Cells at the Edge: The Dentin-Bone Interface in Zebrafish Teeth

Vertebrate mineralized tissues: A modular structural analysis

Vertebrate mineralized tissues, present in bones, teeth and scales, have complex 3D hierarchical structures. As more of these tissues are characterized in 3D using mainly FIB SEM at a resolution that reveals the mineralized collagen fibrils and their organization into collagen fibril bundles, highly complex and diverse structures are being revealed. In this perspective we propose an approach to analyzing these tissues based on the presence of modular structures: material textures, pore shapes and sizes, as well as extents of mineralization. This modular approach is complimentary to the widely used hierarchical approach for describing these mineralized tissues. We present a series of case studies that show how some of the same structural modules can be found in different mineralized tissues, including in bone, dentin and scales. The organizations in 3D of the various structural modules in different tissues may differ. This approach facilitates the framing of basic questions such as: are the spatial relations between modular structures the same or similar in different mineralized tissues? Do tissues with similar sets of modules carry out similar functions or can similar functions be carried out using a different set of modular structures? Do mineralized tissues with similar sets of modules have a common developmental or evolutionary pathway? STATEMENT OF SIGNIFICANCE: 3D organization studies of diverse vertebrate mineralized tissues are revealing detailed, but often confusing details about the material textures, the arrangements of pores and differences in the extent of mineralization within a tissue. The widely used hierarchical scheme for describing such organizations does not adequately provide a basis for comparing these tissues, or addressing issues such as structural components thought to be characteristic of bone, being present in dermal tissues and so on. The classification scheme we present is based on identifying structural components within a tissue that can then be systematically compared to other vertebrate mineralized tissues. We anticipate that this classification approach will provide insights into structure-function relations, as well as the evolution of these tissues.

Focused ion beam-SEM 3D study of osteodentin in the teeth of the Atlantic wolfish Anarhichas lupus

The palette of mineralized tissues in fish is wide, and this is particularly apparent in fish dentin. While the teeth of all vertebrates except fish contain a single dentinal tissue type, called orthodentin, dentin in the teeth of fish can be one of several different tissue types. The most common dentin type in fish is orthodentin. Orthodentin is characterized by several key structural features that are fundamentally different from those of bone and from those of osteodentin. Osteodentin, the second-most common dentin type in fish (based on the tiny fraction of fish species out of ∼30,000 extant fish species in which tooth structure was so far studied), is found in most Selachians (sharks and rays) as well as in several teleost species, and is structurally different from orthodentin. Here we examine the hypothesis that osteodentin is similar to anosteocytic bone tissue in terms of its micro- and nano-structure. We use Focused Ion Beam-Scanning Electron Microscopy (FIB/SEM), as well as several other high-resolution imaging techniques, to characterize the 3D architecture of the three main components of osteodentin (denteons, inter-denteonal matrix, and the transition zone between them). We show that the matrix of osteodentin, although acellular, is extremely similar to mammalian osteonal bone matrix, both in general morphology and in the three-dimensional nano-arrangement of its mineralized collagen fibrils. We also document the presence of a complex network of nano-channels, similar to such networks recently described in bone. Finally, we document the presence of strings of hyper-mineralized small 'pearls' which surround the denteonal canals, and characterize their structure.

Expression of secretory calcium-binding phosphoprotein (scpp) genes in medaka during the formation and replacement of pharyngeal teeth

BACKGROUND

Analyses of tooth families and tooth-forming units in medaka with regard to tooth replacement cycles and the localization of odontogenic stem cell niches in the pharyngeal dentition clearly indicate that continuous tooth replacement is maintained. The secretory calcium-binding phosphoprotein (scpp) gene cluster is involved in the formation of mineralized tissues, such as dental and bone tissues, and the genes encoding multiple SCPPs are conserved in fish, amphibians, reptiles, and mammals. In the present study, we examined the expression patterns of several scpp genes in the pharyngeal teeth of medaka to elucidate their roles during tooth formation and replacement.

