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.

Noninvasive Prenatal Diagnosis of Hypohidrotic Ectodermal Dysplasia by Tooth Germ Sonography.

PURPOSE: Hypohidrotic ectodermal dysplasia, a potentially life-threatening heritable disorder, may be recognized already in utero by characteristic features such as oligodontia and mandibular hypoplasia. As therapeutic options and prognosis depend on the time point of diagnosis, early recognition was attempted during routine prenatal ultrasound examinations. SUBJECTS AND METHODS: Fetuses of nine pregnant women (one triplet and eight singleton pregnancies) with family histories of hypohidrotic ectodermal dysplasia were investigated by sonography between the 20th and 24th week of gestation. RESULTS: In 4 male and 2 female fetuses reduced amounts of tooth germs were detected, whereas 5 fetal subjects showed the normal amount. Three-dimensional ultrasound evaluation revealed mandibular hypoplasia in 5 of the 6 fetuses with oligodontia. Molecular genetic analysis and/or clinical findings after birth confirmed the prenatal sonographic diagnosis in each subject. CONCLUSION: In subjects with a family history of hypohidrotic ectodermal dysplasia, the diagnosis of this rare condition can be established noninvasively by sonography in the second trimester of pregnancy. Early recognition of the disorder may help to prevent dangerous hyperthermic episodes in infancy and may allow timely therapeutic interventions.

The genetic basis of craniofacial and dental abnormalities.

The embryonic head development, including the formation of dental structures, is a complex and delicate process guided by specific genetic programs. Genetic changes and environmental factors can disturb the execution of these programs and result in abnormalities in orofacial and dental structures. Orofacial clefts and hypodontia/ oligodontia are examples of such abnormalities frequently seen in dental clinics. An insight into the mechanisms and genes involved in the formation of orofacial and dental structures has been gradually gained by genetic analysis of families and by the use of experimental vertebrate models such as the mouse and chick models. The development of novel clinical therapies for orofacial and dental pathological conditions depends very much on a detailed knowledge of the molecular and cellular processes that are involved in head formation.

Erupción y retención del tercer molar en jóvenes entre 17 y 20 años: Antofagasta, Chile / Eruption and retention of third molars in young people between 17 and 20 years of age: Antofagasta, Chile

La retención intraósea y la erupción en malposición de 3Ms han sido muy estudiadas. Son causadas, probablemente, por disminución del tamaño de maxilares por cambios de hábitos alimentarios, reduciendo el espacio retromolar, lo que dificulta la erupción normal entre 15 y 25 años de edad, y produce patologías o molestias por comprometer estructuras orofaciales próximas. Lo anterior, ha promovido la exodoncia profiláctica u ortodóncica, incluso del germen dentario, con altos costos clínicos, hospitalarios, laborales, comprometiendo parte de los recursos destinados a financiar otros procedimientos quirúrgicos orales, además de riesgos durante cirugía, postoperatorio y lesiones iatrogénicas temporales o permanentes. La muestra consistió en 100 jóvenes de 17 a 20 años de edad (50 mujeres y 50 hombres) de la ciudad de Antofagasta, sanos, sin malformaciones general y maxilofacial, sin haber presentado enfermedades infecciosas que alteraran odontogénesis y períodos eruptivos, sin exodoncias de 3M ni tratamientos ortodóncicos previos al examen de la radiografía panorámica y clasificando erupción de 3Ms según tablas de Pell-Gregory y Winter. Se determina 49,1 por ciento de 3Ms retenidos con p<0,05 significativo respecto dientes erupcionados, predominando retención maxilar, especialmente en mujeres. En todos los casos y en mandíbula predominan 3Ms con impactación mesioangulada (p<0,05 significativa) y en maxilares la retención vertical (p<0,05 de significancia). Prevalecen 3Ms distoangulados en maxilares, posición que predispone a complicaciones operatorias y postoperatorias en exodoncias. La retención horizontal se aprecia en mandibula, siendo el segundo tipo de retención (21,5 por ciento en toda la muestra y 30 por ciento en hombres). Sin una decisión clínica que indique la cirugía, se sugiere postegar la exodoncia profiláctica de 3Ms, esperando posible erupción tardía (Hattab, 1997; Ventã et al. 1999, 2004 y Kruger et al.). Meta-análisis demuestra: mínima morbilidad...

