Shaping faces: genetic and epigenetic control of craniofacial morphogenesis

Smartphone applications for facial scanning: A technical and scoping review

INTRODUCTION

Facial scanning through smartphone scanning applications (SSA) is increasingly being used for medical applications as cost-effective, chairside method. However, clinical validation is lacking. This review aims to address: (1) Which SSA could perform facial scanning? (2) Which SSA can be clinically used? (3) Which SSA have been reported and scientifically validated for medical applications?

METHODS

Technical search for SSA designed for face or object scanning was conducted on Google, Apple App Store, and Google Play Store from August 2022 to December 2023. Literature search was performed on PubMed, Cochrane, EMBASE, MEDLINE, Scopus, IEEE Xplore, ACM Digital Library, Clinicaltrials.gov, ICTRP (WHO) and preprints up to 2023. Eligibility criteria included English-written scientific articles incorporating at least one SSA for clinical purposes. SSA selection and data extraction were executed by one reviewer, validated by second, with third reviewer being consulted for discordances.

RESULTS

Sixty-three applications designed for three-dimensional object scanning were retrieved, with 52 currently offering facial scanning capabilities. Fifty-six scientific articles, comprising two case reports, 16 proof-of-concepts and 38 experimental studies were analysed. Thirteen applications (123D Catch, 3D Creator, Bellus 3D Dental Pro, Bellus 3D Face app, Bellus 3D Face Maker, Capture, Heges, Metascan, Polycam, Scandy Pro, Scaniverse, Tap tap tap and Trnio) were reported in literature for digital workflow integration, comparison or proof-of-concept studies.

CONCLUSION

Fifty-two SSA can perform facial scanning currently and can be used clinically, offering cost-effectiveness, portability and user-friendliness. Although clinical validation is crucial, only 13 SSA were scientifically validated, underlying awareness of potential pitfalls and limitations.

The TET-Sall4-BMP regulatory axis controls craniofacial cartilage development

Craniofacial microsomia (CFM) is a congenital defect that usually results from aberrant development of embryonic pharyngeal arches. However, the molecular basis of CFM pathogenesis is largely unknown. Here, we employ the zebrafish model to investigate mechanisms of CFM pathogenesis. In early embryos, tet2 and tet3 are essential for pharyngeal cartilage development. Single-cell RNA sequencing reveals that loss of Tet2/3 impairs chondrocyte differentiation due to insufficient BMP signaling. Moreover, biochemical and genetic evidence reveals that the sequence-specific 5mC/5hmC-binding protein, Sall4, binds the promoter of bmp4 to activate bmp4 expression and control pharyngeal cartilage development. Mechanistically, Sall4 directs co-phase separation of Tet2/3 with Sall4 to form condensates that mediate 5mC oxidation on the bmp4 promoter, thereby promoting bmp4 expression and enabling sufficient BMP signaling. These findings suggest the TET-BMP-Sall4 regulatory axis is critical for pharyngeal cartilage development. Collectively, our study provides insights into understanding craniofacial development and CFM pathogenesis.

[Role of Fibroblast Growth Factor 7 in Craniomaxillofacial Development]

Craniomaxillofacial development involves a series of highly ordered temporal-spatial cellular differentiation processes in which a variety of cell signaling factors, such as fibroblast growth factors, play important regulatory roles. As a classic fibroblast growth factor, fibroblast growth factor 7 (FGF7) serves a wide range of regulatory functions. Previous studies have demonstrated that FGF7 regulates the proliferation and migration of epithelial cells, protects them, and promotes their repair. Furthermore, recent findings indicate that epithelial cells are not the only ones subjected to the broad and powerful regulatory capacity of FGF7. It has potential effects on skeletal system development as well. In addition, FGF7 plays an important role in the development of craniomaxillofacial organs, such as the palate, the eyes, and the teeth. Nonetheless, the role of FGF7 in oral craniomaxillofacial development needs to be further elucidated. In this paper, we summarized the published research on the role of FGF7 in oral craniomaxillofacial development to demonstrate the overall understanding of FGF7 and its potential functions in oral craniomaxillofacial development.

A screen of mutants generated and imaged by the International Mouse Phenotyping Consortium identifies 39 novel genes regulating the development of the secondary palate

The International Mouse Phenotyping Consortium (IMPC) has generated thousands of knockout mouse lines, of which a large proportion is embryonic or early neonatal lethal. The IMPC has generated and imaged embryos from lethal lines and made the three-dimensional image data sets publicly available. In this study, we used this resource to screen homozygous null mutants for defects in the development of the secondary palate. Altogether, we analyzed optical sections from 3216 embryos isolated at embryonic day (E) 15.5 and E18.5 from 478 homozygous mutant lines. Through this analysis, we discovered 39 novel genes important for palatal development. These studies provide new insights into the molecular regulation of palatogenesis and craniofacial disease and offer a useful resource for future exploration.