METHODS

Himedaka (Japanese medaka, Oryzias latipes) of both sexes (body length: 28 to 33 mm) were used in this study. Real-time quantitative reverse transcription-polymerase chain reaction (PCR) (qPCR) data were evaluated using one-way analysis of variance for multi-group comparisons, and the significance of differences was determined by Tukey's comparison test. The expression of scpp genes was examined using in situ hybridization (ISH) with a digoxigenin-labeled, single-stranded antisense probe.

RESULTS

qPCR results showed that several scpp genes were strongly expressed in pharyngeal tissues. ISH analysis revealed specific expression of scpp1, scpp5, and sparc in tooth germ, and scpp5 was continually expressed in the odontoblasts of teeth attached to pedicles, but not in the osteoblasts of pedicles. In addition, many scpp genes were expressed in inner dental epithelium (ide), but not in odontoblasts, and scpp2 consistently showed epithelial-specific expression in the functional teeth. Taken together, these data indicate that specific expression of scpp2 and scpp5 may play a critical role in pharyngeal tooth formation in medaka.

CONCLUSION

We characterized changes in the expression patterns of scpp genes in medaka during the formation and replacement of pharyngeal teeth.

Spatiotemporal characteristics of the pharyngeal teeth in interspecific distant hybrids of cyprinid fish: Phylogeny and expression of the initiation marker genes

As an important feeding organ and taxonomical characteristic, the pharyngeal teeth of cyprinid fish have very high morphological diversity and exhibit species-specific numbers and arrangements. Many genes have been verified to regulate the pharyngeal teeth development and act as the initiation marker for teeth. Six initiation marker genes for pharyngeal teeth were used as RNA probes to investigate the expression pattern, and these genes were further used to construct a phylogenetic tree for cyprinid fish including some distant hybrids. The results from in situ hybridization showed that similarities and differences existed in the expression of dlx2b, dlx4b, dlx5a, pitx2, fth1b, and scpp5 in the pharyngeal region of the hybrids (BT) by the crosses of blunt snout bream (BSB, ♀) × topmouth culter (TC, ♂). Particularly, we found a high specificity marker gene scpp5 for the early development of pharyngeal teeth. The Scpp5 expression pattern established a clear graphic representation on the spatiotemporal characteristics of the early morphogenesis of pharyngeal teeth in BT and BSB. Our results suggested that the scpp5 expression in 4V¹, 3V¹, and 5V¹ in BT occurred earlier than that in BSB, while the replacement rate of pharyngeal teeth (4V², 3V², and 5V²) was faster in BSB. Phylogenetic analysis revealed that the six marker genes were highly conserved and could be used as the molecular marker for identifying the parents of the distant hybrids in cyprinid fish. The expression patterns of the scpp5 gene was examined in various tissues, including the brain, gill, heart, liver, muscle, skin, fins, gonad, eye, and kidney, showing that the scpp5 gene was ubiquitously expressed, indicating its important role in cyprinid fish.

Dental structure and tooth attachment modes in the common fangtooth Anoplogaster cornuta (Valenciennes, 1833) (Actinopterygii; Trachichthyiformes; Anoplogastridae)

We studied the structure and attachment modes of the teeth of adult Anoplogaster cornuta using light- and scanning-electron microscopic techniques. All teeth were monocuspid, composed solely of orthodentin, and lacked a covering enameloid cap. Fourteen teeth were present in the oral jaws, with three teeth each on the left and right premaxilla and four teeth each on the left and right dentary. The anteriormost premaxillary and dentary teeth were considerably larger than the more posteriorly located ones. The oral jaw teeth were transparent, non-depressible and firmly ankylosed to their respective dentigerous bone by a largely anosteocytic bone of attachment. No evidence for replacement of the large oral jaw teeth was found in the analyzed adult specimens. The bone of attachment exhibited lower calcium and phosphorus concentrations and a higher Ca/P ratio than the orthodentin. The connection between dentinal tooth shaft and bone of attachment was stabilized by a collar of mineralized collagen fibers. In contrast to the oral jaw teeth, the pharyngeal teeth exhibited a ring-like fibrous attachment to their supporting bones. This mode of attachment provides the teeth with some lateral mobility and allows their depression relative to their supporting bones, which may facilitate intra-pharyngeal prey transport. In contrast, a firm ankylosis was observed in numerous small teeth located on the branchial arches. The function of these teeth is presumably to increase the tightness of the pharyngeal basket and thereby the retention of small prey items in a species living in a habitat with only sparse food supply. Our findings corroborate earlier statements on the tooth attachment modes of the oral jaw teeth of Anoplogaster cornuta, but provide new findings for the attachment modes of pharyngeal teeth in this species.

