Fine structural study of rat molar cementum

Role of the Sterol Regulatory Element Binding Protein Pathway in Tumorigenesis

Metabolic changes are a major feature of tumors, including various metabolic forms, such as energy, lipid, and amino acid metabolism. Sterol regulatory element binding proteins (SREBPs) are important modules in regulating lipid metabolism and play an essential role in metabolic diseases. In the previous decades, the regulatory range of SREBPs has been markedly expanded. It was found that SREBPs also played a critical role in tumor development. SREBPs are involved in energy supply, lipid supply, immune environment and inflammatory environment shaping in tumor cells, and as a protective umbrella to support the malignant proliferation of tumor cells. Natural medicine and traditional Chinese medicine, as an important part of drug therapy, demonstrates the multifaceted effects of SREBPs regulation. This review summarizes the core processes in the involvement of SREBPs in tumors and provides a comprehensive understanding of the pathways through which natural drugs target the SREBP pathway and regulate tumor progression.

Lactation stage impacts the glycolytic function of bovine CD4+ T cells during ex vivo activation

Dairy cattle undergo dynamic physiological changes over the course of a full lactation into the dry period, which impacts their immunocompetence. During activation, T cells undergo a characteristic rewiring to increase the uptake of glucose and metabolically reprogram to favor aerobic glycolysis over oxidative phosphorylation. To date it remains to be completely elucidated how the altered energetic demands associated with lactation in dairy cows impacts T cell metabolic reprogramming. Thus, in our ex vivo studies we have examined the influence of stage of lactation (early lactation into the dry period) on cellular metabolism in activated bovine CD4⁺ T cells. Results showed higher rates of glycolytic function in activated CD4⁺ T cells from late lactation and dry cows compared to cells from early and mid-lactation cows. Similarly, protein and mRNA expression of cytokines were higher in CD4⁺ T cells from dry cows than CD4⁺ T cells from lactating cows. The data suggest CD4⁺ T cells from lactating cows have an altered metabolic responsiveness that could impact the immunocompetence of these animals, particularly those in early lactation, and increase their susceptibility to infection.

The Cementocyte-An Osteocyte Relative?

Cementum is a mineralized tissue covering the tooth root that functions in tooth attachment and posteruptive adjustment of tooth position. During formation of the apically located cellular cementum, some cementoblasts become embedded in the cementoid matrix and become cementocytes. As apparently terminally differentiated cells embedded in a mineralized extracellular matrix, cementocytes are part of a select group of specialized cells, also including osteocytes, hypertrophic chondrocytes, and odontoblasts. The differentiation and potential function(s) of cementocytes are virtually unknown, and the question may be posed whether the cementocyte is a dynamic actor in cementum in comparable fashion with the osteocyte in the skeleton, responding to changing tooth functions and endocrine signals and actively directing local cementum metabolism. This review summarizes the literature with regard to cementocytes, comparing them to their closest "cousins," the osteocytes, where insights gained from osteocyte studies serve to inform the critical examination of cementocytes. The review identifies important unanswered questions about these cells regarding their origins, differentiation, morphology and lacuno-canalicular system, selective markers, and potential functions.

Isolation and Functional Analysis of an Immortalized Murine Cementocyte Cell Line, IDG-CM6

