Radioautographic study of [3H]mannose utilization during cementoblast differentiation, formation of acellular cementum, and development of periodontal ligament principal fibers

The Roles of SIBLING Proteins in Dental, Periodontal and Craniofacial Development

The majority of dental, periodontal, and craniofacial tissues are derived from the neural crest cells and ectoderm. Neural crest stem cells are pluripotent, capable of differentiating into a variety of cells. These cells can include osteoblasts, odontoblasts, cementoblasts, chondroblasts, and fibroblasts which are responsible for forming some of the tissues of the oral and craniofacial complex. The hard tissue forming cells deposit a matrix composed of collagen and non-collagenous proteins (NCPs) that later undergoes mineralization. The NCPs play a role in the mineralization of collagen. One such category of NCPs is the small integrin-binding ligand, N-linked glycoprotein (SIBLING) family of proteins. This family is composed of dentin sialophosphosprotein (DSPP), osteopontin (OPN), dentin matrix protein 1 (DMP1), bone sialoprotein (BSP), and matrix extracellular phosphoglycoprotein (MEPE). The SIBLING family is known to have regulatory effects in the mineralization process of collagen fibers and the maturation of hydroxyapatite crystals. It is well established that SIBLING proteins have critical roles in tooth development. Recent literature has described the expression and role of SIBLING proteins in other areas of the oral and craniofacial complex as well. The objective of the present literature review is to summarize and discuss the different roles the SIBLING proteins play in the development of dental, periodontal, and craniofacial tissues.

Tooth Tissue Engineering: Potential and Pitfalls

All over the world a large number of people suffer from tooth diseases like dental caries, tooth abscess, and plaques. Tooth loss or damage, which occurs frequently in our society are generally repaired by applying several conventional methods, such as root-canal treatment, direct pulp capping and dental implants. These methods are quite painful, create damage to the surrounding tooth tissues and also may at times have adverse side-effects. The limitations of the conventional methods can be overcome by applying the concept of tooth tissue engineering. Tooth tissue engineering is the application of biosciences and engineering to regenerate a biofunctional tooth, which can be used to replace the missing tooth or repair the damaged tooth. Tissue engineering involves three key elements - cell, scaffold and growth factors, which interact with each other to regenerate a specific tissue. The success of tissue engineering depends on the proper selection of these three key elements and understanding the interactions among them. To bring us close to the realization of a tissue-engineered tooth, immense progress is going on in understanding how tooth is first developed, and there is a good advancement in tooth regeneration. In this review, “tooth tissue engineering” will be discussed, along with the recent advancements and challenges in bring a biofunctional tooth from laboratory out into clinical use.

The KK-Periome database for transcripts of periodontal ligament development

The periodontal ligament (PDL) is a strong connective tissue that surrounds the tooth root, absorbs occlusal forces, and functions as a sense organ. PDL originated from dental follicle (DF), which possessed mesenchymal progenitors in the developing tooth germ. However, as specific marker genes for PDL and DF are currently unavailable, the molecular mechanisms of PDL development are yet to be clarified. To facilitate the identification of such genes, we have previously established a transcriptome database of the human PDL (the KK-Periome database) and screened for specific genes expressed during PDL development. Initial screening of the database revealed two marker genes for distinguishing DF and PDL. The KK-Periome database thus appears to offer a useful resource for investigating genes involved in PDL development.

Cellular and extracellular factors in early root resorption repair in the rat

The aim of this study was to investigate the role of extracellular matrix components, such as collagen type I, fibronectin, and osteopontin (OPN) during cementum repair following experimentally induced tooth movement, and to characterize the cells taking part in the regenerative process. The upper right first molars were moved mesially in 21 three-month-old male Wistar rats using a coil spring with a force of 0.5 N. After 9 days, the appliance was removed and the animals were killed in groups of three immediately after withdrawal of the force and 5, 7, 10, 12, 14, and 17 days later. Three rats served as non-experimental control animals. The maxillae were prepared and processed for histological analysis. Together with the disappearance of the multinucleated odontoclasts from the resorption lacunae, signs of repair were visible 5 days after the release of the orthodontic force. The first signs of cementum repair were seen on day 10. The newly produced cementum was of the acellular extrinsic fibre type (AEFC) and reattachment was achieved with the principal periodontal ligament (PDL) fibres orientated almost perpendicular to the root surface. The initial interface formed between the old and new cementum, as well as the new AEFC, was characterized by a strong immunoreaction with OPN and collagen I antibody, but only a weak immunoreaction with the fibronectin antibody. Only a small number of mononuclear cells, which were involved in the repair process, showed a positive immunoreaction with the osteoblastic lineage markers runt-related transcription factor 2 and osteocalcin. These same cells stained sparsely with muscle segment homeobox homologue 2, but not with the E11 antibody. Thus, most of the cells associated with this reparative activity on the surface of the lacunae were differentiated PDL cells of the fibroblastic phenotype. Cells with a defined osteoblastic phenotype seemed to be of minor importance in this repair process.

Extracellular matrix expression and periodontal wound-healing dynamics following guided tissue regeneration therapy in canine furcation defects

AIM

Temporal and spatial expression pattern of extracellular matrix (ECM) components in furcation defects following guided tissue regeneration (GTR) compared with open-flap debridement (OFD).

MATERIAL AND METHODS

In 21 dogs, mandibular second and fourth pre-molars were treated with one non-resorbable and three different resorbable membranes. Third pre-molars were treated by OFD. After 2, 4, 8 weeks and 3, 6, and 12 months, tissues were analysed by immunohistochemistry for collagen I (Col-I) and III (Col-III), fibronectin (FN), bone sialoprotein (BSP), and osteopontin (OPN).

RESULTS

At 2 weeks, the defect was mainly occupied by FN+ granulation tissue (GT), which was sequentially replaced by new connective tissue expressing FN, Col-I, and increasingly Col-III. Following superficial resorptions by OPN+ osteoclasts and odontoclasts, cementum and bone formation ensued with strong expression of BSP and OPN along bone and tooth surfaces. Deposition of Col-I, FN, BSP and OPN+ cementoid and osteoid became evident after 4 weeks. Extrinsic fibres of cementum and bone stained intensely for Col-III. The newly formed periodontal ligament expressed FN, Col-I, and Col-III, but no BSP or OPN.

CONCLUSIONS

The spatial ECM expression was similar for OFD and the different GTR methods, although the timing and quantity of ECM expression were influenced by wound stabilization and inflammatory reactions.

Location of putative stem cells in human periodontal ligament

BACKGROUND AND OBJECTIVE

The origin of cells in the mature periodontium, and the location of their progenitors, are still unknown. It is also unknown whether inflammation influences the number and distribution of these cells within the periodontium. Molecules such as STRO-1, CD146 and CD44 have been identified on a variety of mesenchymal stem cells. The aim of this study was to identify and localize putative stem cells in diseased and healthy human periodontal ligament using cell-surface markers for mesenchymal stem cells.

