Altered cartilage phenotype expressed during intramembranous bone formation

Polyethylenimine-alginate nanocomposites based bone morphogenetic protein 2 gene-activated matrix for alveolar bone regeneration

The repair and treatment of lost or damaged alveolar bone is of great significance in dentistry. Gene-activated matrix (GAM) technology provides a new way for bone regeneration. It is a local gene delivery system, which can not only recruit cells, but also influence their fate. For this purpose, we fabricated a bone morphogenetic protein 2 (BMP-2) gene-loaded absorbable gelatin sponge (AGS) and studied its effect on promoting alveolar bone formation and preventing resorption following tooth extraction in rats. In order to obtain better transfection efficiency, polyethylenimine-alginate (PEI-al) nanocomposites were synthesized and used as gene vectors to deliver BMP-2 cDNA plasmids (PEI-al/pBMP-2). The transfection efficiency, BMP-2 protein expression and osteogenic differentiation of the cells were investigated in vitro. In vivo, we established an alveolar bone regeneration model by extracting the rats' left mandibular incisors. The rats were randomly assigned into 3 groups: control group, unfilled sockets; AGS group, sockets filled with PEI-al solution-loaded gelatin sponges; AGS/BMP group, sockets filled with PEI-al/pBMP-2 solution-loaded gelatin sponge. Radiological and histological assays were performed at 4 and 8 weeks later. In vitro transfection assays indicated that PEI-al/pBMP-2 complexes could effectively transfect MC3T3-E1 cells, promoting the secretion of BMP-2 protein for at least 14 days, as well as increasing the expression of osteogenesis-related gene, ALP activity and calcium deposition. In vivo, western blot analysis showed BMP-2 protein was expressed in bone tissues of AGS/BMP group. The relative height of the residual alveolar ridge and bone mineral density (BMD) of the AGS/BMP group were significantly greater than those in the AGS and control groups at 4 and 8 weeks, respectively. Histological examination showed that, at 4 weeks, osteoblasts had grown in a cubic shape around the new bone in the AGS/BMP group, suggesting new bone formation. In conclusion, the combination of PEI-al/pBMP-2 complexes and gelatin sponge could promote alveolar bone regeneration, which may provide an easy and valuable method for alveolar ridge preservation and augmentation.

Polyethylenimine-alginate nanocomposites based bone morphogenetic protein 2 gene-activated matrix may provide an easy and valuable method for alveolar ridge regeneration.

High-Frequency Acceleration: Therapeutic Tool to Preserve Bone following Tooth Extractions

A common problem in clinical dentistry is the significant and rapid bone loss that occurs after tooth extraction. Currently there is no solution for the long-term preservation of alveolar bone. Previously, we showed that high-frequency acceleration (HFA) has an osteogenic effect on healthy alveolar bone. However, it is not known if HFA can preserve alveolar bone after extraction without negatively affecting wound healing. The purpose of this study was to evaluate the effect of HFA on alveolar bone loss and the rate of bone formation after tooth extraction. Eighty-five adult Sprague-Dawley rats were divided into 3 groups: control, static (static load), and HFA. In all groups, the maxillary right third molar was extracted. The HFA group received HFA for 5 min/d, applied through the second molar. The static group received the same magnitude of static load. The control group did not receive any stimulation. Some animals received fluorescent dyes at 26 and 54 d. Samples were collected on days 0, 7, 14, 28, and 56 for fluorescence microscopy, micro-computed tomography, histology, RNA, and protein analyses. We found that HFA increased bone volume in the extraction site and surrounding alveolar bone by 44% when compared with static, while fully preserving alveolar bone height and width long-term. These effects were accompanied by increased expression of osteogenic markers and intramembranous bone formation and by decreased expression of osteoclastic markers and bone resorption activity, as well as decreased expression of many inflammatory markers. HFA is a noninvasive safe treatment that can be used to prevent alveolar bone loss and/or accelerate bone healing after tooth extraction.

