Patterning of palatal rugae through sequential addition reveals an anterior/posterior boundary in palatal development

Minor physical anomalies including palatal rugae pattern and palatal dimensions in children with sickle cell disease: A cross-sectional analytical study

Background

Sickle cell disease (SCD) is the most common hereditary hemoglobinopathy, which delays growth leading to an altered skeleton and craniofacial pattern. Palatal rugae patterning has been considered the regulator of the development of the palate. The purpose of the research work was to study the morphology of the palate, rugae pattern, and its dimensions in SCD children and compare them with healthy normal children, and to evaluate its role as minor physical anomalies (MPAs).

Methods

A cross-sectional case-control study was designed as per STROBE guidelines. The sample comprised 50 children diagnosed with sickle cell disease (Group SCD) and 50 normal healthy children as control (Group C) belonging to the same age group (10-18 years). Dental impressions were made, followed by the pouring of dental casts. The length of the palatal rugae was measured and categorized into primary (>5 mm), secondary (3 mm-5 mm), and fragmentary rugae (<3 mm). The shape of each primary palatal rugae was identified and categorized as curved, wavy, straight, circular and non-specific. Linear and angular measurements of the palatal rugae patterns and palatal dimensions (width, height, area) were measured and recorded.

Results

The total number of palatal rugae and fragmentary rugae was lesser in Group SCD than in Group C (p < 0.05). The depth of the palate was significantly increased, whereas the area of the palate significantly decreased in Group SCD.

Conclusions

The children with SCD showed distinctive palatal rugae patterns and dimensions when compared with normal healthy children that can be attributed as potential MPAs for sickle cell disease. Children with SCD had an under-developed palatal rugae pattern with a deep, narrow and small palate when compared to healthy children.The dimensions of the palatal rugae pattern in SCD showed reduced distance between the incisive papilla and the first and last rugae, indicating a further decrease in the anteroposterior dimensions of the palate. These findings may aid in the early diagnosis and prevention of malocclusion in children with SCD by appropriate interceptive orthodontic treatment.

Palatal segment contributions to midfacial growth in three inbred mouse strains

Following facial prominence fusion, anterior-posterior (A-P) elongation of the palate is a critical aspect of palatogenesis and integrated midfacial elongation. Reciprocal epithelial-mesenchymal interactions drive secondary palate elongation and periodic signaling center formation within the rugae growth zone (RGZ). However, the relationship between RGZ dynamics and the morphogenetic behavior of underlying palatal bone mesenchymal precursors has remained enigmatic. Our results indicate that cellular activity at the RGZ simultaneously drives rugae formation and elongation of the maxillary bone primordium within the anterior secondary palate, which more than doubles in length prior to palatal shelf fusion. The first formed palatal ruga, found just posterior to the RGZ, represents a consistent morphological boundary between anterior and posterior secondary palate bone precursors, being found at the future maxillary-palatine suture. These results suggest a model where changes in RGZ-driven A-P growth of the anterior secondary palate may produce interspecies and intraspecies differences in facial prognathism and differences in the proportional contribution of palatal segment-associated bones to total palate length. An ontogenetic comparison of three inbred mouse strains indicated that while RGZ-driven growth of the anterior secondary palate is critical for early midfacial outgrowth, subtle strain-specific bony contributions to adult palate length are not present until after this initial palatal growth period. This multifaceted illustration of normal midfacial growth dynamics confirms a one-to-one relationship between palatal segments and upper jaw bones during the earliest stages of palatal growth, which may serve as the basis for evolutionary change in upper jaw morphology. Additionally, identified mouse strain-specific differences in palatal segment elongation provide a useful foundation for understanding the impact of background genetic effects on facial morphogenesis.

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

Bat teeth illuminate the diversification of mammalian tooth classes

Tooth classes are an innovation that has contributed to the evolutionary success of mammals. However, our understanding of the mechanisms by which tooth classes diversified remain limited. We use the evolutionary radiation of noctilionoid bats to show how the tooth developmental program evolved during the adaptation to new diet types. Combining morphological, developmental and mathematical modeling approaches, we demonstrate that tooth classes develop through independent developmental cascades that deviate from classical models. We show that the diversification of tooth number and size is driven by jaw growth rate modulation, explaining the rapid gain/loss of teeth in this clade. Finally, we mathematically model the successive appearance of tooth buds, supporting the hypothesis that growth acts as a key driver of the evolution of tooth number and size. Our work reveal how growth, by tinkering with reaction/diffusion processes, drives the diversification of tooth classes and other repeated structure during adaptive radiations.

Gene Regulatory Networks and Signaling Pathways in Palatogenesis and Cleft Palate: A Comprehensive Review

Palatogenesis is a complex and intricate process involving the formation of the palate through various morphogenetic events highly dependent on the surrounding context. These events comprise outgrowth of palatal shelves from embryonic maxillary prominences, their elevation from a vertical to a horizontal position above the tongue, and their subsequent adhesion and fusion at the midline to separate oral and nasal cavities. Disruptions in any of these processes can result in cleft palate, a common congenital abnormality that significantly affects patient's quality of life, despite surgical intervention. Although many genes involved in palatogenesis have been identified through studies on genetically modified mice and human genetics, the precise roles of these genes and their products in signaling networks that regulate palatogenesis remain elusive. Recent investigations have revealed that palatal shelf growth, patterning, adhesion, and fusion are intricately regulated by numerous transcription factors and signaling pathways, including Sonic hedgehog (Shh), bone morphogenetic protein (Bmp), fibroblast growth factor (Fgf), transforming growth factor beta (Tgf-β), Wnt signaling, and others. These studies have also identified a significant number of genes that are essential for palate development. Integrated information from these studies offers novel insights into gene regulatory networks and dynamic cellular processes underlying palatal shelf elevation, contact, and fusion, deepening our understanding of palatogenesis, and facilitating the development of more efficacious treatments for cleft palate.

Revisiting the embryogenesis of lip and palate development

Clefts of the lip and palate (CLP), the major causes of congenital facial malformation globally, result from failure of fusion of the facial processes during embryogenesis. With a prevalence of 1 in 500-2500 live births, CLP causes major morbidity throughout life as a result of problems with facial appearance, feeding, speaking, obstructive apnoea, hearing and social adjustment and requires complex, multi-disciplinary care at considerable cost to healthcare systems worldwide. Long-term outcomes for affected individuals include increased mortality compared with their unaffected siblings. The frequent occurrence and major healthcare burden imposed by CLP highlight the importance of dissecting the molecular mechanisms driving facial development. Identification of the genetic mutations underlying syndromic forms of CLP, where CLP occurs in association with non-cleft clinical features, allied to developmental studies using appropriate animal models is central to our understanding of the molecular events underlying development of the lip and palate and, ultimately, how these are disturbed in CLP.