MATERIAL AND METHODS

Healthy and periodontitis-affected teeth were collected, fixed in 10% neutral-buffered formalin, decalcified and embedded in paraffin in preparation for immunohistochemistry. Antibodies against STRO-1, CD146 and CD44 were used to identify putative stem cells in the periodontal ligament.

RESULTS

Putative stem cells were identified in both healthy and diseased periodontal ligament. They were mainly located in the paravascular region and small clusters of cells were also found in the extravascular region. Wider distributions of these cells were detected in sections of diseased ligament.

CONCLUSION

Within the periodontal ligament of both healthy and diseased teeth, cells have been identified consistent with their identification as putative stem cells. The presence of an inflammatory reaction associated with periodontitis may enhance the number of these cells.

Mineralization process during acellular cementogenesis in rat molars: a histochemical and immunohistochemical study using fresh-frozen sections

This study was designed to detect tissue non-specific alkaline phosphatase (TNSALP) by Azo-dye staining, calcium by glyoxal bis (2-hydroxyanil) (GBHA) staining, bone sialoprotein (BSP) and osteopontin (OPN) by immunoperoxidase staining in developing rat molars, and also to discuss the mineralization process during acellular cementogenesis. To restrain a reduction in histochemical and immunohistochemical reactions, fresh-frozen undemineralized sections were prepared. Where the epithelial sheath was intact, TNSALP reaction was observed in the dental follicle, but not in the epithelial sheath. With the onset of dentin mineralization, the BSP- and OPN-immunoreactive, initial cementum layer appeared. At this point, cementoblasts had shown intense TNSALP reaction and GBHA reactive particles (=calcium-GBHA complex) appeared on the root surface. With further development, the reaction of TNSALP and GBHA became weak on the root surface. Previous studies have shown that the initial cementum is fibril-poor and that matrix vesicles and calciferous spherules appear on the root surface only during the initial cementogenesis. The findings mentioned above suggest that: during the initial cementogenesis, cementoblasts release matrix vesicles which result in calciferous spherules, corresponding to the GBHA reactive particles. The calciferous spherules trigger the mineralization of the initial cementum. After principal fiber attachment, mineralization advances along collagen fibrils without matrix vesicles.

Stem cells in the periodontal ligament

The ability to identify and manipulate stem cells has been a significant advancement in regenerative medicine and has contributed to the development of tissue engineering-based clinical therapies. Difficulties associated with achieving predictable periodontal regeneration, means that novel techniques such as tissue engineering need to be developed in order to regenerate the extensive soft and hard tissue destruction that results from periodontitis. One of the critical requirements for a tissue engineering approach is the delivery of ex vivo expanded progenitor populations or the mobilization of endogenous progenitor cells capable of proliferating and differentiating into the required tissues. By definition, stem cells fulfill these requirements and the recent identification of stem cells within the periodontal ligament represents a significant development in the progress toward predictable periodontal regeneration. In order to explore the importance of stem cells in periodontal wound healing and regeneration, this review will examine contemporary concepts in stem cell biology, the role of periodontal ligament progenitor cells in the regenerative process, recent developments in identifying periodontal stem cells and the clinical implications of these findings.

Cementum and periodontal wound healing and regeneration

The extracellular matrix (ECM) of cementum resembles other mineralized tissues in composition; however, its physiology is unique, and it contains molecules that have not been detected in other tissues. Cementum components influence the activities of periodontal cells, and they manifest selectivity toward some periodontal cell types over others. In light of emerging evidence that the ECM determines how cells respond to environmental stimuli, we hypothesize that the local environment of the cementum matrix plays a pivotal role in maintaining the homeostasis of cementum under healthy conditions. The structural integrity and biochemical composition of the cementum matrix are severely compromised in periodontal disease, and the provisional matrix generated during periodontal healing is different from that of cementum. We propose that, for new cementum and attachment formation during periodontal regeneration, the local environment must be conducive for the recruitment and function of cementum-forming cells, and that the wound matrix is favorable for repair rather than regeneration. How cementum components may regulate and participate in cementum regeneration, possible new regenerative therapies using these principles, and models of cementoblastic cells are discussed.

The initial attachment of cemental fibrils to the root dentin surface in acellular and cellular cementogenesis in rat molars

To elucidate the initial attachment mechanism of cemental fibrils to the root dentin surface in acellular and cellular cementogenesis, developing rat molars were observed by light microscopy and scanning electron microscopy combined with NaOH maceration. The NaOH maceration was used to observe details of the positional association of cemental and dentinal fibrils during cementogenesis. An initial hematoxylin stained, cementum layer began to form on the root dentin surface with the first dentin mineralization in both acellular and cellular cementogenesis. The initial attachment of cemental fibrils to the dentin surface also began at this point. At the initial attachment the intermingling of cemental and dentinal fibrils occurred only in places. With advanced cementogenesis the initial cementum layer became the fibril-poor cemento-dentinal junction. This suggests that cemental fibrils attach on the initial cementum layer, and not directly on dentinal fibrils, so that the layer results in the fibril-poor cemento-dentinal junction. The present study suggests that an intervening adhesive is necessary for the cemento-dentinal attachment at any stage of cementogenesis in rat molars.

Apoptosis in the early developing periodontium of rat molars

Development of the periodontium involves a series of complex steps that result in the formation of root dentine, cementum, bone and fibres of the ligament. These precisely controlled and timed events require the participation of the enamel organ derived epithelial cells of Hertwig's (HRS) and ectomesenchymal cells of the dental follicle. These events involve rapid turnover of the tissues and cells, including disappearance of epithelial cells of HRS. Thus, it seemed likely to us that programmed cell death (apoptosis) may play a role in the development of the periodontium. Fragments of first molars, obtained from 14- and 29-day-old rats, were fixed in glutaraldehyde-formaldehyde and processed for light and electron microscopy. For the TUNEL method for detection of apoptosis, specimens were fixed in 4% formaldehyde and embedded in paraffin. Results confirmed that epithelial cells of HRS maintain a close relationship with the forming dentine root, and that they may become trapped in the dentino-cemental junction. Some of the epithelial cells exhibited ultrastructural features which are consistent with the interpretation that they were undergoing programmed cell death, i.e. apoptosis. Periodontal fibroblast-like cells showed typical images of apoptosis and engulfed apoptotic bodies. TUNEL positive structures were present in all corresponding regions. It seems therefore that apoptosis of epithelial cells of HRS and fibroblast-like cells of the periodontal ligament constitutes an integral part of the developmental process of the tissues of the periodontium.

