reciprocal transfers of explants between complete and low chloride medium show that the rise in ALP activity depends on the length of time explants are cultured with chloride. In contrast, such transfer experiments show that osteogenesis requires chloride only during days 2-3 of culture.

Role of fracture hematoma and periosteum during fracture healing in rats: interaction of fracture hematoma and the periosteum in the initial step of the healing process

To study the mechanisms of fracture healing, we investigated the interaction between fracture hematoma and periosteum during the early phase of fracture healing in rats. Experimentally induced fractures of the tibia in untreated rats were compared histologically with such fractures in rats in which either the bone marrow or the periosteum had been removed. The extent of periosteal cell proliferation and chondrogenesis in the fracture hematoma was evaluated on experimental days 3, 6, 10, and 14. On day 3, periosteal cell proliferation at the tibial fracture site was decreased in the bone marrow-removed rats compared with the proliferation in untreated rats. Little chondrogenesis in the fracture hematoma was seen through day 6 in the periosteum-removed rats. These results suggest that the periosteum is important for mediating the primary steps of chondrogenesis and enchondral ossification in the fracture hematoma and that the fracture hematoma may be essential for periosteal cell proliferation during fracture healing.

Articular cartilage regeneration using periosteum

Periosteum has chondrogenic potential that makes it possible to repair or regenerate cartilage in damaged joints. Whole periosteal explants also can be cultured in vitro for the purpose of studying chondrogenesis. This chondrogenic potential arises because the cambium layer of periosteum contains chondrocyte precursor cells that form cartilage during limb development and growth in utero, and does so once again during fracture healing. The advantages of whole tissue periosteal transplants for cartilage repair include the fact that this tissue meets the three primary requirements for tissue engineering: a source of cells, a scaffold for delivering and retaining them, and a source of local growth factors. Data from in vivo studies show that periosteum transplanted into osteochondral articular defects produce cartilage that can restore the articular cartilage and be replaced by bone in the subchondral region. This capacity is determined by surgical factors such as the orientation of the cambium layer, postoperative factors such as the use of continuous passive motion, and the age and maturity of the experimental animal. In vitro studies have shown that the chondrogenic potential of periosteal explants is determined by culture, donor conditions, and technical factors. Chondrogenesis is optimized by suspension of the explants in agarose under aerobic conditions, with supplementation of the media using fetal calf serum and growth factors, particularly transforming growth factor-beta 1. The role of physical factors currently is being investigated, but studies show that the mechanical environment is important. Donor factors that are important include the harvest site, the size of the periosteal explant, and most importantly the age of the donor. Periosteal chondrogenesis follows a specific time course of events, with proliferation preceding differentiation. The current challenge is to clarify the process of periosteal chondrogenesis and its regulation at the cellular and molecular levels, so that it can be controlled intelligently and optimized for the purpose of cartilage repair and regeneration.

Probing glucocorticoid-dependent osteogenesis in rat and chick cells in vitro by specific blockade of osteoblastic differentiation with progesterone and RU38486

Glucocorticoids and sex-steroids can modulate osteogenesis in vivo and in vitro. Although the effects of glucocorticoids on bone cells in vitro have been described in detail, the role of sex-steroids is not as well defined. We examined whether sex-steroids influence bone metabolism indirectly by regulating glucocorticoid effects on bone. Interactions of the sex-steroid progesterone or its analog RU38486 with the glucocorticoid dexamethasone (dex) were studied in functional assays of osteogenesis. Three osteoblastic models were evaluated: (1) the rat bone marrow stromal cell (RBMC) nodule system; (2) the chick periosteal osteogenesis (CPO) model; and (3) ROS 17/2.8 cells. RU38486, progesterone, and unlabelled dex competitively inhibited 3H-dex uptake by ROS 17/2.8 cells as well as its (3H-dex) binding to cytosol preps. Both RU38486 and progesterone inhibited dex-induced increases in alkaline phosphatase in CPO cultures, in RBMC cultures, and in ROS 17/2.8 cells. Dex-induced decreases in cell proliferation in ROS 17/2.8 cells were reversed by RU38486 but dex-induced increases in proliferation in the CPO model were not affected. In CPO cultures, dex-induced increases in collagen synthesis were inhibited completely by RU38486 and progesterone. Dex-dependent nodule formation in the RBMC was blocked by RU38486. Both RU38486 and dex mediated reduction of calcium uptake in the CPO model but did not affect mineralized tissue area. The data indicate that RU38486 and progesterone competitively inhibit dex-mediated stimulation of osteogenesis in vitro; this inhibition is exerted on early but not late stage differentiation events of osteoprogenitor cells.

