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(1/364) Diverse developing mouse lineages exhibit high-level c-Myb expression in immature cells and loss of expression upon differentiation.

The c-myb gene encodes a sequence specific transactivator that is required for fetal hematopoiesis, but its potential role in other tissues is less clear because of the early fetal demise of mice with targeted deletions of the c-myb gene and incomplete of knowledge about c-myb's expression pattern. In the hematopoietic system, c-Myb protein acts on target genes whose expression is restricted to individual lineages, despite Myb's presence and role in multiple immature lineages. This suggests that c-Myb actions within different cell type-specific contexts are strongly affected by combinatorial interactions. To consider the possibility of similar c-Myb actions could extend into non-hematopoietic systems in other cell and tissue compartments, we characterized c-myb expression in developing and adult mice using in situ hybridization and correlated this with stage-specific differentiation and mitotic activity. Diverse tissues exhibited strong c-myb expression during development, notably tooth buds, the thyroid primordium, developing trachea and proximal branching airway epithelium, hair follicles, hematopoietic cells, and gastrointestinal crypt epithelial cells. The latter three of these all maintained high expression into adulthood, but with characteristic restriction to immature cell lineages prior to their terminal differentiation. In all sites, during fetal and adult stages, loss of c-Myb expression correlated strikingly with the initiation of terminal differentiation, but not the loss of mitotic activity. Based on these data, we hypothesize that c-Myb's function during cellular differentiation is both an activator of immature gene expression and a suppressor of terminal differentiation in diverse lineages.  (+info)

(2/364) Pathological evaluation of the effects of intentional disocclusion and overloading occlusion in odontogenesis disorders in N-methylnitrosourea-treated hamsters.

This study compares the effects of disocclusion and overloading occlusion on dental lesions. Ten-day-old Syrian hamsters were divided into 4 groups: group I, untreated animals; group II, animals whose hemilateral incisors were disoccluded; group III, N-methylnitrosourea (MNU)-treated animals; and group IV, MNU-treated animals whose hemilateral incisors were disoccluded. The ipsilateral maxillary and mandibular incisors were repetitively cut with diamond discs. The hamster is easier to anesthetize. Animals received a 0.2% solution of MNU (10 mg/kg body weight) intragastrically twice a week for 16 wk. All the cut mandibular incisors and the MNU-treated uncut mandibular incisors showed lack of iron deposition on the enamel surface. The eruption rate was significantly higher in the cut disoccluded incisors of groups II and IV (p < 0.05) and significantly lower in the uncut overloaded incisors of groups II and IV (p < 0.05). In the cut mandibular incisors of group IV, the degree of the disturbance of odontogenesis and the atypical proliferation of odontogenic epithelium were more prominent (p < 0.02), and the dental lesions occurred earlier. Histologically, the disturbed Hertwig's epithelial sheath and the Hertwig's epithelial sheath-like transformed U-shaped part and enamel organ seemed to lead to disturbances of amelogenesis and detinogenesis as well as to atypical proliferation of odontogenic epithelium nests. Thus, this method of disocclusion of the incisors of rodents may represent a useful model for the investigation of the effects of various agents on tooth formation over a short experimental period.  (+info)

(3/364) Immunohistological distributions of fibronectin, tenascin, type I, III and IV collagens, and laminin during tooth development and degeneration in fetuses of minke whale, Balaenoptera acutorostrata.

The immunohistological distributions of fibronectin, tenascin, type I, III and IV collagens, and laminin were observed in the tooth buds of fetuses of minke whale, Balaenoptera acutorostrata. Distributions of extracellular matrices (ECMs) examined in this study except for tenascin were generally similar to those of terrestrial mammalian species during development of the tooth bud. Tenascin in the fetuses of minke whale showed characteristic distributions in the dental lamina and the enamel organ in the early tooth developmental stage. In the physiological degeneration stage of tooth bud development, immunoreactivity of the ECMs were very weakly and limitedly detected in the dental papilla and the surrounding mesenchyme. Immunoreactivity of tenascin and type I and III collagens were positively detected in the developing baleen plate germ which was associated with the degenerating tooth bud. These findings suggested that expressions of the ECMs were related to the formation of the tooth bud and baleen plate germ, and that the lack of the ECMs was related to the degeneration of the tooth bud in the fetal minke whale.  (+info)

(4/364) Cbfa1 is required for epithelial-mesenchymal interactions regulating tooth development in mice.

