The developing mouse coronal suture at single-cell resolution

Mesenchymal stem cells in craniofacial reconstruction: a comprehensive review

Craniofacial reconstruction faces many challenges, including high complexity, strong specificity, severe injury, irregular and complex wounds, and high risk of bleeding. Traditionally, the "gold standard" for treating craniofacial bone defects has been tissue transplantation, which involves the transplantation of bone, cartilage, skin, and other tissues from other parts of the body. However, the shape of craniofacial bone and cartilage structures varies greatly and is distinctly different from ordinary long bones. Craniofacial bones originate from the neural crest, while long bones originate from the mesoderm. These factors contribute to the poor effectiveness of tissue transplantation in repairing craniofacial defects. Autologous mesenchymal stem cell transplantation exhibits excellent pluripotency, low immunogenicity, and minimally invasive properties, and is considered a potential alternative to tissue transplantation for treating craniofacial defects. Researchers have found that both craniofacial-specific mesenchymal stem cells and mesenchymal stem cells from other parts of the body have significant effects on the restoration and reconstruction of craniofacial bones, cartilage, wounds, and adipose tissue. In addition, the continuous development and application of tissue engineering technology provide new ideas for craniofacial repair. With the continuous exploration of mesenchymal stem cells by researchers and the continuous development of tissue engineering technology, the use of autologous mesenchymal stem cell transplantation for craniofacial reconstruction has gradually been accepted and promoted. This article will review the applications of various types of mesenchymal stem cells and related tissue engineering in craniofacial repair and reconstruction.

Dynamic enhancer landscapes in human craniofacial development

The genetic basis of human facial variation and craniofacial birth defects remains poorly understood. Distant-acting transcriptional enhancers control the fine-tuned spatiotemporal expression of genes during critical stages of craniofacial development. However, a lack of accurate maps of the genomic locations and cell type-resolved activities of craniofacial enhancers prevents their systematic exploration in human genetics studies. Here, we combine histone modification, chromatin accessibility, and gene expression profiling of human craniofacial development with single-cell analyses of the developing mouse face to define the regulatory landscape of facial development at tissue- and single cell-resolution. We provide temporal activity profiles for 14,000 human developmental craniofacial enhancers. We find that 56% of human craniofacial enhancers share chromatin accessibility in the mouse and we provide cell population- and embryonic stage-resolved predictions of their in vivo activity. Taken together, our data provide an expansive resource for genetic and developmental studies of human craniofacial development.

Transforming growth factor beta signaling and craniofacial development: modeling human diseases in zebrafish

Humans and other jawed vertebrates rely heavily on their craniofacial skeleton for eating, breathing, and communicating. As such, it is vital that the elements of the craniofacial skeleton develop properly during embryogenesis to ensure a high quality of life and evolutionary fitness. Indeed, craniofacial abnormalities, including cleft palate and craniosynostosis, represent some of the most common congenital abnormalities in newborns. Like many other organ systems, the development of the craniofacial skeleton is complex, relying on specification and migration of the neural crest, patterning of the pharyngeal arches, and morphogenesis of each skeletal element into its final form. These processes must be carefully coordinated and integrated. One way this is achieved is through the spatial and temporal deployment of cell signaling pathways. Recent studies conducted using the zebrafish model underscore the importance of the Transforming Growth Factor Beta (TGF-β) and Bone Morphogenetic Protein (BMP) pathways in craniofacial development. Although both pathways contain similar components, each pathway results in unique outcomes on a cellular level. In this review, we will cover studies conducted using zebrafish that show the necessity of these pathways in each stage of craniofacial development, starting with the induction of the neural crest, and ending with the morphogenesis of craniofacial elements. We will also cover human skeletal and craniofacial diseases and malformations caused by mutations in the components of these pathways (e.g., cleft palate, craniosynostosis, etc.) and the potential utility of zebrafish in studying the etiology of these diseases. We will also briefly cover the utility of the zebrafish model in joint development and biology and discuss the role of TGF-β/BMP signaling in these processes and the diseases that result from aberrancies in these pathways, including osteoarthritis and multiple synostoses syndrome. Overall, this review will demonstrate the critical roles of TGF-β/BMP signaling in craniofacial development and show the utility of the zebrafish model in development and disease.

Fractality of Cranial Sutures

It has long been known that skull suture has a typical fractal structure. Although the fractal dimension has been utilized to assess morphology, the mechanism of the fractal structure formation remains to be elucidated. Recent advances in the mathematical modeling of biological pattern formation provided useful frameworks for understanding this mechanism. This chapter describes how various proposed mechanisms tried to explain the formation of fractal structures in cranial sutures.

Concurrent impact of de novo mutations on cranial and cortical development in nonsyndromic craniosynostosis

OBJECTIVE

Nonsyndromic craniosynostosis (nsCS), characterized by premature cranial suture fusion, is considered a primary skull disorder in which impact on neurodevelopment, if present, results from the mechanical hindrance of brain growth. Despite surgical repair of the cranial defect, neurocognitive deficits persist in nearly half of affected children. Therefore, the authors performed a functional genomics analysis of nsCS to determine when, where, and in what cell types nsCS-associated genes converge during development.

METHODS

The authors integrated whole-exome sequencing data from 291 nsCS proband-parent trios with 29,803 single-cell transcriptomes of the prenatal and postnatal neurocranial complex to inform when, where, and in what cell types nsCS-mutated genes might exert their pathophysiological effects.