Intramembranous bone healing process subsequent to tooth extraction in mice: micro-computed tomography, histomorphometric and molecular characterization

Bone tissue has a significant potential for healing, which involves a significant the interplay between bone and immune cells. While fracture healing represents a useful model to investigate endochondral bone healing, intramembranous bone healing models are yet to be developed and characterized. In this study, a micro-computed tomography, histomorphometric and molecular (RealTimePCRarray) characterization of post tooth-extraction alveolar bone healing was performed on C57Bl/6 WT mice. After the initial clot dominance (0h), the development of a provisional immature granulation tissue is evident (7d), characterized by marked cell proliferation, angiogenesis and inflammatory cells infiltration; associated with peaks of growth factors (BMP-2-4-7,TGFβ1,VEGFa), cytokines (TNFα, IL-10), chemokines & receptors (CXCL12, CCL25, CCR5, CXCR4), matrix (Col1a1-2, ITGA4, VTN, MMP1a) and MSCs (CD105, CD106, OCT4, NANOG, CD34, CD146) markers expression. Granulation tissue is sequentially replaced by more mature connective tissue (14d), characterized by inflammatory infiltrate reduction along the increased bone formation, marked expression of matrix remodeling enzymes (MMP-2-9), bone formation/maturation (RUNX2, ALP, DMP1, PHEX, SOST) markers, and chemokines & receptors associated with healing (CCL2, CCL17, CCR2). No evidences of cartilage cells or tissue were observed, strengthening the intramembranous nature of bone healing. Bone microarchitecture analysis supports the evolving healing, with total tissue and bone volumes as trabecular number and thickness showing a progressive increase over time. The extraction socket healing process is considered complete (21d) when the dental socket is filled by trabeculae bone with well-defined medullary canals; it being the expression of mature bone markers prevalent at this period. Our data confirms the intramembranous bone healing nature of the model used, revealing parallels between the gene expression profile and the histomorphometric events and the potential participation of MCSs and immune cells in the healing process, supporting the forthcoming application of the model for the better understanding of the bone healing process.

Genetic networks in osseointegration

Osseointegration-based dental implants have become a well-accepted treatment modality for complete and partial edentulism. The success of this treatment largely depends on the stable integration and maintenance of implant fixtures in alveolar bone; however, the molecular and cellular mechanisms regulating this unique tissue reaction have not yet been fully uncovered. Radiographic and histologic observations suggest the sustained retention of peri-implant bone without an apparent susceptibility to catabolic bone remodeling; therefore, implant-induced bone formation continues to be intensively investigated. Increasing numbers of whole-genome transcriptome studies suggest complex molecular pathways that may play putative roles in osseointegration. This review highlights genetic networks related to bone quality, the transient chondrogenic phase, the vitamin D axis, and the peripheral circadian rhythm to elute the regulatory mechanisms underlying the establishment and maintenance of osseointegration.

Effects of low-amplitude, high-frequency vibrations on proliferation and differentiation of SAOS-2 human osteogenic cell line

The aim of the work was to understand the consequences of low-amplitude, high-frequency vibrations on proliferation and differentiation of SAOS-2 cells (sarcoma osteogenetic), an osteoblastic and tumorigenic cell line. We realized a bioreactor composed of an eccentric motor that produces a displacement of 11 mm at frequencies between 1 and 120 Hz on a plate connected to the motor. The cultures of SAOS-2 cells were fixed on the plate, and the linear acceleration provoked by the motor to the cultures was measured. We used 30 Hz as stimulating frequency after a preliminary test on the effect of different frequencies on differentiation of cells. Afterward, SAOS-2 cells were stimulated with 30 Hz for different durations, every day for 4 days. The expression of some genes involved in the differentiation process was analyzed first with a reverse transcriptase-polymerase chain reaction and afterward with a real-time polymerase chain reaction on the most expressed genes. Moreover, the proliferation of cells was evaluated. The results suggest a strong increase in the expression of the genes involved in tissue differentiation in the treated groups with respect to the controls. On the other hand, the proliferation seems to be slowed down, so probably the acceleration perceived by the mechanosensors of the cells changes the cellular cycle by blocking the duplication to early differentiate toward bone tissue.