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underlies facial shape variation, yet how those networks in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest, even during the late migratory phase, results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both TFAP2 family members dysregulates numerous midface GRN components involved in midface morphogenesis, patterning and differentiation. Notably, Alx1, Alx3 and Alx4 (ALX) transcript levels are reduced, whereas ChIP-seq analyses suggest TFAP2 family members directly and positively regulate ALX gene expression. Tfap2a, Tfap2b and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a zebrafish mutants present with abnormal alx3 expression patterns, Tfap2a binds ALX loci and tfap2a-alx3 genetic interactions are observed. Together, these data demonstrate TFAP2 paralogs regulate vertebrate midfacial development in part by activating expression of ALX transcription factor genes.

Cationic Biopolymeric Scaffold of Chelating Nanohydroxyapatite Self-Regulates Intraoral Microenvironment for Periodontal Bone Regeneration

Periodontal bone defect is a common but longstanding healthcare issue since traditional bone grafts have limited functionalities in regulating complex intraoral microenvironments. Here, a porous cationic biopolymeric scaffold (CSC-g-nHAp) with microenvironment self-regulating ability was synthesized by chitosan-catechol chelating the Ca2+ of nanohydroxyapatite and bonding type I collagen. Chitosan-catechol's inherent antibacterial and antioxidant abilities endowed this scaffold with desirable abilities to eliminate periodontal pathogen infection and maintain homeostatic balances between free radical generation and elimination. Meanwhile, this scaffold promoted rat bone marrow stromal cells' osteogenic differentiation and achieved significant ectopic mineralization after 4 weeks of subcutaneous implantation in nude mice. Moreover, after 8 weeks of implantation in the rat critical-sized periodontal bone defect model, CSC-g-nHAp conferred 5.5-fold greater alveolar bone regeneration than the untreated group. This cationic biopolymeric scaffold could regulate the local microenvironment through the synergistic effects of its antibacterial, antioxidant, and osteoconductive activities to promote solid periodontal bone regeneration.

ESCRT-dependent control of craniofacial morphogenesis with concomitant perturbation of NOTCH signaling

Craniofacial development is orchestrated by transcription factor-driven regulatory networks, epigenetic modifications, and signaling pathways. Signaling molecules and their receptors rely on endo-lysosomal trafficking to prevent accumulation on the plasma membrane. ESCRT (Endosomal Sorting Complexes Required for Transport) machinery is recruited to endosomal membranes enabling degradation of such endosomal cargoes. Studies in vitro and in invertebrate models established the requirements of the ESCRT machinery in membrane remodeling, endosomal trafficking, and lysosomal degradation of activated membrane receptors. However, investigations during vertebrate development have been scarce. By ENU-induced mutagenesis, we isolated a mouse line, Vps25ENU/ENU, carrying a hypomorphic allele of the ESCRT-II component Vps25, with craniofacial anomalies resembling features of human congenital syndromes. Here, we assessed the spatiotemporal dynamics of Vps25 and additional ESCRT-encoding genes during murine development. We show that these genes are ubiquitously expressed although enriched in discrete domains of the craniofacial complex, heart, and limbs. ESCRT-encoding genes, including Vps25, are expressed in both cranial neural crest-derived mesenchyme and epithelium. Unlike constitutive ESCRT mutants, Vps25ENU/ENU embryos display late lethality. They exhibit hypoplastic lower jaw, stunted snout, dysmorphic ear pinnae, and secondary palate clefting. Thus, we provide the first evidence for critical roles of ESCRT-II in craniofacial morphogenesis and report perturbation of NOTCH signaling in craniofacial domains of Vps25ENU/ENU embryos. Given the known roles of NOTCH signaling in the developing cranium, and notably the lower jaw, we propose that the NOTCH pathway partly mediates the craniofacial defects of Vps25ENU/ENU mouse embryos.

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.

TFAP2 paralogs regulate midfacial development in part through a conserved ALX genetic pathway

Cranial neural crest development is governed by positional gene regulatory networks (GRNs). Fine-tuning of the GRN components underly facial shape variation, yet how those in the midface are connected and activated remain poorly understood. Here, we show that concerted inactivation of Tfap2a and Tfap2b in the murine neural crest even during the late migratory phase results in a midfacial cleft and skeletal abnormalities. Bulk and single-cell RNA-seq profiling reveal that loss of both Tfap2 members dysregulated numerous midface GRN components involved in midface fusion, patterning, and differentiation. Notably, Alx1/3/4 (Alx) transcript levels are reduced, while ChIP-seq analyses suggest TFAP2 directly and positively regulates Alx gene expression. TFAP2 and ALX co-expression in midfacial neural crest cells of both mouse and zebrafish further implies conservation of this regulatory axis across vertebrates. Consistent with this notion, tfap2a mutant zebrafish present abnormal alx3 expression patterns, and the two genes display a genetic interaction in this species. Together, these data demonstrate a critical role for TFAP2 in regulating vertebrate midfacial development in part through ALX transcription factor gene expression.