The dental cementum covering the tooth root is similar to bone in several respects but remains poorly understood in terms of development and differentiation of cementoblasts, as well as the potential function(s) of cementocytes residing in the cellular cementum. It is not known if the cementocyte is a dynamic actor in cementum metabolism, comparable to the osteocyte in the bone. Cementocytes exhibit irregular spacing and lacunar shape, with fewer canalicular connections compared with osteocytes. Immunohistochemistry and quantitative PCR (qPCR) revealed that the in vivo expression profile of cementocytes paralleled that of osteocytes, including expression of dentin matrix protein 1 (Dmp1/DMP1), Sost/sclerostin, E11/gp38/podoplanin, Tnfrsf11b (osteoprotegerin [OPG]), and Tnfsf11 (receptor activator of NF-κB ligand [RANKL]). We used the Immortomouse(+/-); Dmp1-GFP(+/-) mice to isolate cementocytes as Dmp1-expressing cells followed by immortalization using the interferon (IFN)-γ-inducible promoter driving expression of a thermolabile large T antigen to create the first immortalized line of cementocytes, IDG-CM6. This cell line reproduced the expression profile of cementocytes observed in vivo, including alkaline phosphatase activity and mineralization. IDG-CM6 cells expressed higher levels of Tnfrsf11b and lower levels of Tnfsf11 compared with IDG-SW3 osteocytes, and under fluid flow shear stress, IDG-CM6 cells significantly increased OPG while decreasing RANKL, leading to a significantly increased OPG/RANKL ratio, which would inhibit osteoclast activation. These studies indicate similarities yet potentially important differences in the function of cementocytes compared with osteocytes and support cementocytes as mechanically responsive cells.

Analysis of avian bone response to mechanical loading—Part One: Distribution of bone fluid shear stress induced by bending and axial loading

Mechanical loading-induced signals are hypothesized to be transmitted and integrated by a bone-connected cellular network (CCN) before reaching the bone surfaces where adaptation occurs. Our objective is to establish a computational model to explore how bone cells transmit the signals through intercellular communication. In this first part of the study the bone fluid shear stress acting on every bone cell in a CCN is acquired as the excitation signal for the computational model. Bending and axial loading-induced fluid shear stress is computed in transverse sections of avian long bones for two adaptation experiments (Gross et al. in J Bone Miner Res 12:982-988, 1997 and Judex et al. in J Bone Miner Res 12:1737-1745, 1997). The computed fluid shear stress is found to be correlated with the radial strain gradient but not with bone formation. These results suggest that the radial strain gradient is the driving force for bone fluid flow in the radially distributed lacunar-canalicular system and that bone formation is not linearly related to the loading-induced local stimulus.

The modulatory role of cementum matrix on osteoblastic cells in vitro

The formation of new cementum is an important issue in clinical periodontology, as cementum is required to provide attachment for newly formed periodontal tissues to the root surface. In this study a model of cementogenesis in vitro was used in order to test the effects of root surface demineralization on the migration, attachment and formation of a cementum-like tissue by osteoblastic cells cultured on cementum and to test the specificity of cementum matrix in modulating those effects by comparison of root co-cultures with bone co-cultures. It was demonstrated that root surface demineralization did not significantly alter the orientation, number and attachment of cells to the root co-cultures. The results also demonstrated that cementum and bone matrix appear to behave differently in culture, as seen by their distinct action on the morphological profile of the attached cells and the extracellular matrix deposited by these cells. These results demonstrate that although cementum matrix appears to stimulate the production of cementum-like tissue, this action is not confined to cementum matrix alone, since a similar material was also deposited on dentine and bone surfaces. Thus, these results do not support a specific action of cementum matrix on the modulation of the cementoblast phenotype. The use of co-cultures of neonatal rat calvaria cells with root slices represents a promising model of cementogenesis in vitro; however, studies should be undertaken towards the identification of markers to distinguish between cementoblast and osteoblast phenotypes in order to further validate this model.

Origins of cementum

There are diverging opinions about the role of Hertwig's epithelial root sheath in the formation of cementum. In the present review this role is discussed. There is increasing evidence that Hertwig's epithelial root sheath is actively involved in the formation of both acellular and cellular cementum. The development of acellular cementum seems to be associated with secretion of enamel-related proteins by cells of the epithelial root sheath. Formation of the matrix for cellular cementum appears to be induced by exposure of the inner layer of the epithelial root sheath to the mesenchymal cells in the dental follicle. Experimental studies with 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP) indicate that the formation of acellular cementum must be preceded by mineralization of the mantle dentin. If the mineralization is inhibited by means of HEBP, there is precocious separation of the two layers of the epithelial root sheath after which matrix for a cellular type of cementum is formed.

