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

Palatal segment contributions to midfacial anterior-posterior growth

Anterior-posterior (A-P) elongation of the palate is a critical aspect of integrated midfacial morphogenesis. Reciprocal epithelial-mesenchymal interactions drive secondary palate elongation that is coupled to the periodic formation of signaling centers within the rugae growth zone (RGZ). However, the relationship between RGZ driven morphogenetic processes, the differentiative dynamics of underlying palatal bone mesenchymal precursors, and the segmental organization of the upper jaw has remained enigmatic. A detailed ontogenetic study of these relationships is important, because palatal segment growth is a critical aspect of normal midfacial growth, can be modified to produce dysmorphology, and is a likely basis for evolutionary differences in upper jaw morphology. Variation in palatal-segment specific growth may also underlie known differences in palatal segment proportions between inbred mouse strains. We completed a combined whole mount gene expression and morphometric analysis of normal murine palatal growth dynamics and their association with palatal segment elongation and resulting upper jaw morphology. Our results demonstrated that the first formed palatal ruga (ruga 1), found just posterior to the RGZ, maintained an association with important nasal, neurovascular and palatal structures throughout early midfacial development; suggesting that these features are positioned at a proximal source of embryonic midfacial directional growth. Our detailed characterization of midfacial morphogenesis revealed a one-to-one relationship between palatal segments and upper jaw bones during the earliest stages of palatal elongation. Growth of the maxillary anlage within the anterior secondary palate is uniquely coupled to RGZ-driven morphogenesis that more than doubles the length of this palatal segment prior to palatal shelf fusion. Our results also demonstrate that the future maxillary-palatine suture, approximated by the position ruga 1 and consistently associated with the palatine anlage, forms predominantly via the posterior differentiation of the maxilla within the expanding anterior secondary palate. Our complementary ontogenetic comparison of three inbred mouse strains identified small but significant strain-specific differences in early embryonic palatal segment contributions to the upper jaw. Although early palatal segment specific growth is not primarily responsible for adult differences in upper jaw morphology between these strains, our ontogenetic series of measurements provide a useful foundation for understanding the impact of background genetic effects on facial shape and elongation. In combination, our results provide a novel and particularly detailed picture of the earliest spatiotemporal dynamics of intramembranous midfacial skeletal specification and differentiation within the context of the surrounding palatal segment A-P elongation and associated rugae formation.

Ultrasonic mapping of midpalatal suture - An ex-vivo study

OBJECTIVE

Rapid maxillary expansion is a common orthodontic procedure to correct maxillary constriction. Assessing the midpalatal suture (MPS) expansion plays a crucial role in treatment planning to determine its effectiveness. The objectives of this preliminary investigation are to demonstrate a proof of concept that the palatal bone underlying the rugae can be clearly imaged by ultrasound (US) and the reconstructed axial view of the US image accurately maps the MPS patency.

METHODS

An ex-vivo US scanning was conducted on the upper jawbones of two piglet's carcasses before and after the creation of bone defects, which simulated the suture opening. The planar images were processed to enhance bone intensity distribution before being orderly stacked to fuse into a volume. Graph-cut segmentation was applied to delineate the palatal bone to generate a bone volume. The accuracy of the reconstructed bone volume and the suture opening was validated by the micro-computed tomography (µCT) data used as the ground truth and compared with cone beam computed tomography (CBCT) data as the clinical standard. Also included in the comparison is the rugae thickness. Correlation and Bland-Altman plots were used to test the agreement between the two methods: US versus µCT/CBCT.

RESULTS

The reconstruction of the US palatal bone volumes was accurate based on surface topography comparison with a mean error of 0.19 mm for pre-defect and 0.15 mm and 0.09 mm for post-defect models of the two samples, respectively when compared with µCT volumes. A strong correlation (R2 ≥ 0.99) in measuring MPS expansion was found between US and µCT/CBCT with MADs of less than 0.05 mm, 0.11 mm and 0.23 mm for US, µCT and CBCT, respectively.