RESULTS

The authors found that nsCS-mutated genes converged in cranial osteoprogenitors and pial fibroblasts and their transcriptional networks that regulate both skull ossification and cerebral neurogenesis. Nonsyndromic CS-mutated genes also converged in inhibitory neurons and gene coexpression modules that overlapped with autism and other developmental disorders. Ligand-receptor cell-cell communication analysis uncovered crosstalk between suture osteoblasts and neurons via the nsCS-associated BMP, FGF, and noncanonical WNT signaling pathways.

CONCLUSIONS

These data implicate a concurrent impact of nsCS-associated de novo mutations on cranial morphogenesis and cortical development via cell- and non-cell-autonomous mechanisms in a developmental nexus of fetal osteoblasts, pial fibroblasts, and neurons. These results suggest that neurodevelopmental outcomes in nsCS patients may be driven more by mutational status than surgical technique.

Connecting the dots towards precision orthodontics

Precision orthodontics entails the use of personalized clinical, biological, social and environmental knowledge of each patient for deep individualized clinical phenotyping and diagnosis combined with the delivery of care using advanced customized devices, technologies and biologics. From its historical origins as a mechanotherapy and materials driven profession, the most recent advances in orthodontics in the past three decades have been propelled by technological innovations including volumetric and surface 3D imaging and printing, advances in software that facilitate the derivation of diagnostic details, enhanced personalization of treatment plans and fabrication of custom appliances. Still, the use of these diagnostic and therapeutic technologies is largely phenotype driven, focusing mainly on facial/skeletal morphology and tooth positions. Future advances in orthodontics will involve comprehensive understanding of an individual's biology through omics, a field of biology that involves large-scale rapid analyses of DNA, mRNA, proteins and other biological regulators from a cell, tissue or organism. Such understanding will define individual biological attributes that will impact diagnosis, treatment decisions, risk assessment and prognostics of therapy. Equally important are the advances in artificial intelligence (AI) and machine learning, and its applications in orthodontics. AI is already being used to perform validation of approaches for diagnostic purposes such as landmark identification, cephalometric tracings, diagnosis of pathologies and facial phenotyping from radiographs and/or photographs. Other areas for future discoveries and utilization of AI will include clinical decision support, precision orthodontics, payer decisions and risk prediction. The synergies between deep 3D phenotyping and advances in materials, omics and AI will propel the technological and omics era towards achieving the goal of delivering optimized and predictable precision orthodontics.

Enhanced osteoblastic differentiation of parietal bone in a novel murine model of mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II, OMIM 309900) is an X-linked disorder caused by a deficiency of lysosomal enzyme iduronate-2-sulfatase (IDS). The clinical manifestations of MPS II involve cognitive decline, bone deformity, and visceral disorders. These manifestations are closely associated with IDS enzyme activity, which catalyzes the stepwise degradation of heparan sulfate and dermatan sulfate. In this study, we established a novel Ids-deficient mice and further assessed the enzyme's physiological role. Using DNA sequencing, we found a genomic modification of the Ids genome, which involved the deletion of a 138-bp fragment spanning from intron 2 to exon 3, along with the insertion of an adenine at the 5' end of exon 3 in the mutated allele. Consistent with previous data, our Ids-deficient mice showed an attenuated enzyme activity and an enhanced accumulation of glycosaminoglycans. Interestingly, we noticed a distinct enlargement of the calvarial bone in both neonatal and young adult mice. Our examination revealed that Ids deficiency led to an enhanced osteoblastogenesis in the parietal bone, a posterior part of the calvarial bone originating from the paraxial mesoderm and associated with an enhanced expression of osteoblastic makers, such as Col1a and Runx2. In sharp contrast, cell proliferation of the parietal bone in these mice appeared similar to that of wild-type controls. These results suggest that the deficiency of Ids could be involved in an augmented differentiation of calvarial bone, which is often noticed as an enlarged head circumference in MPS II-affected individuals.

Genetic and Gene Expression Resources for Osteoporosis and Bone Biology Research

PURPOSE OF REVIEW

The integration of data from multiple genomic assays from humans and non-human model organisms is an effective approach to identify genes involved in skeletal fragility and fracture risk due to osteoporosis and other conditions. This review summarizes genome-wide genetic variation and gene expression data resources relevant to the discovery of genes contributing to skeletal fragility and fracture risk.

RECENT FINDINGS

Genome-wide association studies (GWAS) of osteoporosis-related traits are summarized, in addition to gene expression in bone tissues in humans and non-human organisms, with a focus on rodent models related to skeletal fragility and fracture risk. Gene discovery approaches using these genomic data resources are described. We also describe the Musculoskeletal Knowledge Portal (MSKKP) that integrates much of the available genomic data relevant to fracture risk. The available genomic resources provide a wealth of knowledge and can be analyzed to identify genes related to fracture risk. Genomic resources that would fill particular scientific gaps are discussed.

Joint multi-ancestry and admixed GWAS reveals the complex genetics behind human cranial vault shape

The cranial vault in humans is highly variable, clinically relevant, and heritable, yet its genetic architecture remains poorly understood. Here, we conduct a joint multi-ancestry and admixed multivariate genome-wide association study on 3D cranial vault shape extracted from magnetic resonance images of 6772 children from the ABCD study cohort yielding 30 genome-wide significant loci. Follow-up analyses indicate that these loci overlap with genomic risk loci for sagittal craniosynostosis, show elevated activity cranial neural crest cells, are enriched for processes related to skeletal development, and are shared with the face and brain. We present supporting evidence of regional localization for several of the identified genes based on expression patterns in the cranial vault bones of E15.5 mice. Overall, our study provides a comprehensive overview of the genetics underlying normal-range cranial vault shape and its relevance for understanding modern human craniofacial diversity and the etiology of congenital malformations.