Intermediate cementum. Development, structure, composition, and potential functions

In the roots and root analogs of mammalian teeth a narrow zone of highly calcified tissue occupies the junction between cementum and dentin. This zone is referred to as "intermediate cementum," despite evidence that it is not a product of cementogenesis (or of dentinogenesis). Studies conducted on teeth of human and nonhuman primates indicate that the intermediate cementum layer contains enamel matrix proteins and is likely a product of Hertwig's epithelial root sheath. The available evidence regarding the development (origin), structure, and composition of intermediate cementum is reviewed, and the potential functions of this layer are assessed to include a possible role in wound healing.

Epithelial remnants in the crestal periodontium of the dog

The purpose of this study was to access the effect of age on the epithelial remnants (EPRs) of the crestal periodontal tissues of beagle dogs. The material consisted of 10 beagle dogs; 5 were 1-year-old (young dogs) and 5 were 9-years-old (old dogs). All animals belonged to the same dog colony and had been raised under similar conditions. Since birth, the dogs had been subjected to professional prophylaxis 4x /per year. Biopsies were obtained from the mandibular 3rd and 4th premolar regions and were prepared for histologic analysis. 2 areas, (1) the supracrestal region and (2) the marginal periodontal ligament region, were identified. The supracrestal region was further divided into 4 compartments of equal height. The histologic parameters studied included: the (i) number of EPRs/mm root length; EPR frequency; (ii) size of EPR; (iii) distance between the root surface and the EPR; (iv) distance between the alveolar bone and the EPR; (v) epithelial cell area. The supracrestal region (total and various compartments) of old dogs harbored significantly fewer yet larger EPRs than epithelial remnants of young dogs. Similarly, the EPR's of the periodontal ligament region were significantly larger and closer to the root surface in old dogs compared to young dogs. No difference was noted in the cell area of EPRs between young and old dogs for either region. Epithelial remnants of the supracrestal region in both groups of dogs were somewhat more frequent, larger and positioned further from the root surface than the EPRs of the periodontal ligament region.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Differences in the transport systems between cementocytes and osteocytes in rats using microperoxidase as a tracer

Microperoxidase (MP) tracer was injected intravenously into rats to investigate any differences in transport pathways of tissue fluids in the lacunae and canaliculi of cementum and bone. Light microscopically, in deep cementum lacunae, pericellular spaces contained a large amount of MP, while close to the cementum surface, the spaces contained scarcely any. In bone, MP was detected throughout all pericellular spaces. MP was detected intracellularly as granular reaction products in most cementocytes and osteocytes. Electron microscopically, MP was found in the pericellular spaces of cementum and bone lacunae, particularly on collagen fibrils and amorphous material. MP deposits were also intense along the plasma membrane of cementocytes in the deep cementum and along the innermost edge of the deep cementum matrix and bone matrix. In uptake of MP by cementocytes, although extracellular tracer was deposited extensively along the plasma membrane of the deeply positioned cementocytes, uptake by these deep cementocytes was less than that of those close to the surface. However, in bone, most osteocytes showed uniform uptake. These results suggest that the transport pathways for tissue fluids in cementum are in the pericellular spaces, but that cementum has an uneven circulation of tissue fluid. In cementum, although there seems to be a well-developed canalicular system to transport tissue fluid into the deep regions, the deep cementocytes had less endocytotic ability than those close to the surface.