CONCLUSIONS

It was possible to axially image the MPS opening and rugae thickness accurately using high-frequency ultrasound.

CLINICAL SIGNIFICANCE

This study introduces an ionizing radiation-free, low-cost, and portable technique to accurately image a difficult part of oral cavity anatomy. The advantages of conceivable visualization could promise a successful clinical examination of MPS to support the predictable treatment outcome of maxillary transverse deficiency.

Periodic pattern formation during embryonic development

During embryonic development many organs and structures require the formation of series of repeating elements known as periodic patterns. Ranging from the digits of the limb to the feathers of the avian skin, the correct formation of these embryonic patterns is essential for the future form and function of these tissues. However, the mechanisms that produce these patterns are not fully understood due to the existence of several modes of pattern generation which often differ between organs and species. Here, we review the current state of the field and provide a perspective on future approaches to studying this fundamental process of embryonic development.

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.

Balancing WNT signalling in early forebrain development: The role of LRP4 as a modulator of LRP6 function

The specification of the forebrain relies on the precise regulation of WNT/ß-catenin signalling to support neuronal progenitor cell expansion, patterning, and morphogenesis. Imbalances in WNT signalling activity in the early neuroepithelium lead to congenital disorders, such as neural tube defects (NTDs). LDL receptor-related protein (LRP) family members, including the well-studied receptors LRP5 and LRP6, play critical roles in modulating WNT signalling capacity through tightly regulated interactions with their co-receptor Frizzled, WNT ligands, inhibitors and intracellular WNT pathway components. However, little is known about the function of LRP4 as a potential modulator of WNT signalling in the central nervous system. In this study, we investigated the role of LRP4 in the regulation of WNT signalling during early mouse forebrain development. Our results demonstrate that LRP4 can modulate LRP5- and LRP6-mediated WNT signalling in the developing forebrain prior to the onset of neurogenesis at embryonic stage 9.5 and is therefore essential for accurate neural tube morphogenesis. Specifically, LRP4 functions as a genetic modifier for impaired mitotic activity and forebrain hypoplasia, but not for NTDs in LRP6-deficient mutants. In vivo and in vitro data provide evidence that LRP4 is a key player in fine-tuning WNT signalling capacity and mitotic activity of mouse neuronal progenitors and of human retinal pigment epithelial (hTERT RPE-1) cells. Our data demonstrate the crucial roles of LRP4 and LRP6 in regulating WNT signalling and forebrain development and highlight the need to consider the interaction between different signalling pathways to understand the underlying mechanisms of disease. The findings have significant implications for our mechanistic understanding of how LRPs participate in controlling WNT signalling.

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.

Distal extension of palatal rugae as a limitation for donor soft tissue grafts in a Jordanian population: A cross-sectional study

Background

The purpose of the cross sectional study was to investigate the distal extension of the rugae area in a Jordanian (Middle Eastern) population, as an anatomical limitation influencing the surgical decision of harvesting a palatal soft tissue graft. Factors that may influence or predict the extension were also assessed.

Methods

Sixty periodontally healthy participants (29 males and 31 females) were included. Maxillary alginate impressions were made and casts were poured. The measurements were highlighted from the origin of the rugae (near mid palatine raphe) to the terminal end with a sharp graphite pencil on the cast and a magnification lens was used for identification. The most posterior extension of the rugae were marked on the casts and determined by a standardized periodontal probe. The normal approximation test for binomial distribution was used to determine the proportion of the subjects with rugael extensions beyond the mesial end of the upper second premolar and fisher’s exact test for the purpose of analysis of the association of this extension with other factors.