A single-cell transcriptomic atlas characterizes age-related changes of murine cranial stem cell niches

The craniofacial bones provide structural support for the skull and accommodate the vulnerable brain tissue with a protective cavity. The bone tissue undergoes constant turnover, which relies on skeletal stem cells (SSCs) and/or mesenchymal stem cells (MSCs) and their niches. SSCs/MSCs and their perivascular niche within the bone marrow are well characterized in long bones. As for cranial bones, besides bone marrow, the suture mesenchyme has been identified as a unique niche for SSCs/MSCs of craniofacial bones. However, a comprehensive study of the two different cranial stem cell niches at single-cell resolution is still lacking. In addition, during the progression of aging, age-associated changes in cranial stem cell niches and resident cells remain uncovered. In this study, we investigated age-related changes in cranial stem cell niches via single-cell RNA sequencing (scRNA-seq). The transcriptomic profiles and cellular compositions have been delineated, indicating alterations of the cranial bone marrow microenvironment influenced by inflammaging. Moreover, we identified a senescent mesenchymal cell subcluster and several age-related immune cell subclusters by reclustering and pseudotime trajectory analysis, which might be closely linked to inflammaging. Finally, differentially expressed genes (DEGs) and cell-cell communications were analyzed during aging, revealing potential regulatory factors. Overall, this work highlights the age-related changes in cranial stem cell niches, which deepens the current understanding of cranial bone and suture biology and may provide therapeutic targets for antiaging and regenerative medicine.

Dissecting calvarial bones and sutures at single-cell resolution

Cranial bones constitute a protective shield for the vulnerable brain tissue, bound together as a rigid entity by unique immovable joints known as sutures. Cranial sutures serve as major growth centres for calvarial morphogenesis and have been identified as a niche for mesenchymal stem cells (MSCs) and/or skeletal stem cells (SSCs) in the craniofacial skeleton. Despite the established dogma of cranial bone and suture biology, technological advancements now allow us to investigate these tissues and structures at unprecedented resolution and embrace multiple novel biological insights. For instance, a decrease or imbalance of representation of SSCs within sutures might underlie craniosynostosis; dural sinuses enable neuroimmune crosstalk and are newly defined as immune hubs; skull bone marrow acts as a myeloid cell reservoir for the meninges and central nervous system (CNS) parenchyma in mediating immune surveillance, etc. In this review, we revisit a growing body of recent studies that explored cranial bone and suture biology using cutting-edge techniques and have expanded our current understanding of this research field, especially from the perspective of development, homeostasis, injury repair, resident MSCs/SSCs, immunosurveillance at the brain's border, and beyond.

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.

Loss of Twist1 and balanced retinoic acid signaling from the meninges causes cortical folding in mice

Secondary lissencephaly evolved in mice due to effects on neurogenesis and the tangential distribution of neurons. Signaling pathways that help maintain lissencephaly are still poorly understood. We show that inactivating Twist1 in the primitive meninges causes cortical folding in mice. Cell proliferation in the meninges is reduced, causing loss of arachnoid fibroblasts that express Raldh2, an enzyme required for retinoic acid synthesis. Regionalized loss of Raldh2 in the dorsolateral meninges is first detected when folding begins. The ventricular zone expands and the forebrain lengthens at this time due to expansion of apical radial glia. As the cortex expands, regionalized differences in the levels of neurogenesis are coupled with changes to the tangential distribution of neurons. Consequentially, cortical growth at and adjacent to the midline accelerates with respect to more dorsolateral regions, resulting in cortical buckling and folding. Maternal retinoic acid supplementation suppresses cortical folding by normalizing forebrain length, neurogenesis and the tangential distribution of neurons. These results suggest that Twist1 and balanced retinoic acid signaling from the meninges are required to maintain normal levels of neurogenesis and lissencephaly in mice.

A multi-stem cell basis for craniosynostosis and calvarial mineralization

Craniosynostosis is a group of disorders of premature calvarial suture fusion. The identity of the calvarial stem cells (CSCs) that produce fusion-driving osteoblasts in craniosynostosis remains poorly understood. Here we show that both physiologic calvarial mineralization and pathologic calvarial fusion in craniosynostosis reflect the interaction of two separate stem cell lineages; a previously identified cathepsin K (CTSK) lineage CSC1 (CTSK+ CSC) and a separate discoidin domain-containing receptor 2 (DDR2) lineage stem cell (DDR2+ CSC) that we identified in this study. Deletion of Twist1, a gene associated with craniosynostosis in humans2,3, solely in CTSK+ CSCs is sufficient to drive craniosynostosis in mice, but the sites that are destined to fuse exhibit an unexpected depletion of CTSK+ CSCs and a corresponding expansion of DDR2+ CSCs, with DDR2+ CSC expansion being a direct maladaptive response to CTSK+ CSC depletion. DDR2+ CSCs display full stemness features, and our results establish the presence of two distinct stem cell lineages in the sutures, with both populations contributing to physiologic calvarial mineralization. DDR2+ CSCs mediate a distinct form of endochondral ossification without the typical haematopoietic marrow formation. Implantation of DDR2+ CSCs into suture sites is sufficient to induce fusion, and this phenotype was prevented by co-transplantation of CTSK+ CSCs. Finally, the human counterparts of DDR2+ CSCs and CTSK+ CSCs display conserved functional properties in xenograft assays. The interaction between these two stem cell populations provides a new biologic interface for the modulation of calvarial mineralization and suture patency.