Intraoseous retention and anomalous eruption position of 3Ms had been widely studied. They are probably produced by reducing the size of jaws by changes in eating habits, reducing the retro molar space, making it difficult normal eruption between 15 and 25 years old, producing pathologies or discomfort by compromising nearly orofacial structures. The above, has promoted the extraction or prophylactic orthodontic, even from the dental germ, with expensives clinical cost, hospital surgery, compromising part of the resources to finance other surgical oral procedures, in addition to risks during surgery, postoperative and iatrogenic injuries temporary or permanent. The sample consisted in 100 young people aged 17 to 20 years of age (50 women and 50 men) of the city of Antofagasta, healthy, without general and maxillofacial malformation, without having submitted infectious diseases that distort odontogenesis and periods of eruption, without extractions of 3M nor orthodontic treatments prior to the examination of the x-ray overview and classifying eruption of 3Ms as tables of Pell-Gregory and Winter. It was determined that 49.1 percent of 3Ms were retained, with p<0.05 significant with regard to erupted teeth, predominate maxillary retention, especially in women. In all cases and in maxilla predominate 3Ms with mesiangular position (p<0.05 of significance) and in maxillary vertical retention (p<0.05 of significance). Prevailing 3Ms maxilla with distoangular position, a position that predisposes surgical complications and postoperative extractions. The horizontal retention is apparent only in mandible, being the second type of retention (21.5 percent in all cases and 30 percent in men). Without a clinic decision to indicate surgery, we suggest to postpone the prophylactic extraction of 3Ms, awaiting possible late eruption (Hattab, 1997; Ventã et al., 1999, 2004; Kruger et al., 2001). Meta-analyzes show minimum morbidity in 3Ms extractions in...

Agenesia del tercer molar en pacientes atendidos en la clínica odontológica de la Universidad de Antofagasta, Chile / Third molar agenesis in patients of dental clinic of the Universidad de Antofagasta, Chile

Se define agenesia como la ausencia de dientes por alteraciones genéticas aisladas o sindrómicas. La agenesia del tercer molar está asociada a malformaciones y es considerada por diversos autores, consecuencia de la evolución humana (Larmour et al., 2005). Son los dientes con mayor prevalencia de agenesia junto con segundos premolares e incisivos laterales (Fuller & Denehy, 1984). La prevalencia varía entre 9% y 37% (McNamara & Foley, 2006); Arboleda et al. (2006) señalan una prevalencia del 20%. La literatura indica variables estadísticas porcentuales, por género, por arcada dentaria, por lado y por diente, con escasos artículos sobre grupos originarios de Chile. La población en estudio consistió en 52 hombres y 48 mujeres, de 14 a 26 años de edad, pacientes de la Clínica Odontológica de la Universidad de Antofagasta. Todos los individuos eran sanos, sin ninguna malformación general o maxilofacial, no habían presentado enfermedades infecciosas que afectaran la odontogénesis y los períodos de erupción dentarios, sin exodoncias de ningún tercer molar y tratamiento ortodóncico previo al examen radiográfico panorámico. Se determina un 20,0% de casos de agenesia, con 8,25% de agenesia, respecto a número total de terceros molares y 1,03% de agenesia de terceros molares en relación al total de dientes. No se determina ninguna diferencia estadística significativa al 95% de confianza, predominando la agenesia en el género femenino, a nivel maxilar, en el lado izquierdo, de tipo simple, siendo el tercer molar superior izquierdo el diente que presenta el mayor número de casos de agenesia.