Results

In almost half (41.7%) of the sample, the rugae extended distal to the upper second premolar, 23.3% extended to the mid-palatal of the upper second premolar, and 11.7% extended to the mesial of the upper second premolar. The implication is that 90.0% of the rugae reached the upper second premolar and 78.3% extended beyond its mesial aspect. The normal approximation test performed with 95% CI with the "rugael extension proximal to the mesial end of the upper 2nd premolar" considered to be the "success" category revealed that the proportion of the subjects with rugael extensions proximal to the mesial end of the upper second premolar was significantly lower than the proportion beyond the mesial end of the upper second premolar (95% CI of 11.2–32.0%, p = .00001). There was no significant difference between gender, smoking status, gingival phenotype and palatal shape with the posterior extension of palatal rugae.

Conclusions

Palatal rugae in a sample of a Jordanian population extends beyond the mesial aspect of the upper second premolar which may cause a substantial limitation for graft harvesting from the palate. The hard palate of Jordanian patients may not be a reliable source of soft tissue grafts required for aesthetic mucogingival surgery. No significant association existed between the most posterior extent of palatal rugae and gender, gingival phenotype or palatal shape. Other possible sources should be explored.

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.

LRPs in WNT Signalling

The WNT/β-catenin signalling pathway is a rich and complex network of cellular proteins that orchestrates diverse short-range cell-to-cell communication in metazoans and is essential for both embryonic development and adult homeostasis. Due to its fundamental importance in controlling cell behaviour at multiple levels, its deregulation is associated with a wide range of diseases in humans and identification of drugs targeting the pathway has attracted strong interest in the pharmaceutical sector. Transduction of WNT signals across the plasma membrane of cells involves a staggering degree of complexity and variety with respect to ligand-receptor, receptor-receptor and receptor-co-receptor interactions (Niehrs, Nat Rev Mol Cell Biol 13:767-779, 2012). Although the low-density-lipoprotein-receptor-related-protein (LRP) family is best known for its role in binding and endocytosis of lipoproteins, specific members appear to have additional roles in cellular communication. Indeed, for WNT/β-catenin signalling one apparently universal requirement is the presence of either LRP5 or LRP6 in combination with one of the ten Frizzled (FZD) WNT receptors (FZD_1-10). In the 20 years since their discovery as WNT/FZD co-receptors, research on the LRP family has contributed greatly to our understanding of WNT signalling and LRPs have emerged as central players in WNT/β-catenin signalling. LRP5/6 are highly similar and represent the least redundant class of WNT receptor that transduce WNT/β-catenin signalling from a wide range of different WNT and FZD subtypes. This apparent simplicity however belies the complex arrangement of binding sites in the extracellular domain (ECD) of LRP5/6, which regulate interaction not only with WNTs but also with several inhibitors of WNT signalling. This chapter provides a historical overview, chronologically charting this remarkable progress in the field during the last 20 years of research on LRPs and their role in WNT/-catenin signalling. A more focused overview of the structural, functional and mechanistic aspects of LRP biology is also provided, together with the implications this has for pharmacological targeting of this notoriously intractable pathway.

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.

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.

Wnt signaling in orofacial clefts: crosstalk, pathogenesis and models

Diverse signaling cues and attendant proteins work together during organogenesis, including craniofacial development. Lip and palate formation starts as early as the fourth week of gestation in humans or embryonic day 9.5 in mice. Disruptions in these early events may cause serious consequences, such as orofacial clefts, mainly cleft lip and/or cleft palate. Morphogenetic Wnt signaling, along with other signaling pathways and transcription regulation mechanisms, plays crucial roles during embryonic development, yet the signaling mechanisms and interactions in lip and palate formation and fusion remain poorly understood. Various Wnt signaling and related genes have been associated with orofacial clefts. This Review discusses the role of Wnt signaling and its crosstalk with cell adhesion molecules, transcription factors, epigenetic regulators and other morphogenetic signaling pathways, including the Bmp, Fgf, Tgfβ, Shh and retinoic acid pathways, in orofacial clefts in humans and animal models, which may provide a better understanding of these disorders and could be applied towards prevention and treatments.