Trabecular bone deterioration in col9a1+/- mice associated with enlarged osteoclasts adhered to collagen IX-deficient bone

INTRODUCTION

Short collagen IX, the exclusive isoform expressed by osteoblasts, is synthesized through alternative transcription of the col9a1 gene. The function of short collagen IX in bone was characterized in col9a1-null mutant mice.

MATERIALS AND METHODS

Trabecular bone morphometry of lumbar bones and tibias was evaluated by muCT and nondecalcified histology. Osteoblastic and osteoclastic activities were evaluated by PCR- and microarray-based gene expression assays and TRACP-5b and C-terminal telopeptide (CTX) assays, as well as in vitro using bone marrow stromal cells and splenocytes. The effect of col9a1(+/-) mutation on osteoclast morphology was evaluated using RAW264.7-derived osteoclastic cells cultured on the mutant or wildtype calvarial bone substrates.

RESULTS

Col9a1 knockout mutation caused little effects on the skeletal development; however, young adult female col9a1(-/-) and col9a1(+/-) mice exhibited significant loss of trabecular bone. The trabecular bone architecture was progressively deteriorated in both male and female heterozygous col9a1(+/-) mice while aging. The aged mutant mice also exhibited signs of thoracic kyphosis and weight loss, resembling the clinical signs of osteoporosis. The col9a1(+/-) osteoblasts synthesized short col9a1 transcripts at decreased rates. Whereas bone formation activities in vitro and in vivo were not affected, the mutant osteoblast expressed the elevated ratio of RANKL/osteoprotegerin. Increased serum TRACP-5b and CTX levels were found in col9a1(+/-) mice, whose bone surface was associated with osteoclastic cells that were abnormally flattened and enlarged. The mutant and wildtype splenocytes underwent similar osteoclastogenesis in vitro; however, RAW264.7-derived osteoclastic cells, when cultured on the col9a1(+/-) calvaria, widely spread over the bone surface and formed large resorption pits. The surface of col9a1(+/-) calvaria was found to lack the typical nanotopography.

CONCLUSIONS

The mineralized bone matrix deficient of short collagen IX may become susceptible to osteoclastic bone resorption, possibly through a novel non-cell-autonomous mechanism. The data suggest the involvement of bone collagen IX in the pathogenesis of osteoporosis.

Regulation of skeletogenic differentiation in cranial dermal bone

Although endochondral ossification of the limb and axial skeleton is relatively well-understood, the development of dermal (intramembranous) bone featured by many craniofacial skeletal elements is not nearly as well-characterized. We analyzed the expression domains of a number of markers that have previously been associated with endochondral skeleton development to define the cellular transitions involved in the dermal ossification process in both chick and mouse. This led to the recognition of a series of distinct steps in the dermal differentiation pathways, including a unique cell type characterized by the expression of both osteogenic and chondrogenic markers. Several signaling molecules previously implicated in endochondrial development were found to be expressed during specific stages of dermal bone formation. Three of these were studied functionally using retroviral misexpression. We found that activity of bone morphogenic proteins (BMPs) is required for neural crest-derived mesenchyme to commit to the osteogenic pathway and that both Indian hedgehog (IHH) and parathyroid hormone-related protein (PTHrP, PTHLH)negatively regulate the transition from preosteoblastic progenitors to osteoblasts. These results provide a framework for understanding dermal bone development with an aim of bringing it closer to the molecular and cellular resolution available for the endochondral bone development.

Collagen IX is indispensable for timely maturation of cartilage during fracture repair in mice

