Early development of the lower deciduous dentition and oral vestibule in human embryos

Mechanisms driving vestibular lamina formation and opening in the mouse

The vestibular lamina (VL) forms as an epithelial outgrowth parallel to the dental lamina (DL) in the oral cavity. During late development, it opens to create a furrow that divides the dental tissue from the cheeks and lips and is known as the vestibule. Defects in this process lead to failure in the separation of the teeth from the lips and cheeks, including the presence of multiple frenula. In this paper, the development of the VL is followed in the mouse, from epithelial placode in the embryo to postnatal opening and vestibule formation. During early outgrowth, differential proliferation controls the curvature of the VL as it extends under the forming incisors. Apoptosis plays a role in thinning the deepest part of the lamina, while terminal differentiation of the epithelium, highlighted by the expression of loricrin and flattening of the nuclei, predates the division of the VL into two to create the vestibule. Development in the mouse is compared to the human VL, with respect to the relationship of the VL to the DL, VL morphology and mechanisms of opening. Overall, this paper provides insight into an understudied part of the oral anatomy, shedding light on how defects could form in this region.

Reawakening of Ancestral Dental Potential as a Mechanism to Explain Dental Pathologies

During evolution, there has been a trend to reduce both the number of teeth and the location where they are found within the oral cavity. In mammals, the formation of teeth is restricted to a horseshoe band of odontogenic tissue, creating a single dental arch on the top and bottom of the jaw. Additional teeth and structures containing dental tissue, such as odontogenic tumors or cysts, can appear as pathologies. These tooth-like structures can be associated with the normal dentition, appearing within the dental arch, or in nondental areas. The etiology of these pathologies is not well elucidated. Reawakening of the potential to form teeth in different parts of the oral cavity could explain the origin of dental pathologies outside the dental arch, thus such pathologies are a consequence of our evolutionary history. In this review, we look at the changing pattern of tooth formation within the oral cavity during vertebrate evolution, the potential to form additional tooth-like structures in mammals, and discuss how this knowledge shapes our understanding of dental pathologies in humans.

Development of the Vestibular Lamina in Human Embryos: Morphogenesis and Vestibule Formation

The vestibular lamina (VL) is a transient developmental structure that forms the lip furrow, creating a gap between the lips/cheeks and teeth (oral vestibule). Surprisingly, little is known about the development of the VL and its relationship to the adjacent dental lamina (DL), which forms the teeth. In some congenital disorders, such as Ellis-van Creveld (EVC) syndrome, development of the VL is disrupted and multiple supernumerary frenula form, physically linking the lips and teeth. Here, we assess the normal development of the VL in human embryos from 6.5 (CS19) to 13 weeks of development, showing the close relationship between the VL and DL, from initiation to differentiation. In the anterior lower region, the two structures arise from the same epithelial thickening. The VL then undergoes complex morphogenetic changes during development, forming a branched structure that separates to create the vestibule. Changing expression of keratins highlight the differentiation patterns in the VL, with fissure formation linked to the onset of filaggrin. Apoptosis is involved in removal of the central portion of the VL to create a broad furrow between the future cheek and gum. This research forms an essential base to further explore developmental defects in this part of the oral cavity.

Early development of the human dentition revisited

In this review, classical data on the early steps in human odontogenesis are summarized and updated with specific insights into the development of the upper and lower embryonic jaws to help in understanding some oral pathologies. The initial step of human odontogenesis is classically characterized by two parallel horseshoe-shaped epithelial laminae. These originate from the oral epithelium and an ingrowth into the jaw mesenchyme: the internal dental lamina gives rise to deciduous tooth primordia, while the external vestibular lamina represents the developmental base of the oral vestibule. However, a more complex situation was revealed by recent studies combining analyses of the dental and adjacent oral epithelia on histological sections and computer-aided three-dimensional (3D) reconstructions during the 2nd month of human embryonic development. The dental epithelium forms a mound, where swellings appear later, corresponding to the individual primordia of deciduous teeth. External to the developing deciduous dentition, the 3D reconstructions do not show any continuous vestibular lamina but instead a complex of discontinuous epithelial bulges and ridges. The patterns of these epithelial structures and their relationship to the dental epithelium differ not only between the upper and lower jaws but also between the lip and cheek segments in each jaw. Knowledge of early odontogenesis may help in understanding some oral pathologies. For example, the human lateral incisor has a dual origin: it arises in the area of fusion between the medial nasal and maxillary facial processes and involves material from these two regions. Such a dual origin at the site of fusion of facial processes represents a predisposition to developmental vulnerability for the upper lateral incisor, resulting in its frequent anomalies (absence, hypoplasia, duplication), especially in patients with a cleft lip and/or jaw. Other pathologies, such as a minute supernumerary tooth, desmoplastic ameloblastoma or extraosseous odontogenic cysts are located external to the upper or lower dentition, and might be derived from structures that transiently appear during early development of the oral vestibule in humans.

