Bone morphogenetic protein-7 (osteogenic protein-1, OP-1) and tooth development

Bone morphogenetic proteins (BMPs) form a family of growth factors originally isolated from extracellular bone matrix that are capable of inducing bone formation ectopically. We studied the expression, tissue localization, and function of BMP-7 (OP-1) during tooth development in rodents. Patterns of BMP-7 gene expression and peptide distribution indicated that BMP-7 was present in dental epithelium during the dental lamina, bud, and cap stages. During the bell stage, BMP-7 mRNA expression and protein distribution shifted from dental epithelium toward the dental mesenchyme. With advancing differentiation of odontoblasts, BMP-7 protein staining in the dental papilla became restricted to the layer of fully functional odontoblasts in the process of depositing (pre)dentin. Secretory-stage ameloblasts exhibited weak immunostaining for BMP-7. A restricted pattern of staining in ameloblasts became apparent in post-secretory stages of amelogenesis. Also, cells of the forming periodontal ligament were immunopositive. Histological analysis of tooth development in neonatal BMP-7-deficient mice did not reveal obvious changes compared with wild-type mice. We conclude that, in developing dental tissues, BMP-7 has distribution and expression patterns similar to those of other BMP members but is not an essential growth factor for tooth development, possibly because of functional redundancy with other BMP members or related growth factors.

Msx1 is required for the induction of Patched by Sonic hedgehog in the mammalian tooth germ

We have used the mouse developing tooth germ as a model system to explore the transmission of Sonic hedgehog (Shh) signal in the induction of Patched (Ptc). In the early developing molar tooth germ, Shh is expressed in the dental epithelium, and the transcripts of Shh downstream target genes Ptc and Gli1 are expressed in dental epithelium as well as adjacent mesenchymal tissue. The homeobox gene Msx1 is also expressed in the dental mesenchyme of the molar tooth germ at this time. We show here that the expression of Ptc, but not Gli1, was downregulated in the dental mesenchyme of Msx1 mutants. In wild-type E11.0 molar tooth mesenchyme SHH-soaked beads induced the expression of Ptc and Gli1. However, in Msx1 mutant dental mesenchyme SHH-soaked beads were able to induce Gli1 but failed to induce Ptc expression, indicating a requirement for Msx1 in the induction of Ptc by SHH. Moreover, we show that another signaling molecule, BMP4, was able to induce Ptc expression in wild-type dental mesenchyme, but induced a distinct expression pattern of Ptc in the Msx1 mutant molar mesenchyme. We conclude that in the context of the tooth germ Msx1 is a component of the Shh signaling pathway that leads to Ptc induction. Our results also suggest that the precise pattern of Ptc expression in the prospective tooth-forming region is controlled and coordinated by at least two inductive signaling pathways.

Use of marrow-cancellous bone grafts in maxillary alveolar and palatal clefts

Autogenous particulate cancellous bone and marrow grafts were studied in bilateral, surgically produced palatal and alveolar clefts in six young rhesus monkeys. Fluorescence and routine light microscopy indicated complete osseous regeneration of the cleft areas. Tooth buds on either side of the surgical area were not disturbed by the grafting procedure.

Temporal and spatial patterns of transforming growth factor-beta 1 expression in developing rat molars

Regulatory peptides of the TGF-beta family affect various aspects of embryonic development. Recent immunolocalization and in situ hybridization studies have demonstrated a specific time- and tissue-dependent expression of TGF-beta 1 in the developing mouse embryo. The purpose of this study was to evaluate the distribution of TGF-beta 1 within rat molars at different stages of development, using a well-characterized antibody, highly specific for TGF-beta 1, and immunohistochemical methods of detection. TGF-beta 1 was immunolocalized intensely within the ectodermally derived stellate reticulum and the mesenchyme of the dental papilla at the bell stage of development. Marked immunostaining was also evident in the papillary layer and the reduced dental organ subjacent to ameloblasts in the differentiation and secretory phases of amelogenesis. During the formation of coronal tissues and in the pre-eruptive phase, immunoreactive TGF-beta 1 was localized conspicuously within the dental follicle overlying the tooth germ. This temporospatial pattern of expression of TGF-beta 1 appears to correlate with specific events in morphogenesis, histogenesis and cytodifferentiation during tooth development.