Fracture repair recapitulates in adult organisms the sequence of cell biological events of endochondral ossification during skeletal development and growth. After initial inflammation and deposition of granulation tissue, a cartilaginous callus is formed which, subsequently, is remodeled into bone. In part, bone formation is influenced also by the properties of the extracellular matrix of the cartilaginous callus. Deletion of individual macromolecular components can alter extracellular matrix suprastructures, and hence stability and organization of mesenchymal tissues. Here, we took advantage of the collagen IX knockout mouse model to better understand the role of this collagen for organization, differentiation and maturation of a cartilaginous template during formation of new bone. Although a seemingly crucial component of cartilage fibrils is missing, collagen IX-deficient mice develop normally, but are predisposed to premature joint cartilage degeneration. However, we show here that lack of collagen IX alters the time course of callus differentiation during bone fracture healing. The maturation of cartilage matrix was delayed in collagen IX-deficient mice calli as judged by collagen X expression during the repair phase and the total amount of cartilage matrix was reduced. Entering the remodeling phase of fracture healing, Col9a1(-/-) calli retained a larger percentage of cartilage matrix than in wild type indicating also a delayed formation of new bone. We concluded that endochondral bone formation can occur in collagen IX knockout mice but is impaired under conditions of stress, such as the repair of an unfixed fractured long bone.

Characterization of two new zebrafish members of the hedgehog family: atypical expression of a zebrafish indian hedgehog gene in skeletal elements of both endochondral and dermal origins

We have characterized two new members of the Hedgehog (Hh) family in zebrafish, ihha and dhh, encoding for orthologues of the tetrapod Indian Hedgehog (Ihh) and Desert Hedgehog (Dhh) genes, respectively. Comparison of ihha and Type X collagen (col10a1) expression during skeletal development show that ihha transcripts are located in hypertrophic chondrocytes of cartilaginous elements of the craniofacial and fin endoskeleton. Surprisingly, col10a1 expression was also detected in cells forming intramembranous bones of the head and in flat cells surrounding cartilaginous structures. The expression of col10a1 in both endochondral and intramembranous bones reflects an atypical composition of the extracellular matrix of the zebrafish craniofacial skeleton. In addition, during fin ray regeneration, both ihha and col10a1 are detected in scleroblasts, osteoblast-like cells secreting the matrix of the dermal bone fin ray. The presence of cartilage markers suggests that the dermal fin ray possesses an intermediate phenotype between cartilage and bone.

Impact of Human Genome Project on treatment of frail and edentulous patients*

OBJECTIVE

Because of ongoing increases in life expectancy and deferment of edentulousness to older age, dentists are facing a different challenge to satisfy elderly denture wearers with a higher prevalence of chronic diseases. This discussion introduces the Human Genome databases as novel and powerful resources to re-examine the core problems experienced by frail and edentulous patients.

BACKGROUND

Recent studies demonstrated that mandibular implant overdentures do not necessarily increase masticatory function, perception and satisfaction in denture wearers with adequate edentulous residual ridges. It has been demonstrated that the rate of edentulous residual ridge resorption significantly varies among individuals. The prognosis and cost-effectiveness of denture treatment, with or without implants, may largely depend on how the edentulous ridge is maintained. However, reliable clinical methods permitting dentists to predict the long-term health of the edentulous residual ridge are lacking.

MATERIALS AND METHODS

With the completion of the Human Genome Project, the genomic sequence database from this multinational consortium will provide a unique resource to determine the genetic basis of similarity and diversity of humans.

RESULTS

One base pair in every 100 to 300 base pairs of the genome sequence varies among humans, suggesting that genetic diagnosis using the single nucleotide polymorphisms (SNPs) may provide a novel opportunity to differentiate our edentulous patients.

CONCLUSIONS

Future dental service for the elderly will require a personalized care paradigm, using highly sensitive diagnostic technology such as SNP genomic analysis, for recommending the treatment with greatest potential benefit.

Overexpression of Nell-1, a craniosynostosis-associated gene, induces apoptosis in osteoblasts during craniofacial development

We studied the cellular function of Nell-1, a craniosynostosis-related gene, in craniofacial development. Nell-1 modulates calvarial osteoblast differentiation and apoptosis pathways. Nell-1 overexpression disrupts these pathways resulting in craniofacial anomalies such as premature suture closure.

INTRODUCTION

Craniosynostosis (CS), one of the most common congenital craniofacial deformities, is the premature closure of cranial sutures. Previously, we reported NELL-1 as a novel molecule overexpressed during premature cranial suture closure in patients with CS. Nell-1 overexpression induced calvarial overgrowth and resulted in premature suture closure in a rodent model. On a cellular level, Nell-1 is suggested to promote osteoblast differentiation.

