Total Spinal Anesthesia Following an Interscalene Block Treated with Intravenous Lipid Emulsion.

Total spinal anesthesia following interscalene block is a rare and life-threatening complication of regional anesthesia. A 56-year-old woman underwent an uncomplicated left shoulder bone spur removal under general anesthesia with an interscalene nerve block at an outpatient surgical center. Subsequently, she developed bilateral mydriasis, paralysis of all extremities, and respiratory arrest. She was intubated and transferred to the emergency department (ED) where she was given intravenous lipid emulsion (ILE) with complete recovery of neurological function. ILE therapy may be considered as a rescue treatment in addition to supportive therapy for total spinal anesthesia.

Porous media investigation before and after hydrochloric acid injection on a pre-salt carbonate coquinas sample.

Porous space characterization of carbonate rocks is an important aid in petroleum exploration from carbonate reservoir. In this study, X-ray microtomography technique was applied to evaluate total porosity of a coquina sample extracted from pre-salt reservoir, in Brazil, before and after acid injection. Two image processing program were used in order to assess performance. The results showed that microtomography has potential to compute porosity of coquina samples and provides information about rock porous network.

H-7 and fetal calf serum (FCS) act synergistically to increase apoptosis in the KB line of human oral carcinoma cells.

There is a high incidence of oral squamous cell carcinoma (SCC) worldwide. The survival rate is among the lowest of the major cancers and has not improved significantly over the past two decades. The KB line of human oral carcinoma cells is a useful experimental system for studies of the biology of oral SCC. In a previous study, we reported inhibition of KB cell proliferation and stimulation of desmosome formation in confluent cultures treated with 20 microM H-7 (1-(5-isoquinolinylsulfonyl)-2-methylpiperazine). In the present study, the effects of this protein kinase C (PKC) inhibitor on the survival of KB cells were investigated. Apoptotic cells were detected using a combination of Hoechst 33258 nuclear stain, TUNEL technique and ultrastructural analysis. Our results indicated that H-7 significantly increased apoptosis in KB cells in a dose-dependent manner. Maximal stimulation occurred at 100 microM, the highest dose of H-7 tested. Apoptotic cells exhibited nuclear fragmentation, chromatin condensation and apoptotic bodies. Interestingly, H-7 and fetal calf serum (FCS) acted synergistically to increase apoptosis in KB cells, suggesting that there is a serum activated subpopulation of H-7 target cells in the cultures. The underlying mechanism of activation remains to be elucidated. Our study suggests that the PKC inhibitor H-7 is a potentially useful cytostatic agent for oral carcinoma cells.

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.

Association of enhanced expression of gap junctions with in vitro chondrogenic differentiation of rat nasal septal cartilage-released cells following their dedifferentiation and redifferentiation.

The nasal septum is an important centre of endochondral ossification during the development of the facial region. Previous studies have shown that it is possible to recapitulate the differentiation programme of 21-day-old rat nasal chondrocytes in vitro. The purpose now was to investigate, in vitro, the cell condensation phase that represents the earliest morphological event associated with cartilage differentiation in skeletal development. The study focuses on the ability of the cells to form condensations before overt differentiation, with special emphasis on gap-junction expression. The gap-junction protein connexin 43 was localized by indirect immunofluorescence as primarily intracellular and, on day 5, at the condensation stage, as spot-like contacts between cells. Intracellular injection of the permeable dye Lucifer yellow led to the staining of up to 20 neighbouring cells, indicating functional gap junctions and coupling. In contrast, treatment of cultures with the gap-junction blocker glycyrrhetinic acid inhibited dye coupling and reduced cartilage differentiation. Northern blotting of connexin 43 mRNA showed a faint band during the first days of culture, with a striking increase after day 4. In addition, the mRNA of the homeodomain-containing gene Cart-1 began to be expressed in prechondrogenic condensations and corresponded to the expression of type II collagen mRNA. These data indicate that the early stage of in vitro chondrocyte differentiation is the formation of cell condensations and the ability to establish cell-to-cell communication. Connexin 43, together with other molecular mechanisms, mediates the condensation phase of chondrogenesis and sets up the optimal environment in which nasal septal cells may terminally differentiate into chondrocytes.

Cloning, characterization and immunolocalization of human ameloblastin.