Expression of TGF-beta isoforms and their receptors during mineralized nodule formation by rat periodontal ligament cells in vitro

Transforming growth factor-betas (TGF-beta s) and bone morphogenetic proteins (BMPs), members of a TGF-beta superfamily, are known to play an important role in osteogenic cell differentiation and consequently bone formation. We have reported previously that periodontal ligament (PDL) cells differentiate and form mineralized nodules when cultured in the presence of dexamethasone (Dex), beta-glycerophosphate (GP) and ascorbic acid (AA). To understand the roles of TGF-beta isoforms (TGF-beta 1, 2 and 3) and TGF-beta type I receptors (activin receptor-like kinase (ALK)-2, -3, -5 and -6) in PDL cell differentiation, their expression was investigated using Northern blot analysis. Rat PDL cells, derived from coagulum in the tooth socket, were cultured in the presence of Dex (5 microM), GP (10 mM) and AA (50 micrograms/ml) for up to 21 d. Total RNA was isolated from PDL cells after 0, 7, 14 and 21 d and used for northern blot analysis of mRNAs for matrix proteins, TGF-beta isoforms and their receptors using 32P-labeled cDNAs as probes. Four stages showing distinct morphological characteristics and matrix expression during development of mineralized nodules were identified. Type I collagen (Col I) and SPARC (secreted protein, acidic and rich in cysteine) mRNAs were expressed at the confluent stage, but decreased during the mineralization stage. Osteopontin (OPN) and alkaline phosphatase (ALP) transcripts were initially observed at multilayer stage, while bone sialoprotein (BSP) and osteocalcin (OC) at the nodule stage and all 4 were expressed thereafter. TGF-beta 1 mRNA expression increased with the progression of PDL cell differentiation, while a relatively high level of TGF-beta 3 transcript decreased slightly during their differentiation. TGF-beta 2 mRNA was not expressed. The expression of TGF beta-RI mRNA decreased, whereas that of TGF beta-RIII increased dramatically with PDL cell differentiation. TGF beta-RII gene activities remained high throughout all stages. ALK-2, ALK-3 and ALK-6 mRNA expression increased with the progression of PDL cell differentiation, suggesting that these receptors may play important roles in Dex-induced PDL cell differentiation and mineralized nodule formation.

Cyclosporin A-induced gingival overgrowth in the rat: a histological, ultrastructural and histomorphometric evaluation

This investigation was undertaken to further study cyclosporin A (CsA)-induced gingival overgrowth. Thirty mg/kg/d of vehicle or CsA solutions were given orally to 6-wk-old male Sprague-Dawley rats. After 4, 9, 14 and 19 wk 2 control and 2 experimental rats were anaesthetized, tissues fixed by intracardiac perfusion of fixative solution and jaws processed for Epon inclusion. Histological and ultrastructural studies conducted in a gingival portion (free gingiva) revealed the presence of hyalinization areas and of multinucleated cells (MCs) containing collagen fibrils (connective tissue), of amorphous areas and disorders of keratinization (epithelia). Histomorphometric evaluation indicated that in the CsA rats the mean cross-sectional area of the free gingiva was 2.52-fold increased compared to the controls. The connective tissue comprised 41.43% of this area (instead of 31.49% in controls). Additional histomorphometric evaluation was performed in 3 groups of free gingival portions: control (C group), CsA-non-respondent (CsA-nR) and CsA-respondent (CsA-R). The cross-sectional gingival areas studied were slightly lower than the mean area of all the control sites previously defined (groups C and CsA-nR) or showed the higher degrees of enlargement (CsA-R). In the CsA-R group the mean cross-sectioned area of the vessel profiles was increased and the number of fibroblast profiles decreased. In the CsA-nR group the number of vessel profiles and that of MCs profiles were increased. In the epithelia of the CsA-R group were increased (a) keratinized epithelia: thickness; thickness of the inner and of the outer compartments; surface area of spinous cell profiles; (b) oral gingival epithelium: number of cell layers (inner compartment); (c) oral sulcular epithelium: surface area of granular cell profiles; (d) junctional epithelium: thickness; number of cell layers. These results indicate that (a) the CsA induced modifications are not limited to enlarged gingiva (b) the overgrowth of the GCT is the result of a vasodilatation and of an increase in the volume of the extracellular matrix and (c) the increase of the epithelial thickness is mainly the result of a cell hypertrophy in the keratinized epithelia and of a cell hyperplasia in the junctional epithelium.

Immunolocalization of glycosaminoglycans in ageing, healthy and periodontally diseased human cementum

The distribution of glycosaminoglycans in the extracellular matrix of human cementum was investigated in periodontally involved and periodontal disease-free teeth separated into eight different age groups (from 12 to 90 years), to investigate possible changes in the distribution of glycosaminoglycan species associated with ageing and periodontal disease. A standard indirect immunoperoxidase technique was used, with a panel of monoclonal antibodies, 2B6, 3B3, 5D4, and 7D4, that recognize epitopes in chondroitin-4-sulphate/dermatan sulphate (C-4S/DS), chondroitin-6-sulphate (C-6S), keratan sulphate (KS) and a novel sulphated chondroitin sulphate (CS) epitope, respectively. Intense positive staining for C4-S/DS was observed at the margins and lumina of almost all the lacunae and canaliculi in cellular cementum in all sections. Immunoreactivity to C6-S, KS and novel CS epitopes was limited to a proportion of lacunae and canaliculi in all sections, although C6-S and the novel CS epitopes were more widely distributed than KS. In acellular cementum, there was no demonstrable staining for any of the glycosaminoglycans except where periodontal ligament (Sharpey's) fibres insert; periodontal ligament fibres inserting in cellular cementum also demonstrated positive immunoreactivity. In addition, the cementoblasts on the outer root surface, as well as the pericellular areas around a proportion of these cells, demonstrated positive immunoreactivity. These results indicate that glycosaminoglycan species present in human cementum include C4-S, DS, C6-S, and novel sulphated CS epitopes. KS is also present in cementum but is limited to a more restricted proportion of lacunae and canaliculi. Regional differences in the distribution of glycosaminoglycans exist between the two cementum types, but no qualitative differences in that distribution were observed between the various age groups or between periodontally involved and periodontal disease-free teeth. The immunoreactivity observed in a proportion of lacunae after staining for C6-S, KS, and novel sulphated CS epitopes could suggest the existence of different cementocyte subpopulations.

Developmental appearance and distribution of bone sialoprotein and osteopontin in human and rat cementum

BACKGROUND

Bone sialoprotein (BSP) and osteopontin (OPN), two major noncollagenous proteins (NCPs) in collagen-based mineralized tissues, have been implicated in mineral deposition and cell- and matrix-matrix interactions during root development. However, their role in cementogenesis is still a subject of debate. Since distribution of proteins is indicative of function, we have analyzed their temporo-spatial appearance in relation to that of cementum collagen.