Spatiotemporal Expression of Wnt/β-catenin Signaling during Morphogenesis and Odontogenesis of Deciduous Molar in Miniature Pig

The canonical Wnt/β-catenin signaling pathway has been shown to play essential roles in tooth initiation and early tooth development. However, the role of Wnt/β-catenin signaling in cusp patterning and crown calcification in large mammals are largely unknown. In our previous study, miniature pigs were used as the animal model due to the similarity of tooth anatomy and replacement pattern between miniature pig and human. Dynamic gene expression of third deciduous molar (DM3) in miniature pig at early stages was profiled using microarray method and expression of Wnt genes was significantly correlate with odontogenesis. In the present study, dynamic expression patterns of Wnt/β-catenin signaling genes of DM3 at cap, early bell and late bell (secretory) stage were identified. We found that Lef1 and Axin2 were expressed in the enamel knot and underlying mesenchyme regions. Meanwhile, Dkk1 was expressed in the peripheral and lower parts of dental papilla, thus forming the potential Wnt signaling gradient. We also found that β-Catenin, Axin2 and Lef1 were expressed strongly in undifferentiated cells of the inner enamel epithelium (IEE), but weakly in differentiated ameloblasts. Furthermore, we found that both Wnt signaling read-out gene Lef1 and the inhibitor Dkk1 were co-expressed in the pre-odontoblasts. In conclusion, the spatiotemporal distribution and potential gradient of Wnt signaling may contribute to cusp patterning and crown calcification. These data may yield insight into future study of precise control of crown morphogenesis and regeneration in large mammals.

The Biology Underlying Abnormalities of Tooth Number in Humans

In past decades, morphologic, molecular, and cellular mechanisms that govern tooth development have been extensively studied. These studies demonstrated that the same signaling pathways regulate development of the primary and successional teeth. Mutations of these pathways lead to abnormalities in tooth development and number, including aberrant tooth shape, tooth agenesis, and formation of extra teeth. Here, we summarize the current knowledge on the development of the primary and successional teeth in animal models and describe some of the common tooth abnormalities in humans.

One Odontogenic Cell-Population Contributes to the Development of the Mouse Incisors and of the Oral Vestibule

The area of the oral vestibule is often a place where pathologies appear (e.g., peripheral odontomas). The origin of these pathologies is not fully understood. In the present study, we traced a cell population expressing Sonic hedgehog (Shh) from the beginning of tooth development using Cre-LoxP system in the lower jaw of wild-type (WT) mice. We focused on Shh expression in the area of the early appearing rudimentary incisor germs located anteriorly to the prospective incisors. The localization of the labelled cells in the incisor germs and also in the inner epithelial layer of the vestibular anlage showed that the first very early developmental events in the lower incisor area are common to the vestibulum oris and the prospective incisor primordia in mice. Scanning electron microscopic analysis of human historical tooth-like structures found in the vestibular area of jaws confirmed their relation to teeth and thus the capability of the vestibular tissue to form teeth. The location of labelled cells descendant of the early appearing Shh expression domain related to the rudimentary incisor anlage not only in the rudimentary and functional incisor germs but also in the externally located anlage of the oral vestibule documented the odontogenic potential of the vestibular epithelium. This potential can be awakened under pathological conditions and become a source of pathologies in the vestibular area.

A Large Extragnathic Keratocystic Odontogenic Tumour

Odontogenic keratocysts (OKCs) are developmental cysts which occur typically in the jawbones. They present more commonly in the posterior mandible of young adults than the maxilla. OKCs have been reclassified under odontogenic tumours in 2005 by the WHO and have since been termed as keratocystic odontogenic tumours (KCOTs). Here we report a case of a recurrent buccal lesion in a 62-year-old man which was provisionally diagnosed as a space infection (buccal abscess) but surprisingly turned out to be a soft tissue KCOT in an unusual location on histopathologic examination.

Three-dimensional analysis of the early development of the dentition

Tooth development has attracted the attention of researchers since the 19th century. It became obvious even then that morphogenesis could not fully be appreciated from two-dimensional histological sections. Therefore, methods of three-dimensional (3D) reconstructions were employed to visualize the surface morphology of developing structures and to help appreciate the complexity of early tooth morphogenesis. The present review surveys the data provided by computer-aided 3D analyses to update classical knowledge of early odontogenesis in the laboratory mouse and in humans. 3D reconstructions have demonstrated that odontogenesis in the early stages is a complex process which also includes the development of rudimentary odontogenic structures with different fates. Their developmental, evolutionary, and pathological aspects are discussed. The combination of in situ hybridization and 3D reconstruction have demonstrated the temporo-spatial dynamics of the signalling centres that reflect transient existence of rudimentary tooth primordia at loci where teeth were present in ancestors. The rudiments can rescue their suppressed development and revitalize, and then their subsequent autonomous development can give rise to oral pathologies. This shows that tooth-forming potential in mammals can be greater than that observed from their functional dentitions. From this perspective, the mouse rudimentary tooth primordia represent a natural model to test possibilities of tooth regeneration.

