Intercellular junctions in odontoblasts of the rat incisor studied with freeze-fracture

Cell polarization: From epithelial cells to odontoblasts

Cell polarity identifies the asymmetry of a cell. Various types of cells, including odontoblasts and epithelial cells, polarize to fulfil their destined functions. Odontoblast polarization is a prerequisite and fundamental step for tooth development and tubular dentin formation. Current knowledge of odontoblast polarization, however, is very limited, which greatly impedes the development of novel approaches for regenerative endodontics. Compared to odontoblasts, epithelial cell polarization has been extensively studied over the last several decades. The knowledge obtained from epithelia polarization has been found applicable to other cell types, which is particularly useful considering the remarkable similarities of the morphological and compositional features between polarized odontoblasts and epithelia. In this review, we first discuss the characteristics, the key regulatory factors, and the process of epithelial polarity. Next, we compare the known facts of odontoblast polarization with epithelial cells. Lastly, we clarify knowledge gaps in odontoblast polarization and propose the directions for future research to fill the gaps, leading to the advancement of regenerative endodontics.

Dentin sialoprotein facilitates dental mesenchymal cell differentiation and dentin formation

Dentin sialoprotein (DSP) is a dentin extracellular matrix protein. It is involved in dental mesenchymal cell lineages and dentin formation through regulation of its target gene expression. DSP mutations cause dentin genetic diseases. However, mechanisms of DSP in controlling dental mesenchymal cell differentiation are unknown. Using DSP as bait, we screened a protein library from mouse odontoblastic cells and found that DSP is a ligand and binds to cell surface receptor, occludin. Further study identified that the C-terminal DSP domain^{aa 363–458} interacts with the occludin extracellular loop 2^{aa 194–241}. The C-terminal DSP domain induced phosphorylation of occludin Ser⁴⁹⁰ and focal adhesion kinase (FAK) Ser⁷²² and Tyr⁵⁷⁶. Coexpression of DSP, occludin and FAK was detected in dental mesenchymal cells during tooth development. Occludin physically interacts with FAK, and occludin and FAK phosphorylation can be blocked by DSP and occludin antibodies. This DSP domain facilitates dental mesenchymal cell differentiation and mineralization. Furthermore, transplantation and pulp-capping procedures revealed that this DSP domain induces endogenous dental pulp mesenchymal cell proliferation, differentiation and migration, while stimulating blood vessel proliferation. This study elucidates the mechanism of DSP in dental mesenchymal lineages and implies that DSP may serve as a therapeutic agent for dentin-pulp complex regeneration in dental caries.

Glial network responses to polymicrobial invasion of dentin

This study investigated the distribution patterns of glial networks disclosed by reactivity for glial fibrillary acidic protein (GFAP) and S100B in healthy and carious human teeth. The objective was to determine the assembly and collapse of glial networks in response to encroaching infection. 15 healthy and 37 carious posterior teeth from adults were studied. Immediately after extraction, teeth were cleaned and vertically split and the half with pulp fixed and prepared for resin or frozen sections. Sections were stained with toluidine blue and for immunofluorescence, with observation by confocal laser microscopy and analysis by ImageJ software. Carious teeth were subdivided into three groups according to degree of carious involvement: microbial penetration through enamel (stage A), extension into dentin (stage B) and advanced penetration into dentin but without invasion of underlying pulp tissue (stage C). In stage A lesions there was marked increase in glial networks in dental pulp tissue that extended beyond the zone of microbial invasion. This response was maintained in stage B lesions. In advanced stage C lesions these networks were degraded in the zone of invasion in association with failure to contain infection. Cells expressing the glial markers GFAP and S100B showed a response to initial microbial invasion of dentin by increase in number and altered anatomical arrangement. The late stage of dentinal caries was marked by collapse of these networks in the region adjacent to advancing bacteria. This behaviour is important for understanding and explaining the defensive response of the neurosensory peripheral dental pulp apparatus to infection.

Immunoelectron microscopic observation of connexin43 in rat odontoblasts

Gap junctions play an important role in differentiation of odontoblasts. Gap junction protein, connexin 43 is expressed in odontoblast. However, the detailed localization in odontoblasts has yet to be fully investigated. We investigated the localization of connexin43 in rat odontoblasts immuno-electron microscopically. The rats were transcardially fixed with 1% paraformaldehyde in 0.1M phosphate buffer, and mandibles were decalcified with 10% ethylenediamine tetraacetic acid. Pre-embedding method was carried out for immuno-electron microscopic analysis. Microscopically, gap junctions were localized between bodies of odontoblasts, and between bodies and processes of odontoblasts. The gap junctions were labeled with gold particles that indicated connexin43. These results suggest that gap junctions between odontoblasts are definitely composed of connexin43 in rats, and our methods used in this study is useful to investigate localization of connexin43 immuno-electron microscopically.