METHODS

Human premolars and rat molars at various stages of root development characterized by differing rates of formation were fixed in aldehyde and embedded in epoxy and LR White resin. Sections were processed for ultrastructural analysis and postembedding colloidal gold (immuno)cytochemistry.

RESULTS

Incubations with antibodies against BSP and OPN and with lectins recognizing prominent sugars in these proteins generally revealed similar labeling patterns in both human and rat teeth, with gold particles accumulating mainly in the interfibrillar spaces. The lectin Helix pomatia, specific for N-acetyl-D-galactosamine, was distinctive in that it consistently reacted with human cementum, but only sporadically labeled rat cementum. Regardless of both the species and the stage of root development, mineralization initiated in mantle predentin in association with distinct foci immunoreactive for BSP and OPN. In human teeth, the deposition of cementum collagen began before the start of dentin mineralization and thus prior to any detectable labeling for BSP and OPN. However, at early stages of root formation in the rat, cementum collagen appeared after BSP and OPN accumulated on the root surface, whereas at advanced stages the deposition of cementum collagen, BSP and OPN coincided.

CONCLUSIONS

The temporo-spatial differences in the appearance of BSP and OPN relative to cementum collagen correlate well with known differences in the speed of root elongation and explain the variable appearance of the dentino-cemental junction. The data reveal no causal relationship between BSP and OPN and the differentiation of cementoprogenitor cells and indicate that the distribution of collagen fibrils ultimately determines the amount and pattern of accumulation of these NCPs. There also is no consistent planar accumulation of BSP and OPN between dentin and cementum such as the cement lines found between "old" and "new" bone. It is concluded that the interlacement of collagen fibrils at the dentino-cemental junction, across which mineralization spreads, represents the primary attachment mechanism between cementum and dentin.

Recombinant human bone morphogenetic protein-2 promotes wound healing in rat periodontal fenestration defects

Although there is considerable interest in the use of bone morphogenetic protein (BMP) to promote periodontal regeneration, little is known of its effects on the early stages of wound healing. The aim of this study was to investigate the effects of recombinant human bone morphogenetic protein 2 (rhBMP-2) on an early stage of post-operative wound healing and following complete healing (10 and 38 days, respectively) in a rat model of periodontal regeneration. The buccal aspects of molar roots were carefully denuded of their periodontal ligament through a bony window created in the mandibles of Wistar rats under general anesthesia. After the root surfaces were acid-conditioned, a 10-microL quantity of 50 microg/mL rhBMP-2 in a collagen gel solution was placed into the surgically created defect in test animals; in controls, either a 10-microL quantity of only collagen gel was received, or the defect was untreated. Animals were killed 10 days or 38 days after surgery and the tissues processed for histological examination. Transverse 5-microm sections were stained for the identification of new bone, cementum, and collagen fiber formation. In the 10-day study groups, new bone formation over the second molar and beyond the defect was significantly increased in the test group (p < 0.02), although there was no evidence of increased ankylosis. RhBMP-2 stimulated more than twice the area of cementum growth coronally compared with controls (712 +/- 286 microm2 and 258 +/- 57 microm2, respectively). Connective tissue attachment, including the number and width of collagen bundles, was similar in both test and controls. Complete healing without any evidence of ankylosis had occurred in all animals 38 days post-operatively, and no significant differences were observed between test and control groups. In conclusion, a single dose of rhBMP-2 increased the rate of normal intramembranous bone formation and selectively enhanced cementum formation coronally during early wound healing. However, the finding that rhBMP-2 induced bone formation at some distance from the defect suggests the importance of developing a suitable delivery system to maintain the concentration of BMP-2 at the site of implantation for potential therapeutic use.

New cementum formation induced by cyclosporin A: a histological, ultrastructural and histomorphometric study in the rat

Cyclosporin A (CsA), a widely used immunosuppressive agent, is known to induce gingival overgrowth; 30 mg/kg/d of CsA were administrated orally in young and adult male Sprague-Dawley rats. The same number of rats received oil-based vehicle solution. After 4, 9, 14 and 19 wk of CsA or vehicle administration 3 control and 3 experimental rats were anaesthetized and tissues fixed by an intracardiac perfusion of fixative solution. Upper and lower jaws were dissected, demineralized and processed for Epon inclusion. Histological examination revealed the presence of large amounts of new cementum (NC) covering extensive areas of the acellular extrinsic fibre cementum (AEFC) in all the root surfaces. NC was particularly abundant at the cervical third of the roots facing the gingival connective tissue, where it occurred as layers, spurs or in both configurations. NC was characterized by its irregular outline, globular body content and infrequent presence of incremental lines. Histomorphometric evaluation by semi-automatic image analysis indicated that the volume and the external surface of NC spurs were 2.86-6.49 and 1.29-1.97-fold increased comparative to those of the AEFC covering the same root areas. Electron microscopy revealed that NC was a functional tissue with insertion of collagen fibres perpendicularly to the long axis of the root. It can be concluded that under some experimental conditions formation of abundant amounts of NC can be achieved and that these results must be taken into account for a new approach in the treatment of periodontal disease.

Cells with osteoblastic phenotypes can be explanted from human gingiva and periodontal ligament

Considerable phenotypic heterogeneity has been reported in gingival fibroblasts. Similarly, cells from the periodontal ligament (PDL) can be isolated with different phenotypes. Although it has been suggested that cells from the gingiva do not contribute to the formation of hard tissue, it is theoretically possible that under appropriate stimuli, immature mesenchymal cells in gingiva could differentiate along an osteoblastic pathway. Differentiation of immature mesenchymal cells into osteoblasts following stimulation with osteoinductive factors has been demonstrated in muscle. We undertook experiments to establish whether cells with osteoblastic characteristics could be identified from human gingiva as well as from human periodontal ligament. Some cell populations from each of these tissues were found to have high basal alkaline phosphatase activity, to release osteocalcin in response to 1,25(OH)2 VitD3, and to form a mineralized matrix. Thus, cells can be isolated from the gingiva and PDL that exhibit phenotypic markers, which taken together are characteristic of osteoblastic cells. Other cell populations derived from the PDL and gingival connective tissue were isolated that had fibroblastic characteristics. These studies support the concept that gingival tissue can give rise to cells which may differentiate along either a fibroblastic or an osteoblastic pathway.

Is fibroblast heterogeneity relevant to the health, diseases, and treatments of periodontal tissues?

