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.

Dual embryonic origin of maxillary lateral incisors: clinical implications in patients with cleft lip and palate.

INTRODUCTION: Cleft lip and palate are craniofacial anomalies highly prevalent in the overall population. In oral clefts involving the alveolar ridge, variations of number, shape, size and position are observed in maxillary lateral incisors. The objective of this manuscript is to elucidate the embryonic origin of maxillary lateral incisors in order to understand the etiology of these variations.Contextualization: The hypothesis that orofacial clefts would split maxillary lateral incisor buds has been previously reported. However, recent studies showed that maxillary lateral incisors have dual embryonic origin, being partially formed by both the medial nasal process and the maxillary process. In other words, the mesial half of the lateral incisor seems to come from the medial nasal process while the distal half of the lateral incisor originates from the maxillary process. In cleft patients, these processes do not fuse, which results in different numerical and positional patterns for lateral incisors relating to the alveolar cleft. In addition to these considerations, this study proposes a nomenclature for maxillary lateral incisors in patients with cleft lip and palate, based on embryology and lateral incisors position in relation to the alveolar cleft. CONCLUSION: Embryological knowledge on the dual origin of maxillary lateral incisors and the use of a proper nomenclature for their numerical and positional variations renders appropriate communication among professionals and treatment planning easier, in addition to standardizing research analysis.

Dual embryonic origin of maxillary lateral incisors: clinical implications in patients with cleft lip and palate

Introduction:Cleft lip and palate are craniofacial anomalies highly prevalent in the overall population. In oral clefts involving the alveolar ridge, variations of number, shape, size and position are observed in maxillary lateral incisors. The objective of this manuscript is to elucidate the embryonic origin of maxillary lateral incisors in order to understand the etiology of these variations.Contextualization: The hypothesis that orofacial clefts would split maxillary lateral incisor buds has been previously reported. However, recent studies showed that maxillary lateral incisors have dual embryonic origin, being partially formed by both the medial nasal process and the maxillary process. In other words, the mesial half of the lateral incisor seems to come from the medial nasal process while the distal half of the lateral incisor originates from the maxillary process. In cleft patients, these processes do not fuse, which results in different numerical and positional patterns for lateral incisors relating to the alveolar cleft. In addition to these considerations, this study proposes a nomenclature for maxillary lateral incisors in patients with cleft lip and palate, based on embryology and lateral incisors position in relation to the alveolar cleft.Conclusion:Embryological knowledge on the dual origin of maxillary lateral incisors and the use of a proper nomenclature for their numerical and positional variations renders appropriate communication among professionals and treatment planning easier, in addition to standardizing research analysis.

Introdução:as fissuras de lábio e palato são malformações de alta prevalência na população. Nas fissuras que envolvem o rebordo alveolar, o incisivo lateral superior mostra variações de número, forma, tamanho e posição, o que o torna objeto de estudo, na tentativa de elucidar sua origem embrionária para compreender a etiologia dessas alterações.Contextualização:existia a hipótese de que a fissura orofacial seria capaz de segmentar o botão embrionário do incisivo lateral. No entanto, estudos recentes evidenciaram que o incisivo lateral superior possui dupla origem embrionária, sendo formado parcialmente pelo processo nasal medial e pelo processo maxilar. Em outras palavras, a metade mesial do incisivo lateral provém do processo nasal medial, enquanto a metade distal do incisivo lateral origina-se do processo maxilar. No paciente com fissura, não há fusão desses processos, o que resulta nos diferentes padrões numéricos e posicionais do incisivo lateral em relação à fissura. Além dessas considerações, propõe-se também uma nomenclatura para o incisivo lateral em pacientes com fissura labiopalatina, com embasamento na Embriologia, considerando-se sua posição em relação à fissura alveolar.Conclusão:o conhecimento embriológico da dupla origem do incisivo lateral superior e o emprego de uma nomenclatura adequada para as suas variações numéricas e posicionais facilita a comunicação entre profissionais, o planejamento dos casos e possibilita a realização de estudos clínicos comparativos.