Essential role of Msx1 in regulating anterior-posterior patterning of the secondary palate in mice

Development of the secondary palate displays molecular heterogeneity along the anterior-posterior axis; however, the underlying molecular mechanism remains largely unknown. MSX1 is an anteriorly expressed transcription repressor required for palate development. Here, we investigate the role of Msx1 in regional patterning of the secondary palate. The Wnt1-Cre-mediated expression of Msx1 (RosaMsx1^Wnt1-Cre) throughout the palatal mesenchyme leads to cleft palate in mice, associated with aberrant cell proliferation and cell death. Osteogenic patterning of the hard palate in RosaMsx1^Wnt1-Cre mice is severely impaired, as revealed by a marked reduction in palatine bone formation and decreased expression of the osteogenic regulator Sp7. Overexpression and knockout of Msx1 in mice show that the transcription repressor promotes the expression of the anterior palate-specific Alx1 but represses the expression of the medial-posterior palate genes Barx1, Meox2, and Tbx22. Furthermore, Tbx22 constitutes a direct Msx1 target gene in the secondary palate, suggesting that Msx1 can directly repress the expression of medial-posterior specific genes. Finally, we determine that Sp7 is downstream of Tbx22 in palatal mesenchymal cells, suggesting that a Msx1/Tbx22/Sp7 axis participates in the regulation of palate development. Our findings unveil a novel role for Msx1 in regulating the anterior-posterior growth and patterning of the secondary palate.

Reawakening of Ancestral Dental Potential as a Mechanism to Explain Dental Pathologies

During evolution, there has been a trend to reduce both the number of teeth and the location where they are found within the oral cavity. In mammals, the formation of teeth is restricted to a horseshoe band of odontogenic tissue, creating a single dental arch on the top and bottom of the jaw. Additional teeth and structures containing dental tissue, such as odontogenic tumors or cysts, can appear as pathologies. These tooth-like structures can be associated with the normal dentition, appearing within the dental arch, or in nondental areas. The etiology of these pathologies is not well elucidated. Reawakening of the potential to form teeth in different parts of the oral cavity could explain the origin of dental pathologies outside the dental arch, thus such pathologies are a consequence of our evolutionary history. In this review, we look at the changing pattern of tooth formation within the oral cavity during vertebrate evolution, the potential to form additional tooth-like structures in mammals, and discuss how this knowledge shapes our understanding of dental pathologies in humans.

Genetic variants in bone morphogenetic proteins signaling pathway might be involved in palatal rugae phenotype in humans

This study investigated, if genetic variants in BMP2, BMP4 and SMAD6 are associated with variations in the palatal rugae pattern in humans. Dental casts and genomic DNA from 75 patients were evaluated. Each patient was classified as follows: total amount of rugae; bilateral symmetry in the amount, length and shape of the palatal rugae; presence of secondary or fragmentary palatal rugae; presence of unifications; predominant shape; and predominant direction of the palatal rugae. The genetic variants in BMP2 (rs1005464 and rs235768), BMP4 (rs17563) and SMAD6 (rs2119261 and rs3934908) were genotyped. Genotype distribution was compared between palatal rugae patterns using the chi-square test (alpha = 0.05). The allele A was associated with the presence of secondary or fragmentary rugae for rs1005464 (OR = 2.5, 95%CI 1.1–6.3; p = 0.014). Secondary or fragmentary rugae were associated with the G allele in rs17563 (OR = 2.1, 95%CI 1.1–3.9; p = 0.017). rs17563 was also associated with rugae unification (p = 0.017 in the additive model). The predominant shape (wavy) was associated with rs2119261 (p = 0.023 in the additive model). The left–right symmetry of the length of primary rugae was associated with rs3934908 in the recessive model (OR = 3.6, 95%CI 1.2–11.7; p = 0.025). In conclusion, genetic variants in the BMP pathway impacted on palatal rugae pattern.

Wavelet Screening identifies regions highly enriched for differentially methylated loci for orofacial clefts

DNA methylation is the most widely studied epigenetic mark in humans and plays an essential role in normal biological processes as well as in disease development. More focus has recently been placed on understanding functional aspects of methylation, prompting the development of methods to investigate the relationship between heterogeneity in methylation patterns and disease risk. However, most of these methods are limited in that they use simplified models that may rely on arbitrarily chosen parameters, they can only detect differentially methylated regions (DMRs) one at a time, or they are computationally intensive. To address these shortcomings, we present a wavelet-based method called 'Wavelet Screening' (WS) that can perform an epigenome-wide association study (EWAS) of thousands of individuals on a single CPU in only a matter of hours. By detecting multiple DMRs located near each other, WS identifies more complex patterns that can differentiate between different methylation profiles. We performed an extensive set of simulations to demonstrate the robustness and high power of WS, before applying it to a previously published EWAS dataset of orofacial clefts (OFCs). WS identified 82 associated regions containing several known genes and loci for OFCs, while other findings are novel and warrant replication in other OFCs cohorts.

Structural and Ultrastructural Characteristics of the Red-Rumped Agouti (Dasyprocta leporina-Linnaeus, 1758) Palatine Epithelium

The palate is a fundamental region in food swallowing and presents different adaptations in species. This research aimed to describe structural and ultrastructural characteristics of the palatine epithelium and the connective tissue cores (CTCs) of ten red-rumped agoutis (Dasyprocta leporina—Linnaeus, 1758) using macroscopic, light microscopy, scanning electron microscopy, and transmission electron microscopy. We found nine palatine ridges in the diastema and hard palate, and a smooth surface in the soft palate. Stratified squamous keratinized epithelium with projections of lamina propria and soft palate had gland clusters. Epithelial removal revealed CTCs with a conical shape with high density in the hard palate and the sides of the soft palate. Near the CTCs were nerve fibers in the hard palate, and the soft palate had muscular tissue below the gland clusters. The structural and ultrastructural characteristics enable stability of the hard palate and fixation to the soft palate sides, while the soft palate center has greater mobility thus assisting in food swallowing. We concluded that structural characteristics are similar to other mammals, although the morphology of agouti's palate differs in the amount and disposition of palatine ridges, and the conical CTC's morphology.

Oral biosciences: The annual review 2020

BACKGROUND

The Journal of Oral Biosciences is devoted to the advancement and dissemination of fundamental knowledge concerning every aspect of oral biosciences.

HIGHLIGHT

This review featured the review articles in the fields of "Microbiology," "Palate," "Stem Cells," "Mucosal Diseases," "Bone Cell Biology," "MicroRNAs," "TRPV1 Cation Channels," and "Interleukins" in addition to the review article by prize-winners of the "Rising Members Award" ("DKK3 expression and function in head and neck squamous cell carcinoma and other cancers"), presented by the Japanese Association for Oral Biology.

CONCLUSION

These reviews in the Journal of Oral Biosciences have inspired the readers of the journal to broaden their knowledge regarding the various aspects of oral biosciences. The current editorial review introduces these exciting review articles.