Agenesis is defined as the absence of teeth by genetic alterations isolated or syndromic. Agenesis of third molar is associated to malformations and is considerate by diverse authors as a consequence of human evolution (Larmour et al, 2005). The third molars are teeth with higher prevalence of agenesis together with seconds premolars and lateral incisive (Fuller & Denehy, 1984). The prevalence varies between 9 percent to 37% (McNamara & Foley, 2006). Arboleda et al. (2006) indicated a prevalence of 20%. The literature notes statistical variables percentage by gender, dental arch, side, and tooth, with few articles on groups originating from Chile. The population in study consisted of 52 men and 48 women between 14 and 26 years old, patients of the dental clinic of the Universidad de Antofagasta. All individuals were healthy, without any general or maxillofacial malformation without infectious diseases affecting the odontogenesis and dental eruption, without extractions of third molar and orthodontic treatment prior to the panoramic x-ray. A 20% of individuals with agenesis was determined, with 8.25% of agenesis respect the total number of third molars and 1.03% agenesis respect the total number of teeth. Statistical analyses did not show significant differences at 95% level, with agenesis of third molar prevalence in females, in maxilla, in the left side, simple, being the left maxillary third molar the tooth that present many number of agenesis.

Tooth development: 1. Generating teeth in the embryo.

Teeth are organs that develop in the embryo via a series of interactions between oral epithelium and neural crest-derived ectomesenchyme of the early jaws. These interactions are initiated by the regional production of signalling molecules in the oral epithelium and the transfer of information to the underlying mesenchyme via homeobox gene transcription. This article describes how these interactions are co-ordinated in the embryo during development of the dentition and provides a theoretical basis for the second article in this series; understanding how biologists are attempting to generate teeth artificially in the laboratory.

A nonsense mutation in MSX1 causes Witkop syndrome.

Witkop syndrome, also known as tooth and nail syndrome (TNS), is a rare autosomal dominant disorder. Affected individuals have nail dysplasia and several congenitally missing teeth. To identify the gene responsible for TNS, we used candidate-gene linkage analysis in a three-generation family affected by the disorder. We found linkage between TNS and polymorphic markers surrounding the MSX1 locus. Direct sequencing and restriction-enzyme analysis revealed that a heterozygous stop mutation in the homeodomain of MSX1 cosegregated with the phenotype. In addition, histological analysis of Msx1-knockout mice, combined with a finding of Msx1 expression in mesenchyme of developing nail beds, revealed that not only was tooth development disrupted in these mice, but nail development was affected as well. Nail plates in Msx1-null mice were defective and were thinner than those of their wild-type littermates. The resemblance between the tooth and nail phenotype in the human family and that of Msx1-knockout mice strongly supports the conclusions that a nonsense mutation in MSX1 causes TNS and that Msx1 is critical for both tooth and nail development.

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.

Agenesis of permanent incisors in a mediaeval maxilla and mandible: aetiological aspects.

A paleopathological maxilla and mandible with tooth agenesis were analyzed, focussing on the aetiology of the condition. The jaw material, derived from an adult mediaeval male, was examined by standard anthropological analyses, including radiography. In the maxilla there was agenesis of three permanent incisors and one premolar, and in the mandible of one permanent incisor and two permanent molars. Absence or marked reduction of the incisive foramen and the nasopalatine canal was found. The premaxillary area was reduced without general alveolar bone resorption. The pattern of tooth agenesis was similar to the pattern observed in contemporary individuals, except for the agenesis of one permanent maxillary central incisor. It is suggested that the pronounced lack of teeth in the maxillary anterior region is connected with deficient development of the premaxillary area of the nasopalatine canals and the incisive foramen. As the condition can be ascribed to deviations in the prenatal development, this investigation shows that embryological developmental patterns, which form the basis for the pattern of tooth agenesis, should be taken into account when evaluating dry bone pathology.