Odontoblastic syncytium through electrical coupling in the human dental pulp

We have previously reported a dye-coupling network between odontoblasts (OBs). However, it is still unclear how the information detected by the odontoblasts is transmitted. The aim of this study was to characterize the odontoblastic syncytium electrophysiologically in the human dental pulp. Pulpal cells were freshly isolated from human premolars immediately after extraction. Under a light microscope, coupled or small clusters (3-20) of odontoblasts, each of which had a monopolar process (95-280 µm) and an oval cell body, were easily observed to be lined up in parallel. Cells were used for electrophysiological recording within 3 hrs in the dual patch-clamp configuration. Electrical couplings were found between odontoblasts (37/40 pairs). Voltage gating showed directional independence between pairs of odontoblasts. The time constant to a current decay increased with the number of clustered odontoblasts. Nine of 37 pairs isolated from young patients were electrically coupled, but could not be voltage-clamped. Transjunctional currents were blocked by octanol. These results suggest that odontoblasts form a syncytium that is directionally independent via symmetric gap junction channels in the odontoblastic layer. Young odontoblasts with a high electrical conductance to neighboring cells may be related to high potential of information transmission or calcification.

Disruption of Nfic causes dissociation of odontoblasts by interfering with the formation of intercellular junctions and aberrant odontoblast differentiation

We reported previously that Nfic-deficient mice exhibit short and abnormal molar roots and severely deformed incisors. The objective of this study is to address the mechanisms responsible for these changes using morphological, IHC, and RT-PCR analysis. Nfic-deficient mice exhibited aberrant odontoblasts and abnormal dentin formation in molar roots and the labial crown analog of incisors. The most striking changes observed in these aberrant odontoblasts were the loss of intercellular junctions and the decreased expression of ZO-1 and occludin. As a result, they became dissociated, had a round shape, and lost their cellular polarity and arrangement as a sheet of cells. Furthermore, the dissociated odontoblasts became trapped in dentin-like mineralized tissue, resembling osteodentin in the overall morphology. These findings suggest that loss of the Nfic gene interferes with the formation of intercellular junctions that causes aberrant odontoblast differentiation and abnormal dentin formation. Collectively, these changes in odontoblasts contributed to development of molars with short and abnormal roots in Nfic-deficient mice.

Claudin rather than occludin is essential for differentiation in rat incisor odontoblasts

Many morphological and developmental studies have demonstrated the characteristics of tight junctions (TJs) between odontoblasts. However, detailed localization of TJ-associated proteins in odontoblasts and their functions has not yet been clarified. To elucidate the relationship between the establishment of TJ structures and the differentiation of odontoblasts during early dentinogenesis, we studied the expression and localization of constituent proteins of TJs (claudin-1, occludin, ZO-1 and ZO-2) between odontoblasts in rat lower incisors using Western blotting, immunofluorescence and immunoelectron microscopy. When the expression of claudin-1 increases at the distal portion of mature odontoblasts, the TJs form complex networks of strands, and odontoblasts differentiated by developing distal membrane domains and by secreting specific molecules for mineralization. We conclude that the TJs of odontoblasts may play an important role in the differentiation of odontoblasts in rat lower incisors during early dentinogenesis.

Inhibition of connexin 43 expression and function in cultured rat dental pulp cells by antisense oligonucleotide

Connexins are gap-junction proteins forming hexameric structures in the plasma membranes of adjacent cells, thereby creating intercellular channels. Connexin 43 (CX43) is expressed in pulp tissue. However, its function in dental pulp tissue has yet to be fully investigated. We have employed antisense oligonucleotides (AS) against rat CX43 to study the role of CX43 in dental pulp cells. Cultured dental pulp cells were treated with AS or sense (S) oligonucleotides. The number of cells in the AS-treated groups was approximately 1.3-fold that in the S-treated controls. Growth rates were significantly different between the AS- and S-treated groups at 48 h (P < 0.01). An alkaline phosphatase assay revealed that AS-treated pulp cells dramatically decreased at 48 h after AS incorporation, whereas S-treated pulp cells showed no marked changes. Western blot analysis revealed that heat-shock protein 25 was highly expressed in S-treated cells but was only weakly expressed in AS-treated cells at 48 h. Furthermore, AS-treated cells highly expressed CX45, whereas S-treated cells exhibited high expression of CX32. These results suggest that CX43 is involved in cell growth, mineralization, and differentiation to odontoblasts in rat pulp cells, and that CX43 plays the opposite role to that of CX45.

