Reassessing the embryonic origin and potential of craniofacial ectomesenchyme

Craniofacial developmental biology in the single-cell era

The evolution of a unique craniofacial complex in vertebrates made possible new ways of breathing, eating, communicating and sensing the environment. The head and face develop through interactions of all three germ layers, the endoderm, ectoderm and mesoderm, as well as the so-called fourth germ layer, the cranial neural crest. Over a century of experimental embryology and genetics have revealed an incredible diversity of cell types derived from each germ layer, signaling pathways and genes that coordinate craniofacial development, and how changes to these underlie human disease and vertebrate evolution. Yet for many diseases and congenital anomalies, we have an incomplete picture of the causative genomic changes, in particular how alterations to the non-coding genome might affect craniofacial gene expression. Emerging genomics and single-cell technologies provide an opportunity to obtain a more holistic view of the genes and gene regulatory elements orchestrating craniofacial development across vertebrates. These single-cell studies generate novel hypotheses that can be experimentally validated in vivo. In this Review, we highlight recent advances in single-cell studies of diverse craniofacial structures, as well as potential pitfalls and the need for extensive in vivo validation. We discuss how these studies inform the developmental sources and regulation of head structures, bringing new insights into the etiology of structural birth anomalies that affect the vertebrate head.

Single cell RNA sequencing reveals mesenchymal heterogeneity and critical functions of Cd271 in tooth development

BACKGROUND

Accumulating evidence suggests that the maxillary process, to which cranial crest cells migrate, is essential to tooth development. Emerging studies indicate that Cd271 plays an essential role in odontogenesis. However, the underlying mechanisms have yet to be elucidated.

AIM

To establish the functionally heterogeneous population in the maxillary process, elucidate the effects of Cd271 deficiency on gene expression differences.

METHODS

p75NTR knockout (Cd271^-/-) mice (from American Jackson laboratory) were used to collect the maxillofacial process tissue of p75NTR knockout mice, and the wild-type maxillofacial process of the same pregnant mouse wild was used as control. After single cell suspension, the cDNA was prepared by loading the single cell suspension into the 10x Genomics Chromium system to be sequenced by NovaSeq6000 sequencing system. Finally, the sequencing data in Fastq format were obtained. The FastQC software is used to evaluate the quality of data and CellRanger analyzed the data. The gene expression matrix is read by R software, and Seurat is used to control and standardize the data, reduce the dimension and cluster. We search for marker genes for subgroup annotation by consulting literature and database; explore the effect of p75NTR knockout on mesenchymal stem cells (MSCs) gene expression and cell proportion by cell subgrouping, differential gene analysis, enrichment analysis and protein-protein interaction network analysis; understand the interaction between MSCs cells and the differentiation trajectory and gene change characteristics of p75NTR knockout MSCs by cell communication analysis and pseudo-time analysis. Last we verified the findings single cell sequencing in vitro.

RESULTS

We identified 21 cell clusters, and we re-clustered these into three subclusters. Importantly, we revealed the cell–cell communication networks between clusters. We clarified that Cd271 was significantly associated with the regulation of mineralization.

CONCLUSION

This study provides comprehensive mechanistic insights into the maxillary- process-derived MSCs and demonstrates that Cd271 is significantly associated with the odontogenesis in mesenchymal populations.

Ap-2β regulates cranial osteogenic potential via the activation of Wnt/β-catenin signaling pathway