MATERIALS AND METHODS

Different levels of Nell-1 were introduced into osteoblastic cells by viral infection and recombinant protein. Apoptosis and gene expression assays were performed. Mice overexpressing Nell-1 were examined for apoptosis.

RESULTS

In this report, we further showed that overexpression of Nell-1 induced apoptosis along with modulation of apoptosis-related genes. The induction of apoptosis by Nell-1 was observed only in osteoblastic cells and not in NIH3T3 or primary fibroblasts. The CS mouse model overexpressing Nell-1 showed increased levels of apoptosis in the calvaria.

CONCLUSION

We show that Nell-1 expression modulates calvarial osteoblast differentiation and apoptosis pathways. Nell-1 overexpression disrupts these pathways resulting in craniofacial anomalies such as premature suture closure.

A short isoform of Col9a1 supports alveolar bone repair

Bone wound created in intramembranous alveolar bone heals without the formation of cartilage precursor tissue. However, the expression of cartilage collagen mRNAs has been suggested. In this report, we examined the expression and the potential role of type IX collagen in bone restoration and remodeling. The sequence specific polymerase chain reaction demonstrated the exclusive expression of short transcriptional isoform of alpha1(IX) collagen (Col9a1) in alveolar bone wound healing, while the long isoform of Col9a1 transcript was absent. Type IX collagen was immunolocalized in the preliminary matrix organized in granulation tissue before trabecular bone formation in tooth extraction socket. In Col9a1-null mutant mice, there were considerable variations in alveolar bone wound healing with the absence of or abnormally organized trabecular bone. Occasionally, unusual apposition of cortical-bone-like layers in bone marrow space was observed. The Col9a1-null mice indicated no growth retardation, and their facial and long bones maintained the normal size and shape. However, the primary spongiosa region of adult Col9a1 mutant mice showed an abnormal trabecular bone structure associated with abnormal immunostaining with the hypertrophic cartilage specific type X collagen antibody. These data suggest that type IX collagen short transcriptional variant is involved in the restoration and remodeling processes of trabecular bone.

Human NELL-1 expressed in unilateral coronal synostosis

Surgical correction of unilateral coronal synostosis offers a unique opportunity to examine the molecular differences between an abnormal and a normal cranial suture. We isolated and identified a cDNA fragment whose expression was up-regulated in the premature fusing and fused coronal sutures, as compared with normal coronal sutures. The nucleotide sequence of the full-length cDNA of this gene, human NELL-1, has approximately 61% homology with the chicken Nel gene. Both chicken Nel and human NELL-1 are comprised of six epidermal growth factor-like repeats. The human NELL-1 messages were localized primarily in the mesenchymal cells and osteoblasts at the osteogenic front, along the parasutural bone margins, and within the condensing mesenchymal cells of newly formed bone in sites of premature sutural fusion. Human multiorgan tissue mRNA blot showed that NELL-1 was specifically expressed in fetal brain but not in fetal kidney, liver, or lung. We also showed that Nell-1 was expressed in rat calvarial osteoprogenitor cells and was largely absent in rat tibiae and fibroblast cell cultures. In conclusion, our data suggest that the NELL-1 gene is preferentially expressed in cranial intramembranous bone and neural tissue (both of neural crest cell origin) and is up-regulated during unilateral premature closure of the coronal suture. The precise role of this gene is unknown.

Biologic determinants of bone formation for osseointegration: clues for future clinical improvements

STATEMENT OF PROBLEM

Further improvement in and expansion of the application of dental implants requires control and improvement of bone mass for implant support.

PURPOSE

Although osseointegration involves both the formation and the maintenance of bone at implant surfaces, the aim of this article is to identify cellular and molecular determinants of bone formation that may be used in clinical attempts to enhance or expand the application of endosseous implants for dental and craniofacial prosthetics.

METHODS

A review of bone biology and dental and orthopedic implant literature was performed using Medline and published monographs.

RESULTS

This spectrum of information indicates that molecular and cellular approaches to creating and maintaining bone mass may be used to expand the application of dental implants and to improve dental implant success in bone-deficient sites.