Chondrogenesis in periosteal explants. An organ culture model for in vitro study

Periosteal grafts have chondrogenic potential and have been used to repair defects in articular cartilage. We studied the effects of the culture conditions and of transforming growth factor-beta 1 on chondrogenesis in rabbit periosteal explants that were cultured in vitro. A total of 390 periosteal explants were obtained from the anteromedial sides of the proximal parts of the tibiae of eleven rabbits that were two weeks, two months, or six months old. The culture medium (alpha minimum essential medium or Dulbecco minimum essential medium) contained fetal calf serum, with or without transforming growth factor-beta 1, at a concentration of one or ten nanograms per milliliter for the first two weeks of culture. Three hundred and twenty-one explants were submerged in liquid medium and sixty-nine were suspended in an agarose gel; they were then evaluated histochemically, histomorphometrically, and by collagen-typing. In the media without agarose, in the presence of ten nanograms of transforming growth factor-beta 1 per milliliter, chondrogenesis was commonly seen after two to four weeks with use of safranin-O staining and histomorphometry. In the agarose gels, chondrogenesis from the periosteum was observed at four and six weeks and was enhanced by the presence of one or ten nanograms of transforming growth factor-beta 1 per milliliter. The combination of agarose with transforming growth factor-beta 1 most favored the formation of cartilage, which was maximum at six weeks in the presence of ten nanograms of transforming growth factor-beta 1 per milliliter. Under these conditions, chondrogenesis occurred in almost every explant, with 50 +/- 30 per cent of the tissue being composed of cartilage. Type-II collagen was present in the explants that had undergone chondrogenesis.

1,25-Dihydroxy vitamin D3 stimulation of TGF-beta expression in chick embryonic calvarial bone

Bone is a highly active producer of the cytokine, transforming growth factor-beta (TGF-beta), which is likely to be functionally involved in the regulation and maintenance of bone development and growth. In addition, bone functions are also regulated by the major calciotropic hormone, 1,25-dihydroxy vitamin D3 (1,25(OH)2D3). This investigation aims to examine the possible relationship between TGF-beta and 1,25(OH)2D3 using an unique calcium-deficient chick embryonic model. By means of long-term culture without the eggshell (shell-less or SL culture), chick embryos may be rendered severely calcium-deficient with gross undermineralization of the skeleton. We have previously observed that the calvaria of these SL embryos develop abnormal chondrogenic phenotype, with production of collagen type II, and elevated TGF-beta expression. Administration of 1,25(OH)2D3 to the SL embryos in vivo on incubation days 10 and 12 (SL + D embryos) resulted in near-normal serum calcium on day 14 and improved calvarial calcification. However, TGF-beta expression in the SL + D calvaria was further increased compared to untreated SL calvaria, when analyzed at both the mRNA and protein levels. Histolocalization of gene expression by immunohistochemistry and in situ hybridization revealed that cells in the less mineralized orbital and temporal zones of the calvarium are particularly affected by the 1,25(OH)2D3 treatment. Interestingly, the increased TGF-beta expression resulting from 1,25(OH)2D3 treatment did not correct the aberrant collagen phenotype in the SL calvaria. These observations suggest that TGF-beta expression by bone cells in situ is stimulated by 1,25(OH)2D3, and that normal cellular differentiation and morphogenesis of the embryonic calvaria are dependent on proper and balanced TGF-beta expression as well as the state of tissue mineralization.

Mineralization and bone formation on microcarrier beads with isolated rat calvaria cell population

Using enzymatically isolated rat bone cells in the presence of cytodex microcarrier beads, osteoblastic cell differentiation and bone nodule formation were studied at the optical and electron microscopic level. Cytochemical method showed an intense alkaline phosphatase activity mainly around the microcarriers where the cells have formed multilayers on day 4 of cultures. On day 7 of experiment cultures, Von Kossa method stained positively only the cytodex microcarriers. During the following days, bone nodule formation was closely associated with cytodex microcarriers. In contrast, in control cultures with negatively charged glass beads, cells failed to pile up around the glass beads, and bone nodule formation occurred randomly in the culture dishes with 24 hour delay. Light microscopy observations of experiment cultures revealed the formation of nodular structures, with active osteoblastic cells forming a mineralized matrix in which osteocytes were present. Transmission electron microscopy revealed first, a mineralization process of the surface of the cytodex microcarriers which appeared like a granular electron-dense, collagen-free layer followed by the deposit of a collagenous matrix. These results indicated that cytodex microcarriers provided an excellent matrix for bone cell differentiation and mineralization.