Possible role of cementoblasts in the resorbant organ of human deciduous teeth during root resorption

Human deciduous teeth undergoing physiologic root resorption were extracted and fixed with a mixture of formaldehyde and glutaraldehyde and processed for scanning (SEM) and transmission (TEM) electron microscopy, and for acid (ACPase) and alkaline phosphatase (ALPase) cytochemistry. The resorbant organ, rich in odontoclasts, cementoblasts, fibroblasts, and macrophages, formed prominent resorption lacunae in root dentin. SEM observations of resorption lacunae treated with trypsin solution showed islands of newly-formed cementum matrix in part of the resorbing dentin surfaces. Such cementum consisted of bundles of densely-arranged collagen fibrils and, in part, contained forming cementocytic lacunae and canaliculi. Active cementoblasts adjacent to odontoclasts on resorbing dentin surfaces showed cuboidal outlines and were characterized by the presence of numerous cisterns of rough endoplasmic reticulum, well-developed Golgi complexes, secretion granules, and many mitochondria. They sometimes formed a thin layer of cementoid and/or cementum matrix upon the resorbing dentin surface. These cementoblasts had ACPase-positive lysosomes in the cell bodies and exhibited intense ALPase activity along the plasma membranes of whole cell surfaces. These results suggest that, during root resorption, 1) active cementoblasts are present adjacent to active odontoclasts and 2) these cementoblasts are involved in remodeling the resorbing dentin surfaces.

Endocytosis in odontoclasts and osteoclasts using microperoxidase as a tracer

Microperoxidase (MP, a peptide tracer) was intravenously injected into rats after six days of tooth movement by the Waldo method. Bone resorption was seen along the distal bone surface of the inter-radicular septum of the upper first molar, and tooth root resorption occurred along the mesial and distal surfaces of the distal root of the same molar. Odontoclasts were smaller in size and numbers than osteoclasts, but had the same organelles [multiple nuclei, ruffled borders (RB), clear zones, vacuoles, mitochondria, and specific granules] as osteoclasts. MP was deposited on the resorbed area through the clear zone and was transported into the vacuoles along the channels of the RB. The uptake of MP by odontoclasts was small, compared with that by osteoclasts. Collagen fibrils were found in the channels of the RB but were not detected in the vacuoles. Instead, filament-like structures were seen in the vacuoles and were located very near the collagen fibrils in the channels. Fibroblasts outside the resorbed lacunae had endocytosed collagen. In contrast, some cells close to odontoclasts (osteoclasts) in the dentin (bone)-resorbed lacunae had not endocytosed any collagen fibrils. These findings suggest (1) that odontoclasts resorb the dentin or cementum just as osteoclasts do in bone resorption, (2) that the resorbed area connects with the extracellular spaces of the odontoclasts or osteoclasts, and (3) that the organic components (e.g., collagen fibrils) of dentin or cementum are endocytosed through the channels of the RB in the same manner as that for MP. However, the endocytotic ability of odontoclasts appears inferior to that of osteoclasts.

Cellular roles in physiological root resorption of deciduous teeth in the cat

This study has attempted to assess the importance of mesenchymal cells, fibroblasts, cementoblasts, and mononuclear phagocytes (i.e., macrophages) in physiological root resorption of feline deciduous teeth. Deciduous incisors of three- to six-month-old kittens undergoing root resorption were investigated by means of electron microscopy. In an early phase of root resorption, the resorption organ consisted of many fibroblasts and relatively few macrophages and odontoclasts, the last with a wide, clear zone and narrow, immature, ruffled border. In the active phase of root resorption, the resorption organ contained many odontoclasts with a well-developed ruffled border and a reduced clear zone, cementoblasts, fibroblasts, macrophages, neutrophils, and many blood vessels. Cementoblasts were present usually on the resorbing dentin surface adjacent to odontoclasts and, in many cases, these cells communicated with each other via gap junctions. Cementoblasts frequently extended broad cell processes with secretion granules and with phagosomes containing collagen fibrils into the dentinal tubules exposed to resorption lacunae. Some macrophages exhibiting a clear zone-like structure also appeared on resorbing dentin surfaces. In the resting phase of root resorption, the dentin surface was covered mostly with cementoblasts resembling bone lining cells. There was an occasional macrophage, but no odontoclasts were observed during this phase. During removal of the periodontal ligament concomitant with root resorption, many fibroblasts phagocytosed mature collagen fibrils, as well as amorphous fluffy material. These results suggest that these mesenchymal cells, as well as odontoclasts, are essential for the cellular removal of dental hard and soft tissues during shedding of feline deciduous teeth.