Osteoblasts and odontoblasts, cells that are responsible for the formation of bone and dentin matrices respectively, share several molecular characteristics. Recently, Cbfa1 was shown to be a critical transcriptional regulator of osteoblast differentiation. Mutations in this gene cause cleidocranial dysplasia (CCD), an autosomal dominant disorder in humans and mice characterized by defective bone formation. CCD also results in dental defects that include supernumerary teeth and delayed eruption of permanent dentition. The dental abnormalities in CCD suggest an important role for this molecule in the formation of dentition. Here we describe results of studies aimed at understanding the functions of Cbfa1 in tooth formation. RT-PCR and in situ hybridization analyses show that Cbfa1 has a unique expression pattern in dental mesenchyme from the bud to early bell stages during active epithelial morphogenesis. Unlike that observed in osteoblast differentiation, Cbfa1 is downregulated in fully differentiated odontoblasts and is surprisingly expressed in ectodermally derived ameloblasts during the maturation phase of enamel formation. The role of Cbfa1 in tooth morphogenesis is further illustrated by the misshapen and severely hypoplastic tooth organs in Cbfa1-/- mice. These tooth organs lacked overt odontoblast and ameloblast differentiation and normal dentin and enamel matrices. Epithelial-mesenchymal recombinants demonstrate that dental epithelium regulates mesenchymal Cbfa1 expression during the bud and cap stages and that these effects are mimicked by the FGFs but not by the BMPs as shown by our bead implantation assays. We propose that Cbfa1 regulates the expression of molecules in mesenchyme that act reciprocally on dental epithelium to control its growth and differentiation. Taken together, our data indicate a non-redundant role for Cbfa1 in tooth development that may be distinct from that in bone formation. In odontogenesis, Cbfa1 is not involved in the early signaling networks regulating tooth initiation and early morphogenesis but regulates key epithelial-mesenchymal interactions that control advancing morphogenesis and histodifferentiation of the epithelial enamel organ.  (+info)

(5/364) The heritability of malocclusion: Part 1--Genetics, principles and terminology.

The relative contribution of genes and the environment to the aetiology of malocclusion has been a matter of controversy throughout the twentieth century. Genetic mechanisms are clearly predominant during embryonic craniofacial morphogenesis, but environment is also thought to influence dentofacial morphology postnatally, particularly during facial growth. Orthodontic and orthopaedic techniques are used in the treatment of malocclusion and other dentofacial deformities, but with limited effectiveness. The key to the determination of the aetiology of malocclusion, and its treatability lies in the ability to differentiate the effect of genes and environment on the craniofacial skeleton in a particular individual. Our ability to do this is limited by our lack of knowledge on the genetic mechanisms that control facial growth and lack of scientific evidence for the influence of environmental factors on human craniofacial morphogenesis.  (+info)

(6/364) Nerve growth factor (NGF) supports tooth morphogenesis in mouse first branchial arch explants.