Amelogenesis imperfecta is a broad classification of hereditary enamel defects, exhibiting both genetic and clinical diversity. Most amelogenesis imperfecta cases are autosomal dominant disorders, yet only the local hypoplastic form has been mapped to human chromosome 4q between D4S242 1 and the albumin gene. An enamel protein cDNA, termed ameloblastin (also known as amelin and sheathlin), has been isolated from rat, mouse and pig. Its human homolog has been mapped to chromosome 4q21 between markers D4S409 and D4S400, flanking the local hypoplastic amelogenesis imperfecta critical region. Therefore, ameloblastin is a strong candidate gene for this form of amelogenesis imperfecta. To facilitate genetic studies related to this dental disease, we isolated and characterized a human ameloblastin cDNA. A human third molar cDNA library was screened and two ameloblastin clones identified. Nucleotide sequencing of these cDNAs indicated alternative splicing of the putative open reading frame, use of different polyadenylation signals, and a high degree of similarity to reported rat, mouse and porcine cDNAs. Immunohistochemistry studies on embryonic human teeth using an antibody to recombinant ameloblastin indicated ameloblastin expression by ameloblasts with localization in the enamel matrix associated with the sheath structures.

Phenotypic modulation of nasal septal chondrocytes by cytoskeleton modification.

The reversion to the initial round shape of chondrocytes in monolayer cultures is one of the initial events required for the expression of cartilage-specific macromolecules. 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. These morphological changes occurred parallel to the expression of type II collagen, marker of the chondrocytes phenotype, strongly expressed in experimental cultures, but relatively weak in control cultures, and only restricted on areas of polygonal cellular aggregates. Furthermore, [35S]-sulphate incorporation into sulphated 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.

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 beta-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.

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. (J Histochem Cytochem 48:277-283, 2000)

In vitro bone formation on a bone-like apatite layer prepared by a biomimetic process on a bioactive glass-ceramic.

In this study we have investigated the behavior of fetal rat osteoblasts, cultured up to 23 days, on a bioactive apatite-wollastonite (AW) glass-ceramic and on the same material on which a carbonated apatite layer had been formed by a biomimetic process (AWa). At the last day of culture, the specific activity of alkaline phosphatase activity, as determined biochemically, was about 30% greater on AWa compared with AW disks. After the cell layers had been scraped off, scanning electron microscopic (SEM) observations of the materials' surfaces revealed that mineralized bone nodules remained attached to both surfaces but in larger amounts on AWa. X-ray microanalysis indicated the presence of calcium (Ca) and phosphorus (P) in the bone tissue throughout the AWa surface and Ca, P, and silicon (Si) on the AW surface. The AW/ and AWa/bone interfaces also were analyzed after fracturing of the disks. The interfacial analysis showed firm bone bonding to the AW and AWa surfaces, confirmed by the X-ray microanalytic mappings. These results indicate the importance of surface composition in supporting differentiation of osteogenic cells and the subsequent apposition of bone matrix, which allows a strong bond of the bioactive materials to the bone. Furthermore, prefabrication of a biologic apatite layer by a method that mimics biomineralization could find application to bone-repairing materials.

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, Meckel's 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 microdissected 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 biomineralization 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. (J Histochem Cytochem 47:1541-1552, 1999)

Involvement of desmoplakin phosphorylation in the regulation of desmosomes by protein kinase C, in HeLa cells.

In the present study, we have examined how modulation of protein kinase C (PKC) activity affected desmosome organization in HeLa cells. Immunofluorescence and electron microscopy showed that PKC activation upon short exposure to 12-O-tetradecanoylphorbol 13-acetate (TPA) resulted in a reduction of intercellular contacts, splitting of desmosomes and dislocation of desmosomal components from the cell periphery towards the cytoplasm. As determined by immunoblot analysis of Triton X-100-soluble and -insoluble pools of proteins, these morphological changes were not correlated with modifications in the extractability of both desmoglein and plakoglobin, but involved almost complete solubilization of the desmosomal plaque protein, desmoplakin. Immunoprecipitation experiments and immunoblotting with anti-phosphoserine, antiphosphothreonine and anti-phosphotyrosine antibodies revealed that desmoplakin was mainly phosphorylated on serine and tyrosine residues in both treated and untreated cells. While phosphotyrosine content was not affected by PKC activation, phosphorylation on serine residues was increased by about two-fold. This enhanced serine phosphorylation coincided with the increase in the protein solubility, suggesting that phosphorylation of desmoplakin may be a mechanism by which PKC mediates desmosome disassembly. Consistent with the loss of PKC activity, we also showed that down-modulation of the kinase (in response to prolonged TPA treatment) or its specific inhibition (by GF 109203X) had opposite effects and increased desmosome formation. Taken together, these results clearly demonstrate an important role for PKC in the regulation ofdesmosomal junctions in HeLa cells, and identify serine phosphorylation of desmoplakin as a crucial event in this pathway.

