Primary tooth size asymmetry in twins and singletons.

OBJECTIVES: To explore asymmetry values of antimeric deciduous tooth crown dimensions in three types of twins: monozygotic (MZ), dizygotic same-sex (DZ) and opposite-sex (OS) vs. single-born controls. SETTING AND SAMPLE POPULATION: Mesiodistal and labio-lingual crown dimensions of second deciduous molars and mesiodistal canine and first molar crown dimensions of 2159 children at 6-12 years of age were evaluated, originating from the US cross-sectional Collaborative Perinatal Study from the 1970s, including altogether MZ (n = 28), DZ same-sex (n = 33) and OS (n = 39) pairs. Single born (n = 1959) were used as controls. MATERIAL AND METHODS: Dental casts were measured for comparison of variance relationships calculated from antimeric teeth, exhibiting fluctuating (FA), and directional (DA) asymmetry using anova. RESULTS: Significant differences appeared in MZ and OS girls in DA of deciduous canines, which gain size in the first and second trimester, and deciduous second molars, which finally stop crown growth during the early post-natal period. Significantly, increased FA values appeared for lower deciduous canines and second molars, indicating greatest environmental stress in OS girls, MZ girls and DZ boys. Twin girls had more fluctuating and directional crown asymmetry than twin boys, but in some dimensions, the twins were more symmetric than controls. CONCLUSIONS: Transmembrane hormonal influence between opposite-sex twins, and late gestational stress factors, caused by placental malfunction and/or monochorionicity, may be involved in asymmetric growth of antimers, during critical periods of crown size gain.

YAP overexpression affects tooth morphogenesis and enamel knot patterning.

Teeth develop through distinct morphological stages. At the cap stage, a compactly clustered and concentrically arranged cell mass, the enamel knot, appears at the tip of the enamel organ. Cells in this knot express sets of key molecules, and as such have been proposed to act as a signaling center directing tooth morphogenesis and tooth cusp formation. YAP is a transcriptional co-activator of the Hippo signaling pathway that is essential for the proper regulation of organ growth. In this study, we analyzed the tooth phenotype in transgenic mice that overexpressed a constitutively active form of YAP in the dental epithelium. We found that overexpression of YAP resulted in deformed tooth morphogenesis with widened dental lamina. In addition, the enamel knot was mislocated to the upper portion of the enamel organ, where it remained devoid of proliferating cells and contained apoptotic cells with intense Edar transcripts and reduced E-cadherin expression. Interestingly, some signaling molecules, such as Shh, Fgf4, and Wnt10a, were not expressed in this mislocated enamel knot, but remained at the tip of the enamel organ. Analysis of these data suggests that the signaling center is induced by reciprocal epithelial-mesenchymal interactions, and its induction may be independent of the enamel knot.

The morphology and mineralization of dental hard tissue in the offspring of passive smoking rats.

OBJECTIVE: To study the effects of maternal passive smoking on the morphology and mineralization of dental hard tissue in offspring rats. DESIGN: We have established a maternal passive smoking model. Offspring rats were sacrificed on the 20th day of gestation (E20) or the 3rd (D3) or 10th day (D10) after birth. We observed hard tissue morphology using Haematoxylin-Eosin (H&E) staining sections, used micro computer tomography (Micro-CT) to measure hard tissue thickness and volume on the mandibular first molars of the offspring rats, and used Micro-CT and energy dispersive X-ray spectroscopy with scanning electron microscopy (SEM/EDS) to determine the hard tissue mineral density and the ratio of calcium atom number/calcium atom+phosphorus atom number (Ca(2+)/P(3-)+Ca(2+)). RESULTS: Overall, the development of dental hard tissue was delayed in the offspring of passive smoking rats. The thickness and volume of hard tissue were lower in the offspring of the maternal passive smoking group than in the offspring of the control group. Mineral density of the hard tissue and the ratio of (Ca(2+)/P(3-)+Ca(2+)) were also reduced in the offspring of the maternal passive smoking group. CONCLUSION: Maternal passive smoking inhibits the morphological development and mineralization level of hard tissue on the mandibular first molars of offspring rats.

Temporal and spatial expression of TGF-beta2 in tooth crown development in mouse first lower molar.

Transforming Growth Factor beta2 (TGF-beta2) is involved in the regulation of many important cellular processes during tooth development. In this study we systematically characterized the expression pattern of TGF-beta2 in vivo and further analyzed its possible roles during different developmental stages of mouse first lower molar using immunofluorescence histochemical method with confocal microscopy. TGF-beta2 signaling was detected in different developing stages in both dental epithelium and surrounding dental mesenchyme. For the first time, we found that the basement membrane and epithelial cells in the basal layer showed no immunostaining from embryonic day 11 to 13; the primary enamel knot and secondary enamel knot exhibited pronounced immunostaining with different expression patterns at embryonic day 14 and 16. In addition, the mature ameloblast lost immunoreactivity, but the secretory ameloblast still exhibited positive immunoreaction at day 2 of postnatal development. Collectively, the temporospatial distribution patterns of TGF- beta2, especially in the basement membrane, epithelial cells in the basal layer, enamel knot, mature odontoblast and ameloblast, suggested a close association between TGF-beta2 signaling and tooth crown development, and indicated that TGF-beta2 might participate in tooth initiation, epithelial morphogenesis, formation of dentine matrix, and ameloblast differentiation.