Palatal rugae morphology is associated with variation in tooth number

This observational study compared palatal rugae morphology in adolescent subjects with normal tooth number and tooth agenesis. Maxillary dental study casts were used to compare rugae number, length and shape. Each study group contained 60 subjects (30 females and 30 males) mean age 13.4 (SD, 1.55) in control and 13.56 (SD, 1.54) years in tooth agenesis groups (p = 0.576). Mean number of missing tooth units in the tooth agenesis group was 2.1. Mean number of primary rugae in the whole sample was 4.35 (SD, 0.98) on the right and 4.33 (SD, 0.92) on the left with no significant differences (p = 0.236 and p = 0.404, respectively). However, the number of secondary rugae on the left (p = 0.006) and fragmentary rugae on the right (p = 0.004) was significantly increased in the tooth agenesis group. The shape of left primary rugae 2 and 3 also differed between groups, tending towards a wavy pattern in the control group and curved in the tooth agenesis group (p = 0.012 and p = 0.004, respectively). In addition, primary rugae 3 was more convergent (p = 0.008) whilst left primary rugae 3 and 5 were orientated in an antero-posterior direction (p = 0.04 for both rugae) in the tooth agenesis group. Subgroup analysis also identified significant associations between patterns of tooth agenesis and rugae number, in addition to shape of primary rugae. The identification of significant differences in rugae pattern between subjects with normal tooth number and agenesis suggests potential commonality in signal pathway disruption during establishment of these structures.

Genetics and signaling mechanisms of orofacial clefts

Craniofacial development involves several complex tissue movements including several fusion processes to form the frontonasal and maxillary structures, including the upper lip and palate. Each of these movements are controlled by many different factors that are tightly regulated by several integral morphogenetic signaling pathways. Subject to both genetic and environmental influences, interruption at nearly any stage can disrupt lip, nasal, or palate fusion and result in a cleft. Here, we discuss many of the genetic risk factors that may contribute to the presentation of orofacial clefts in patients, and several of the key signaling pathways and underlying cellular mechanisms that control lip and palate formation, as identified primarily through investigating equivalent processes in animal models, are examined.

Morphological patterns of the palatal rugae: A review

BACKGROUND

Palatal rugae are asymmetric ridges of connective tissue located behind the incisive papilla over the anterior hard palate. They serve as stable superimposition landmarks to assess tooth movement in orthodontics and as identification aids in forensic odontology. However, the stability of palatal rugae remains controversial. This review aimed to describe the genetic, growth, and environmental factors that may influence the palatal rugae patterns. A broad search of PubMed and ScienceDirect databases was conducted. A total of 193 articles were identified, of which 73 met the selection criteria. Data were extracted into a table that presented the details of the study, sample description, and changes in the palatal rugae patterns.

HIGHLIGHT

There were conflicting results regarding sexual dimorphism and population characterization of the palatal rugae patterns. All rugae showed positional changes, increased lengths, and lower numbers, but no significant shape changes with growth. The lengths, numbers, and positions of the rugae were affected by orthodontic treatment, especially their lateral points, but their individual characteristics did not change.

CONCLUSION

The diversity in rugae patterns and their potential for sex discrimination among different populations showed differing results due to individual variations and the complex influence of genetic, growth, and environmental factors on their morphology.

Pattern formation in reaction-diffusion systems with piecewise kinetic modulation: An example study of heterogeneous kinetics

The study of pattern emergence together with exploration of the exemplar Turing model is enjoying a renaissance both from theoretical and experimental perspective. Here, we implement a stability analysis of spatially dependent reaction kinetics by exploring the effect of a jump discontinuity within piecewise constant kinetic parameters, using various methods to identify and confirm the diffusion-driven instability conditions. Essentially, the presence of stability or instability in Turing models is a local property for piecewise constant kinetic parameters and, as such, may be analyzed locally. In particular, a local assessment of whether parameters are within the Turing space provides a strong indication that for a large enough region with these parameters, an instability can be induced.

Shox2 regulates osteogenic differentiation and pattern formation during hard palate development in mice

During mammalian palatogenesis, cranial neural crest-derived mesenchymal cells undergo osteogenic differentiation and form the hard palate, which is divided into palatine process of the maxilla and the palatine. However, it remains unknown whether these bony structures originate from the same cell lineage and how the hard palate is patterned at the molecular level. Using mice, here we report that deficiency in Shox2 (short stature homeobox 2), a transcriptional regulator whose expression is restricted to the anterior palatal mesenchyme, leads to a defective palatine process of the maxilla but does not affect the palatine. Shox2 overexpression in palatal mesenchyme resulted in a hyperplastic palatine process of the maxilla and a hypoplastic palatine. RNA sequencing and assay for transposase-accessible chromatin-sequencing analyses revealed that Shox2 controls the expression of pattern specification and skeletogenic genes associated with accessible chromatin in the anterior palate. This highlighted a lineage-autonomous function of Shox2 in patterning and osteogenesis of the hard palate. H3K27ac ChIP-Seq and transient transgenic enhancer assays revealed that Shox2 binds distal-acting cis-regulatory elements in an anterior palate-specific manner. Our results suggest that the palatine process of the maxilla and palatine arise from different cell lineages and differ in ossification mechanisms. Shox2 evidently controls osteogenesis of a cell lineage and contributes to the palatine process of the maxilla by interacting with distal cis-regulatory elements to regulate skeletogenic gene expression and to pattern the hard palate. Genome-wide Shox2 occupancy in the developing palate may provide a marker for identifying active anterior palate-specific gene enhancers.

Primary cilia in murine palatal rugae development

Periodic patterning of iterative structures is a fundamental process during embryonic development, since these structures are diverse across the animal kingdom. Therefore, elucidating the molecular mechanisms in the formation of these structures promotes understanding of the process of organogenesis. Periodically patterned ridges, palatal rugae (situated on the hard palate of mammals), are an excellent experimental model to clarify the molecular mechanisms involved in the formation of periodic patterning of iterative structures. Primary cilia are involved in many biological events, including the regulation of signaling pathways such as Shh and non-canonical Wnt signaling. However, the role of primary cilia in the development of palatal rugae remains unclear. We found that primary cilia were localized to the oral cavity side of the interplacode epithelium of the palatal rugae, whereas restricted localization of primary cilia could not be detected in other regions. Next, we generated mice with a placodal conditional deletion of the primary cilia protein Ift88, using ShhCre mice (Ift88^fl/fl;ShhCre). Highly disorganized palatal rugae were observed in Ift88^fl/fl;ShhCre mice. Furthermore, by comparative in situ hybridization analysis, many Shh and non-canonical Wnt signaling-related molecules showed spatiotemporal expression patterns during palatal rugae development, including restricted expression in the epithelium (placodes and interplacodes) and mesenchyme. Some of these expression were found to be altered in Ift88^fl/fl;ShhCre mice. Primary cilia is thus involved in development of palatal rugae.