Effects of bisphosphonates and inorganic pyrophosphate on osteogenesis in vitro

The bisphosphonates, which are chemically related to pyrophosphate, have been studied extensively both in vivo and in vitro to elucidate their effects on bone tissues and cells. However, because these agents have important effects on bone resorption, the majority of investigations have focused on this area. Few studies regarding direct bisphosphonate effects on bone formation have been carried out in the past and, thus, we chose to use the chick periosteal osteogenesis (CPO) in vitro model system to test the direct effects of pyrophosphate and the bisphosphonates ethane-1-hydroxy-1,1-diphosphate (HEBP) and disodium-1-hydroxy-1-amino-propylidine (APD) on various parameters of osteogenesis in vitro. The data show that the bisphosphonate HEBP inhibits bone mineralization reversibly while APD, at low doses, may actually enhance mineralization of bone. Similarly, pyrophosphate (PPi) will prevent mineralization in CPO cultures. However, CPO cultures can circumvent PPi-mediated blockage of mineralization with longer-term, continuous (10-day) incubation, whereas this does not occur if cultures are incubated continuously with bisphosphonates. Both drugs appear to be able to reverse beta-glycerophosphate-induced changes in alkaline phosphatase activity, but do not appear on their own to regulate the activity of this enzyme. The findings show that in addition to their well-known effects on resorption, bisphosphonates have significant and direct effects on mineralization in bone-forming cultures. Their direct effects on osteoblastic activity and differentiation remain to be determined.

The extracellular matrix in cartilage organoid culture: biochemical, immunomorphological and electron microscopic studies

Limb bud mesenchymal cells obtained from day-12 mouse embryos were grown at high density on a membrane filter (pore size 0.2 micron) at the medium/air interphase. Chondrogenesis in this so-called cartilage organoid culture was monitored quantitatively by immunological estimation of type I and type II collagen and qualitatively by indirect immunofluorescence and electron microscopy in the course of a 36 days culture period. Three stages of cartilage development could be substantiated: 1. Formation of cartilage between days 2 and 7; 2. maturation of cartilage between days 9 and 13; 3. degeneration of cartilage beginning at day 20. Differentiation in cell aggregates and a loose mesenchymal tissue occurred during the first two days of the culture period. Type II collagen synthesis started in cell aggregates two days after plating and after 6 days in culture distinct cartilage nodules had developed which were embedded in loose connective tissue that contained type I collagen. During this period the type II collagen content increased progressively from 2.3 micrograms (day 3) to nearly 40 micrograms (day 7) per mg dry weight, whereas the type I collagen level increased more linearly from 2.7 to 21.3 micrograms/mg dry weight. The second period was characterized by enlargement and fusion of cartilage nodules and a diminished increase in type II collagen content from 45 to 60 micrograms/mg dry weight. Enlargement and fusion occurred by matrix production as well as by transformation of perichondrial cells into chondroblasts. Type I collagen synthesis enhanced from 29 to 54 micrograms/mg. Hypertrophic chondrocytes could be demonstrated ultrastructurally. At the third stage a nearly continuous layer of cartilage on the membrane filter covered by noncartilagenous tissue had developed. To some extent chondrocytes lost their matrix capsule and changed into fibroblast-like cells accompanied by a switch of collagen synthesis from type II to type I collagen. Quantitative studies yielded a constant level of about 60 micrograms/mg type II collagen and a further increase in type I collagen from 77 to 116 micrograms/mg dry weight. This study reveals an in vitro model of a prolonged, but almost identical image of chondrogenesis in vivo prior to endochondral mineralization which may be useful for investigations on cartilage differentiation, maturation and degeneration.

c-fos oncogene expression in dexamethasone stimulated osteogenic cells in chick embryo periosteal cultures