Intracellular Distribution of Desmoplakin in Human Odontoblasts

Coexpression of desmosomal proteins and vimentin has been reported in a specific mesenchymal phenotype. This study investigated the expression of vimentin-binding desmosomal proteins in human dental pulp fibroblasts (DPF) and odontoblasts. The dental pulp has no cells expressing desmocollin (DSC) 1–3, desmoglein (DSG) 1–3, junction plakoglobin (JUP), or desmoplakin (DPK) 1 and 2 except for odontoblasts expressing DPK. A confocal image by laser-scanning microscopy demonstrated the diffuse distribution of DPK in the cytoplasm throughout the odontoblast processes. In culture, the mRNA expression of JUP and DPK1, but not DSC1–3 and DSG1–3, was detected in all DPF clones tested and also in odontoblast-like cells (OB) expressing osteocalcin and dentin sialophosphoprotein mRNAs established in the differentiation medium. The DPF having the potential to differentiate into OB expressed vimentin, but not DPK before culturing in the differentiation medium, whereas OB expressed vimentin-binding DPK1. These results suggest that DPF usually expresses DPK1 mRNA, and that the DPK1 production and the bonding of vimentin to DPK1 occur in DPF with the differentiation into odontoblasts.

Development of tight junctions between odontoblasts in early dentinogenesis as revealed by freeze-fracture

BACKGROUND

Mature odontoblasts possess junctional structures constituted by adherens, gap, and tight junctions. Although adherens and gap junctions appear early between odontoblasts, there is no information on the appearance and development of tight junctions between odontoblasts. In this study, we have examined freeze-fracture replicas of early dentinogenesis to study the development of tight junctions between odontoblasts and to determine whether these junctions are of zonular or macular type.

METHODS

Upper first molar tooth germs of Wistar rats between 1 and 3 days old were fixed in buffered 4% glutaraldehyde/4% formaldehyde and subsequently cryoprotected with cacodylate-buffered glycerol. Freeze-fracture replicas were obtained in a Balzers 301 apparatus, and early stages of dentinogenesis were examined in a Jeol 100 CX II electron microscope.

RESULTS

In the stage of early dentine matrix prior to mineralization, odontoblasts exhibit only gap junctions. With the progression of development, the distal plasma membranes of odontoblasts show numerous short tight junctions formed by fused particles and grooves. In the stage of advanced mineralization, branched and continuous rows of fused particles or grooves constitute tight junctions of the focal or macular type.

CONCLUSIONS

The present study shows that tight junctions of focal or macular type appear on distal plasma membrane of early odontoblasts during differentiation. Formation of tight junctions indicates the establishment of a distal membrane domain and maturation of odontoblasts. These events occur as mantle dentine formation ceases and circumpulpar dentine formation begins.

Effects of a functional agar surface on in vitro dentinogenesis induced in proteolytically isolated, agar-coated dental papillae in rat mandibular incisors

In an attempt to study the effects of a three-dimensional agar surface on in vitro dentinogenesis both in the growing end and in incisally cross-cut pulp, the possible expression of odontoblast phenotype was investigated morphologically, autoradiographically and immunohistochemically. Explants were incubated for 8 days. In the growing end, during the last 4 days, mitotic cells differentiated into [3H]-thymidine-labelled, tubular matrix-forming cells. In cross-cut pulp, however, during the first 4 days, mitotic cells differentiated into [3H]-thymidine-labelled, tubular matrix-forming cells. Electron microscopy demonstrated that, in both regions, tubular matrix-forming cells had characteristics similar to those of primary odontoblasts. When agar was incubated alone, exogenous fibronectin was deposited on it rapidly. After 12 h, endogenous fibronectin appeared on explant peripheral cells. Collagen and materials reacting positively to periodic acid-Schiff (PAS) were first interposed between agar and explant after 4 days. After 8 days, an inner immunonegative layer corresponding to materials reacting positively to PAS or toluidine blue and an outer immunopositive layer of fibronectin or collagen were visible adjacent to the rows of elongated columnar cells. In the presence of Gly-Arg-Gly-Asp-Ser-Pro (GRGDSP), a competitive inhibitor of attachment of cells to fibronectin, explants became detached from the agar surface, and no dentinogenesis occurred. These results indicate that, when in contact with an agar surface that becomes modified by fibronectin and/or by a complex of fibronectin with deposited matrix, dental mesenchymal cells progressively differentiate into tubular matrix-forming cells. Possibly the functional agar surface has the important role of providing a foothold for cell attachment, which is the first step towards in vitro odontoblast differentiation. This system of inducing tubular matrix-forming cells constitutes a useful model for the study of in vitro dentinogenesis.