Craniofacial and dental development in Costello syndrome.

Costello syndrome (CS) is a RASopathy characterized by a wide range of cardiac, musculoskeletal, dermatological, and developmental abnormalities. The RASopathies are defined as a group of syndromes caused by activated Ras/mitogen-activated protein kinase (MAPK) signaling. Specifically, CS is caused by activating mutations in HRAS. Although receptor tyrosine kinase (RTK) signaling, which is upstream of Ras/MAPK, is known to play a critical role in craniofacial and dental development, the craniofacial and dental features of CS have not been systematically defined in a large group of individuals. In order to address this gap in our understanding and fully characterize the CS phenotype, we evaluated the craniofacial and dental phenotype in a large cohort (n = 41) of CS individuals. We confirmed that the craniofacial features common in CS include macrocephaly, bitemporal narrowing, convex facial profile, full cheeks, and large mouth. Additionally, CS patients have a characteristic dental phenotype that includes malocclusion with anterior open bite and posterior crossbite, enamel hypo-mineralization, delayed tooth development and eruption, gingival hyperplasia, thickening of the alveolar ridge, and high palate. Comparison of the craniofacial and dental phenotype in CS with other RASopathies, such as cardio-facio-cutaneous syndrome (CFC), provides insight into the complexities of Ras/MAPK signaling in human craniofacial and dental development.

Tbx1 regulates progenitor cell proliferation in the dental epithelium by modulating Pitx2 activation of p21.

Tbx1(-/-) mice present with phenotypic effects observed in DiGeorge syndrome patients however, the molecular mechanisms of Tbx1 regulating craniofacial and tooth development are unclear. Analyses of the Tbx1 null mice reveal incisor microdontia, small cervical loops and BrdU labeling reveals a defect in epithelial cell proliferation. Furthermore, Tbx1 null mice molars are lacking normal cusp morphology. Interestingly, p21 (associated with cell cycle arrest) is up regulated in the dental epithelium of Tbx1(-/-) embryos. These data suggest that Tbx1 inhibits p21 expression to allow for cell proliferation in the dental epithelial cervical loop, however Tbx1 does not directly regulate p21 expression. A new molecular mechanism has been identified where Tbx1 inhibits Pitx2 transcriptional activity and decreases the expression of Pitx2 target genes, p21, Lef-1 and Pitx2c. p21 protein is increased in PITX2C transgenic mouse embryo fibroblasts (MEF) and chromatin immunoprecipitation assays demonstrate endogenous Pitx2 binding to the p21 promoter. Tbx1 attenuates PITX2 activation of endogenous p21 expression and Tbx1 null MEFs reveal increased Pitx2a and activation of Pitx2c isoform expression. Tbx1 physically interacts with the PITX2 C-terminus and represses PITX2 transcriptional activation of the p21, LEF-1, and Pitx2c promoters. Tbx1(-/+)/Pitx2(-/+) double heterozygous mice present with an extra premolar-like tooth revealing a genetic interaction between these factors. The ability of Tbx1 to repress PITX2 activation of p21 may promote cell proliferation. In addition, PITX2 regulation of p21 reveals a new role for PITX2 in repressing cell proliferation. These data demonstrate new functional mechanisms for Tbx1 in tooth morphogenesis and provide a molecular basis for craniofacial defects in DiGeorge syndrome patients.

Implications for tooth development on ENU-induced ectodermal dysplasia mice.