Current perspectives in residual ridge remodeling and its clinical implications: a review

PURPOSE

This article reviews the current understanding of the biology of tooth extraction wound healing and residual ridge remodeling.

METHODS

The review of the biology of tooth extraction wound healing involves a discussion of the different cells populating the tooth extraction wound, the matrix formation, and the control of the repair process in the short-term. Defects in socket matrix formation or cellular activity will lead to stalled healing. The review of residual ridge remodeling describes the long-term result of tooth extraction and formation of residual ridges, in which the quantity of bone tissue continuously decreases. This may suggest that any potential regulatory factors of residual ridge resorption should have an adverse effect either on the increased catabolic activity by osteoclasts or on the decreased anabolic activity by osteoblasts. Both short-term tooth extraction healing and long-term residual ridge remodeling processes are interdependent. Furthermore, any potential genetic and environmental regulatory factors can affect the quality and quantity of bone by altering the gene expression events taking place in bone cells.

RESULTS

The intent of this article was to review the current progresses of biologic research on residual ridge remodeling and to relate the changes at molecular, cellular, and tissue levels. The understanding of residual ridge remodeling may provide a sound scientific basis for improved restorative and therapeutic treatments of the edentulous population.

Human COL9A1 and COL9A2 genes. Two genes of 90 and 15 kb code for similar polypeptides of the same collagen molecule

Here we report the complete structure for the human COL9A1 and the complete sequence for the human COL9A2 genes. The COL9A1 gene is about 90 kb and consists of 38 exons. The COL9A2 gene is only about 15 kb, and it contains 32 exons. Sequence analysis of the promoter regions for the human COL9A2, the mouse Col9a2 and the human COL2A1 genes identified a conserved 14 bp sequence. The data also indicated that the alternative exon 1* found in intron 6 of the COL9A1 gene is separated from exon 7 only by a short intron in the chick, human, mouse and rat genes probably explaining why transcripts from exon 1* are spliced directly to exon 8.

Developmental regulation of mRNA species for types II, IX and XI collagens during mouse embryogenesis

Several techniques were used to study the co-ordination of mRNA levels for five constituent chains of cartilage collagen fibrils during mouse development. Short cDNA clones were first constructed for mouse and human alpha3(IX) and for mouse proalpha1(XI) collagen mRNA species. Northern analysis of developing mouse embryos revealed that the mRNA species for alpha1, alpha2 and alpha3 chains of type IX collagen peaked earlier than those for proalpha1(II) and proalpha1(XI) collagen chains. Quantification of these mRNA species by slot-blot hybridization confirmed this developmental regulation: the mRNA ratios for type II/type IX/type XI collagens changed from 5.7:1:0.6 (at embryonic day 12.5) to 10.6:1:0.9 (in newborn mice). However, the genes coding for the three chains of type IX collagen seemed to be under more co-ordinated regulation during mouse development. In addition to high mRNA levels in cartilages and the eye, low levels of type IX collagen transcripts were identified in brain and skin of newborn mouse using RNase protection and reverse transcriptase-PCR assays. Finally, hybridization in situ revealed identical tissue distributions of the three type IX collagen mRNA species during early chondrogenesis but somewhat more widespread expression of the alpha1(IX) and alpha3(IX) mRNA species during endochondral ossification at day 16.5 of embryonic development. These results suggest a relatively tight co-ordination of the alpha1(IX), alpha2(IX), and alpha3(IX) collagen mRNA species in chondrocytes, but a lack of co-ordination in several non-cartilaginous tissues.

Effect of ovariectomy on the local residual ridge remodeling

STATEMENT OF PROBLEM

Osteoporosis and edentulism are two disease processes that affect a large group of elderly people in the United States (24 and 25 million, respectively). These two diseases are independent of each other; however, they have several pathologic symptoms in common, such as reduction in bone mass.

PURPOSE

The purpose of this study was to determine whether estrogen deficiency or its replacement therapy have any effect on the phenomenon of residual ridge remodeling.