Autotransplantation and replantation of tooth germs in monkeys. Effect of damage to the dental follicle and position of transplant in the alveolus

The effect of damage to the follicle and of superficial positioning of tooth germs after replantation or autotransplantation was studied in green Vervet monkeys. Radiographs were taken immediately after surgery and 3, 6 and 9 months postoperatively, whereafter the animals were sacrificed. The tissue blocks were sectioned in step serial sections along a frontal plane and histologic and radiographic evaluation was made. Tooth germs whose follicle was either damaged or removed showed no sign of eruption, while tooth germs which were replanted with an intact follicle were fully erupted within 3 months. Non-operated control teeth were fully erupted within 6 months. Histologic examination revealed that roots of replanted teeth were only slightly shorter than the non-operated control teeth. Transplantation of tooth germs in different positions in relation to the alveolar crest showed that teeth placed in their original position attained an average tooth length of 12.5 mm, whereas teeth placed in a semi-erupted position achieved an average length of only 9.7 mm. A control group of non-operated incisors demonstrated an average tooth length of 13.7 mm. This study indicates that damage to the follicle at the time of replantation of tooth germs is of major importance for tooth eruption and that placing tooth germs in a semi-erupted position adversely influences later root development.

A histological and histochemical study of developing teeth in Polypterus (Pisces, Actinopterygii)

The tooth cap matrix contained proteins of ectodermal and mesodermal origin which extended the data already available on teleost teeth and showed that a combined origin for the tooth cap should be regarded as a primitive feature of actinopterygian fish. Furthermore, the dual nature of the Polypterus tooth cap matrix suggested that evolution of tetrapod enamel did not occur within the actinopterygians. The collar tissue was an unmodified dentine, in contrast to its enameloid nature in many other actinopterygian teeth. The presence of a range of developmental states from unmodified dentine to hypermineralized enameloid within Polypterus teeth themselves and also in other parts of the dermal skeleton, as well as the combined origin of the tooth cap, demonstrates that the enamel/enameloid/dentine system forms a continuum of tissues that have diverged from one another by changes in the relative timing of developmental events and matrix production.

Defects of enamel in sheep resulting from trauma during tooth development

Defects of enamel were produced by inserting an instrument into the tooth crypt of developing permanent incisor teeth in sheep. The activity of the labial ameloblasts was deduced from tooth length at the time of trauma. Missing enamel (hypoplasia) resulted when secretory ameloblasts were damaged. Opacities (hypocalcification) followed trauma to the cells in their maturation phase.

Development and physiological degradation of tooth buds and development of rudiment of baleen plate in southern minke whale, Balaenoptera acutorostrata

The development and degradation of temporary tooth buds and the development of rudiment of baleen plate were observed by gross-anatomical and histological examinations in twenty-four fetuses of the southern minke whale, Balaenoptera acutorostrata. The primary patterns of development of tooth buds were similar to those of deciduous tooth buds in the terrestrial species. Degradation of tooth buds was observed in the fetuses more than 615 mm body length (BL) and might proceed throughout the dental surface of the tooth buds. That degradation pattern was a little different from that of deciduous tooth buds in terrestrial species, which has a limited degradation area at the root of the tooth buds. In the fetuses with 135 and 153 mm BL, the upper jaw had a larger number of tooth buds than the lower jaw, although the number of buds varied in different individuals. Formation of rudiment of baleen plate was observed with degraded tooth buds in the fetus of 903 mm BL and it may be induced by the degradation of tooth buds.