Sonic Hedgehog Signaling Is Required for Cyp26 Expression during Embryonic Development

Deciphering how signaling pathways interact during development is necessary for understanding the etiopathogenesis of congenital malformations and disease. In several embryonic structures, components of the Hedgehog and retinoic acid pathways, two potent players in development and disease are expressed and operate in the same or adjacent tissues and cells. Yet whether and, if so, how these pathways interact during organogenesis is, to a large extent, unclear. Using genetic and experimental approaches in the mouse, we show that during development of ontogenetically different organs, including the tail, genital tubercle, and secondary palate, Sonic hedgehog (SHH) loss-of-function causes anomalies phenocopying those induced by enhanced retinoic acid signaling and that SHH is required to prevent supraphysiological activation of retinoic signaling through maintenance and reinforcement of expression of the Cyp26 genes. Furthermore, in other tissues and organs, disruptions of the Hedgehog or the retinoic acid pathways during development generate similar phenotypes. These findings reveal that rigidly calibrated Hedgehog and retinoic acid activities are required for normal organogenesis and tissue patterning.

Lrp4/Wise regulates palatal rugae development through Turing-type reaction-diffusion mechanisms

Periodic patterning of iterative structures is diverse across the animal kingdom. Clarifying the molecular mechanisms involved in the formation of these structure helps to elucidate the process of organogenesis. Turing-type reaction-diffusion mechanisms have been shown to play a critical role in regulating periodic patterning in organogenesis. Palatal rugae are periodically patterned ridges situated on the hard palate of mammals. We have previously shown that the palatal rugae develop by a Turing-type reaction-diffusion mechanism, which is reliant upon Shh (as an inhibitor) and Fgf (as an activator) signaling for appropriate organization of these structures. The disturbance of Shh and Fgf signaling lead to disorganized palatal rugae. However, the mechanism itself is not fully understood. Here we found that Lrp4 (transmembrane protein) was expressed in a complementary pattern to Wise (a secreted BMP antagonist and Wnt modulator) expression in palatal rugae development, representing Lrp4 expression in developing rugae and Wise in the inter-rugal epithelium. Highly disorganized palatal rugae was observed in both Wise and Lrp4 mutant mice, and these mutants also showed the downregulation of Shh signaling, which was accompanied with upregulation of Fgf signaling. Wise and Lrp4 are thus likely to control palatal rugae development by regulating reaction-diffusion mechanisms through Shh and Fgf signaling. We also found that Bmp and Wnt signaling were partially involved in this mechanism.

Ectopic Hedgehog Signaling Causes Cleft Palate and Defective Osteogenesis

Cleft palate is a common birth defect that frequently occurs in human congenital malformations caused by mutations in components of the Sonic Hedgehog (S HH) signaling cascade. Shh is expressed in dynamic, spatiotemporal domains within epithelial rugae and plays a key role in driving epithelial-mesenchymal interactions that are central to development of the secondary palate. However, the gene regulatory networks downstream of Hedgehog (Hh) signaling are incompletely characterized. Here, we show that ectopic Hh signaling in the palatal mesenchyme disrupts oral-nasal patterning of the neural crest cell-derived ectomesenchyme of the palatal shelves, leading to defective palatine bone formation and fully penetrant cleft palate. We show that a series of Fox transcription factors, including the novel direct target Foxl1, function downstream of Hh signaling in the secondary palate. Furthermore, we demonstrate that Wnt/bone morphogenetic protein (BMP) antagonists, in particular Sostdc1, are positively regulated by Hh signaling, concomitant with downregulation of key regulators of osteogenesis and BMP signaling effectors. Our data demonstrate that ectopic Hh-Smo signaling downregulates Wnt/BMP pathways, at least in part by upregulating Sostdc1, resulting in cleft palate and defective osteogenesis.

Alteration of palatine ruga pattern in subjects with oligodontia: A pilot study

INTRODUCTION

The aim of this study was to compare the palatal ruga patterns in subjects with oligodontia and normal tooth numbers.

METHODS

An observational investigation was conducted by using maxillary dental study casts to compare ruga numbers, lengths, and shapes in subjects with diagnosed oligodontia or normal tooth numbers.

RESULTS

A total of 32 subjects comprised both the oligodontia (mean age, 14.0 years; SD, 5.0 years) and the control (mean age, 14.5 years; SD, 5.1 years) groups. The mean number of missing teeth in the oligodontia group was 8.7. The mean number of rugae in the whole sample was 7.36 (SD, 1.16), with no significant difference between the groups (P = 0.264). For ruga pattern, no differences were found for right-sided rugae; however, on the left side, a significant difference existed in shape frequency associated with ruga 2. Specifically, a curved shape was seen more frequently in ruga 2 of the oligodontia group (P = 0.01).

CONCLUSIONS

The identification of subtle differences in ruga patterns between subjects with oligodontia and normal tooth numbers suggests potentially shared pathways during the development of these oral structures. Further large-scale investigations are warranted.

Evolutionary aspects of the development of teeth and baleen in the bowhead whale

In utero, baleen whales initiate the development of several dozens of teeth in upper and lower jaws. These tooth germs reach the bell stage and are sometimes mineralized, but toward the end of prenatal life they are resorbed and no trace remains after birth. Around the time that the germs disappear, the keratinous baleen plates start to form in the upper jaw, and these form the food-collecting mechanism. Baleen whale ancestors had two generations of teeth and never developed baleen, and the prenatal teeth of modern fetuses are usually interpreted as an evolutionary leftover. We investigated the development of teeth and baleen in bowhead whale fetuses using histological and immunohistochemical evidence. We found that upper and lower dentition initially follow similar developmental pathways. As development proceeds, upper and lower tooth germs diverge developmentally. Lower tooth germs differ along the length of the jaw, reminiscent of a heterodont dentition of cetacean ancestors, and lingual processes of the dental lamina represent initiation of tooth bud formation of replacement teeth. Upper tooth germs remain homodont and there is no evidence of a secondary dentition. After these germs disappear, the oral epithelium thickens to form the baleen plates, and the protein FGF-4 displays a signaling pattern reminiscent of baleen plates. In laboratory mammals, FGF-4 is not involved in the formation of hair or palatal rugae, but it is involved in tooth development. This leads us to propose that the signaling cascade that forms teeth in most mammals has been exapted to be involved in baleen plate ontogeny in mysticetes.