Initiation of teeth from the dental lamina in the ferret

Mammalian tooth development is characterized by formation of primary teeth that belong to different tooth classes and are later replaced by a single set of permanent teeth. The first primary teeth are initiated from the primary dental lamina, and the replacement teeth from the successional dental lamina at the lingual side of the primary teeth. An interdental lamina connects the primary tooth germs together. Most mammalian tooth development research is done on mouse, which does not have teeth in all tooth classes, does not replace its teeth, and does not develop an interdental lamina. We have used the ferret (Mustela putorius furo) as a model animal to elucidate the morphological changes and gene expression during the development of the interdental lamina and the initiation of primary teeth. In addition we have analyzed cell-cell signaling taking place in the interdental lamina as well as in the successional lamina during tooth replacement. By 3D reconstructions of serial histological sections we observed that the morphogenesis of the interdental lamina and the primary teeth are intimately linked. Expression of Pitx2 and Foxi3 in the interdental lamina indicates that it has odontogenic identity, and there is active signaling taking place in the interdental lamina. Bmp4 is coexpressed with the stem cell factor Sox2 at its lingual aspect suggesting that the interdental lamina may retain competence for tooth initiation. We show that when tooth replacement is initiated there is Wnt pathway activity in the budding successional lamina and adjacent mesenchyme but no active Fgf or Eda signaling. Genes associated with human tooth replacement phenotypes, including Runx2 and Il11rα, are mostly expressed in the mesenchyme around the successional lamina in the ferret. Our results highlight the importance of the dental lamina in the mammalian tooth development during the initiation of both primary and replacement teeth.

A keratocyst in the buccal mucosa with the features of keratocystic odontogenic tumor

A 74-year-old male patient consulted us for an elastic firm mass in the right buccal mucosa. CT examination revealed a well-circumscribed oval cystic lesion in the anterior region of the masseter muscle. On MRI, the lesion showed a low signal on T1-weighted image and a high signal on T2-weighted image. Aspiration biopsy demonstrated the presence of squamous cells in whitish liquid. Under the diagnosis of epidermoid cyst, the lesion was intraorally extirpated under general anesthesia. The lesion was cystic at the size of 30 × 25mm. Histologically, the cyst wall was lined with parakeratinized squamous epithelium corrugated on its surface, the basal layer of which consisted of cuboidal cells showing palisading of the nuclei. Immunohistochemically, the lining epithelium was positive for CK17 and negative for CK10. The basal and suprabasal cells were labeled for Ki-67 at a relatively high rate. These features are compatible with those of keratocystic odontogenic tumor.

Shh expression in a rudimentary tooth offers new insights into development of the mouse incisor

For teeth as for any organ, knowledge of normal development is essential for the proper interpretation of developmental anomalies in mutant mice. It is generally accepted that tooth formation is initiated with a single signaling center that, in the incisor region, is exclusively related to the development of the functional adult incisor. Here, using a unique combination of computer-aided three-dimensional reconstructions and whole mount in situ hybridization of mandibles from finely staged wild-type mouse embryos, we demonstrate that several Sonic hedgehog (Shh) expression domains sequentially appear in the lower incisor region during early development. In contrast to the single Shh expression domain that is widely assumed to be present in each lower incisor area at ED12.5-13.5, we identified two spatially distinct regions of Shh expression that appear in an anterior-posterior sequence during this period. The initial anterior, more superficially located Shh expression region represented the rudimentary (so-called deciduous) incisor, whereas only the later posterior deeper situated region corresponded to the prospective functional incisor. In the more advanced embryos, only this posterior Shh expression in the incisor bud was detectable as a precursor of the enamel knot. This study offers a new interpretation of published molecular data on the mouse incisor from initiation through ED13.5. We suggest that, as with Shh expression, other molecular data that have been ascribed to the progressive development of the mouse functional incisor at early stages, in fact, correspond to a rudimentary incisor whose development is aborted.

Spatial and temporal distribution of Ki-67 proliferation marker, Bcl-2 and Bax proteins in the developing human tooth

OBJECTIVE

To investigate the spatial and temporal expression of proliferation Ki-67 marker, pro-apoptotic Bax and anti-apoptotic Bcl-2 proteins during early development of the human tooth.