Down-regulation of desmosomal molecules in oral and pharyngeal squamous cell carcinomas as a marker for tumour growth and distant metastasis.

Down-regulation of adhesion molecules has been observed in a number of squamous cell carcinomas (SCC) and is considered to be associated with tumour invasiveness and lymph node metastasis. The present prospective investigation aimed at analyzing the expression patterns of desmosomal markers in oral and pharyngeal SCC and correlations that may exist between these patterns and tumour behaviour. Two constitutive desmosomal molecules, desmoplakin (Dp) and plakoglobin (Pg), were examined in 26 samples of primary carcinoma of the head and neck. The correlation between Dp and Pg expression was only moderate, reflecting functional differences between the two proteins. Whereas decreased Dp and Pg expression was closely associated with distant metastasis formation, reduced Pg expression was correlated to the development of large tumours. There were also variable relationships between the expression of these markers and lymph node invasion, histological differentiation, or survival of the patients. Biochemical analysis of cytoskeletal fractions confirmed the decrease in desmosomal proteins, particularly in tumours which later developed metastases. Down-regulation of Dp and Pg in oral and pharyngeal SCC may represent a reliable marker for extensive tumour growth and the risk of distant metastasis formation, Dp and Pg apparently having metastasis- and tumour-suppressor properties, respectively.

Expression of DLX5 during human embryonic craniofacial development.

Dlx (distal-less gene) homeogenes encode transcription factors that are involved in the patterning of orofacial skeleton derived from cephalic neural crest cells. In order to study the role of DLX genes during embryonic development in human, DLX5 expression pattern was investigated in 6- to 11-week-old human embryos. A DLX5 PCR fragment was amplified from a human dental cDNA library subcloned and used for in situ hybridization investigations. DLX5 gene expression was primarily detected in the mandible at 6 weeks and then, after in the maxilla. DLX5 gene expression became restricted to progenitor cells of developing tooth germs, bones and cartilages of mandible and maxilla. During odontogenesis from bud to late cap stages, DLX5 transcripts were present in both dental epithelium and mesenchyme tissues. DLX5 expression was restricted to few cells in the vestibular aspect of the dental epithelium, while DLX5 mRNA signal was more widely distributed in dental mesenchyme. The observed expression pattern of DLX5 homeogene extends the proposed site-specific combination of homeogene expression in neural crest derived cells to human specific dentition. Furthermore, during the bud and cap stages of tooth morphogenesis, the asymmetric expression of DLX5 in the dental epithelium and dental mesenchyme may contribute to the complex patterning of human tooth shape.

Evidence for regulation of amelogenin gene expression by 1,25-dihydroxyvitamin D(3) in vivo.

The unique hereditary enamel defect clearly related to the disturbance of one enamel matrix protein is X-linked amelogenesis imperfecta (AI), in which several mutations of amelogenin gene have been identified. The clinical phenotype of many of these subjects shows similarities with enamel defects related to rickets. Therefore, we hypothesized that rachitic dental dysplasia is related to disturbances in the amelogenin pathway. In order to test this hypothesis, combined qualitative and quantitative studies in experimental vitamin D-deficient (-D) rat model systems were performed. First, Western blot analysis of microdissected enamel matrix (secretion and maturation stages) showed no clear evidence of dysregulation of amelogenin protein processing in -D rats as compared with the controls. Second, the ultrastructural investigation permitted identification of the internal tissular defect of rachitic enamel, the irregular absence of intraprismatic enamel observed in -D animals, suggesting a possible link between prism morphogenesis and vitamin D. In addition, the steady-state levels of amelogenin mRNAs measured in microdissected dental cells was decreased in -D rats and up-regulated by an unique injection of 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)). The present study shows evidences that amelogenin expression is regulated by vitamin D. This is the first study of an hormonal regulation of tooth-specific genes.

