Nadine Forest

Bioactive Glass Stimulates In Vitro Osteoblast Differentiation and Creates a Favorable Template for Bone Tissue Formation

In this study, we have investigated the behavior of fetal rat osteoblasts cultured on bioactive glasses with 55 wt% silica content (55S) and on a bioinert glass (60S) used either in the form of granules or in the form of disks. In the presence of Bioglass granules (55 wt% silica content), phase contrast microscopy permitted step‐by‐step visualization of the formation of bone nodules in contact with the particles. Ultrastructural observations of undecalcified sections revealed the presence of an electron‐dense layer composed of needle‐shaped crystals at the periphery of the material that seemed to act as a nucleating surface for biological crystals. Furthermore, energy dispersive X‐ray (EDX) analysis and electron diffraction patterns showed that this interface contains calcium (Ca) and phosphorus (P) and was highly crystalline. When rat bone cells were cultured on 55S disks, scanning electron microscopic (SEM) observations revealed that cells attached, spread to all substrata, and formed multilayered nodular structures by day 10 in culture. Furthermore, cytoenzymatic localization of alkaline phosphatase (ALP) and immunolabeling with bone sialoprotein antibody revealed a positive staining for the bone nodules formed in cultures on 55S. In addition, the specific activity of ALP determined biochemically was significantly higher in 55S cultures than in the controls. SEM observations of the material surfaces after scraping off the cell layers showed that mineralized bone nodules remained attached on 55S surfaces but not on 60S. X‐ray microanalysis indicated the presence of Ca and P in this bone tissue. The 55S/bone interfaces also were analyzed on transverse sections. The interfacial analysis showed a firm bone bonding to the 55S surface through an intervening apatite layer, confirmed by the X‐ray mappings. All these results indicate the importance of the surface composition in supporting differentiation of osteogenic cells and the subsequent apposition of bone matrix allowing a strong bond of the bioactive materials to bone.

Cytochalasin D induces changes in cell shape and promotes in vitro chondrogenesis: A morphological study

Summary— One of the initial events required for the expression of cartilage‐specific macromolecules in monolayer cultures is the reversion to the initial round shape of chondrocytes. Thus, considerable research efforts have focused on developing reliable procedures to maintain a round morphology of cultured chondrocytes. Our study focuses on evaluating the response of dedifferentiated fetal rat chondrocytes to cytochalasin D, an actin‐disrupting agent, with special emphasis on the morphological events. Immediately after exposure to the drug, cells round up but flatten again after removing the agent. However, immunocytochemical procedures revealed a disorganization of microfilaments and intermediate filaments. Phase‐contrast and scanning electron microscopic observations revealed that on day 6 of culture, cells located at the top of the cell layer adopted a spherical morphology. Prominent differences were noted in control cultures where cells had to aggregate prior to overt chondrogenesis. Transmission electron microscopy confirmed the round morphology of the cells situated at the top layer but also revealed the presence of cell contacts between the cells. In addition, cells located at the central part of the cell layer displayed a typical morphology of mature chondrocytes, separated by an extensive extracellular matrix. These morphological changes occurred parallel to the expression of type II collagen and chondroitin sulfate, both hallmarks of the chondrocyte phenotype strong in experimental cultures, relatively weak in control cultures, and only restricted on areas of polygonal cellular aggregates. Furthermore, [35S]‐sulfate incorporation into sulfated glycosaminoglycans increased rapidly with the period of culture to a maximum after 7 days and was then two‐fold in treated cultures. Taken together, these findings indicated that cytochalasin D stimulates chondrogenesis in response to modification of cytoskeleton architecture and the subsequent rounding up of the cells.

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

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

Cartilage formation by fetal rat chondrocytes cultured in alginate beads: A proposed model for investigating tissue-biomaterial interactions

Chondrocytes from 21-day-old rat fetal nasal cartilage were cultured in alginate beads for up to 20 days. It was found that chondrocytes retained their spherical shape and typical chondrocytic appearance. During the culture time, chondrocytes underwent differentiation, as demonstrated by the alkaline phosphatase-specific activity and rate of proteoglycan synthesis. Morphological data confirmed chondrocyte differentiation with the appearance of hypertrophic chondrocytes scattered in the alginate gel and a dense extracellular matrix containing filamentous structures and matrix vesicles. In addition, Northern blot analysis performed on day 8 of culture showed that chondrocytes cultured in alginate beads expressed type II collagen mRNA. The alginate bead method also appeared to be suitable for testing biomaterials, and the ready dissolution of the alginate beads by chelating agents provided a simple means for the rapid recovery of encapsulated chondrocytes. Powdered glass-ceramic particles entrapped in the alginate gel were colonized by chondrocytes, which then proliferated and formed a tissue similar to a true calcified cartilaginous structure. These results indicate that the alginate system represents a relevant model for studies of chondrogenesis and endochondral ossification. Furthermore, the encapsulation method could prove useful for studies of tissue-biomaterial interactions in an in vitro environment which more closely mirrors the cartilage matrix than other culture methods.