The importance of basonuclin 2 in adult mice and its relation to basonuclin 1

BNC2 is an extremely conserved zinc finger protein with important functions in the development of craniofacial bones and male germ cells. Because disruption of the Bnc2 gene in mice causes neonatal lethality, the function of the protein in adult animals has not been studied. Until now BNC2 was considered to have a wider tissue distribution than its paralog, BNC1, but the precise cell types expressing Bnc2 are largely unknown. We identify here the cell types containing BNC2 in the mouse and we show the unexpected presence of BNC1 in many BNC2-containing cells. BNC1 and BNC2 are colocalized in male and female germ cells, ovarian epithelial cells, sensory neurons, hair follicle keratinocytes and connective cells of organ capsules. In many cell lineages, the two basonuclins appear and disappear synchronously. Within the male germ cell lineage, BNC1 and BNC2 are found in prospermatogonia and undifferentiated spermatogonia, and disappear abruptly from differentiating spermatogonia. During oogenesis, the two basonuclins accumulate specifically in maturing oocytes. During the development of hair follicles, BNC1 and BNC2 concentrate in the primary hair germs. As follicle morphogenesis proceeds, cells possessing BNC1 and BNC2 invade the dermis and surround the papilla. During anagen, BNC1 and BNC2 are largely restricted to the basal layer of the outer root sheath and the matrix. During catagen, the compartment of cells possessing BNC1 and BNC2 regresses, and in telogen, the two basonuclins are confined to the secondary hair germ. During the next anagen, the BNC1/BNC2-containing cell population regenerates the hair follicle. By examining Bnc2(-/-) mice that have escaped the neonatal lethality usually associated with lack of BNC2, we demonstrate that BNC2 possesses important functions in many of the cell types where it resides. Hair follicles of postnatal Bnc2(-/-) mice do not fully develop during the first cycle and thereafter remain blocked in telogen. It is concluded that the presence of BNC2 in the secondary hair germ is required to regenerate the transient segment of the follicle. Postnatal Bnc2(-/-) mice also show severe dwarfism, defects in oogenesis and alterations of palatal rugae. Although the two basonuclins possess very similar zinc fingers and are largely coexpressed, BNC1 cannot substitute for BNC2. This is shown incontrovertibly in knockin mice expressing Bnc1 instead of Bnc2 as these mice invariably die at birth with craniofacial abnormalities undistinguishable from those of Bnc2(-/-) mice. The function of the basonuclins in the secondary hair germ is of particular interest.

Hedgehog receptor function during craniofacial development

The Hedgehog signalling pathway plays a fundamental role in orchestrating normal craniofacial development in vertebrates. In particular, Sonic hedgehog (Shh) is produced in three key domains during the early formation of the head; neuroectoderm of the ventral forebrain, facial ectoderm and the pharyngeal endoderm; with signal transduction evident in both ectodermal and mesenchymal tissue compartments. Shh signalling from the prechordal plate and ventral midline of the diencephalon is required for appropriate division of the eyefield and forebrain, with mutation in a number of pathway components associated with Holoprosencephaly, a clinically heterogeneous developmental defect characterized by a failure of the early forebrain vesicle to divide into distinct halves. In addition, signalling from the pharyngeal endoderm and facial ectoderm plays an essential role during development of the face, influencing cranial neural crest cells that migrate into the early facial processes. In recent years, the complexity of Shh signalling has been highlighted by the identification of multiple novel proteins that are involved in regulating both the release and reception of this protein. Here, we review the contributions of Shh signalling during early craniofacial development, focusing on Hedgehog receptor function and describing the consequences of disruption for inherited anomalies of this region in both mouse models and human populations.

A Shh-Foxf-Fgf18-Shh Molecular Circuit Regulating Palate Development

Cleft palate is among the most common birth defects in humans. Previous studies have shown that Shh signaling plays critical roles in palate development and regulates expression of several members of the forkhead-box (Fox) family transcription factors, including Foxf1 and Foxf2, in the facial primordia. Although cleft palate has been reported in mice deficient in Foxf2, whether Foxf2 plays an intrinsic role in and how Foxf2 regulates palate development remain to be elucidated. Using Cre/loxP-mediated tissue-specific gene inactivation in mice, we show that Foxf2 is required in the neural crest-derived palatal mesenchyme for normal palatogenesis. We found that Foxf2 mutant embryos exhibit altered patterns of expression of Shh, Ptch1, and Shox2 in the developing palatal shelves. Through RNA-seq analysis, we identified over 150 genes whose expression was significantly up- or down-regulated in the palatal mesenchyme in Foxf2^-/- mutant embryos in comparison with control littermates. Whole mount in situ hybridization analysis revealed that the Foxf2 mutant embryos exhibit strikingly corresponding patterns of ectopic Fgf18 expression in the palatal mesenchyme and concomitant loss of Shh expression in the palatal epithelium in specific subdomains of the palatal shelves that correlate with where Foxf2, but not Foxf1, is expressed during normal palatogenesis. Furthermore, tissue specific inactivation of both Foxf1 and Foxf2 in the early neural crest cells resulted in ectopic activation of Fgf18 expression throughout the palatal mesenchyme and dramatic loss of Shh expression throughout the palatal epithelium. Addition of exogenous Fgf18 protein to cultured palatal explants inhibited Shh expression in the palatal epithelium. Together, these data reveal a novel Shh-Foxf-Fgf18-Shh circuit in the palate development molecular network, in which Foxf1 and Foxf2 regulate palatal shelf growth downstream of Shh signaling, at least in part, by repressing Fgf18 expression in the palatal mesenchyme to ensure maintenance of Shh expression in the palatal epithelium.

Cleft lip and/or cleft palate (CL/P) are among the most common birth defects in humans, occurring at a frequency of about 1 in 500–2500 live births. The etiology and pathogenesis of CL/P are complex and poorly understood. Generation and analysis of mice carrying targeted null and conditional mutations in many genes have revealed that functional disruption of each of more than 100 genes could cause cleft palate. However, how these genes work together to regulate palate development is not well understood. In this study, we identify a novel molecular circuit consisting of two critical molecular pathways, the fibroblast growth factor (FGF) and Sonic hedgehog (SHH) signaling pathways, and the Forkhead family transcription factors Foxf1 and Foxf2, mediating reciprocal epithelial-mesenchymal signaling interactions that control palatogenesis. As mutations affecting each of multiple components of both the FGF and SHH signaling pathways have been associated with CL/P in humans, our results provide significant new insight into the mechanisms regulating palatogenesis and cleft palate pathogenesis.

Cellular and Molecular Mechanisms of Palatogenesis

Palatogenesis involves the initiation, growth, morphogenesis, and fusion of the primary and secondary palatal shelves from initially separate facial prominences during embryogenesis to form the intact palate separating the oral cavity from the nostrils. The palatal shelves consist mainly of cranial neural crest-derived mesenchymal cells covered by a simple embryonic epithelium. The growth and patterning of the palatal shelves are controlled by reciprocal epithelial-mesenchymal interactions regulated by multiple signaling pathways and transcription factors. During palatal shelf outgrowth, the embryonic epithelium develops a "teflon" coat consisting of a single, continuous layer of periderm cells that prevents the facial prominences and palatal shelves from forming aberrant interepithelial adhesions. Palatal fusion involves not only spatiotemporally regulated disruption of the periderm but also dynamic cellular and molecular processes that result in adhesion and intercalation of the palatal medial edge epithelia to form an intershelf epithelial seam, and subsequent dissolution of the epithelial seam to form the intact roof of the oral cavity. The complexity of regulation of these morphogenetic processes is reflected by the common occurrence of cleft palate in humans. This review will summarize major recent advances and discuss major remaining gaps in the understanding of cellular and molecular mechanisms controlling palatogenesis.