Cytokeratin alterations as diagnostic and prognostic markers of oral and pharyngeal carcinomas. A prospective study.

Cytokeratin (CK) alterations have been reported in carcinomas from different anatomical sites, and these have been associated with specific aspects of tumour behaviour. In order to assess the relationships between CK modifications and future tumour behaviour, we conducted the present prospective study on 26 squamous cell carcinomas (SCC) of oral and pharyngeal mucosae and corresponding controls. Cytokeratins were investigated using two-dimensional gel electrophoresis and immunofluorescence techniques. All healthy tissues, oral lining and oropharyngeal mucosae, expressed the oesophageal type CKs, including CK 19. Other simple epithelial CKs (7, 8, 17 and 18) were not detected. In carcinomas originating from corresponding sites, expression of oesophageal CKs varied widely from one specimen to another, and simple epithelial keratins were often found. Statistical analysis indicated correlations between CK expression and the clinicopathological data of SCC patients. Small tumour size was strongly associated with the expression of CKs 10 and 19. Interestingly, an absence of lymph node involvement was significantly associated with CK 18 expression. Tumours giving rise to recurrences, metachronous tumours, and distant metastasis were significantly associated with an absence of CK 13. These results suggest that CKs 10, 19, 18 and 13 could be reliable diagnostic and prognostic markers in the assessment of oral and pharyngeal squamous carcinomas.

H-7 stimulates desmosome formation and inhibits growth in KB oral carcinoma cells.

Protein kinase inhibitor H-7 was reported to stimulate desmosome formation in normal keratinocytes and to inhibit proliferation of neural cell lines. In the present study, the effects of this inhibitor on adhesion and growth of KB human oral carcinoma cells were investigated. H-7 was found to enhance desmosome assembly, as evidenced by an increased punctate labeling for the major desmosomal markers. Immunogold labeling confirmed the formation of desmosomes both at the cell surface and in the cytoplasm. In order to assess cell proliferation and possible correlation with adhesion, confluent cultures were treated and both adherert and detached cell fractions were counted. Under serum-free conditions, H-7 significantly reduced cell detachment. In contrast, EGF stimulated cell detachment, and this effect was abolished when cells were simultaneously treated with both EGF and H-7. Total cell counts were also significantly reduced by H-7, both in the presence and absence of EGF. Using the TUNEL technique, labeled cells were increased after H-7 treatment, thus implicating protein kinase inhibition in cell death. These results indicate that H-7 inhibits growth and stimulates adhesion of KB carcinoma cells.

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.

Retinoic acid suppresses the osteogenic differentiation capacity of murine osteoblast-like 3/A/1D-1M cell cultures.

Bone is a target tissue for action of retinoids though their precise role remains unclear. This study investigated the effects of retinoic acid (RA) on the marrow stromal 3/A/1D-1M osteoblast-like cells, derived from the in vivo transplantation of 3/A/1D-1 chondroprogenitor cells. Long-term treatment with 1 microM RA for 7 weeks induced a marked decrease in bone-like nodule number and ultrastructural alterations in the striation and the size of the collagen fibres. RA at concentrations varying from 10 nM to 3.16 microM had a dose-dependent inhibition effect on alkaline phosphatase (AP) activity with an IC50 of 0.7 microM. Treatment with 1 microM RA for up to 17 days induced a time-dependent inhibition of AP activity, while the beginning of RA treatment (4 or 52 h of culture) produced a differential magnitude of inhibition. These variations were unrelated to modifications of the expression of RAR receptor at the protein level, as assessed by Western blot analysis. Exposure to 1 microM RA for 6 or 24 h administered at day 14 produced an inhibition of AP activity, which reached a maximum after 48 h, with a recovery time of 8 days in both cases. Long-term treatment with RA at 1 microM completely abolished the level of osteocalcin mRNA on both days 12 and 16, as revealed by Northern blot analysis. However, such RA-treated cells retained the constitutive expression of type II procollagen transcripts. These results suggest that RA inhibits several aspects of osteogenic differentiation capacity, a loss of phenotype, which, in association with the maintenance of type II procollagen cartilage-related characteristic, could be a prerequisite step for cellular plasticity.