There are wide variations of gene expression and strikingly different responses to extracellular signals among different fibroblast populations. This has prompted a large number of in vitro studies which suggest that fibroblasts are not homogeneous but instead comprise multiple subpopulations with extensive site-to-site and intra-site variations. Conceivably, either fibroblasts are not all created equal, or, alternatively, discrete subpopulations may emerge in development, inflammatory lesions, or wound healing. While the heterogeneous nature of cultured fibroblasts has been known for some time, are these variations relevant to our understanding of the biology of oral tissues, their involvement in disease, and their response to therapy? Since fibroblasts are the predominant cell type in soft connective tissue matrices, the regulation of their proliferative, synthetic, and degradative behavior is likely to be important in tissue physiology and pathology. In this review, we use the current literature to assess whether fibroblast subpopulations really make a difference in the health and disease of periodontal tissues. We address the following questions: (1) Is fibroblast heterogeneity a real in vivo phenomenon? (2) How can we advance our knowledge of phenotypic variations and the regulation of fibroblast differentiation? (3) Could a knowledge of fibroblast heterogeneity have an impact on the development of new approaches to pathogenesis and the treatment of periodontal tissues?

Extracellular matrix in tooth cementum and mantle dentin: localization of osteopontin and other noncollagenous proteins, plasma proteins, and glycoconjugates by electron microscopy

BACKGROUND

Noncollagenous proteins (NCPs) are considered to have multiple functions related to the formation, turnover, and repair of the collagen-based mineralized tissues. Collectively, they comprise a class of generally acidic, mineral-binding proteins showing extensive posttranslational modifications, including glycosylation, phosphorylation, and sulfation. METHODS. We have used colloidal-gold immunocytochemistry and lectin-gold cytochemistry, together with transmission electron microscopy, to examine the organic matrix composition of tooth cementum and the subjacent mantle dentin in rodent molar teeth. Molars were processed for immunocytochemistry using antibodies against osteopontin (OPN), osteocalcin (OC), bone sialoprotein (BSP), bone acidic glycoprotein-75 (BAG-75), albumin (ALB), and alpha 2HS-glycoprotein (alpha 2HS-GP), or for glycoconjugate cytochemistry using lectin-gold complexes.

RESULTS

Ultrastructurally, at the advancing root edge in developing molars, OPN and BSP initially were associated with small calcification foci in the mantle dentin. With progressing mineralization, OC and alpha 2HS-GP appeared diffusely distributed throughout the calcified mantle dentin, and diminished as a gradient toward the circumpulpal dentin. Immediately following disruption of Hertwig's epithelial root sheath, cementum deposition commenced at the root surface occasionally with the appearance of a cement line rich in OPN. Cementum matrix proper contained abundant OPN, BSP, OC, and alpha 2HS-GP, but no or little BAG-75 or ALB. Protein immunolabeling, as well as lectin labeling for beta-D-galactose and N-acetyl-neuraminic acid and/or N-glycolyl-neuraminic acid, both being prominent sugars of certain NCPs, was primarily concentrated between, and at the surface of, collagen fibrils in acellular extrinsic fiber cementum. OPN, BSP, OC, and alpha 2HS-GP were also prominent components of cellular cementum and of Sharpey's fibers. In cellular cementum, laminae limitantes sometimes present delimiting cementocyte lacunae and cell process-containing canaliculi were also rich in OPN. Along the root surface, occasional cementoblasts exhibited intracellular labeling for OPN over the Golgi apparatus and secretory granules.

CONCLUSIONS

We have identified OPN, BSP, OC, and alpha 2HS-GP as being prominent organic constituents of both mantle dentin and acellular and cellular cementum, and, have elucidated the details of their distribution at the ultrastructural level. The temporal appearance and spatial distribution of these organic moieties in the teeth root are similar to those seen during bone formation and are consistent with proposals that certain NCPs may be involved in regulating calcification and/or participating in cell-matrix and matrix-matrix/mineral adhesion events.

Periodontal ligament cell population: the central role of fibroblasts in creating a unique tissue

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament (PL) and have important roles in the development, function, and regeneration of the tooth support apparatus. Biological processes initiated during the formation of the PL contribute to the long-lasting homeostasic properties exhibited by PL fibroblast populations.

DEVELOPMENT

The formation of the PL is likely controlled by epithelial-mesenchymal and epithelial hard tissue interactions, but the actual mechanisms that contribute to the development of cellular lineages in the PL are unknown. Fibroblasts in the normally functioning PL migrate through the tissue along collagen fibres to cementum and bone and in an apico-coronal direction during tooth eruption. ADULT TISSUE: Cell kinetic experiments have shown that PL fibroblasts comprise a renewal cell system in steady-state and the progenitors can generate multiple types of more differentiated, specialized cells. Progenitor cell populations of the PL are enriched in locations adjacent to blood vessels and in contiguous endosteal spaces. In normally functioning periodontal tissues, there is a relatively modest turnover of cells in which apoptotic cell death balances proliferation. Large increases of cell formation and cell differentiation occur after application of orthodontic forces or wounding. As PL cells comprise multiple cellular phenotypes, it has been postulated that after wounding, the separate phenotypes repopulating the site will ultimately dictate the tissue form and type.

CONCLUSIONS

PL fibroblasts play an essential role in responses to mechanical force loading of the tooth by remodelling and repairing effete or damaged matrix components. In consideration of the important roles played by fibroblasts in PL homeostasis, they could be described as "the architect, builder, and caretaker" of the periodontal ligament.

Cementogenesis reviewed: a comparison between human premolars and rodent molars

BACKGROUND

Cementum continues to be the least-known mineralized tissue. Although recent advances in the field of molecular biology have contributed to an understanding of the involvement of molecular factors in cementum formation during development and regeneration, cementogenesis on a cell biological basis is still poorly understood. Virtually nothing is known about cementoblast origin, differentiation, and the cell dynamics during normal development, repair, and regeneration. This review describes the recent findings of cementogenesis on roots of human premolars and opposes them to those of teeth from other mammals, particularly the rodent molar.

METHODS

Using light and electron microscopy, light microscopic radioautography, and various measurements, a comprehensive insight into the development and repair of cementum during and after root formation and tooth eruption has been achieved for human premolars.

RESULTS

Cementum is a highly responsive mineralized tissue. This biological activity is necessary for root integrity and for bringing and maintaining the tooth in its proper position. With regard to cementum formation and periodontal fiber attachment, considerable species-particularities exist that are mainly based on differences in growth rates and tooth sizes. Since root development and initial cementogenesis last on the average 5-7 years in human premolars, cementum formation in these teeth is characterized by along-lasting phase of prefunctional development, with occurs independent of principal periodontal fiber attachment to the root and which may take 5 years or more. The first molar of the rat, however, is in functional occlusion 3 1/2 weeks after the onset of root formation. Since initial cementum formation and periodontal fiber attachment to the root occur almost at the same time in this tooth, the distinction between cells associated with one or the other process is very difficult to achieve, and cementogenesis cannot be described independent of periodontal fiber attachment to the root. Therefore, the determination of cementoblast origin in the rodent molar may be intricate.