Structure and organization of odontoblasts

Differentiation of odontoblasts involves cell-to-cell recognition, contact stabilization involving the formation of attachment specializations, cytoplasmic polarization, development of the protein synthetic and secretory apparatus, and the active transport of mineral ions. The secretory odontoblast is characterized by an extensive rough-surfaced endoplasmic reticulum, a highly developed Golgi complex, and the presence of specific secretion granules. Type I collagen, a major constituent of dentin matrix, appears to be secreted by the odontoblast into predentin at the proximal portion of the odontoblast process, the major cytoplasmic process extending from the odontoblast cell body into the dentin. The odontoblast process contains a rich network of microtubules and microfilaments. The proximal portion of the process is also a site of fluid-phase endocytosis. Adjacent odontoblasts are held together by numerous macula adherens junctions and a well-developed distal junctional complex adjacent to be predentin. Junctional strands of the occludens type have been observed to be a component of this junctional complex. Tracer studies employing horseradish peroxidase indicate that this junctional complex does not form a tight barrier to the diffusion of tissue fluid from the interodontoblast spaces into the predentin. Many well-developed gap junctions are formed between adjacent odontoblasts and between odontoblasts and the fibroblasts that make up the subodontoblastic layer. Ca-ATPase activity is demonstrated in the Golgi complex and mitochondrial cristae and along the distal plasma membranes of odontoblasts. ALPase activity is also intense along the entire odontoblast cell surface. The osmium tetroxide-pyroantimonate technique for calcium localization demonstrates prominent reaction precipitates in mitochondria of odontoblasts. Energy-dispersive x-ray microanalysis of anhydrously fixed and processed odontoblasts detected Ca and P peaks throughout the cytoplasm. A sulfur peak is noted in the distal cytoplasm of odontoblasts and in matrix vesicles. Together, these results demonstrate the complexity and variety of cell functions involved in dentinogenesis.

Dentinogenesis

The formation of dentin, dentinogenesis, comprises a sophisticated interplay between several factors in the tissue, cellular as well as extracellular. Dentin may be regarded as a calcified connective tissue. In this respect, as well as in its mode of formation, it is closely related to bone. Using dentinogenesis as an experimental model to study biomineralization provides several practical advantages, and the results may be extrapolated to understand similar processes in other tissues, primarily bone. After describing dentin structure and composition, this review discusses items such as the morphology of dentinogenesis; the dentinogenically active odontoblast, transport, and concentrations of mineral ions; the constituents of the dentin organic matrix; and the presumed mechanisms involved in mineral formation.

The carboxy-terminal extension of the collagen binding domain of fibronectin mediates interaction with a 165 kDa membrane protein involved in odontoblast differentiation

Terminal differentiation of the odontoblast is characterized by an elongation and a polarization of the cell. The change in the cell shape and the reorganization of the cytoplasm involve the microfilament system. An immunological approach has previously implicated a transmembrane interaction between fibronectin and vinculin in the control of odontoblast differentiation. A 165 kDa protein localized on the cell-surface of odontoblasts mediated this interaction. In order to define the nature of the interaction of the 165 kDa protein with fibronectin, peptides were prepared by proteolytic cleavage of fibronectin with alpha-chymotrypsin. The results indicate that the 165 kDa protein interacted with a 62 kDa peptide located towards the amino-terminal extremity of fibronectin, but not with a 47 kDa related fragment. Both these 62 kDa and 47 kDa peptides included the collagen-binding domain and were retarded on a heparin-Ultrogel column. Microsequences demonstrated that the 62 kDa and 47 kDa fragments had the same amino-terminal extremity and that the larger fragment was extended in the carboxy-terminal direction. This carboxy-terminal extension of the collagen binding domain of fibronectin is implicated in the interaction of this molecule with the 165 kDa protein. On the other hand, odontoblasts differentiated normally when tooth germs were cultured in the presence of GRGDS synthetic peptide, suggesting that RGD-dependent integrins were not involved in odontoblast differentiation. Staining of dental mesenchymal cells in primary culture and of differentiated odontoblasts in situ with antibodies directed against the beta 1-subunit of integrins confirmed previous observations and showed that although beta 1 integrins are involved in the attachment of cultured dental cells, they are not implicated in the process of odontoblast differentiation.