Larval vs. adult pharyngeal dentition in some Japanese cyprinid fishes

The development of the pharyngeal dentition was examined in 13 species of five subfamilies of the family Cyprinidae. A previously described difference between the larval and adult dentition in Gnathopogon caerulescens was observed in 11 species of Gobioinae, Cyprininae, Leuciscinae, and Danioinae, but was not seen in two species of Acheilognathinae. The adult dentition is usually complete by the later larval or juvenile period. In the adult dentition, the functioning teeth are generally shed at each position, when the replacement tooth has been ankylosed to the bone, or when the germ of that tooth has grown sufficiently. In the larval dentition, however, the replacement tooth is ankylosed medial to the functioning tooth, so the older functioning tooth is retained after ankylosis of the replacement tooth. That is why the dentigerous surface of the bone grows rapidly during the larval period. Each tooth row represents a single replacement wave. The distribution and pattern of the central teeth are exactly the same in all types of the larval dentition. However, those of the anterior teeth vary between and within different species. The larval dentition develops into the major row of the adult dentition through tooth replacements. The number of tooth families in the larval dentition is completely consistent with the tooth number of the major row in the adult dentition. Therefore, the number of major row teeth owes its variation to that in the appearance pattern of the anterior teeth.

Tooth replacement without a dental lamina: the search for epithelial stem cells in Polypterus senegalus

Most actinopterygians replace their teeth continuously throughout life. To address the question of where and how replacement teeth form in actinopterygians, it is advisable to investigate well-chosen representatives within the lineage. The African bichir, Polypterus senegalus, belongs to the earliest diverged group of the actinopterygian lineage with currently living representatives. Its well characterized dentition, together with its phylogenetic position, make this species an attractive model to answer following questions: (1) when and where does the replacement tooth form and how is it connected with the dental organ of the predecessor, and (2) is there any evidence for the presence of epithelial stem cells, hypothesized to play a role in replacement? Serial sections show that one tooth family can contain up to three members, which are all interconnected by dental epithelium. Replacement teeth develop without the presence of a successional dental lamina. We propose that this is the plesiomorphic condition for tooth replacement in actinopterygians. BrdU pulse-chase experiments reveal cells in the outer and middle dental epithelium, proliferating at the time of initiation of a new replacement tooth. It is tempting to assume that these cell layers provide a stem cell niche. The observed absence of label-retaining cells after long chase times (up to 8 weeks) is held against the light of divergent views on cell cycling properties of stem cells. At present, our data do not support, neither reject, the hypothesis on involvement of epithelial stem cells within the process of continuous tooth replacement.

Origin and developmental fate of vestigial tooth primordia in the upper diastema of the field vole (Microtus agrestis, Rodentia)

OBJECTIVE

Odontogenesis in voles is a convenient model to test hypotheses on tooth development generated from investigations in the mouse. Similar to other rodents, the functional dentition of the vole includes a toothless diastema. At its mesial end, a vestigial tooth bud has been found in the upper jaw of vole embryos. The aim of this study was to analyse the developmental dynamics of vestigial tooth structures in the upper diastema of the field vole and to compare it with the situation in the mouse.

DESIGN

The development of odontogenic structures in the upper diastema of the field vole was investigated using serial histological sections and three-dimensional (3D) computer-aided reconstruction.

RESULTS

A transient continuous dental lamina in the upper diastema of the field vole extended mesially to the first molar primordium, but was not continuous with the dental lamina in the incisor region. At its mesial limit, a large vestigial tooth primordium was regularly present. A further distinct vestigial bud was located mesially to the first molar primordium. The segmentation of the dental lamina suggested a potential to give rise to further vestiges in the upper diastema of the vole.

CONCLUSIONS

In the prospective diastema of the vole exists as in the mouse a continuous dental lamina. Beside the prominent vestigial tooth bud in the mesial diastema, a further large bud was transiently located in front of the molars. The incorporation of dental epithelium into the first upper molar (M(1)) primordium in the vole differs from that in the mouse.