CONCLUSIONS

Taking into account these species differences, the current description on the origin and differentiation of cementoblasts is inconsistent and the description of cementogenesis is still incomplete. This review calls into question the currently held concept of cementogenesis and offers a possible alternative.

Implications of cellular and molecular biology advances in periodontal regeneration

BACKGROUND

The mechanisms by which new periodontium is established on root surfaces previously exposed to periodontal disease has been an area of active research interest for the past decade.

METHODS

Recently, histological examination of periodontal regeneration has revealed a complex process orchestrated by temporo-spatial specific cell-matrix interactions.

RESULTS

Advances in cell and molecular biology techniques have provided invaluable tools to begin investigating the cascade of events occurring periodontal regeneration.

CONCLUSION

This report summarizes current understanding of the cellular and molecular aspects of periodontal regeneration and determines the clinical relevance of these findings.

The role of epithelium in the development, structure and function of the tissues of tooth support

The roles of epithelium in the development, structure and function of the tissues of tooth support are reviewed. Epithelium is involved in initiating odontogenesis which includes the tissues of tooth support and this role is discussed. Particular attention is paid to Hertwig's epithelial root sheath and its participation in the formation of the hyaline layer on the root surface as well as its possible role in initiating the differentiation of cementoblasts. The possible functions of the epithelial cell rests are reviewed and it is concluded that as yet no function can be ascribed to them. Evidence for an increasing role for dental epithelium in tooth eruption is presented and the role of dental epithelium in establishing the dentogingival junction is discussed, with the conclusion drawn that this role temporary.

Platelet-derived growth factor-modulated guided tissue regenerative therapy

The goal of this study was to develop an effective regenerative therapy capable of achieving periodontal regeneration of Class III furcation defects. We attempted to achieve this goal by combining three therapeutic approaches. First, the lesion was protected by an expanded polytetrafluoroethylene barrier membrane that prevents migration of gingival fibroblasts as well as osteogenic cells from the mucoperiosteal flaps. Second, platelet-derived growth factor-BB (PDGF-BB), which has potent chemotactic and mitogenic effects on periodontal ligament fibroblasts (PDL), was used to promote migration of fibroblasts and their proliferation on the root surface. Third, the root surface, demineralized by citric acid conditioning, was chosen as the primary site for PDGF-BB application. The demineralized root surface appeared to have the capability of providing a sustained release of the applied growth factor. This seemed to facilitate rapid repopulation of PDL fibroblasts on the root surface and new PDL formation in the early stages of repair, which contributed to complete periodontal regeneration without root resorption and ankylosis in later stages. Combining these approaches, we developed a therapy referred to as "PDGF-modulated guided tissue regenerative therapy." Unlike guided tissue regenerative therapy alone (without PDGF-BB), this therapy effectively promoted periodontal regeneration of Class III furcation defects in the beagle dog without significant ankylosis or root resorption.

Periodontal regeneration in class III furcation defects of beagle dogs using guided tissue regenerative therapy with platelet-derived growth factor

We developed an effective regenerative therapy, referred to as platelet-derived growth factor-BB (PDGF-BB)-modulated guided tissue regenerative (GTR) therapy (P-GTR), capable of achieving periodontal regeneration of horizontal (Class III) furcation defects in the beagle dog. To determine its efficacy, repair and regeneration of horizontal furcation defects by P-GTR therapy and GTR therapy were compared. Chronically inflamed horizontal furcation defects were created around the second (P2) and fourth mandibular premolars (P4). After demineralization of the root surfaces with citric acid, the surfaces of left P2 and P4 were treated with PDGF-BB (P-GTR therapy) and those of contralateral teeth were treated with vehicle only (GTR therapy). Periodontal membranes were placed and retained 0.5 mm above the cemento-enamel junction for both groups. The mucoperiosteal flap was sutured in a coronal position and plaque control was achieved by daily irrigation with 2% chlorhexidine gluconate. At 5, 8, and 11 weeks, two animals each were sacrificed by perfusion with 2.5% glutaraldehyde through the carotid arteries, and the lesions were sliced mesio-distally, demineralized, dehydrated, and embedded. Periodontal healing and regeneration after GTR and P-GTR therapy were compared by histomorphometric as well as morphological analysis. Morphometric analysis for each time period was performed on the pooled samples of P2 and P4. Five weeks after both therapies, the lesions were filled primarily by tissue-free area, epithelium, inflamed tissue, and a small amount of newly formed fibrous connective tissue. At 8 and 11 weeks after P-GTR therapy, there was a statistically greater amount of bone and periodontal ligament formed in the lesions. The newly formed bone filled 80% of the lesion at 8 weeks and 87% at 11 weeks with P-GTR therapy, compared to 14% of the lesion at 8 weeks and 60% at 11 weeks with GTR therapy. Also, with P-GTR therapy there was less epithelium and tissue-free area, less inflamed tissue, and less connective tissue. Morphological analysis indicated that the defects around P2 revealed faster periodontal repair and regeneration than those around P4. While the lesions around P2 were effectively regenerated by 11 weeks even after GTR therapy, those around P4 failed to regenerate. On the other hand, P-GTR therapy further promoted periodontal repair and regeneration so that at 8 weeks the lesions around P2 and P4 demonstrated complete and nearly complete regeneration, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of two mineral-associated adhesion molecules, osteopontin and bone sialoprotein, during cementogenesis

Adhesion molecules and their cell membrane receptors are known to play important regulatory roles in cell differentiation. Consequently, the following experiments were conducted to determine the role of two adhesion molecules, bone sialoprotein (BSP) and osteopontin (OPN) in tooth root formation. Developing murine molar tooth germs at sequential stages of development (developmental days 21-42) were analyzed using immunohistochemical and in situ hybridization techniques. While BSP was localized to alveolar bone and odontoblasts early in development, BSP was distinctly localized to the cemental root surface at latter periods coincident with the initiation of root formation and cementogenesis. Conversely, OPN was distributed in a nonspecific fashion throughout the PDL and the eruption pathway of the forming tooth. In situ hybridization confirmed that cells lining the root surface express BSP. The fact that BSP is specifically localized to the cemental surface suggests that this protein is involved in cementoblast differentiation and/or early mineralization of the cementum matrix. Localization of OPN to non-mineralized tissues further suggests that OPN functions as an inhibitor of mineralization during periodontal ligament formation. These findings collectively suggest that BSP and OPN are intimately involved in the sequence of cellular and molecular events accompanying cementogenesis.