Characterizing expression pattern of Six2Cre during mouse craniofacial development

Craniofacial development is a complex process involving diverse cell populations. Various transgenic Cre lines have been developed to facilitate studying gene function in specific tissues. In this study, we have characterized the expression pattern of Six2Cre mice at multiple stages during craniofacial development. Our data revealed that Six2Cre lineage cells are predominantly present in frontal bone, mandible, and secondary palate. Using immunostaining method, we found that Six2Cre triggered reporter is co-expressed with Runx2. In summary, our data showed Six2Cre can be used to study gene function during palate development and osteogenesis in mouse models.

Pathogenic variants in the paired-related homeobox 1 gene (PRRX1) cause craniosynostosis with incomplete penetrance

PURPOSE

Studies have previously implicated PRRX1 in craniofacial development, including demonstration of murine Prrx1 expression in the preosteogenic cells of the cranial sutures. We investigated the role of heterozygous missense and loss-of-function (LoF) variants in PRRX1 associated with craniosynostosis.

METHODS

Trio-based genome, exome, or targeted sequencing were used to screen PRRX1 in patients with craniosynostosis; immunofluorescence analyses were used to assess nuclear localization of wild-type and mutant proteins.

RESULTS

Genome sequencing identified 2 of 9 sporadically affected individuals with syndromic/multisuture craniosynostosis, who were heterozygous for rare/undescribed variants in PRRX1. Exome or targeted sequencing of PRRX1 revealed a further 9 of 1449 patients with craniosynostosis harboring deletions or rare heterozygous variants within the homeodomain. By collaboration, 7 additional individuals (4 families) were identified with putatively pathogenic PRRX1 variants. Immunofluorescence analyses showed that missense variants within the PRRX1 homeodomain cause abnormal nuclear localization. Of patients with variants considered likely pathogenic, bicoronal or other multisuture synostosis was present in 11 of 17 cases (65%). Pathogenic variants were inherited from unaffected relatives in many instances, yielding a 12.5% penetrance estimate for craniosynostosis.

CONCLUSION

This work supports a key role for PRRX1 in cranial suture development and shows that haploinsufficiency of PRRX1 is a relatively frequent cause of craniosynostosis.

Temporal transcriptome features identify early skeletal commitment during human epiphysis development at single-cell resolution

Human epiphyseal development has been mainly investigated through radiological and histological approaches, uncovering few details of cellular temporal genetic alternations. Using single-cell RNA sequencing, we investigated the dynamic transcriptome changes during post-conception weeks (PCWs) 15-25 of human distal femoral epiphysis cells. We find epiphyseal cells contain multiple subtypes distinguished by specific markers, gene signatures, Gene Ontology (GO) enrichment analysis, and gene set variation analysis (GSVA). We identify the populations committed to cartilage or ossification at this time, although the secondary ossification centers (SOCs) have not formed. We describe the temporal alternation in transcriptional expression utilizing trajectories, transcriptional regulatory networks, and intercellular communication analyses. Moreover, we find the emergence of the ossification-committed population is correlated with the COL2A1-(ITGA2/11+ITGB1) signaling. NOTCH signaling may contribute to the formation of cartilage canals and ossification via NOTCH signaling. Our findings will advance the understanding of single-cell genetic changes underlying fetal epiphysis development.

Single-cell analysis of lizard blastema fibroblasts reveals phagocyte-dependent activation of Hedgehog-responsive chondrogenesis

Lizards cannot naturally regenerate limbs but are the closest known relatives of mammals capable of epimorphic tail regrowth. However, the mechanisms regulating lizard blastema formation and chondrogenesis remain unclear. Here, single-cell RNA sequencing analysis of regenerating lizard tails identifies fibroblast and phagocyte populations linked to cartilage formation. Pseudotime trajectory analyses suggest spp1+-activated fibroblasts as blastema cell sources, with subsets exhibiting sulf1 expression and chondrogenic potential. Tail blastema, but not limb, fibroblasts express sulf1 and form cartilage under Hedgehog signaling regulation. Depletion of phagocytes inhibits blastema formation, but treatment with pericytic phagocyte-conditioned media rescues blastema chondrogenesis and cartilage formation in amputated limbs. The results indicate a hierarchy of phagocyte-induced fibroblast gene activations during lizard blastema formation, culminating in sulf1+ pro-chondrogenic populations singularly responsive to Hedgehog signaling. These properties distinguish lizard blastema cells from homeostatic and injury-stimulated fibroblasts and indicate potential actionable targets for inducing regeneration in other species, including humans.

Bacterial meningitis in the early postnatal mouse studied at single-cell resolution

Bacterial meningitis is a major cause of morbidity and mortality, especially among infants and the elderly. Here, we study mice to assess the response of each of the major meningeal cell types to early postnatal E. coli infection using single nucleus RNA sequencing (snRNAseq), immunostaining, and genetic and pharamacologic perturbations of immune cells and immune signaling. Flatmounts of the dissected leptomeninges and dura were used to facilitiate high-quality confocal imaging and quantification of cell abundances and morphologies. Upon infection, the major meningeal cell types - including endothelial cells (ECs), macrophages, and fibroblasts - exhibit distinctive changes in their transcriptomes. Additionally, ECs in the leptomeninges redistribute CLDN5 and PECAM1, and leptomeningeal capillaries exhibit foci with reduced blood-brain barrier integrity. The vascular response to infection appears to be largely driven by TLR4 signaling, as determined by the nearly identical responses induced by infection and LPS administration and by the blunted response to infection in Tlr4^-/- mice. Interestingly, knocking out Ccr2, encoding a major chemoattractant for monocytes, or acute depletion of leptomeningeal macrophages, following intracebroventricular injection of liposomal clodronate, had little or no effect on the response of leptomeningeal ECs to E. coli infection. Taken together, these data imply that EC responses to infection are largely driven by the intrinsic EC response to LPS.