Epithelial-mesenchymal cell ratios can determine the crown morphogenesis of dental pulp stem cells.

Although dental pulp stem cells (DPSC) have been isolated from adult dental pulp tissues, knowledge on how to use them to make teeth lags behind. To date, little is known about the effects of epithelial-mesenchymal cell ratios on the bioengineered odontogenesis mediated by DPSCs. In this study, we investigated the effects of apical bud cells (ABC) from dental epithelial stem cell niche of rat incisors on the differentiation and morphogenesis of molar DPSCs at different proportions (DPSC/ABC cell ratios=1:10, 1:3, 1:1, 3:1, 10:1, respectively). In vitro mixed DPSCs/ABCs at 1:1, 1:3, and 3:1 ratios displayed several crucial characteristics of odontoblast/ameloblast lineages, as indicated by accelerated mineralization, upregulated alkaline phosphatase activity, protein/gene expression for dentin sialophosphoprotein and ameloblastin. In vivo transplantation of reassociated DPSC and ABC pellets at different ratios was also carried out. Histological analyses demonstrated that only DPSC/ABC recombinants at 1:1 ratio generated typical molar crown-shaped structures, whereas recombinations at other ratios presented an atypical crown morphogenesis with unbalanced distribution of amelogenesis and dentinogenesis. Together, these findings revealed that the proportions of dental epithelial and mesenchymal cell populations can determine the odontogenic differentiation of DPSCs/ABCs in vitro as well as the bioengineered tooth morphogenesis in vivo.

[Effect of the Tabby mutation on the dentition of mice]. / Effets de la mutation Tabby sur la dentition chez la souris.

The X-linked hypohidrotic ectodermal dysplasia in man leads to dental defects and is homologous to the Tabby (Ta) mutation in mouse. We currently investigate the effects of the Ta mutation on odontogenesis. The incisor germ of Ta showed an abnormal size and shape, a change in the balance between prospective crown- and root-analogue tissues and retarded cytodifferentiation. Although the enamel organ in Ta incisors was smaller, a larger proportion of the dental papilla was covered by preameloblasts-ameloblasts. The independent development of the labial and lingual parts of the enamel organ in rodent lower incisor might reflect their heterogeneous origin, as demonstrated for the upper incisor. The mandibular cheek dentition in Ta mice exhibits large variations classified in five morphotypes, based on the tooth number, shape, size and position. In Ta embryos, the mesio-distal extent of the dental epithelium was similar to that in WT, but its segmentation was altered. These morphotypes could be explained by a tentative model suggesting that 1) the positions of tooth boundaries differ in Ta and WT molars and among the Ta morphotypes; 2) the tooth patterns are determined by the distal boundary of the most mesial tooth primordium while the distal teeth take advantage of the remaining dental epithelium; 3) one tooth primordium in Ta mice might derive from adjacent parts of two primordia in WT.

Bone morphogenetic protein 4 is involved in cusp formation in molar tooth germ of mice.

In order to clarify the role of BMP4 in the development of the tooth crown, we employed the antisense technique on molar tooth germs removed from the mandibles of embryonic 13.5-d-old mice. In the tooth germ explants incubated for 14 d with antisense oligodeoxynucleotide (AS-ODN) against Bmp4 (a) cusps were not formed, whereas dentin matrix was secreted in the whole region of the crown, (b) inner enamel epithelial (IEE) cells remained in the undifferentiated state in the occlusal region of the crown, though they differentiated in the proximal region (lateral surface region of tooth crown), and (c) insufficient growth of the dental papilla was observed. A 5-bromo-2'-deoxyuridine (BrdU) uptake experiment showed that, although a site-specific proliferation of IEE cells occurred in the occlusal region in the control explants, it was not found in the AS-ODN-treated explants. In the proximal region, however, the proliferation of IEE cells was detected evenly in all explants treated with or without AS-ODNs. These results suggest that AS-ODN against Bmp4 inhibited the differentiation and the site-specific proliferation of IEE cells in the occlusal region of molar tooth germs, resulting in the suppression of cusp formation. Our data thus suggest that BMP4 is involved in cusp formation and differentiation of ameloblasts in the occlusal region of molars.

Effects of hepatocyte growth factor anti-sense oligodeoxynucleotides or met D/D genotype on mouse molar crown morphogenesis.

Hepatocyte growth factor (HGF) is considered to be one of the mediators of epithelio-mesenchymal interactions during early organogenesis and to be also involved in the development of murine molars. In the developing tooth, HGF is expressed in the cells of the dental papillae, and c-Met, its receptor, in the cells of dental epithelia. In order to study the functional role played by HGF in tooth development, we tested the effects of HGF translation arrest by anti-sense phosphorothioate oligodeoxynucleotides on E-14 molars cultured in vitro. We also analyzed the histo-morphogenesis and crown cytodifferentiation of transgenic met E-14 molars cultured in vitro. 3D reconstructions revealed perturbations of the cusp pattern. However, histo-morphogenesis and crown cytodifferentiation were normal at the histological level.