Dentin formation in so-called "fibro-osteo-cemental" lesions of the jaw: histologic, electron microscopic, and immunohistochemical investigations

Two cases of the so-called fibro-osteo-cemental lesions of the jaws, containing unusual deposits of hard tissue, are described. There were peculiar spheroid calcifications or larger masses forming small cavities with a radial arrangement and discontinuous blasts extending with axonlike cell processes toward an acellular core. For further definition, electron microscopic and immunohistologic studies were done. The most conspicuous features were abundant intracytoplasmic vimentin filaments in the blasts, tight junctions, matrix vesicle formation, a globular accretion pattern, and so-called matrix maturation. These findings militate against an osseous or cementous nature of this hard tissue. Rather, both the light and electron microscopic findings are highly compatible with the assumption that the blasts are odontoblasts and that an immature type of dentin is formed. In addition to fibroblasts, osteoblasts, and cementoblasts, apparently also a further descendant of the ectomesenchyme--the odontoblasts with dentin formation--may participate in the so-called fibro-osteo-cemental lesions.

Light and scanning electron microscopic observations of the canalicular system in human cellular cementum

A scanning electron microscopic cast technique was used to determine the nature of the canalicular system in human cellular cementum. Prior light microscopic observations suggested the presence of two distinct types of lacunae: bone-like and a large irregular type generally confined to the interradicular region. Only the bone-like lacunae were visualized in the SEM cast preparations. The canalicular system associated with the bone-like lacunae was usually continuous from the surface of the dentin to the surface of the cementum in newly-erupted teeth. Casts having a sponge-like configuration were observed near the cementodentinal junction in some of the specimens from the interradicular region. The presence of these casts could not be predicted from prior light microscopic observations and it was concluded that they may represent infiltration of hypomineralized matrix rather than lacunae.

Ultrastructural and morphometric analyses of human cementoblasts and periodontal fibroblasts

In order to elucidate the cytological characteristics of human cementoblasts which distinguish them from periodontal fibroblasts, the periodontal ligament and gingival connective tissue attached to 37 extracted teeth from 27 patients (ages 10-67) were analyzed by electron microscopy coupled with a morphometric procedure. The cementoblasts largely consisted of either immature or resting types of collagen-producing cells (CPC), both of which were poor in the rough endoplasmic reticulum and Golgi complex; these organelles were well-developed in the relatively less common active cementoblasts. The cementoblasts consistently revealed a higher mean-volume-density per tissue unit than the fibroblasts and sometimes were grouped into clusters with the formation of junctional apparatuses. The relative volume of glycogen particles per cytoplasm was significantly higher in the cementoblasts, whereas the rough endoplasmic reticulum was higher in the fibroblasts. From the present study, it is suggested that the cementoblasts are functionally less active CPC than the periodontal fibroblasts.

Location and ultrastructure of the first cementum formed in rabbit incisors

The morphology of the continuously erupting rabbit incisor differs characteristically from that of the rodent incisor and is incompletely known. In this study, permanent incisors from fetal New Zealand white rabbits aged 26-30 days in utero were processed for transmission electron microscopy. Examination of longitudinal and cross-sections revealed that disintegration of the epithelial sheath on root analog surfaces was followed by cementogenesis. The first layer of cementum matrix was seen on the mesial aspect and consisted of a finely granular ground substance and a few embedded collagen fibrils which extended to the cementodentinal junction. Within the period 27-30 days in utero, acellular cementum was formed on all root-analog surfaces and, in addition, formed a fine coating over the enamel. Although the initially formed cementum layer contained fewer collagen fibrils than subsequent layers, a layer of so-called intermediate cementum was not observed.