Although the complex effects of glucocorticoids on bone cells have been studied extensively in vitro, little is known about the molecular mechanisms of glucocorticoid responses in osteogenic cells. As c-fos and its protein product are believed to play a key role in intracellular signal transduction, and since their role in regulation of bone formation is well-recognized, we studied the effect of the glucocorticoid analogue dexamethasone (DEX) on the expression of c-fos oncogene in the chick periosteal osteogenesis (CPO) model. C-fos mRNA expression was determined by in situ hybridization at various time points after 10(-7) M DEX treatment. Prior to DEX treatment, the cultures had been synchronized with 2 mM thymidine. The mean area of positively hybridized cells in experimental (DEX-treated) and control (DEX-free) cultures was quantitated by computer assisted morphometry. In DEX-treated cultures c-fos mRNA could be detected transiently and mainly in the osteogenic layer at 30, 45 and 60 min after treatment whereas no c-fos expression could be detected above background level in the control groups. Differences between experimental and control groups were significant (P less than 0.01) as determined by a general linear model (GLM) analysis of variance. These data indicate that in the CPO culture system, DEX (10(-7) M) induces c-fos expression. The findings are compatible with the hypothesis which states that glucocorticoid-induced phenotypic changes in osteogenic cells may be mediated by c-fos.

Patterns of mineralization in vitro

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

The concept of osseointegration and bone matrix expression

Osseointegration has been defined as the direct structural and functional connection between ordered, living bone and the surface of a load-carrying implant. To date, this concept has been described by descriptive histological and ultrastructural criteria but not by biochemical means. This review evaluates the basic science work performed on this concept and then applies the concept to the principle of osseous healing. Specific studies are cited where alterations in the healing response are due to clinical management of implant placement and how studies of surface properties may lead to further insights on implant design and prognosis. In addition, a review of bone expression as a function of in vitro stress applications is given. This is followed by an indepth review of the collagens and noncollagenous proteins, described to date, within isolated bone matrix. It is this collagenous matrix (especially type I) that is described as being close to and oriented with a glycoprotein component next to the implant surface. In turn, the large family of noncollagenous proteins are important in mediating bone proliferation, matrix accumulation, orientation, mineralization, and turnover. This section is followed by a discussion of specific growth factors as they may relate to osseous healing around an implant.

In vitro bone formation on coral granules

We investigated the ability of fetal rat bone cells isolated after collagenase digestion to differentiate in vitro and to produce a mineralized matrix on coral granules. Scanning electron microscopy examination of the surface of the seeded coral granules revealed that cells attached, spread, and proliferated on the material surface. Bone nodule formation was studied in this in vitro system by direct examination under an inverted phase contrast microscope. The initial event observed was the appearance of cells with phosphatase alkaline activity arranged in several layers and forming a three-dimensional organization around the coral particles. By Day 7, nodule formation began and a refringent material appeared and extended to the background cells during the following days. By Day 15, some coral granules were embedded in a mineralized matrix. Histologic results demonstrated the formation of a mineralized tissue with the appearance of woven bone.

Osteogenic cell lineage analysis is facilitated by organ cultures of embryonic chick periosteum

Monoclonal antibodies against the surface of embryonic osteogenic cells (SB-1, SB-2, SB-3, and SB-5) have been used to characterize the sequence of transitions involved in the osteogenic cell lineage. In the present study, immunohistochemical analyses of the expression of osteogenic cell surface antigens in organ cultures of folded chick periosteum were performed. Unlike traditional culture methods using isolated osteoblastic cells, periosteal explants form a mineralized bone tissue in 4 to 6 days which is virtually identical to the in vivo counterpart. Examination of fresh explants confirm that no mature osteoblastic cells were present, although a discontinuous layer of preosteoblasts was evident. As the wave of osteodifferentiation swept through the cultured tissue, antibody SB-1 reacted with the surface of a large family of cells associated with the developing bone. Antibodies SB-3 and SB-2 reacted with progressively smaller subsets of cells, namely those in successively closer physical association with the newly formed and mineralizing bone. Cells recently encased in bone matrix were stained by both SB-2 and SB-5 antibodies, while those cells deep within the matrix reacted only with antibody SB-5. Analysis of this culture model allows dissection of the discrete cellular transition steps of osteogenesis, and reveals that osteogenic precursor cells proceed through the unique lineage stages which have been documented for in vivo osteogenesis. This culture system has furthermore provided evidence which is used to refine our understanding of the osteogenic cell lineage.