Demonstration of physiological barrier between pulpal odontoblasts and its perturbation following routine restorative procedures: a horseradish peroxidase tracing study in the rat

Vascular injection of the macromolecular tracer, horseradish peroxidase (HRP), was used to study the permeability of the odontoblast cell layer in developing and mature rat molar teeth, and to investigate the effect of cavity preparations on the permeability of this epithelioid cell layer in adult animals. HRP injected into the vascular system of normal animals 28 days of age and older was localized histochemically (from 5 to 90 min after injection) throughout the extracellular spaces of the maxillary dental pulps; however, the tracer did not penetrate beyond the tight junctions at the apical region of the odontoblast cell layer, and was absent from the predentin and dentin. In contrast, HRP injected into very young neonatal animals (e.g., day 3) resulted in free passage of HRP between odontoblasts and into the overlying predentin and dentin. When Class V cavities had been prepared in adult maxillary molars after HRP was injected into the blood stream, HRP reaction product penetrated the predentin and dentin immediately beneath the cavity preparation; however, adjacent, untraumatized areas of predentin and dentin in the operated teeth were devoid of reaction product. These results provide evidence that: (1) a physiological barrier develops between the distal segments of odontoblast cell bodies in normal rat molar teeth between days 15 and 28 of postnatal life, and this barrier prevents the passage of macromolecules from the pulp into the predentin and dentin; and (2) this barrier is perturbed following routine restorative procedures in adult animals.

Ultrastructure of a new generation of odontoblasts in grafted coronal tissues of mouse molar tooth germs

Third molar tooth germs were removed from 14-day-old mice and freed from the enamel organ and follicle. After section of the apical tissues, including Hertwig's sheath, they were transplanted in 1-day-old newborn mice of the same lineage. Electron microscopy of grafts removed 7, 14 and 21 days later showed that, following the disappearance of the initial layer of odontoblasts and a period of adaptation, 14 days after transplantation newly differentiated odontoblasts deposited tubular dentine. The dentine matrix production was increased over that of controls, demonstrating that synthesis was accelerated, possibly because of lack of nerves in the grafts. Numerous characteristic structures that might be involved in the transit of proteoglycans from the Golgi apparatus were seen, as far as the extremity of the odontoblast processes. The particular experimental conditions allowed the observation in the neck region of the odontoblast of a concentration of coated vesicles which might be involved in cellular lengthening. Thus, in the presence of a fine and regular vascular network, a new generation of odontoblasts may differentiate, even in the absence of epithelial and nervous elements, and so predentine may contain inductive factors that allow the odontoblastic differentiation of pulp cells in contact with it.

Morphological changes of intercellular junctions in the rat submandibular gland treated by long-term repeated administration of isoproterenol

Long-term repeated administration of isoproterenol (IPR) 2 mg/100 g bw, once daily for ten days, resulted in morphological changes in the intercellular junctions of rat submandibular glands, which were investigated by means of the freeze-fracture technique. A significantly increased number of tight-junctional strands was present. These junctional strands extended much deeper toward the basal membrane than those in normal acinar cells. The basal frontier strands that branched from the networks of tight junctions were elongated and had either free-endings or terminal loops, which were more frequently observed in the IPR-treated acinar cells than in untreated acinar cells. Some of the strands of tight junctions were connected to small gap junctions. The diameters of gap junctions were not significantly different from those of control acinar cells. However, smooth areas devoid of particles were found intermingling with the usual packed particles in irregularly shaped small gap junctions. There was no significant difference between the desmosomes of IPR-treated and untreated acinar cells, in terms of either morphology or distribution. These changes in junctional morphology in the IPR-treated acinar cells resemble those seen in salivary glands during development, and in some experimental conditions including tumorous changes.