BACKGROUND: In this study, the mutated phenotypes were produced by treatment of chemical mutagen, N-ethyl-N-nitrosourea (ENU). We analyzed the mutated mice showing the specific phenotype of ectodermal dysplasia (ED) and examined the affected gene. METHODS: Phenotypes, including size, bone formation, and craniofacial morphology of ENU-induced ED mice, were focused. Tooth development and expression of several molecules were analyzed by histologic observations and immunohistochemistry. We carried out genome-wide screening and quantitative real-time PCR to define the affected and related genes. RESULTS: As examined previously in human ectodermal dysplasia, ENU-induced ED mice showed the specific morphologic deformities in tooth, hair, and craniofacial growth. Tooth development in the ENU-induced ED mice ceased at early cap stage. In addition, skeletal staining showed retardation in craniofacial development. Finally, the affected gene, which would be involved in the mechanism of ED, was located between the marker D3Mit14 and D3Mit319 on chromosome 3. CONCLUSIONS: The affected gene in ENU-induced ED mice showed several defects in ectodermal organogenesis and these results indicate that this gene plays an important role in mouse embryogenesis.

Mouse models of tooth abnormalities.

Tooth number is abnormal in about 20% of the human population. The most common defect is agenesis of the third molars, followed by loss of the lateral incisors and loss of the second premolars. Tooth loss appears as both a feature of multi-organ syndromes and as a non-syndromic isolated character. Apart from tooth number, abnormalities are also observed in tooth size, shape, and structure. Many of the genes that underlie dental defects have been identified, and several mouse models have been created to allow functional studies to understand, in greater detail, the role of particular genes in tooth development. The ability to manipulate the mouse embryo using explant culture and genome targeting provides a wealth of information that ultimately may pave the way for better diagnostics, treatment or even cures for human dental disorders. This review aims to summarize recent knowledge obtained in mouse models, which can be used to gain a better understanding of the molecular basis of human dental abnormalities.

Influencia del surco palato-gingival sobre la anatomía radicular y canalicular / The influence of palato-gingival groove on radicular and canal anatomy

El surco palato-gingival es una anomalía de desarrollo que produce cambios morfológicos y estructurales en la cara palatina de la corona y raíz de incisivos permanentes superiores. La magnitud de la lesión es muy variada y en ciertos casos provoca importantes deformaciones en la raíz y cavidad pulpar, con graves complicaciones pulpares y periodontales que pueden ocasionar la pérdida del diente. Ante esta situación anatómica un tanto compleja, y no siempre de fácil diagnóstico, nos propusimos como objetivo estudiar con una visión endodóntica, la influencia que esta anomalía tiene sobre la macroy microestructura radicular y, en consecuencia, sobre la cavidad pulpar. En cortes transversales de raíces que presentaban esta patología, se observó que los defectos podían ser muy variados y que en los casos donde la lesión era muy acentuada, se producían importantes cambios, que evidenciaban un intento de bifurcación tanto enl a raíz como en el conducto.

The genetic basis of tooth development and dental defects.

More than 300 genes have so far been associated with tooth development, mainly in mouse embryos. The majority of them are associated with conserved signaling pathways mediating cellular communication, in particular between epithelial and mesenchymal tissues. Necessary functions of many signals, receptors and transcription factors have been demonstrated in mice, and mutations causing dental defects in humans have been identified in several genes.

Zebrafish acvr2a and acvr2b exhibit distinct roles in craniofacial development.

To examine the roles of activin type II receptor signaling in craniofacial development, full-length zebrafish acvr2a and acvr2b clones were isolated. Although ubiquitously expressed as maternal mRNAs and in early embryogenesis, by 24 hr postfertilization (hpf), acvr2a and acvr2b exhibit restricted expression in neural, hindbrain, and neural crest cells (NCCs). A morpholino-based targeted protein depletion approach was used to reveal discrete functions for each acvr2 gene product. The acvr2a morphants exhibited defects in the development of most cranial NCC-derived cartilage, bone, and pharyngeal tooth structures, whereas acvr2b morphant defects were largely restricted to posterior arch structures and included the absence and/or aberrant migration of posterior NCC streams, defects in NCC-derived posterior arch cartilages, and dysmorphic pharyngeal tooth development. These studies revealed previously uncharacterized roles for acvr2a and acvr2b in hindbrain and NCC patterning, in NCC derived pharyngeal arch cartilage and joint formation, and in tooth development.

In utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects tooth development in rhesus monkeys.

We thought to validate the current tolerable daily intake (TDI) value for dioxin (4 pg/kg) in Japan. Pregnant rhesus monkeys received an initial dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 0, 30, or 300 ng/kg subcutaneously) on day 20 of gestation; the dams received additional injection of 5% of the initial dose every 30 days until day 90 after delivery. The teeth of stillborn, postnatally dead, and surviving offspring (now approximately 4 years old) were evaluated. None of the offspring in the 0 and 30 ng/kg groups (n=17 and 15, respectively) had tooth abnormalities, whereas 10 of 17 in the 300 ng/kg had them. These findings suggest the lowest-observed adverse-effect level (LOAEL) for TCDD in the rhesus monkey is between 30 and 300 ng/kg, and probably is close to that for rodents (86 ng/kg) on which the current TDI was based. It is reasonable to conclude that the current TDI needs no immediate modification.

Tooth and jaw: molecular mechanisms of patterning in the first branchial arch.

The mammalian jaw apparatus is ultimately derived from the first branchial arch derivatives, the maxillary and mandibular processes, and composed of a highly specialised group of structures. Principle amongst these are the skeletal components of the mandible and maxilla and the teeth of the mature dentition. Integral to the development of these structures are signalling interactions between the stomodeal ectoderm and underlying neural crest-derived ectomesenchymal cells that populate this region. Recent evidence suggests that in the early mouse embryo, regionally restricted expression of homeobox-containing genes, such as members of the Dlx, Lhx and Gsc classes, are responsible for generating early polarity in the first branchial arch and establishing the molecular foundations for patterning of the skeletal elements. Teeth also develop on the first branchial arch and are derived from both ectoderm and the underlying ectomesenchyme. Reciprocal signalling interactions between these cell populations also control the odontogenic developmental programme, from early patterning of the future dental axis to the initiation of tooth development at specific sites within the ectoderm. In particular, members of the Fibroblast growth factor (Fgf), Bmp, Hedgehog and Wnt families of signalling molecules induce regionally restricted expression of downstream target genes in the odontogenic ectomesenchyme. Finally, the processes of morphogenesis and cellular differentiation ultimately generate a tooth of specific class. Many of the same genetic interactions that are involved in early tooth development mediate these effects through the activity of localised signalling centres within the developing tooth germ.

Different morphotypes of the tabby (EDA) dentition in the mouse mandible result from a defect in the mesio-distal segmentation of dental epithelium.

OBJECTIVES: Prenatal identification of the different dentition morphotypes, which exist in the lower molar region of tabby (Ta) adult mice, and investigation of their origin. The mouse Ta syndrome and its counterpart anhidrotic (hypohidrotic) ectodermal dysplasia (EDA) in human are characterized by absence or hypoplasia of sweat glands, hair and teeth. DESIGN: Analysis of tooth morphogenesis using serial histological sections and 3D computer aided reconstructions of the dental epithelium in the cheek region of the mandible. SETTING AND SAMPLE POPULATION: Institute of Experimental Medicine, Academy of Sciences, Prague. Heads of 75 Ta homozygous and hemizygous mice and 40 wild type (WT) control mice aged from embryonic day (ED) 14.0-20.5 (newborns), harvested during 1995-2001. OUTCOME MEASURE: Prenatal identification of five distinct morphotypes of Ta dentition on the basis of differences in tooth number, size, shape, position and developmental stage and of the morphology of the enamel knot in the most mesial tooth primordium. RESULTS: The mesio-distal length of the dental epithelium was similar in the lower cheek region in Ta and WT mice. In Ta embryos, there was altered the mesio-distal segmentation of the dental epithelium giving rise to the individual tooth primordia. Prenatally, two basic morphotypes I and II and their particular subtypes (Ia, Ib, Ic, and IIa, IIb, respectively) of the developing dentition were identified from day 15.5. The incidence of the distinct morphotypes in the present sample did not differ from postnatal data. The proportion of the morphotype I and II was dependent on mother genotype. CONCLUSION: The different dentition morphotypes in Ta mice originate from a defect in the mesio-distal segmentation of the dental epithelium in mouse embryos. This defect presumably leads to variable positions of tooth boundaries that do not correspond to those of the WT molars. One tooth primordium of Ta mice might be derived from adjacent parts of two molar primordia in WT mice.