Bone formation by osteoblast-like cells in a three-dimensional cell culture

Cells of the clonal osteogenic cell line MC3T3-E1 were seeded onto a three-dimensional matrix of denatured collagen type 1 and cultured for a period of up to 8 weeks. Specimens were analyzed by histological, enzyme histochemical, immunocytochemical, and ultrastructural methods and by in situ hybridization between day 7 and day 56 after seeding. In 56-day cultures, the MC3T3-E1 cells were arranged in a three-dimensional network and formation of bone-like tissue was indicated by calcification of a newly synthesized collagen type I matrix resembling osteoid and surrounding osteocyte-like cells. The differentiating culture showed high expression of osteocalcin and alkaline phosphatase activity. NIH3T3 fibroblasts used as control cells passed through the network of the substrate forming a confluent monolayer underneath. This culture system offers a potentially powerful model for bone formation in vitro and for investigating the osteogenic potential of bone-derived cells.

Long-term organ culture of embryonic chick femora: a system for investigating bone and cartilage formation at an intermediate level of organization

Bone organ culture is an experimental system in which skeletal cells remain within their extracellular matrix but are removed from systemic influences. Femurs from 14-day-old chick embryos, which contain bone and cartilage matrix in approximately equal proportions, were cultured for up to 9 days in a serum-free medium. Cell proliferation, differentiation into chondrocytes and osteoblasts, formation of bone and cartilage matrix, and in vitro mineralization as well as bone and cartilage resorption were assessed using histologic and analytic methods. Particular attention was paid to the differences between cartilage and bone growth and to interpreting analytic data in the light of histologic observations. The first 2 days of culture represented an "adaptation" period, characterized by the release of intracellular enzymes into the culture medium, probably as a consequence of cell breakdown. Days 3-9 in culture represented a period of "steady growth" during which skeletal cells continued to multiply in the absence of fetal serum and to secrete large amounts of bone and cartilage matrix. De novo mineralization could be induced by Ca-beta-glycerophosphate, but calcium deposits in tissues other than bone and cartilage were also induced. Resorption of bone or cartilage matrix was virtually absent. Bone organ culture facilitates the study of bone and cartilage formation at an intermediate level of organization and thereby provides the necessary link between in vivo studies and investigations at the cellular level.

Microcinematographic and autoradiographic kinetic studies of bone cell differentiation in vitro: matrix formation and mineralization

Matrix formation and mineralization have been reported in vitro with cells isolated from rat calvaria bones by collagenase digestion (Nefussi et al., 1985). In the current study, kinetics of bone nodule formation and osteoblastic cell differentiation were studied in this in vitro system using an improved microcinematographic device and flash and follow-up labeling autoradiographic techniques. Microcinematographic analysis showed the formation of bone nodules within 24 h. The initial event observed was the change in the top cells layer which became alkaline phosphatase positive. Matrix synthesis occurred a few hours after this. The autoradiographic results demonstrated the formation of an integrated system where osteoblasts and osteocytes were active and synthesized a collagen matrix and mineralized it in a similar time sequence than in vivo.

Bone formation by rat calvarial cells grown at high density in organoid culture

Calvarial cells from day 21 rat fetuses were isolated by enzymatic digestion and grown at high density in an organoid culture system at the medium/air interface. In this type of culture, mineralization occurred as early as 7 days in vitro, as revealed by light and electron microscopic means. After about 18 days in vitro, most of the culture consisted of mineralized tissue. Mineralization was also achieved without beta-glycerophosphate, but it was delayed by 2 to 3 days. Maximal alkaline phosphatase activity occurred at days 8 to 12 in vitro and then declined continuously during further cultivation. Two types of mineralization could be observed: (1) mineralization of a collagen-rich osteoid by typical apatite crystals; (2) mineralization of a nearly collagen-free matrix by amorphous material which was possibly secreted by the cells. The importance of higher cell densities for cell differentiation and formation of histotypic tissue in vitro is apparent, and it is indicated that cell-cell contacts and cell-matrix interactions may be prerequisites for the development of histotypic conditions similar to the in vivo situation.

Microcarriers facilitate mineralization in MC3T3-E1 cells

MC3T3-E1 cells showed mineral deposits after about 1 week of culture when incubated in the presence of microcarrier beads. These deposits appeared as white spots on the dish surface, and under light microscopy the cells showed multiple cell layers and mineralization around the microcarriers. The deposits stained positive with calcium-specific Von Kossa's method. Using conventional assay, alkaline phosphatase activity (ALP) and parathyroid hormone-stimulated intracellular cAMP production were lower in the microcarrier cultures than in the control, but using cytochemical methods, high alkaline phosphatase activity was found around the microcarriers. These results indicate that microcarriers facilitated the formation of multiple cell layers and provided a culture environment for mineralization.