Altered FGF Signaling Pathways Impair Cell Proliferation and Elevation of Palate Shelves

In palatogenesis, palatal shelves are patterned along the mediolateral axis as well as the anteroposterior axis before the onset of palatal fusion. Fgf10 specifically expressed in lateral mesenchyme of palate maintains Shh transcription in lateral epithelium, while Fgf7 activated in medial mesenchyme by Dlx5, suppressed the expansion of Shh expression to medial epithelium. How FGF signaling pathways regulate the cell behaviors of developing palate remains elusive. In our study, we found that when Fgf8 is ectopically expressed in the embryonic palatal mesenchyme, the elevation of palatal shelves is impaired and the posterior palatal shelves are enlarged, especially in the medial side. The palatal deformity results from the drastic increase of cell proliferation in posterior mesenchyme and decrease of cell proliferation in epithelium. The expression of mesenchymal Fgf10 and epithelial Shh in the lateral palate, as well as the Dlx5 and Fgf7 transcription in the medial mesenchyme are all interrupted, indicating that the epithelial-mesenchymal interactions during palatogenesis are disrupted by the ectopic activation of mesenchymal Fgf8. Besides the altered Fgf7, Fgf10, Dlx5 and Shh expression pattern, the reduced Osr2 expression domain in the lateral mesenchyme also suggests an impaired mediolateral patterning of posterior palate. Moreover, the ectopic Fgf8 expression up-regulates pJak1 throughout the palatal mesenchyme and pErk in the medial mesenchyme, but down-regulates pJak2 in the epithelium, suggesting that during normal palatogenesis, the medial mesenchymal cell proliferation is stimulated by FGF/Erk pathway, while the epithelial cell proliferation is maintained through FGF/Jak2 pathway.

Spindle orientation processes in epithelial growth and organisation

This review focuses on the role of orientated cell division (OCD) in two aspects of epithelial growth, namely layer formation and growth in the epithelial plane. Epithelial stratification is invariably associated with fate asymmetric cell divisions. We discuss this through the example of epidermal stratification where cell division plane regulation facilitates concomitant thickening and cell differentiation. Embryonic neuroepithelia are considered as a special case of epithelial stratification. We highlight early ectodermal layer specification, which sets the epidermal versus neuronal fates, as well as later neurogenesis in vertebrates and mammals. We also discuss the heart epicardium as an example of coordinating OCDs with delamination and subsequent differentiation. Epithelial planar growth is examined both in the context of uniform growth, such as in Xenopus epiboly, the Drosophila wing disc and the mammalian intestinal crypt as well as in anisotropic growth, or elongation, such as Drosophila and vertebrate axial elongation and the mouse palate. Coupling between growth perpendicular to and within epithelial planes is recognised, but so are exceptions, as is the often passive role of spindle orientation sometimes hitherto considered to be an active driver of directional growth.

Palatoscopy: An adjunct to forensic odontology: A comparative study among five different populations of India

Objectives:

This study was conducted to analyze and identify differences in the palatal rugae patterns and to identify gender wise changes in the palatal rugae shapes in populations of five different states of India.

Study Design:

Study was conducted in five different Indian states. 500 sample subjects from Andhra Pradesh, Tamil Nadu, Karnataka, Madhya Pradesh and Maharashtra were included. Rugae patterns with predominant shapes were analyzed and categorized according to different states and both genders, data was statistically analyzed using SPSS software 15.0 and the results were obtained by Chi-square analysis.

Results:

“Wavy” type of palatal rugae pattern is the most predominant variant in five different study groups in both the genders.

Conclusion:

This study could identify variations in distribution of various palatal rugae pattern in five different states and confirmed the “wavy” type of palatal rugae patterns to be the most predominant variant in five different study groups.

Analysis of PRICKLE1 in human cleft palate and mouse development demonstrates rare and common variants involved in human malformations

Palate development is shaped by multiple molecular signaling pathways, including the Wnt pathway. In mice and humans, mutations in both the canonical and noncanonical arms of the Wnt pathway manifest as cleft palate, one of the most common human birth defects. Like the palate, numerous studies also link different Wnt signaling perturbations to varying degrees of limb malformation; for example, shortened limbs form in mutations of Ror2,Vangl2 (looptail) and, in particular, Wnt5a. We recently showed the noncanonical Wnt/planar cell polarity (PCP) signaling molecule Prickle1 (Prickle like 1) also stunts limb growth in mice. We now expanded these studies to the palate and show that Prickle1 is also required for palate development, like Wnt5a and Ror2. Unlike in the limb, the Vangl2looptail mutation only aggravates palate defects caused by other mutations. We screened Filipino cleft palate patients and found PRICKLE1 variants, both common and rare, at an elevated frequency. Our results reveal that in mice and humans PRICKLE1 directs palate morphogenesis; our results also uncouple Prickle1 function from Vangl2 function. Together, these findings suggest mouse and human palate development is guided by PCP-Prickle1 signaling that is probably not downstream of Vangl2.

Pax9 regulates a molecular network involving Bmp4, Fgf10, Shh signaling and the Osr2 transcription factor to control palate morphogenesis

Cleft palate is one of the most common birth defects in humans. Whereas gene knockout studies in mice have shown that both the Osr2 and Pax9 transcription factors are essential regulators of palatogenesis, little is known about the molecular mechanisms involving these transcription factors in palate development. We report here that Pax9 plays a crucial role in patterning the anterior-posterior axis and outgrowth of the developing palatal shelves. We found that tissue-specific deletion of Pax9 in the palatal mesenchyme affected Shh expression in palatal epithelial cells, indicating that Pax9 plays a crucial role in the mesenchyme-epithelium interactions during palate development. We found that expression of the Bmp4, Fgf10, Msx1 and Osr2 genes is significantly downregulated in the developing palatal mesenchyme in Pax9 mutant embryos. Remarkably, restoration of Osr2 expression in the early palatal mesenchyme through a Pax9(Osr2KI) allele rescued posterior palate morphogenesis in the absence of Pax9 protein function. Our data indicate that Pax9 regulates a molecular network involving the Bmp4, Fgf10, Shh and Osr2 pathways to control palatal shelf patterning and morphogenesis.