MATERIAL AND METHODS

Three animal groups were formed that consisted of six female Sprague-Dawley rats each. The two groups had ovariectomy and received either a vehicle solution or a daily dose (1.5 micrograms/day) of 17 beta-estradiol delivered through osmotic pumps. The control group underwent sham surgery and received a vehicle solution. Animals were pair fed throughout the experiment. Unilateral molar extraction was performed in the maxilla, which produced a suitable site for examination of histologic characteristics and molecular biologic analyses. At the 4-week postextraction period the bone remodeling activity was noted at the surface of the residual ridge in the control group.

RESULTS

The ovariectomized group showed increased bone resorption activity, whereas the surface of the residual ridge alveolar bone of the ovariectomized and estrogen-treated group was covered by a layer of hyaline tissue. Poly(A)+ ribonucleic acid samples were isolated from the remodeling residual ridge tissues. Expression of alpha 2(I), alpha 1(II), alpha 1(IX), and alpha 2(X) collagens were examined by ribonucleic acid transfer dot blots. Compared with the control group, ovariectomized animals showed a reduction in bone formation with decreased expressions of type I and II collagens. In contrast, the estrogen-treatment group showed decreased formation of type I collagen with a much increased expression of type II collagen. Further examination of type II collagen formation on the ovariectomized and estrogen-treated group by means of in situ hybridization revealed the notable labeling by the type IIA collagen probe, which was associated with the surface tissue of the residual ridge alveolar bone.

CONCLUSION

These findings suggest that estrogen deficiency and its replacement therapy seem to affect the activity of residual ridge bone remodeling at the molecular level.

Establishment of bone morphogenetic protein 2 responsive chondrogenic cell line

A clonal cell line named RMD-1 was established from the skeletal muscle of a 20-day fetal rat. RMD-1 represents a morphologically homogeneous population of undifferentiated mesenchymal cells, expressing alpha-smooth muscle actin and type I collagen, but no cartilage-associated genes. When cultured in agarose gel containing 100 ng/ml of recombinant human bone morphogenetic protein 2 (rhBMP-2; BMP-2), RMD-1 cells formed colonies and showed chondrocyte-like features as assessed by their ultrastructure, metachromatic staining with toluidine blue, and the production of large hydrodynamic-size proteoglycans. RMD-1 cells also differentiated into chondrocytes when the cells were plated at high density (over 2.5 x 10(5) cells/cm2) on type I collagen and incubated in medium containing 0.5% fetal bovine serum and 100 ng/ml of BMP-2. This chondrogenic differentiation was evidenced by a distinct morphological change into spherical cells, an increase in the levels of sulfated glycosaminoglycans, a decrease in type I collagen mRNA and the expression of cartilage-associated genes, including type II collagen, type IX collagen, aggrecan and alkaline phosphatase. In the presence of ascorbic acid and 10% serum, RMD-1 cells increased in size and expressed type X collagen as well as high alkaline phosphatase activity, then induced matrix mineralization. Thus, RMD-1 is a unique cell line that can differentiate from undifferentiated mesenchymal cells into hypertrophic chondrocytes.

Localization of pro-collagen type II mRNA and collagen type II in the healing tooth socket of the rat

Sprague-Dawley rats (50 days old) were anaesthetized and the maxillary right molars extracted. The rats were killed at 2, 3, 6, 8 and 10 days after extraction. The maxillae were dissected and prepared for either routine histology, in situ hybridization for pro-collagen type II mRNA, or immunohistochemical detection of collagen type II. Pro-collagen type II mRNA was expressed maximally in the healing tooth socket at 8 days after the extractions, but the protein was not expressed at any time. This suggests that the translation of pro-collagen type II mRNA does not occur in osteoblasts following tooth extraction. Ossification was present in the socket at 6 days after the extractions, which is consistent with the suggestion that an early feature of osteoblastic differentiation may be the expression of type II pro-collagen mRNA.