Formation of afibrillar acellular cementum-like layers induced by alkaline phosphatase activity from periodontal ligament explants maintained in vitro

Fibroblasts of the periodontal ligament, by their alkaline phosphatase (ALP) activity, are considered to play a role in the formation of acellular cementum. As a means of exploring this hypothesis, periodontal ligament explants from rat incisors were cultured in direct contact with bovine dentin slices in the presence of 10 mmol/L beta-glycerophosphate. Periosteal and pericardial tissue explants were maintained under similar conditions. After two weeks, the slices were harvested and processed for electron microscopic examination. Controls included periodontal ligament explants to which the ALP-inhibitor levamisole was added. The results suggest that only ALP-positive cultures from periodontal ligament and periosteum form mineralized layers along the dentin. After demineralization, layers consisted of fine filamentous or granular material of moderate electron-density and resembled afibrillar acellular cementum. Our findings support the hypothesis that periodontal ligament fibroblasts, by means of their ALP activity, play a pivotal role in the formation of acellular cementum.

Ultrastructure of cementogenesis as affected by growth hormone in the molar periodontium of the hypophysectomized rat

To document the effect of hypophysectomy and growth hormone replacement on the ultrastructure of cementogenesis in the developing rat third molar, 12 female Wistar rats were randomly allocated to normal control, hypophysectomized or hypophysectomized plus human growth hormone (for 10 days) treatment groups. The results of this study by electron and light microscopy and morphometry have shown that qualitative and quantitative changes occur in the organelles of cementoblasts forming cellular cementum as a result of hypophysectomy and growth hormone replacement. After hypophysectomy, the changes of less prominent nucleoli and nuclear pores, less prominent Golgi apparatuses and decreased endoplasmic reticulum can be interpreted as diminished cementum matrix biosynthesis--an interpretation that can be confirmed morphometrically by less cellular cementum formation. Growth hormone replacement for 10 days reactivates protein synthesis and cementogenesis as evidenced by ultrastructural changes in cementoblasts and a greater production of cementum.

Cellular origins and differentiation control mechanisms during periodontal development and wound healing

In the context of cellular origins, odontogenic epithelium and oral epithelium are the sources for junctional epithelium during development and during wound healing respectively. In contrast, both odontogenic and non-odontogenic mesenchyme contain the progenitors for gingival fibroblasts in developing tissues while in wounded tissues, gingival fibroblasts are derived from gingival connective tissues and comprise a heterogeneous population of cells with diverse properties and functions. Periodontal ligament, bone and cementum cell populations apparently originate from dental follicle progenitor cells during development, but during wound healing derive from ancestral cells in periodontal ligament and bone. Cellular differentiation in developing periodontium is governed in part by epithelial-mesenchymal interactions that generate specific signals which regulate selective cell populations in time and space. On the other hand, differentiation during wound healing and regeneration is regulated by a vast array of extracellular matrix informational molecules and by cytokines that induce both selective and non-selective responses in the different cell lineages and their precursors. Further, several important signalling systems are irretrievably lost after development is complete. Thus, in the context of cellular origins and differentiation, developing and wounded periodontal tissues exhibit fundamental differences. Future prospects for improved healing and regeneration of periodontal tissues may derive from identification and isolation of informational molecules that are stored in connective tissue matrices. These molecules and elucidation of their functions may open new perspectives in our understanding of the biology of periodontal wound healing and may provide novel approaches to periodontal regeneration.

A quantitative enzyme histochemical analysis of the distribution of alkaline phosphatase activity in the periodontal ligament of the rat incisor

The spatial distribution of alkaline phosphatase (ALP) activity was examined in the periodontal ligament of the continuously growing rat incisor. With the indoxyl-tetrazolium salt method, enzyme activity was demonstrated in undecalcified cryosections, and the amount of reaction product was quantified. ALP activity appeared to be distributed heterogeneously. Its highest activity was found in the bone-related compartment of the ligament. In the tooth-related compartment and the supracrestal extension of the ligament, enzyme activity was significantly lower, but still higher than in the lamina propria of the gingiva. In the part of the ligament bordering the cementum, highest activity was found in the apical region just occlusal to Hertwig's epithelial root sheath, where formation of acellular cementum begins. From there toward the incisal edge, the activity of the enzyme gradually decreased. It is suggested that differences among the various parts of the periodontal ligament are related to local variations in phosphate metabolism and cementum deposition.

Isolation of human tumor cells that produce cementum proteins in culture

We have cultured cells from explants of a human cementum tumor. The cells obtained were multipolar, they formed network-like structures and they were alkaline-phosphatase positive. Immunostaining and Western blots using specific antibodies revealed that these cells produced bone sialoprotein and collagen types I and V, and they also mineralized in vitro. Conditioned medium was mitogenic to fibroblasts and mitogens present were separated by heparin-affinity chromatography. Based on affinity to heparin and antibody-inhibition studies, the heparin fractions were shown to contain cementum-derived growth factor, platelet-derived growth factor and fibroblast growth factors. The cementum tumor cells, but not gingival fibroblasts, were stained positively by an antibody to cementum-derived attachment protein. The attachment protein was separated by immunoaffinity chromatography, and Western blots revealed that the preparation contained 56-kDa and 43-kDa proteins as major bands. Cells pulse-labeled with radioactive amino acids contained a 43-kDa protein as the major component; however, this protein was absent after a cold chase in the presence of cycloheximide, but 56-kDa, 39-kDa and 26-kDa species became prominent. These data indicated that the 56-kDa cementum attachment protein is derived from a 43-kDa precursor. Our data show that the cells cultured from the cementum tumor represent cementum cells capable of synthesizing and secreting cementum proteins in culture.

Expression of growth hormone receptor by immunocytochemistry in rat molar root formation and alveolar bone remodeling

Growth hormone (GH) may regulate tooth formation and bone remodeling associated with tooth eruption. This study reports the distribution of growth hormone receptor/binding protein in developing rat molars and adjacent alveolar bone by immunocytochemistry using well-characterized anti-growth hormone receptor monoclonal antibodies. These tissues represent an excellent model for studying the ontogenic changes that occur in odontogenic and osteogenic cells, as these cells are found in linear arrays displaying the various stages of morphological and functional differentiation, and differentiated function. Immunoreactivity was first seen in precementoblasts in contact with the epithelial root sheath, and preodontoblasts. However, growth hormone receptor immunoreactivity was associated primarily with the cytoplasm of odontogenic and osteogenic cells forming their respective matrices. Thus, cementoblasts and odontoblasts at sites of new matrix formation showed intense immunoreactivity whereas cementocytes and mature odontoblasts at later stages of tooth development were nonreactive. Osteoblasts engaged in intramembranous ossification in the alveolar bone were positive, although osteocytes and endosteal cells were immunonegative. Osteoclasts at sites of alveolar bone remodeling resorption were also immunopositive. These patterns of receptor expression parallel the ontogenic sequences of odontogenic and osteogenic cells and suggest that GH promotes the functional state of these cells. Our results also imply that GH may influence differentiation or differentiated functions associated with odontogenesis, osteogenesis, and bone remodeling independent of systemic insulin-like GF-I.