Immunohistochemical localization of plakophilins (PKP1, PKP2, PKP3, and p0071) in primary oropharyngeal tumors: correlation with clinical parameters

Plakophilins (PKPs) are members of the armadillo multigene family. Armadillo-related proteins function in both cell adhesion and signal transduction, and also play a central role in tumorigenesis. Here we report the immunohistochemical localization of PKPs in 37 cases of human primary squamous cell carcinoma of the oropharynx lacking overt distant metastases that were followed clinically for 3 years. Immunoreactivity for the PKPs PKP1, PKP2, PKP3, and p0071 (also known as PKP4) was assessed on frozen unfixed sections using a semiquantitative scoring system. Results were correlated with tumor grade, clinicopathologic parameters, and patient survival. Only p0071 was associated with tumor growth, demonstrating an inverse correlation with tumor size. PKP1 and PKP3 immunoreactivity was inversely correlated with tumor histological grade and was observed only in tumors that did not metastasize. In contrast, strong PKP2 immunoreactivity was observed in 85.7% of metastatic tumors. Interestingly, patients with tumors in which PKP1 and PKP3 immunoreactivity was reduced or absent exhibited local recurrences or metastases, or both, as well as poor survival. Correlation of the subcellular localization of PKPs with routine histological and clinical parameters suggests that these proteins may serve as useful markers for predicting the clinical outcome of the disease. Although the 4 PKPs displayed different levels and patterns of subcellular distribution in tumors, there was a positive correlation between immunoreactivity for PKP2 and PKP3, as well as for PKP2 and p0071, suggesting possible functional similarities associated with differentiation, tumor growth, and disease prognosis. Nevertheless, the mechanisms involved in altering the subcellular localization in tumors compared with normal epithelium are unknown, and further investigation is needed to determine whether PKPs are causative factors for oral carcinogenesis or are merely characteristic of the phenotype.

Epithelial Dlx-2 Homeogene Expression and Cementogenesis

The Dlx-2 (distal-less gene) homeoprotein transcription factor controls early tooth development but has not been studied during the late stages of biomineralization. Transgenic mice containing a Dlx-2/LacZ reporter construct were used to map the Dlx-2 expression pattern in cementoblasts, the dental cells most closely related to bone cells and therefore suggested to be uniquely positioned osteoblasts. During initial root formation, marked expression of Dlx-2 was evident in molar and incisor root epithelium, whereas dental papilla and follicle were negative. Dlx-2 was expressed in this epithelium from the apical loop to the area of its disruption. During acellular cementum formation in both incisors and molars, Dlx-2 expression was observed in the majority of differentiated cementoblasts from the apical region to the erupting zones. During cellular cementum formation, the presence of which characterizes growth-limited molars, Dlx-2 expression was restricted to the innermost cementoblasts and entrapped cementocytes. These data further support the hypothesis of a complex origin and fate of cementum-forming cells, as previously suggested by the expression patterns of a set of mesenchymal and epithelial markers, notably ameloblastin as shown here. Dlx-2 expression might constitute a landmark of cementoblast subpopulations of epithelial origin.

Comparative study of MSX-2, DLX-5, and DLX-7 gene expression during early human tooth development.

Msx and Dlx family transcription factors are key elements of craniofacial development and act in specific combinations with growth factors to control the position and shape of various skeletal structures in mice. In humans, the mutations of MSX and DLX genes are associated with specific syndromes, such as tooth agenesis, craniosynostosis, and tricho-dento-osseous syndrome. To establish some relationships between those reported human syndromes, previous experimental data in mice, and the expression patterns of MSX and DLX homeogenes in the human dentition, we investigated MSX-2, DLX-5, and DLX-7 expression patterns and compared them in orofacial tissues of 7.5- to 9-wk-old human embryos by using in situ hybridization. Our data showed that MSX-2 was strongly expressed in the progenitor cells of human orofacial skeletal structures, including mandible and maxilla bones, Meckels cartilage, and tooth germs, as shown for DLX-5. DLX-7 expression was restricted to the vestibular lamina and, later on, to the vestibular part of dental epithelium. The comparison of MSX-2, DLX-5, and DLX-7 expression patterns during the early stages of development of different human tooth types showed the existence of spatially ordered sequences of homeogene expression along the vestibular/lingual axis of dental epithelium. The expression of MSX-2 in enamel knot, as well as the coincident expression of MSX-2, DLX-5, and DLX-7 in a restricted vestibular area of dental epithelium, suggests the existence of various organizing centers involved in the control of human tooth morphogenesis.