Plicidentine in the oral fangs of parrotfish (Scarinae, Labriformes)

Parrotfish play important ecological roles in coral reef and seagrass communities across the globe. Their dentition is a fascinating object of study from an anatomical, functional and evolutionary point of view. Several species maintained non-interlocked dentition and browse on fleshy algae, while others evolved a characteristic beak-like structure made of a mass of coalesced teeth that they use to scrape or excavate food off hard limestone substrates. While parrotfish use their highly specialized marginal teeth to procure their food, they can also develop a series of large fangs that protrude from the upper jaw, and more rarely from the lower jaw. These peculiar fangs do not participate in the marginal dentition and their function remains unclear. Here we describe the morphology of these fangs and their developmental relationship to the rest of the oral dentition in the marbled parrotfish (Leptoscarus vaigiensis), the star-eye parrotfish (Calotomus carolinus), and the palenose parrotfish (Scarus psittacus). Through microtomographic and histological analyses, we show that some of these fangs display loosely folded plicidentine along their bases, a feature that has never been reported in parrotfish. Plicidentine is absent from the marginal teeth and is therefore exclusive to the fangs. Parrotfish fangs develop a particular type of simplexodont plicidentine with a pulpal infilling of alveolar bone at later stages of dental ontogeny. The occurrence of plicidentine and evidence of extensive tooth wear, and even breakage, lead us to conclude that the fangs undergo frequent mechanical stress, despite not being used to acquire food. This strong mechanical stress undergone by fangs could be linked either to forced contact with congeners or with the limestone substrate during feeding. Finally, we hypothesize that the presence of plicidentine in parrotfish is not derived from a labrid ancestor, but is probably a recently evolved trait in some parrotfish taxa, which may even have evolved convergently within this subfamily.

Osteodentin in the Atlantic wolffish (Anarhichas lupus): Dentin or bone?

The teeth of actinopterygian fish, like those of mammals, consist of a thin outer hyper-mineralized layer (enamel or enameloid) that surrounds a core of dentin. While all mammalian species have a single type of dentin (called orthodentin), various dentin types have been reported in the teeth of actinopterygian fish. The most common type of actinopterygian fish dentin is orthodentin. However, the second most common type of actinopterygian fish dentin, called osteodentin, found in several teleost species and in many Selachians, is structurally radically different from orthodentin. Osteodentin, comprising denteons and inter-denteonal matrix, is characterized by an appearance that is similar to mammalian osteonal bone, however, it lacks cells and a lacuno-canalicular system. The current consensus is that although osteodentin is morphologically different from orthodentin, it is a true dentinal material, the product of odontoblast cells. We present the results of a study of osteodentin found in the teeth of the Atlantic wolffish, Anarhichas lupus. Using a variety of microscopy techniques, high-resolution microCT scans, and micro-indentation we describe the three-dimensional structure of both its components (denteons and inter-denteonal matrix), as well as their mineral density distribution and mechanical properties, at several length-scales. We show that wolffish osteodentin is remarkably similar to the anosteocytic bone of the swords of several swordfish species. We also describe the three-dimensional network of canals found in mature osteodentin. The high density of these canals in a metabolically inactive, acellular tissue casts doubt upon the accepted paradigm, that the canals house a vascular network.