FGF signaling in cranial suture development and related diseases

Suture mesenchymal stem cells (SMSCs) are a heterogeneous stem cell population with the ability to self-renew and differentiate into multiple cell lineages. The cranial suture provides a niche for SMSCs to maintain suture patency, allowing for cranial bone repair and regeneration. In addition, the cranial suture functions as an intramembranous bone growth site during craniofacial bone development. Defects in suture development have been implicated in various congenital diseases, such as sutural agenesis and craniosynostosis. However, it remains largely unknown how intricate signaling pathways orchestrate suture and SMSC function in craniofacial bone development, homeostasis, repair and diseases. Studies in patients with syndromic craniosynostosis identified fibroblast growth factor (FGF) signaling as an important signaling pathway that regulates cranial vault development. A series of in vitro and in vivo studies have since revealed the critical roles of FGF signaling in SMSCs, cranial suture and cranial skeleton development, and the pathogenesis of related diseases. Here, we summarize the characteristics of cranial sutures and SMSCs, and the important functions of the FGF signaling pathway in SMSC and cranial suture development as well as diseases caused by suture dysfunction. We also discuss emerging current and future studies of signaling regulation in SMSCs.

Development of Erf-Mediated Craniosynostosis and Pharmacological Amelioration

ETS2 repressor factor (ERF) insufficiency causes craniosynostosis (CRS4) in humans and mice. ERF is an ETS domain transcriptional repressor regulated by Erk1/2 phosphorylation via nucleo-cytoplasmic shuttling. Here, we analyze the onset and development of the craniosynostosis phenotype in an Erf-insufficient mouse model and evaluate the potential of the residual Erf activity augmented by pharmacological compounds to ameliorate the disease. Erf insufficiency appears to cause an initially compromised frontal bone formation and subsequent multisuture synostosis, reflecting distinct roles of Erf on the cells that give rise to skull and facial bones. We treated animals with Mek1/2 and nuclear export inhibitors, U0126 and KPT-330, respectively, to increase Erf activity by two independent pathways. We implemented both a low dosage locally over the calvaria and a systemic drug administration scheme to evaluate the possible indirect effects from other systems and minimize toxicity. The treatment of mice with either the inhibitors or the administration scheme alleviated the synostosis phenotype with minimal adverse effects. Our data suggest that the ERF level is an important regulator of cranial bone development and that pharmacological modulation of its activity may represent a valid intervention approach both in CRS4 and in other syndromic forms of craniosynostosis mediated by the FGFR-RAS-ERK-ERF pathway.

Expansion of the sagittal suture induces proliferation of skeletal stem cells and sustains endogenous calvarial bone regeneration

In newborn humans, and up to approximately 2 y of age, calvarial bone defects can naturally regenerate. This remarkable regeneration potential is also found in newborn mice and is absent in adult mice. Since previous studies showed that the mouse calvarial sutures are reservoirs of calvarial skeletal stem cells (cSSCs), which are the cells responsible for calvarial bone regeneration, here we hypothesized that the regenerative potential of the newborn mouse calvaria is due to a significant amount of cSSCs present in the newborn expanding sutures. Thus, we tested whether such regenerative potential can be reverse engineered in adult mice by artificially inducing an increase of the cSSCs resident within the adult calvarial sutures. First, we analyzed the cellular composition of the calvarial sutures in newborn and in older mice, up to 14-mo-old mice, showing that the sutures of the younger mice are enriched in cSSCs. Then, we demonstrated that a controlled mechanical expansion of the functionally closed sagittal sutures of adult mice induces a significant increase of the cSSCs. Finally, we showed that if a calvarial critical size bone defect is created simultaneously to the mechanical expansion of the sagittal suture, it fully regenerates without the need for additional therapeutic aids. Using a genetic blockade system, we further demonstrate that this endogenous regeneration is mediated by the canonical Wnt signaling. This study shows that controlled mechanical forces can harness the cSSCs and induce calvarial bone regeneration. Similar harnessing strategies may be used to develop novel and more effective bone regeneration autotherapies.

Stuck on you: Meninges cellular crosstalk in development

The spatial and temporal development of the brain, overlying meninges (fibroblasts, vasculature and immune cells) and calvarium are highly coordinated. In particular, the timing of meningeal fibroblasts into molecularly distinct pia, arachnoid and dura subtypes coincides with key developmental events in the brain and calvarium. Further, the meninges are positioned to influence development of adjacent structures and do so via depositing basement membrane and producing molecular cues to regulate brain and calvarial development. Here, we review the current knowledge of how meninges development aligns with events in the brain and calvarium and meningeal fibroblast "crosstalk" with these structures. We summarize outstanding questions and how the use of non-mammalian models to study the meninges will substantially advance the field of meninges biology.

Differential regulation of skeletal stem/progenitor cells in distinct skeletal compartments

Skeletal stem/progenitor cells (SSPCs), characterized by self-renewal and multipotency, are essential for skeletal development, bone remodeling, and bone repair. These cells have traditionally been known to reside within the bone marrow, but recent studies have identified the presence of distinct SSPC populations in other skeletal compartments such as the growth plate, periosteum, and calvarial sutures. Differences in the cellular and matrix environment of distinct SSPC populations are believed to regulate their stemness and to direct their roles at different stages of development, homeostasis, and regeneration; differences in embryonic origin and adjacent tissue structures also affect SSPC regulation. As these SSPC niches are dynamic and highly specialized, changes under stress conditions and with aging can alter the cellular composition and molecular mechanisms in place, contributing to the dysregulation of local SSPCs and their activity in bone regeneration. Therefore, a better understanding of the different regulatory mechanisms for the distinct SSPCs in each skeletal compartment, and in different conditions, could provide answers to the existing knowledge gap and the impetus for realizing their potential in this biological and medical space. Here, we summarize the current scientific advances made in the study of the differential regulation pathways for distinct SSPCs in different bone compartments. We also discuss the physical, biological, and molecular factors that affect each skeletal compartment niche. Lastly, we look into how aging influences the regenerative capacity of SSPCs. Understanding these regulatory differences can open new avenues for the discovery of novel treatment approaches for calvarial or long bone repair.