In vitro formation of mineralized nodules by periodontal ligament cells from the rat

The purposes of this study were to determine whether periodontal ligament (PDL) cells are capable of producing mineralized nodules in vitro and to analyze ultrastructural features of the nodules. Rat PDL cells were obtained from coagulum in the socket at 2 days after tooth extraction and cultured at confluence in standard medium containing Dulbecco's Modified Eagle's Medium supplemented with 10% FBS and antibiotics. To test mineralized nodule formation, cells were further cultured for an additional 3 weeks in the standard medium containing (1) ascorbic acid (50 micrograms/ml) and sodium beta-glycerophosphate (10 mM), (2) ascorbic acid, sodium beta-glycerophosphate, and dexamethasone (5 microM), or (3) ascorbic acid alone. Cells were then fixed in 2.5% glutaraldehyde, postfixed in 1% OsO4, and prepared for light and electron microscopy. Three-dimensional nodules containing mineralized matrices were formed only when the cells were cultured in the presence of ascorbic acid and dexamethasone. They were composed of multilayered fibroblasts (up to 13 layers), and highly organized collagen fibrils with 64 nm cross-banding patterns between the cell layers. The fibroblasts in the nodules exhibited an elongated shape with a high degree of cytoplasmic polarity throughout the nodule, and have the morphological features of PDL fibroblasts as seen in vivo. Mineral deposition with needle-like crystals was initiated on collagen fibrils located in intercellular spaces of the upper cell layers and became increasingly heavier towards the bottom half of the nodules. X-ray microanalysis and electron diffraction analysis confirmed that mineral deposition contained calcium and phosphate in the form of immature hydroxyapatite.(ABSTRACT TRUNCATED AT 250 WORDS)

Bundle formation of principal fibers in rat molars

The bundling of principal fibers was investigated in tangential sections through the tooth-related portion in developing rat molars by light and electron microscopy. When root dentin calcification began, cross sections of principal fibers emerged as fibril aggregates in the narrow intercellular spaces in a densely packed population of periodontal ligament cells. Subsequently, these cells changed shape and location to widen the intercellular spaces. The fibril aggregates became thicker in these spaces. With root development, the collagen fibrils formed loosely aggregated bundles and the periodontal ligament cells extended cell processes between the bundles. The cell processes usually contained microfilaments suggestive of actin filaments, and as the cell processes extended and came in close apposition, they formed delimited compartments. These compartments appeared to be a sheath-like structure, and the loose fibril bundles developed into tight fibril bundles in the compartments. Finally the principal fibers consisted of many tight fibril bundles, which were partially or entirely surrounded by cell processes and cell bodies. The findings suggest that the sheath-like, cellular compartments cause the tight bundling of the principal fibers.

Occurrence of epidermal growth factor-binding sites during differentiation of cementoblasts and periodontal ligament fibroblasts of the young rat: a light and electron microscopic radioautographic study

Occurrence of epidermal growth factor (EGF)-binding sites during differentiation of cementoblasts and periodontal ligament (PDL) fibroblasts was investigated using radioautography after I. V. injection of 125I-EGF to 14-day-old rats. During differentiation of cementoblasts, a very low level of EGF-binding sites was present on the mesenchymal cells in dental follicle proper, precementoblasts, and cementoblasts. On the other hand, during differentiation of PDL fibroblasts, numerous EGF-binding sites were observed on the undifferentiated paravascular cells and on the perifollicular mesenchymes representing the major source of PDL fibroblast precursor cells. Also heavy labeling was observed throughout their differentiation to PDL fibroblasts, as well as during full synthetic activity as mature cells. Quantitative analysis of the light microscopic radioautographs revealed that these cells demonstrated approximately 4 grains per 100 microns 2 of cell area. These results suggest that EGF plays an important role in differentiation of PDL fibroblasts, but not in that of cementoblasts. Furthermore, the well-known in vivo effect of EGF in producing precocious eruption of teeth may be a consequence of a more extensive effect of EGF throughout differentiation of PDL fibroblasts as well as during full synthetic activity as mature cells.

The development and structure of principal fibers and cellular cementum in rat molars

The principal fibers and cellular cementum were examined in various developmental stages in rat molars by light and electron microscopy, and their development and structure were studied. The principal fibers increased in thickness with root development. At any given stage, the principal fibers consisted of small fiber bundles on the cementum surface and showed a branching structure. Sharpey's fibers also showed a similar branching structure and increased in thickness, as traced superficially in the cementum. It was suggested that the principal fibers increased in thickness by means of the aggregation of fiber bundles. The development of cellular cementum was divided into two (early and late) phases on the basis of the fiber arrangement. The early cementum was believed to contain a few Sharpey's fibers and many randomly arranged intrinsic fibers. The late cementum contained thicker Sharpey's fibers with branches and the intrinsic fibers which appeared to encircle Sharpey's fibers and/or meander among them.

Initiation of acellular extrinsic fiber cementum on human teeth. A light- and electron-microscopic study

The development of acellular extrinsic fiber cementum (AEFC) has never before been studied in human teeth. We have therefore examined the initiation of AEFC in the form of a collagenous fiber fringe and its attachment to the underlying dentinal matrix, in precisely selected, erupting human premolars with roots developed to 50%-60% of their final length. Freshly extracted teeth were prefixed in Karnovsky's fixative, decalcified in EDTA and subdivided into about 10 blocks each, cut from the mesial and distal root surfaces, vertical to and along the root axis. The blocks were postfixed in osmium tetroxide, embedded in Epon and cut for light- and electron-microscopic investigation. Starting at the advancing edge of the root, within a region extending about 1 mm coronal to this edge, fibroblast-like cells were seen closely covering the external root surface. Along the first 100 microns from the root edge, these cells extended cytoplasmic processes and contacted the dentinal collagen fibrils. Between these cells and the dentinal matrix, new collagen fibrils and very short collagen fibers gradually developed. Within the second 100 microns from the root edge, this resulted in the formation of a cell-fiber fringe network. Newly formed fibers of the fringe were directly attached to the non-mineralized matrix containing dentinal collagen fibrils and could be distinguished from the latter by differences in fibril orientation. During the process of dentin mineralization, the transitional zone between the fiber-fringe base and the dentinal matrix, i.e., the future dentino-cemental junction, also mineralized. It is suggested that this fiber fringe is the base of AEFC, which later increases in thickness by fiber extension and subsequent mineralization.