Differential Expression and Activity of Tissue-nonspecific Alkaline Phosphatase (TNAP) in Rat Odontogenic Cells In Vivo:

Among the four existing isoforms of alkaline phosphatase (AP), the present study is devoted to tissue-nonspecific alkaline phosphatase (TNAP) in mineralized dental tissues. Northern blot analysis and measurements of phosphohydrolase activity on micro-dissected epithelium and ectomesenchyme, in situ hybridization, and immunolabeling on incisors confirmed that the AP active in rodent teeth is TNAP. Whereas the developmental pattern of TNAP mRNA and protein and the previously described activity were similar in supra-ameloblastic and mesenchymal cells, they differed in enamel-secreting cells, the ameloblasts. As previously shown for other proteins involved in calcium and phosphate handling in ameloblasts, a biphasic pattern of steady-state TNAP mRNA levels was associated with additional variations in ameloblast TNAP protein levels during the cyclic modulation process. Although the association of TNAP upregulation and the initial phase of bio-mineralization appeared to be a basic feature of all mineralized tissues, ameloblasts (and to a lesser extent, odontoblasts) showed a second selectively prominent upregulation of TNAP mRNA/protein/activity during terminal growth of large enamel crystals only, i.e., the maturation stage. This differential expression/activity for TNAP in teeth vs bone may explain the striking dental phenotype vs bone reported in hypophosphatasia, a hereditary disorder related to TNAP mutation.

Bioactive glass-ceramic containing crystalline apatite and wollastonite initiates biomineralization in bone cell cultures

Rat bone cells were cultured in the presence of bioactive glass-ceramic containing crystalline apatite and wollaston te. Scanning electron microscopy observations of the surface of the seeded ceramic disks revealed that cells attached, spread, and proliferated on the material surface. Soaking in cell-free culture medium showed that no change occurred in the surface structure. However, when cultured with bone cells and observed under a transmission electron microscope, an electron-dense layer was noted initially at the surface of the material, before bone formation occurred. In addition, energy-dispersive X-ray microanalysis demonstrated the presence of calcium and phosphorus in this layer. Progressively, during the following days of culture, active osteoblasts synthetized and laid down an osteoid matrix composed of numerous collagen fibrils arranged either parallel or perpendicularly to the first-formed electron-dense layer. Mineralization initiated on the ceramic surface dispersed then along the collagenous fibrils, leading to a mineralized matrix which surrounded the ceramic particles. These results demonstrate the capacity of apatite-wollastonite glass ceramic to initiate biomineralization in osteoblast cultures and to achieve a direct bond between the surface apatite layer of the bioactive glass-ceramic and the mineralized bone matrix.

Biomineralization, Life-Time of Odontogenic Cells and Differential Expression of the Two Homeobox Genes MSX-1 and DLX-2 in Transgenic Mice

Msx and Dlx homeobox genes encode for transcription factors that control early morphogenesis. More specifically, Msx‐1, Msx‐2, and Dlx‐2 homeobox genes contribute to the initial patterning of the dentition. The present study is devoted to the potential role of those homeobox genes during the late formation of mineralized tissues, using the rodent incisor as an experimental system. The continuously erupting mandibular incisor allows (1) the coinvestigation of the whole sequences of amelogenesis and dentinogenesis, aligned along the main dental axis in a single sample in situ and (2) the differential characterization of transcripts generated by epithelial and ectomesenchymal odontogenic cells. Northern blot experiments on microdissected cells showed the continuing expression of Msx‐2 and Dlx‐2 in the later stages of dental biomineralization, differentially in epithelial and ectomesenchymal compartments. Transgenic mice produced with LacZ reporter constructs for Dlx‐2 and Msx‐1 were used to detect different components of the gene expression patterns with the sensitive β‐galactosidase histoenzymology. The results show a prominent epithelial involvement of Dlx‐2, with stage‐specific variations in the cells involved in enamel formation. Quantitative analyses identified specific modulations of Dlx‐2 expression in ameloblasts depending on the anatomical sites of the incisor, showing more specifically an inverse linear relationship between the Dlx‐2 promoter activity level and enamel thickness. This investigation extends the role of homeoproteins to postmitotic stages, which would control secretory cell activity, in a site‐specific manner as shown here for Dlx‐2.