Review of Recurrently Mutated Genes in Craniosynostosis Supports Expansion of Diagnostic Gene Panels

Craniosynostosis, the premature fusion of the cranial sutures, affects ~1 in 2000 children. Although many patients with a genetically determined cause harbor a variant in one of just seven genes or have a chromosomal abnormality, over 60 genes are known to be recurrently mutated, thus comprising a long tail of rarer diagnoses. Genome sequencing for the diagnosis of rare diseases is increasingly used in clinical settings, but analysis of the data is labor intensive and involves a trade-off between achieving high sensitivity or high precision. PanelApp, a crowd-sourced disease-focused set of gene panels, was designed to enable prioritization of variants in known disease genes for a given pathology, allowing enhanced identification of true-positives. For heterogeneous disorders like craniosynostosis, these panels must be regularly updated to ensure that diagnoses are not being missed. We provide a systematic review of genetic literature on craniosynostosis over the last 5 years, including additional results from resequencing a 42-gene panel in 617 affected individuals. We identify 16 genes (representing a 25% uplift) that should be added to the list of bona fide craniosynostosis disease genes and discuss the insights that these new genes provide into pathophysiological mechanisms of craniosynostosis.

De novo mutations in the BMP signaling pathway in lambdoid craniosynostosis

Lambdoid craniosynostosis (CS) is a congenital anomaly resulting from premature fusion of the cranial suture between the parietal and occipital bones. Predominantly sporadic, it is the rarest form of CS and its genetic etiology is largely unexplored. Exome sequencing of 25 kindreds, including 18 parent-offspring trios with sporadic lambdoid CS, revealed a marked excess of damaging (predominantly missense) de novo mutations that account for ~ 40% of sporadic cases. These mutations clustered in the BMP signaling cascade (P = 1.6 × 10^-7), including mutations in genes encoding BMP receptors (ACVRL1 and ACVR2A), transcription factors (SOX11, FOXO1) and a transcriptional co-repressor (IFRD1), none of which have been implicated in other forms of CS. These missense mutations are at residues critical for substrate or target sequence recognition and many are inferred to cause genetic gain-of-function. Additionally, mutations in transcription factor NFIX were implicated in syndromic craniosynostosis affecting diverse sutures. Single cell RNA sequencing analysis of the mouse lambdoid suture identified enrichment of mutations in osteoblast precursors (P = 1.6 × 10^-6), implicating perturbations in the balance between proliferation and differentiation of osteoprogenitor cells in lambdoid CS. The results contribute to the growing knowledge of the genetics of CS, have implications for genetic counseling, and further elucidate the molecular etiology of premature suture fusion.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell–cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell–cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C–C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

•

Sox9-positive skeletal progenitors contributed to the calvaria bone repair process.

•

scRNA-seq analysis revealed a heterogeneous cell population at bone defect sites.

•

Skeletal progenitors had a bifurcated lineages of osteogenesis and adipogenesis.

•

Ccl9 was identified as an important signaling molecule regulating bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Cranium growth, patterning and homeostasis

Craniofacial development requires precise spatiotemporal regulation of multiple signaling pathways that crosstalk to coordinate the growth and patterning of the skull with surrounding tissues. Recent insights into these signaling pathways and previously uncharacterized progenitor cell populations have refined our understanding of skull patterning, bone mineralization and tissue homeostasis. Here, we touch upon classical studies and recent advances with an emphasis on developmental and signaling mechanisms that regulate the osteoblast lineage for the calvaria, which forms the roof of the skull. We highlight studies that illustrate the roles of osteoprogenitor cells and cranial suture-derived stem cells for proper calvarial growth and homeostasis. We also discuss genes and signaling pathways that control suture patency and highlight how perturbing the molecular regulation of these pathways leads to craniosynostosis. Finally, we discuss the recently discovered tissue and signaling interactions that integrate skull and cerebrovascular development, and the potential implications for both cerebrospinal fluid hydrodynamics and brain waste clearance in craniosynostosis.

Insights into skeletal stem cells

The tissue-resident skeletal stem cells (SSCs), which are self-renewal and multipotent, continuously provide cells (including chondrocytes, bone cells, marrow adipocytes, and stromal cells) for the development and homeostasis of the skeletal system. In recent decade, utilizing fluorescence-activated cell sorting, lineage tracing, and single-cell sequencing, studies have identified various types of SSCs, plotted the lineage commitment trajectory, and partially revealed their properties under physiological and pathological conditions. In this review, we retrospect to SSCs identification and functional studies. We discuss the principles and approaches to identify bona fide SSCs, highlighting pioneering findings that plot the lineage atlas of SSCs. The roles of SSCs and progenitors in long bone, craniofacial tissues, and periosteum are systematically discussed. We further focus on disputes and challenges in SSC research.