Histological and biochemical observations of developing enameloid of the Sea Bream

In order to study changes in the enameloid matrix of the Sea Bream during the course of its development, we selected the developmental tooth germs of this fish as representative of three different developmental stages: "chalk-like", "cheese-like", and "soft" enameloid. The protein, calcium, and phosphate contents of each sample were analyzed. The changes of the total protein content in each sample suggest that a major part of the proteins decreased during maturation, although newly formed enameloid of the Sea Bream contains collagen and noncollagenous proteins. The existence of proteolytic activity was examined by placement of undemineralized cryostat sections of unfixed tooth germs on exposed and processed photographic films and then incubation for 30 min in a water-saturated atmosphere at 37°C. Proteolytic activity could be detected in the enameloid matrix, which appeared to be in a "cheese-like" stage. It is suggested that proteolytic enzymes play an important role in the removal of proteins during the maturation of enameloid, although the detailed mechanism of the process is still obscure.

Presence of third molar germs in orthodontic patients in Japan

The purpose of this investigation was to evaluate the existence of third molar germs in orthodontic patients in Japan and to examine the relationship between the existence of third molars and sagittal maxillomandibular jaw relationships. The subjects comprised 306 patients from the orthodontic clinic of Hokkaido University Dental Hospital who were younger than 15 years. The subjects were divided into 2 groups: 1 group included 144 patients who were born between 1966 and 1969 (60s group), and the other group included 162 patients who were born between 1980 and 1987 (80s group). Assessments were made from panoramic radiographs and lateral cephalograms. The following results were obtained: (1) all 4 third molar germs were present in 77% of subjects, (2) mandibular third molars were present significantly more often than maxillary third molars, and (3) the percentage of skeletal Class III subjects who had all 4 third molars was lower than that of skeletal Class II subjects. The chi(2) test was used to determine statistical significance in differences.

Development of the rabbit craniomandibular joint in association with tooth eruption

The adult rabbit craniomandibular joints (CMJs) are stress-bearing joints. The two CMJs and the teeth form an articular triad. In early fetal life the developing triad consists of the CMJ primordia, the tooth germs for the entire set of deciduous teeth, as well as the posterior extensions of the dental lamina, which will give rise to the permanent teeth with no deciduous predecessors. During postnatal life, before occlusion is established, there is a remodelling stage in which the CMJ builds up its matrix components such as collagenous and elastic fibres, proteoglycans and type II collagen. Remodelling gradually diminishes into the maintenance stage once occlusion is fully established and after eruption of the first and second molars. Chondrocytes first appear in the CMJ articular disc during the second week of postnatal development. These cells localize in the band areas of the disc and establish an extensive cartilaginous matrix 3-4 weeks postnatally. This study supports the concept that the full development of a fibrocartilaginous articular disc, rich in proteoglycans, occurs as adult occlusion is established.

Deciduous tooth chronology in the mandible of the domestic pig

Fetuses of known age, collected from 20 days' gestation to term, were used to characterize the chronology of deciduous tooth development within the right mandible of swine. Tooth development was first observed at 32 days' embryonic development, with the differentiation of the deciduous third molar. Bud stages for the remaining deciduous teeth differentiated within the period of 32 to 38 days of embryonic development. Although the initial histological appearance of these teeth covered a short period of time, it was apparent that each tooth continued to develop at its own rate. The deciduous second incisor and first molar reached a stage of enamel formation by the 80th and 86th day of fetal development. This is a much later stage than previously recorded for beginning enamel formation. The stages of tooth development and enamel formation for each tooth are summarized. A previous report on the distribution of the dental lamina and deciduous tooth development in the mandible of the domestic pig combined with the information presented in this report on tooth chronology provide much of the information required for future studies using the domestic pig in dental research. A fetus observed at the 74th day of development demonstrated a tooth bud for the deciduous first premolar. The development of this tooth was followed closely throughout the remainder of fetal development with the cap stage representing its most definitive form at 110 days' development. The suggested deciduous origin for this tooth could result in a reevaluation of the nomenclature for the dental formula of swine.