Epithelial root-sheath changes during molar formation in the mouse

The ultrastructure of the epithelial root-sheath was examined in the first mandibular molar teeth of 11, 16 and 21-day-old mice. The changing morphology of the sheath was related to root maturity. An initial, predominantly bicellular layer progressively shortened in an apical direction, the outer layer more so than the inner. The changing morphology appeared to involve maintenance of the inner layer largely at the expense of the outer. Some inner-layer cells persisted at spaced intervals adjacent to forming acellular cementum. Some epithelial rests, distant from the root surface, appeared to be formed early in root development at the time of initial root-sheath severance from the cervical loop of the enamel organ. Epithelial rests located along the acellular cementum appeared to arise from inner, rather than outer, epithelial root-sheath cells and at later stages in root development.

Responses of periodontal tissues and cementum following transplantation of teeth

Responses of periodontal tissues, particularly cementum, were assessed histologically in teeth which had been transplanted after root formation was moderately well advanced. When the epithelial root sheath persisted, it was associated with continued root formation. In instances where the sheath disintegrated prematurely, either as a consequence of the surgery or of functional factors such as occlusal forces, normal root formation did not proceed, but there was rapid and extensive production of cellular cementum apically and along the root of the tooth. A zone of imperfect cementum formation was frequently seen between the preoperative cementum and that which developed postoperatively. Dilaceration of the tip of the root, with subsequent envelopment in a bulbous mass of cementum, commonly resulted.

A radiographic assessment of autologous tooth transplants in dogs

This study assessed structural and functional changes that occurred when teeth in dogs were transplanted at various stages of development. Unerupted developing permanent teeth were transferred to other existing bony crypts or to surgically prepared bony sockets. Erupted teeth with varying degrees of root formation were transplanted to other sockets, some with increased functional influences. The development, morphologic features, and relationships of the transplanted teeth were assessed radiographically at intervals ranging from seven to 150 days postoperatively.

Experimental induction of cementogenesis on the enamel of transplanted mouse tooth germs

First and second maxillary molar tooth germs with their surrounding bone were removed from 9-day-old mice, freed of the reduced enamel epithelium, re-inserted crown downwards in their bony crypts and then transplanted in the subcutaneous tissue of hosts of the same age and litter. Grafts were removed 14 days later and prepared for light and electron microscopy. In the areas where the reduced enamel epithelium was missing, a layer of cementum-like tissue was present on the enamel surface, always associated with cells showing the typical features of cementoblasts. A thin electron-lucent layer of fine fibrillar material separated the enamel surface from the new hard tissue which was composed of densely-packed collagen mixed with a ground substance. Where the cementum-like tissue was thick, cells were trapped in a collagenous matrix. The cementogenesis on enamel was strictly dependent on the absence of the reduced enamel epithelium. Thus, when exposed to follicular tissue, the surface of immature enamel appears to exert an influence on follicular cells and stimulate cementogenesis. This hypothesis could explain the presence of overgrowths of cementum in the cervical region of tooth crowns where the reduced enamel epithelium may be particularly vulnerable.

Further observations on the fine structure of cellular cementum from deciduous teeth of pigs

Examination of microradiographs from the deciduous teeth of pigs revealed large lacunae or radiolucent zones close to the cemento-dentinal junction. Electron microscopic studies of the ground sections showed areas of irregularly shaped zones devoid of mineral and filled with collagen fibers. In the wide unmineralized zones, spherical clusters of crystallites were noted. Several cementum lacunae bordered by a broad rim of unmineralized collagen fibers were noted and some lacunae also contained zones of a moderately electron dense material. This material did not yield a diffraction pattern, while the mineralized part of the cementum gave the diffraction pattern typical of hydroxyapatite.