Animal model for evaluating the effect of systemic estrogen deficiency on residual ridge resorption

The rate of resorption of residual ridges after tooth extraction varies among subjects. Despite many human studies, the etiologic factors of severe forms of resorption of residual ridges are unknown and probably multifactorial. To facilitate investigations of resorption of residual ridges, it is preferable to use an animal model with a dentoalveolar component similar to that of human beings. This study developed a rapid, nonhuman primate model in which one could measure resorption of residual ridges at the most critical period, with the least amount of time, minimal operation, and the least interference with normal nutrition. Three female Macaca fascicularis monkeys, 7 to 9 years old, were selected for simple extraction of four maxillary incisors, four mandibular incisors, and two mandibular canines. Lateral cephalographs were taken at weeks 0, 1, 2 to 3, 5 to 6, 8 to 9, 11 to 12, and 26 to 28 after extraction with a newly constructed cephalostat that allowed easy reproducibility of animal placement in the prone position. Height and area measurements were made of maxillary and mandibular anterior residual ridges in the superimposed cephalographs. Sequential mean bone loss data produced bone loss curves similar to those observed in humans. One animal underwent ovariectomy. The plasma levels of both estrogen and progesterone decreased over 25 days and stayed below the detection limits thereafter. Tooth extraction was done 4 weeks after the ovariectomy and the bone resorption pattern was able to be observed longitudinally with the newly established monkey cephalostat. The results indicated that Macaca fascicularis need not be sacrificed when used in longitudinal studies of resorption of residual ridges in a relatively short period of experimental time. This model can provide an excellent opportunity to investigate a causal relationship between the rate of resorption of residual ridges and predisposing systemic factors such as estrogen deficiency.

Chondrogenic potential of chick embryonic calvaria: I. Low calcium permits cartilage differentiation

Calvaria from day-14 calcium-deficient chick embryos produced by long-term maintenance in shell-less culture, exhibit a cartilage-like phenotype (Jacenko and Tuan [1986] Dev. Biol. 115:215-232), which is restored to an osteogenic phenotype upon calcium repletion to the embryo. The expression of cartilage markers in a typically osteogenic tissue under calcium deficiency implies the presence of chondrogenic cells, and questions the conditions associated with calcium deficiency which may cause their divergent pathway of differentiation. In the present study, by explanting normal and shell-less embryonic calvarial pairs in organ culture in vitro and experimentally regulating their calcium supply, the calcium status of the calvaria was modulated as a function of medium calcium. Histological and immunoblotting analyses demonstrated for the first time that calvaria possess cells which can form genuine cartilage. This chondrogenic potential is expressed only in a low-calcium environment, where cartilage forms in both normal and shell-less calvarial pairs; their calcium-supplemented counterparts, however, develop as fully osteogenic tissues. Furthermore, chondrogenesis in both normal and shell-less calvaria indicates that the chondrogenic cells must be endogenous constituents of the calvaria, rather than being derived elsewhere in response to systemic calcium deficiency. Finally, the correlation between matrix under-calcification and cartilage expression in the embryonic calvarium suggests that calcium, perhaps in the form of matrix mineral, may modulate cell differentiation during skeletogenesis.

Altered bone remodeling pattern of the residual ridge in ovariectomized rats

This study investigated the effect of estrogen deficiency on residual ridge remodeling after tooth extraction. Ovariectomy was performed on female Sprague-Dawley rats, and the plasma levels of estrogen and progesterone were monitored. The maxillary molars of the ovariectomized and control rats were extracted and the remodeling residual ridge tissues were harvested at 2, 4, and 8 weeks postextraction. The specimens were examined at the mesiodistal center point of the residual ridge by use of light and scanning electron microscopy. The surface of the residual ridge of the control animals showed a number of osteoclastic lacunae indicating bone resorption activity. In the ovariectomized animals, the surface of the residual alveolar bone was partially covered by a distinct calcified tissue. This tissue contained large cells and a mesh-like structure of thin calcified extracellular matrix consistent with the tissue characteristics of chondroid bone. The chondroid bone-like calcified tissue was found only in the ovariectomized animals throughout the experiment period. This study's data suggest that a systemic condition such as estrogen deficiency due to ovariectomy may alter the phenotypic expression of cells associated with the residual ridge and result in less osteoclastic activity and a different type of calcified tissue.