Spatiotemporal single-cell regulatory atlas reveals neural crest lineage diversification and cellular function during tooth morphogenesis

Cranial neural crest cells are an evolutionary innovation of vertebrates for craniofacial development and function, yet the mechanisms that govern the cell fate decisions of postmigratory cranial neural crest cells remain largely unknown. Using the mouse molar as a model, we perform single-cell transcriptome profiling to interrogate the cell fate diversification of postmigratory cranial neural crest cells. We reveal the landscape of transcriptional heterogeneity and define the specific cellular domains during the progression of cranial neural crest cell-derived dental lineage diversification, and find that each domain makes a specific contribution to distinct molar mesenchymal tissues. Furthermore, IGF signaling-mediated cell-cell interaction between the cellular domains highlights the pivotal role of autonomous regulation of the dental mesenchyme. Importantly, we reveal cell-type-specific gene regulatory networks in the dental mesenchyme and show that Foxp4 is indispensable for the differentiation of periodontal ligament. Our single-cell atlas provides comprehensive mechanistic insight into the cell fate diversification process of the cranial neural crest cell-derived odontogenic populations.

Craniofacial sutures: Signaling centres integrating mechanosensation, cell signaling, and cell differentiation

Cranial sutures are dynamic structures in which stem cell biology, bone formation, and mechanical forces interface, influencing the shape of the skull throughout development and beyond. Over the past decade, there has been significant progress in understanding mesenchymal stromal cell (MSC) differentiation in the context of suture development and genetic control of suture pathologies, such as craniosynostosis. More recently, the mechanosensory function of sutures and the influence of mechanical signals on craniofacial development have come to the forefront. There is currently a gap in understanding of how mechanical signals integrate with MSC differentiation and ossification to ensure appropriate bone development and mediate postnatal growth surrounding sutures. In this review, we discuss the role of mechanosensation in the context of cranial sutures, and how mechanical stimuli are converted to biochemical signals influencing bone growth, suture patency, and fusion through mediation of cell differentiation. We integrate key knowledge from other paradigms where mechanosensation forms a critical component, such as bone remodeling and orthodontic tooth movement. The current state of the field regarding genetic, cellular, and physiological mechanisms of mechanotransduction will be contextualized within suture biology.

Single-cell atlas of craniogenesis uncovers SOXC-dependent, highly proliferative, and myofibroblast-like osteodermal progenitors

The mammalian skull vault is essential to shape the head and protect the brain, but the cellular and molecular events underlying its development remain incompletely understood. Single-cell transcriptomic profiling from early to late mouse embryonic stages provides a detailed atlas of cranial lineages. It distinguishes various populations of progenitors and reveals a high expression of SOXC genes (encoding the SOX4, SOX11, and SOX12 transcription factors) early in development in actively proliferating and myofibroblast-like osteodermal progenitors. SOXC inactivation in these cells causes severe skull and skin underdevelopment due to the limited expansion of cell populations before and upon lineage commitment. SOXC genes enhance the expression of gene signatures conferring dynamic cellular and molecular properties, including actin cytoskeleton assembly, chromatin remodeling, and signaling pathway induction and responsiveness. These findings shed light onto craniogenic mechanisms and SOXC functions and suggest that similar mechanisms could decisively control many developmental, adult, pathological, and regenerative processes.

Angelozzi and colleagues establish a detailed transcriptomic atlas of mouse embryonic craniogenesis and use mutant mice to show that SOXC (SOX4, SOX11, and SOX12 transcription factors) critically support osteogenesis and dermogenesis by promoting the expression of dynamic cellular and molecular properties of progenitor populations. SOXC could similarly affect many other processes.

Molecular and Cellular Analysis of the Repair of Zebrafish Optic Tectum Meninges Following Laser Injury

We studied cell recruitment following optic tectum (OT) injury in zebrafish (Danio rerio), which has a remarkable ability to regenerate many of its organs, including the brain. The OT is the largest dorsal layered structure in the zebrafish brain. In juveniles, it is an ideal structure for imaging and dissection. We investigated the recruited cells within the juvenile OT during regeneration in a Pdgfrβ-Gal4:UAS-EGFP line in which pericytes, vascular, circulating, and meningeal cells are labeled, together with neurons and progenitors. We first performed high-resolution confocal microscopy and single-cell RNA-sequencing (scRNAseq) on EGFP-positive cells. We then tested three types of injury with very different outcomes (needle (mean depth in the OT of 200 µm); deep-laser (depth: 100 to 200 µm depth); surface-laser (depth: 0 to 100 µm)). Laser had the additional advantage of better mimicking of ischemic cerebral accidents. No massive recruitment of EGFP-positive cells was observed following laser injury deep in the OT. This type of injury does not perturb the meninx/brain–blood barrier (BBB). We also performed laser injuries at the surface of the OT, which in contrast create a breach in the meninges. Surprisingly, one day after such injury, we observed the migration to the injury site of various EGFP-positive cell types at the surface of the OT. The migrating cells included midline roof cells, which activated the PI3K-AKT pathway; fibroblast-like cells expressing numerous collagen genes and most prominently in 3D imaging; and a large number of arachnoid cells that probably migrate to the injury site through the activation of cilia motility genes, most likely being direct targets of the FOXJ1a gene. This study, combining high-content imaging and scRNAseq in physiological and pathological conditions, sheds light on meninges repair mechanisms in zebrafish that probably also operate in mammalian meninges.