Posterior midbrain and anterior hindbrain neuroectoderm trans-differentiate into cranial neural crest cells (CNCC), emigrate from the neural folds, and become crest-derived ectomesenchyme within the mandibular and maxillary processes. To investigate the growth factor requirement specific for the initiation of tooth morphogenesis, we designed studies to test whether nerve growth factor (NGF) can support odontogenesis in a first branchial arch (FBA) explant culture system. FBA explants containing neural-fold tissues before CNCC emigration and the anlagen of the FBA were microdissected from embryonic day 8 (E8) mouse embryos, and cultured for 8 days in medium supplemented with 10% fetal calf serum only, or serum-containing medium further supplemented with either NGF or epidermal growth factor (EGF) at three different concentrations: 50, 100, or 200 ng/ml. Morphological, morphometric, and total protein analyses indicated that growth and development in all groups were comparable. Meckel's cartilage and tongue formation were also observed in all groups. However, odontogenesis was only detected in explants cultured in the presence of exogenous NGF. NGF-supplemented cultures were permissive for bud stage (50 ng/ml) as well as cap stage of tooth morphogenesis (100 and 200 ng/ml). Morphometric analyses of the volume of tooth organs showed a significant dose-dependent increase in tooth volume as the concentration of NGF increased. Whole-mount in situ hybridization and semiquantitative reverse transcription-polymerase chain reaction for Pax9, a molecular marker of dental mesenchyme, further supported and confirmed the morphological data of the specificity and dose dependency of NGF on odontogenesis. We conclude that (1) E8 FBA explants contain premigratory CNCC that are capable of emigration, proliferation, and differentiation in vitro; (2) serum-supplemented medium is permissive for CNCC differentiation into tongue myoblasts and chondrocytes in FBA explants; and (3) NGF controls CNCC cell fate specification and differentiation into tooth organs.  (+info)

(7/364) Alterations in the incisor development in the Tabby mouse.

The X-linked tabby (Ta) syndrome in the mouse is homologous to the hypohidrotic ectodermal dysplasia (HED) in humans. As in humans with HED, Ta mice exhibit hypohidrosis, characteristic defects of hairs and tooth abnormalities. To analyze the effects of Ta mutation on lower incisor development, histology, morphometry and computer-aided 3D reconstructions were combined. We observed that Ta mutation had major consequences for incisor development leading to abnormal tooth size and shape, change in the balance between prospective crown- and root-analog tissues and retarded cytodifferentiations. The decrease in size of Ta incisor was observed at ED13.5 and mainly involved the width of the tooth bud. At ED14.5-15.5, the incisor appeared shorter and narrower in the Ta than in the wild type (WT). Growth alterations affected the diameter to a greater extent than the length of the Ta incisor. From ED14.5, changes in the shape interfered with the medio-lateral asymmetry and alterations in the posterior growth of the cervical loop led to a loss of the labio-lingual asymmetry until ED17.0. Although the enamel organ in Ta incisors was smaller than in the WT, a larger proportion of the dental papilla was covered by preameloblasts-ameloblasts. These changes apparently resulted from reduced development of the lingual part of the enamel organ and might be correlated with a possible heterogeneity in the development of the enamel organ, as demonstrated for upper incisors. Our observations suggest independent development of the labial and lingual parts of the cervical loop. Furthermore, it appeared that the consequences of Ta mutation could not be interpreted only as a delay in tooth development.  (+info)

(8/364) Antagonistic signals between BMP4 and FGF8 define the expression of Pitx1 and Pitx2 in mouse tooth-forming anlage.

Members of the Pitx/RIEG family of homeodomain-containing transcription factors have been implicated in vertebrate organogenesis. In this study, we examined the expression and regulation of Pitx1 and Pitx2 during mouse tooth development. Pitx1 expression is detected in early development in a widespread pattern, in both epithelium and mesenchyme, covering the tooth-forming region in the mandible, and is then maintained in the dental epithelium from the bud stage to the late bell stage. Pitx2 expression, on the other hand, is restricted to the dental epithelium throughout odontogenesis. Interestingly, from E9.5 to E10.5, the expression domains of Pitx1 and Pitx2, in the developing mandible, overlap with that of Fgf8 but are exclusive to the zone of Bmp4 expression. Bead implantation experiments demonstrate that ectopic expression of Fgf8 can induce/maintain the expression of both Pitx1 and Pitx2 at E9.5. In contrast, Bmp4-expressing tissues and BMP4-soaked beads were able to repress Pitx1 expression in mandibular mesenchyme and Pitx2 expression in the presumptive dental epithelium, respectively. However, the effects of FGF8 and BMP4 are transient. It thus appears that the early expression patterns of Pitx1 and Pitx2 in the developing mandible are regulated by the antagonistic effects of FGF8 and BMP4 such that the Pitx1 and Pitx2 expression patterns are defined. These results indicate that the epithelial-derived signaling molecules are responsible not only for restricting specific gene expression in the dental mesenchyme, but also for defining gene expression in the dental epithelium.  (+info)