Whole population cell analysis of a landmark-rich mammalian epithelium reveals multiple elongation mechanisms

Tissue elongation is a fundamental component of developing and regenerating systems. Although localised proliferation is an important mechanism for tissue elongation, potentially important contributions of other elongation mechanisms, specifically cell shape change, orientated cell division and cell rearrangement, are rarely considered or quantified, particularly in mammalian systems. Their quantification, together with proliferation, provides a rigorous framework for the analysis of elongation. The mammalian palatal epithelium is a landmark-rich tissue, marked by regularly spaced ridges (rugae), making it an excellent model in which to analyse the contributions of cellular processes to directional tissue growth. We captured confocal stacks of entire fixed mouse palate epithelia throughout the mid-gestation growth period, labelled with membrane, nuclear and cell proliferation markers and segmented all cells (up to ∼20,000 per palate), allowing the quantification of cell shape and proliferation. Using the rugae as landmarks, these measures revealed that the so-called growth zone is a region of proliferation that is intermittently elevated at ruga initiation. The distribution of oriented cell division suggests that it is not a driver of tissue elongation, whereas cell shape analysis revealed that both elongation of cells leaving the growth zone and apico-basal cell rearrangements do contribute significantly to directional growth. Quantitative comparison of elongation processes indicated that proliferation contributes most to elongation at the growth zone, but cell shape change and rearrangement contribute as much as 40% of total elongation. We have demonstrated the utility of an approach to analysing the cellular mechanisms underlying tissue elongation in mammalian tissues. It should be broadly applied to higher-resolution analysis of links between genotypes and malformation phenotypes.

Epigenetic regulation of Sox4 during palate development

AIM

Identification of genes that contribute to secondary palate development provide a better understanding of the etiology of palatal clefts. Gene-expression profiling of the murine palate from gestational days 12-14 (GD12-14), a critical period in palate development, identified Sox4 as a differentially expressed gene. In this study, we have examined if the differential expression of Sox4 in the palate is due to changes in DNA methylation.

MATERIALS & METHODS

In situ hybridization analysis was used to localize the expression of Sox4 in the developing murine secondary palate. CpG methylation profiling of a 1.8-kb upstream region of Sox4 in the secondary palate from GD12-14 and transfection analysis in murine embryonic maxillary mesenchymal cells using Sox4 deletion, mutant and in vitro methylated plasmid constructs were used to identify critical CpG residues regulating Sox4 expression in the palate.

RESULTS

Spatiotemporal analysis revealed that Sox4 is expressed in the medial edge epithelium and presumptive rugae-forming regions of the palate from GD12 to GD13. Following palatal shelf fusion on GD14, Sox4 was expressed exclusively in the epithelia of the palatal rugae, structures that serve as signaling centers for the anteroposterior extension of the palate, and that are thought to serve as neural stem cell niches. Methylation of a 1.8-kb region upstream of Sox4, containing the putative promoter, completely eliminated promoter activity. CpG methylation profiling of the 1.8-kb region identified a CpG-poor region (DMR4) that exhibited significant differential methylation during palate development, consistent with changes in Sox4 mRNA expression. Changes in the methylation of DMR4 were attributed primarily to CpGs 83 and 85.

CONCLUSION

Our studies indicate that Sox4 is an epigenetically regulated gene that likely integrates multiple signaling systems for mediating palatal fusion, palatal extension and/or the maintenance of the neural stem cell niche in the rugae.

Sequential Shh expression in the development of the mouse upper functional incisor

The mouse incisor is a frequently used model in studies of the molecular control of organ development. The appropriate interpretation of data on normogenesis is essential for understanding the data obtained in mutant mice. For this reason, we performed a very detailed investigation of the development of the upper incisor in wild-type mice from embryonic day (ED) 11.5 till 14.5. A combination of histology, whole mount in situ hybridization, computer-aided three-dimensional reconstructions, and fluorescent microscopy, has been used. Several sonic hedgehog (Shh) expression domains have been detected in the upper incisor region during early prenatal development. At ED11.5-13.5, there was a single Shh positive domain present in the anterior part of left or right upper jaw arches, corresponding to the epithelial thickening. More posteriorly, a new Shh expression domain appeared in the incisor bud in the developmentally more advanced ED13.5 embryos. At ED14.5, only this posterior Shh expression in the incisor germ remained detectable. This study brings new insights into the early development of the upper incisor in mice and completes the data on normal mouse incisor development. The temporal-spatial pattern of Shh expression reflects the development of two tooth generations, being detectable in two successive, antero-posteriorly located areas in the prospective incisor region in the upper jaw. The first, anterior and superficial Shh expression domain reflects the rudimentary tooth development suppressed during evolution. Only the subsequent, posterior and deeper Shh expression region, appearing at ED13.5, correlates with the prospective upper functional incisor in wild-type mice.

Molecular signaling along the anterior-posterior axis of early palate development

Cleft palate is a common congenital birth defect in humans. In mammals, the palatal tissue can be distinguished into anterior bony hard palate and posterior muscular soft palate that have specialized functions in occlusion, speech or swallowing. Regulation of palate development appears to be the result of distinct signaling and genetic networks in the anterior and posterior regions of the palate. Development and maintenance of expression of these region-specific genes is crucial for normal palate development. Numerous transcription factors and signaling pathways are now recognized as either anterior- (e.g., Msx1, Bmp4, Bmp2, Shh, Spry2, Fgf10, Fgf7, and Shox2) or posterior-specific (e.g., Meox2, Tbx22, and Barx1). Localized expression and function clearly highlight the importance of regional patterning and differentiation within the palate at the molecular level. Here, we review how these molecular pathways and networks regulate the anterior-posterior patterning and development of secondary palate. We hypothesize that the anterior palate acts as a signaling center in setting up development of the secondary palate.

Deciphering gene expression program of MAP3K1 in mouse eyelid morphogenesis

Embryonic eyelid closure involves forward movement and ultimate fusion of the upper and lower eyelids, an essential step of mammalian ocular surface development. Although its underlying mechanism of action is not fully understood, a functional mitogen-activated protein kinase kinase kinase 1 (MAP3K1) is required for eyelid closure. Here we investigate the molecular signatures of MAP3K1 in eyelid morphogenesis. At mouse gestational day E15.5, the developmental stage immediately prior to eyelid closure, MAP3K1 expression is predominant in the eyelid leading edge (LE) and the inner eyelid (IE) epithelium. We used laser capture microdissection (LCM) to obtain highly enriched LE and IE cells from wild type and MAP3K1-deficient fetuses and analyzed genome-wide expression profiles. The gene expression data led to the identification of three distinct developmental features of MAP3K1. First, MAP3K1 modulated Wnt and Sonic hedgehog signals, actin reorganization, and proliferation only in LE but not in IE epithelium, illustrating the temporal-spatial specificity of MAP3K1 in embryogenesis. Second, MAP3K1 potentiated AP-2α expression and SRF and AP-1 activity, but its target genes were enriched for binding motifs of AP-2α and SRF, and not AP-1, suggesting the existence of novel MAP3K1-AP-2α/SRF modules in gene regulation. Third, MAP3K1 displayed variable effects on expression of lineage specific genes in the LE and IE epithelium, revealing potential roles of MAP3K1 in differentiation and lineage specification. Using LCM and expression array, our studies have uncovered novel molecular signatures of MAP3K1 in embryonic eyelid closure.