The skull is a fundamental bone that protects the development of brain and consists of several bony elements, such as the frontal and parietal bones. Frontal bone exhibited superior in osteogenic potential and regeneration of cranial defects compared to parietal bone. However, how this regional difference is regulated remains largely unknown. In this study, we identified an Ap-2β transcriptional factor with a higher expression in frontal bone, but its molecular function in osteoblasts needs to be elucidated. We found that Ap-2β knockdown in preosteoblasts leads to reduced proliferation, increased cell death and impaired differentiation. Through RNA-seq analysis, we found that Ap-2β influences multiple signaling pathways including the Wnt pathway, and overexpression of Ap-2β showed increased nuclear β-catenin and its target genes expressions in osteoblasts. Pharmacological activation of Wnt/β-catenin signaling using LiCl treatment cannot rescue the reduced luciferase activities of the β-catenin/TCF/LEF reporter in Ap-2β knockdown preosteoblasts. Besides, transient expression of Ap-2β via the lentivirus system could sufficiently rescue the inferior osteogenic potential in parietal osteoblasts, while Ap-2β knockdown in frontal osteoblasts resulted in reduced osteoblast activity, reduced active β-catenin and target genes expressions. Taken together, our data demonstrated that Ap-2β modulates osteoblast proliferation and differentiation through the regulation of Wnt/β-catenin signaling pathway and plays an important role in regulating regional osteogenic potential in frontal and parietal bone.

Nuclear receptor Nr5a2 promotes diverse connective tissue fates in the jaw

Organ development involves the sustained production of diverse cell types with spatiotemporal precision. In the vertebrate jaw, neural-crest-derived progenitors produce not only skeletal tissues but also later-forming tendons and salivary glands. Here we identify the pluripotency factor Nr5a2 as essential for cell-fate decisions in the jaw. In zebrafish and mice, we observe transient expression of Nr5a2 in a subset of mandibular postmigratory neural-crest-derived cells. In zebrafish nr5a2 mutants, nr5a2-expressing cells that would normally form tendons generate excess jaw cartilage. In mice, neural-crest-specific Nr5a2 loss results in analogous skeletal and tendon defects in the jaw and middle ear, as well as salivary gland loss. Single-cell profiling shows that Nr5a2, distinct from its roles in pluripotency, promotes jaw-specific chromatin accessibility and gene expression that is essential for tendon and gland fates. Thus, repurposing of Nr5a2 promotes connective tissue fates to generate the full repertoire of derivatives required for jaw and middle ear function.

Control of cranial ectomesenchyme fate by Nr2f nuclear receptors

Certain cranial neural crest cells are uniquely endowed with the ability to make skeletal cell types otherwise only derived from mesoderm. As these cells migrate into the pharyngeal arches, they downregulate neural crest specifier genes and upregulate so-called ectomesenchyme genes that are characteristic of skeletal progenitors. Although both external and intrinsic factors have been proposed as triggers of this transition, the details remain obscure. Here, we report the Nr2f nuclear receptors as intrinsic activators of the ectomesenchyme program: zebrafish nr2f5 single and nr2f2;nr2f5 double mutants show marked delays in upregulation of ectomesenchyme genes, such as dlx2a, prrx1a, prrx1b, sox9a, twist1a and fli1a, and in downregulation of sox10, which is normally restricted to early neural crest and non-ectomesenchyme lineages. Mutation of sox10 fully rescued skeletal development in nr2f5 single but not nr2f2;nr2f5 double mutants, but the initial ectomesenchyme delay persisted in both. Sox10 perdurance thus antagonizes the recovery but does not explain the impaired ectomesenchyme transition. Unraveling the mechanisms of Nr2f function will help solve the enduring puzzle of how cranial neural crest cells transition to the skeletal progenitor state.

Fascial Nomenclature: Update 2022

The connective tissue or fascia plays key roles in maintaining bodily function and health. The fascia is made up of solid and fluid portions, which interpenetrate and interact with each other, forming a polymorphic three-dimensional network. In the vast panorama of literature there is no univocal thought on the nomenclature and terminology that best represents the concept of fascia. The Foundation of Osteopathic Research and Clinical Endorsement (FORCE) organization brings together various scientific figures in a multidisciplinary perspective. FORCE tries to find a common nomenclature that can be shared, starting from the scientific notions currently available. Knowledge of the fascial continuum should always be at the service of the clinician and never become an exclusive for the presence of copyright, or commodified for the gain of a few. FORCE is a non-profit organization serving all professionals who deal with patient health. The article reviews the concepts of fascia, including some science subjects rarely considered, to gain an understanding of the broader fascial topic, and proposing new concepts, such as the holographic fascia.