MATERIALS AND METHODS

Histological sections of eight human conceptuses, 5-10 postovulatory weeks old, were used for immunolocalization for Ki-67, Bax and Bcl-2 markers. Quantification was performed by calculating the fraction of Ki-67 positive cells, expressed as a mean ± SD, and analysed by Mann-Whitney test, Kruskal-Wallis and Dunn's post hoc test.

RESULTS

In 6th-7th developmental weeks, the tooth germ and dental crest contained 37% of proliferating cells, which increased to 40% in the 8th week, and then decreased to 15% in the 10th week, whilst the proliferation in the ectomesenchyme subsequently dropped from 37% to 23%. Epithelial parts of the enamel organ displayed similar proliferation activity (31-36%), dental crest 10%, whilst enamel knot showed no proliferating activity. The tooth ectomesenchyme contained more proliferating cells (50%) than the jaw ectomesenchyme (35%), and both dropped to 28% in the 10th week. Ectomesenchyme between the tooth germs contained 23%, whilst the jaw ectomesenchyme contained 15% of proliferating cells. Bcl-2 expression had following pattern: strong in proliferating cells, moderate in tooth germs and dental crest, and weak in the ectomesenchyme. Bax co-expressed with Bcl-2 in the tooth germ and dental crest. In the reticulum and inner enamel epithelium Bcl-2 had prevalent expression, whilst Bax prevailed in the outer enamel epithelium and tooth ectomesenchyme.

CONCLUSIONS

Proliferating cells most likely influence growth of the tooth germ, Bcl-2 affects proliferation and differentiation of specific cell lineages, whilst Bax influences process of cell death.

Plate-like permanent dental laminae of upper jaw dentition in adult gobiid fish, Sicyopterus japonicus

Sicyopterus japonicus (Teleostei, Gobiidae) possesses a unique upper jaw dentition different from that known for any other teleosts. In the adults, many (up to 30) replacement teeth, from initiation to attachment, are arranged orderly in a semicircular-like strand within a capsule of connective tissue on the labial side of each premaxillary bone. We have applied histological, ultrastructural, and three-dimensional imaging from serial sections to obtain insights into the distribution and morphological features of the dental lamina in the upper jaw dentition of adult S. japonicus. The adult fish has numerous permanent dental laminae, each of which is an infolding of the oral epithelium at the labial side of the functional tooth and forms a thin plate-like structure with a wavy contour. All replacement teeth of a semicircular-like strand are connected to the plate-like dental lamina by the outer dental epithelium and form a tooth family; neighboring tooth families are completely separated from each other. The new tooth germ directly buds off from the ventro-labial margin of the dental lamina, whereas no distinct free end of the dental lamina is present, even adjacent to this region. Cell proliferation concentrated at the ventro-labial margin of the dental lamina suggests that this region is the site for repeated tooth initiation. During tooth development, the replacement tooth migrates along a semicircular-like strand and eventually erupts through the dental lamina into the oral epithelium at the labial side of the functional tooth. This unique thin plate-like permanent dental lamina and the semicircular-like strand of replacement teeth in the upper jaw dentition of adult S. japonicus probably evolved as a dental adaptation related to the rapid replacement of teeth dictated by the specialized feeding habit of this algae-scraping fish.

Dental lamina as source of odontogenic stem cells: evolutionary origins and developmental control of tooth generation in gnathostomes

This study considers stem cells for odontogenic capability in biological tooth renewal in the broad context of gnathostome dentitions and the derivation of them from oral epithelium. The location of the developmental site and cell dynamics of the dental lamina are parameters of a possible source for odontogenic epithelial stem cells, but the phylogenetic history is not known. Understanding the phylogenetic basis for stem cell origins throughout continuous tooth renewal in basal jawed vertebrates is the ultimate objective of this study. The key to understanding the origin and location of stem cells in the development of the dentition is sequestration of stem cells locally for programmed tooth renewal. We suggest not only the initial pattern differences in each dentate field but local control subsequently for tooth renewal within each family. The role of the specialized odontogenic epithelium (odontogenic band) is considered as that in which the stem cells reside and become partitioned. These regulate time, position and shape in sequential tooth production. New histological data for chondrichthyan fish show first a thickening of the oral epithelium (odontogenic band). After this, all primary and successive teeth are only generated deep to the oral epithelium from a dental lamina. In contrast, in osteichthyan fish the first teeth develop directly within the odontogenic band. In addition, successors are initiated at each tooth site in the predecessor tooth germ (without a dental lamina). We suggest that stem cells specified for each tooth family are set up and located in intermediate cells between the outer and inner dental epithelia.