Periodic stripe formation by a Turing mechanism operating at growth zones in the mammalian palate

We present direct evidence of an activator-inhibitor system in the generation of the regularly spaced transverse ridges of the palate. We show that new ridges, or rugae, marked by stripes of Sonic hedgehog (Shh) expression, appear at two growth zones where the space between previously laid-down rugae increases. However, inter-rugal growth is not absolutely required: new stripes still appear when growth is inhibited. Furthermore, when a ruga is excised new Shh expression appears, not at the cut edge but as bifurcating stripes branching from the neighbouring Shh stripe, diagnostic of a Turing-type reaction-diffusion mechanism. Genetic and inhibitor experiments identify Fibroblast Growth Factor (FGF) and Shh as an activator-inhibitor pair in this system. These findings demonstrate a reaction-diffusion mechanism likely to be widely relevant in vertebrate development.

Palatogenesis: morphogenetic and molecular mechanisms of secondary palate development

Mammalian palatogenesis is a highly regulated morphogenetic process during which the embryonic primary and secondary palatal shelves develop as outgrowths from the medial nasal and maxillary prominences, respectively, remodel and fuse to form the intact roof of the oral cavity. The complexity of control of palatogenesis is reflected by the common occurrence of cleft palate in humans. Although the embryology of the palate has long been studied, the past decade has brought substantial new knowledge of the genetic control of secondary palate development. Here, we review major advances in the understanding of the morphogenetic and molecular mechanisms controlling palatal shelf growth, elevation, adhesion and fusion, and palatal bone formation.

The inductive role of Wnt-β-Catenin signaling in the formation of oral apparatus

Proper patterning and growth of oral structures including teeth, tongue, and palate rely on epithelial-mesenchymal interactions involving coordinated regulation of signal transduction. Understanding molecular mechanisms underpinning oral-facial development will provide novel insights into the etiology of common congenital defects such as cleft palate. In this study, we report that ablating Wnt signaling in the oral epithelium blocks the formation of palatal rugae, which are a set of specialized ectodermal appendages serving as Shh signaling centers during development and niches for sensory cells and possibly neural crest related stem cells in adults. Lack of rugae is also associated with retarded anteroposterior extension of the hard palate and precocious mid-line fusion. These data implicate an obligatory role for canonical Wnt signaling in rugae development. Based on this complex phenotype, we propose that the sequential addition of rugae and its morphogen Shh, is intrinsically coupled to the elongation of the hard palate, and is critical for modulating the growth orientation of palatal shelves. In addition, we observe a unique cleft palate phenotype at the anterior end of the secondary palate, which is likely caused by the severely underdeveloped primary palate in these mutants. Last but not least, we also discover that both Wnt and Shh signalings are essential for tongue development. We provide genetic evidence that disruption of either signaling pathway results in severe microglossia. Altogether, we demonstrate a dynamic role for Wnt-β-Catenin signaling in the development of the oral apparatus.

Shh expression in a rudimentary tooth offers new insights into development of the mouse incisor

For teeth as for any organ, knowledge of normal development is essential for the proper interpretation of developmental anomalies in mutant mice. It is generally accepted that tooth formation is initiated with a single signaling center that, in the incisor region, is exclusively related to the development of the functional adult incisor. Here, using a unique combination of computer-aided three-dimensional reconstructions and whole mount in situ hybridization of mandibles from finely staged wild-type mouse embryos, we demonstrate that several Sonic hedgehog (Shh) expression domains sequentially appear in the lower incisor region during early development. In contrast to the single Shh expression domain that is widely assumed to be present in each lower incisor area at ED12.5-13.5, we identified two spatially distinct regions of Shh expression that appear in an anterior-posterior sequence during this period. The initial anterior, more superficially located Shh expression region represented the rudimentary (so-called deciduous) incisor, whereas only the later posterior deeper situated region corresponded to the prospective functional incisor. In the more advanced embryos, only this posterior Shh expression in the incisor bud was detectable as a precursor of the enamel knot. This study offers a new interpretation of published molecular data on the mouse incisor from initiation through ED13.5. We suggest that, as with Shh expression, other molecular data that have been ascribed to the progressive development of the mouse functional incisor at early stages, in fact, correspond to a rudimentary incisor whose development is aborted.

Importance of region-specific epithelial rearrangements in mouse rugae development

Epithelial appendages on palatal rugae develop during mouse palatogenesis through epithelial thickening and pattern formation. Recently, the patterned formation of nine rugae was observed together with the specific expression patterns of Shh in rodents. However, no crucial evidence was found for a direct association between Shh expression and the distinct structural formation of rugae. In order to reveal possible relationships, we investigated the morphological changes of rugae and expression patterns of Shh directly by in vitro organ culture at embryonic day 13 (E13) for 2 days. To compare and examine the diverse growing aspects of the palate and rugae, we carefully observed the detailed morphogenesis, with cell proliferation of the rugae occurring between E13 and E14.5. After 2 days of cultivation at E13, DiI micro-injections revealed that the middle part of the palate, adjacent to the upper molar-forming region, contributed to the formation of the subsequent structure of rugae by extensive cell rearrangement and proliferation within the epithelium in the preferred anteroposterior direction. The results also defined the intimate relationship between Shh expression and rugae formation.

Bmpr1a signaling plays critical roles in palatal shelf growth and palatal bone formation

Cleft palate, including submucous cleft palate, is among the most common birth defects in humans. While overt cleft palate results from defects in growth or fusion of the developing palatal shelves, submucous cleft palate is characterized by defects in palatal bones. In this report, we show that the Bmpr1a gene, encoding a type I receptor for bone morphogenetic proteins (Bmp), is preferentially expressed in the primary palate and anterior secondary palate during palatal outgrowth. Following palatal fusion, Bmpr1a mRNA expression was upregulated in the condensed mesenchyme progenitors of palatal bone. Tissue-specific inactivation of Bmpr1a in the developing palatal mesenchyme in mice caused reduced cell proliferation in the primary and anterior secondary palate, resulting in partial cleft of the anterior palate at birth. Expression of Msx1 and Fgf10 was downregulated in the anterior palate mesenchyme and expression of Shh was downregulated in the anterior palatal epithelium in the Bmpr1a conditional mutant embryos, indicating that Bmp signaling regulates mesenchymal-epithelial interactions during palatal outgrowth. In addition, formation of the palatal processes of the maxilla was blocked while formation of the palatal processes of the palatine was significantly delayed, resulting in submucous cleft of the hard palate in the mutant mice. Our data indicate that Bmp signaling plays critical roles in the regulation of palatal mesenchyme condensation and osteoblast differentiation during palatal bone formation.