Embriología dentaria: implicaciones clínicas / Dental embryology: clinical implications

El desarrollo de los dientes es un largo proceso que comienza alrededor de la sexta semana de vida intrauterina para los dientes temporales y finaliza con los segundos molares permanentes alrededor de los 14 años. Este largo periodo de formación hace que los dientes esten sometidos a un amplio margen de trastornos potenciales que pueden alterar su normal desarrollo. El conocimiento del desarrollo cronológico de los dientes es un dato importante para poder valorar las anomalias dentarias, poder establecer, en algunos casos, el agente etiologico y el momento en el cual actuó dicho agente. (AU)

Cre-mediated gene inactivation demonstrates that FGF8 is required for cell survival and patterning of the first branchial arch.

In mammals, the first branchial arch (BA1) develops into a number of craniofacial skeletal elements including the jaws and teeth. Outgrowth and patterning of BA1 during early embryogenesis is thought to be controlled by signals from its covering ectoderm. Here we used Cre/loxP technology to inactivate the mouse Fgf8 gene in this ectoderm and have obtained genetic evidence that FGF8 has a dual function in BA1: it promotes mesenchymal cell survival and induces a developmental program required for BA1 morphogenesis. Newborn mutants lack most BA1-derived structures except those that develop from the distal-most region of BA1, including lower incisors. The data suggest that the BA1 primordium is specified into a large proximal region that is controlled by FGF8, and a small distal region that depends on other signaling molecules for its outgrowth and patterning. Because the mutant mice resemble humans with first arch syndromes that include agnathia, our results raise the possibility that some of these syndromes are caused by mutations that affect FGF8 signaling in BA1 ectoderm.

Neuro-osteology.

Neuro-osteology stresses the biological connection during development between nerve and hard tissues. It is a perspective that has developed since associations were first described between pre-natal peripheral nerve tissue and initial osseous bone formation in the craniofacial skeleton (Kjaer, 1990a). In this review, the normal connection between the central nervous system and the axial skeleton and between the peripheral nervous system and jaw formation are first discussed. The early central nervous system (the neural tube) and the axial skeleton from the lumbosacral region to the sella turcica forms a unit, since both types of tissue are developmentally dependent upon the notochord. In different neurological disorders, the axial skeleton, including the pituitary gland, is malformed in different ways along the original course of the notochord. Anterior to the pituitary gland/sella turcica region, the craniofacial skeleton develops from prechordal cartilage, invading mesoderm and neural crest cells. Also, abnormal development in the craniofacial region, such as tooth agenesis, is analyzed neuro-osteologically. Results from pre-natal investigations provide information on the post-natal diagnosis of children with congenital developmental disorders in the central nervous system. Examples of these are myelomeningocele and holoprosencephaly. Three steps are important in clinical neuro-osteology: (1) clinical definition of the region of an osseous or dental malformation, (2) embryological determination of the origin of that region and recollection of which neurological structure has developed from the same region, and (3) clinical diagnosis of this neurological structure. If neurological malformation is the first symptom, step 2 results in the determination of the osseous region involved, which in step 3 is analyzed clinically. The relevance of future neuro-osteological diagnostics is emphasized.