Marsupials and Multi-Omics: Establishing New Comparative Models of Neural Crest Patterning and Craniofacial Development

Studies across vertebrates have revealed significant insights into the processes that drive craniofacial morphogenesis, yet we still know little about how distinct facial morphologies are patterned during development. Studies largely point to evolution in GRNs of cranial progenitor cell types such as neural crest cells, as the major driver underlying adaptive cranial shapes. However, this hypothesis requires further validation, particularly within suitable models amenable to manipulation. By utilizing comparative models between related species, we can begin to disentangle complex developmental systems and identify the origin of species-specific patterning. Mammals present excellent evolutionary examples to scrutinize how these differences arise, as sister clades of eutherians and marsupials possess suitable divergence times, conserved cranial anatomies, modular evolutionary patterns, and distinct developmental heterochrony in their NCC behaviours and craniofacial patterning. In this review, I lend perspectives into the current state of mammalian craniofacial biology and discuss the importance of establishing a new marsupial model, the fat-tailed dunnart, for comparative research. Through detailed comparisons with the mouse, we can begin to decipher mammalian conserved, and species-specific processes and their contribution to craniofacial patterning and shape disparity. Recent advances in single-cell multi-omics allow high-resolution investigations into the cellular and molecular basis of key developmental processes. As such, I discuss how comparative evolutionary application of these tools can provide detailed insights into complex cellular behaviours and expression dynamics underlying adaptive craniofacial evolution. Though in its infancy, the field of "comparative evo-devo-omics" presents unparalleled opportunities to precisely uncover how phenotypic differences arise during development.

A dysmorphic mouse model reveals developmental interactions of chondrocranium and dermatocranium

The cranial endo- and dermal skeletons, which comprise the vertebrate skull, evolved independently over 470 million years ago and form separately during embryogenesis. In mammals, much of the cartilaginous chondrocranium is transient, undergoing endochondral ossification or disappearing, so its role in skull morphogenesis is not well studied and it remains an enigmatic structure. We provide complete three-dimensional (3D) reconstructions of the laboratory mouse chondrocranium from embryonic day 13.5 through 17.5 using a novel methodology of uncertainty-guided segmentation of phosphotungstic enhanced 3D microcomputed tomography images with sparse annotation. We evaluate the embryonic mouse chondrocranium and dermatocranium in 3D and delineate the effects of a Fgfr2 variant on embryonic chondrocranial cartilages and on their association with forming dermal bones using the Fgfr2c^C342Y/+ Crouzon syndrome mouse. We show that the dermatocranium develops outside of and in shapes that conform to the chondrocranium. Results reveal direct effects of the Fgfr2 variant on embryonic cartilage, on chondrocranium morphology, and on the association between chondrocranium and dermatocranium development. Histologically we observe a trend of relatively more chondrocytes, larger chondrocytes, and/or more matrix in the Fgfr2c^C342Y/+ embryos at all timepoints before the chondrocranium begins to disintegrate at E16.5. The chondrocrania and forming dermatocrania of Fgfr2c^C342Y/+ embryos are relatively large, but a contrasting trend begins at E16.5 and continues into early postnatal (P0 and P2) timepoints, with the skulls of older Fgfr2c^C342Y/+ mice reduced in most dimensions compared to Fgfr2c^+/+ littermates. Our findings have implications for the study and treatment of human craniofacial disease, for understanding the impact of chondrocranial morphology on skull growth, and potentially on the evolution of skull morphology.

Elevated Protein Kinase A Activity in Stomach Mesenchyme Disrupts Mesenchymal-epithelial Crosstalk and Induces Preneoplasia

BACKGROUND & AIMS

Mesenchymal-epithelial crosstalk (MEC) in the stomach is executed by pathways such as bone morphogenetic protein (BMP) and extracellular signal-regulated kinase (ERK). Mis-regulation of MEC disrupts gastric homeostasis and causes tumorigenesis. Protein Kinase A (PKA) crosstalks with BMP and ERK signaling; however, PKA function(s) in stomach development and homeostasis remains undefined.

METHODS

We generated a novel Six2-Cre^+/-PKAcαR^fl/wt (CA-PKA) mouse in which expression of constitutive-active PKAcαR was induced in gastric mesenchyme progenitors. Lineage tracing determined spatiotemporal activity of Six2-Cre in the stomach. For phenotyping CA-PKA mice histological, co-immunofluorescence, immunoblotting, mRNA sequencing, and bioinformatics analyses were performed.

RESULTS

Lineage tracing showed that Six2-Cre activity in the stomach is restricted to the mesenchymal compartment. CA-PKA mice showed disruption of gastric homeostasis characterized by aberrant mucosal development and epithelial hyperproliferation; ultimately developing multiple features of gastric corpus preneoplasia including decreased parietal cells, mucous cell hyperplasia, spasmolytic peptide expressing metaplasia with intestinal characteristics, and dysplastic and invasive cystic glands. Furthermore, mutant corpus showed marked chronic inflammation characterized by infiltration of lymphocytes and myeloid-derived suppressor cells along with the upregulation of innate and adaptive immune system components. Striking upregulation of inflammatory mediators and STAT3 activation was observed. Mechanistically, we determined there is an activation of ERK1/2 and downregulation of BMP/SMAD signaling characterized by marked upregulation of BMP inhibitor gremlin 1.

CONCLUSIONS

We report a novel role of PKA signaling in gastric MEC execution and show that PKA activation in the gastric mesenchyme drives preneoplasia by creating a proinflammatory and proproliferative microenvironment associated with the downregulation of BMP/SMAD signaling and activation of ERK1/2.

Single-cell RNA sequencing unravels heterogeneity of skeletal progenitors and cell-cell interactions underlying the bone repair process

Introduction

Activation of skeletal progenitors upon tissue injury and the subsequent cell fate specification are tightly coordinated in the bone repair process. Although known osteoimmunological signaling networks play important roles in the microenvironment of the bone defect sites, the molecular mechanism underlying the bone repair process has not been fully understood.

Methods

To better understand the behavior of the skeletal progenitors and the heterogeneity of the cells during bone repair at the microenvironmental level, we performed a combinatorial analysis consisting of lineage tracing for skeletal progenitors using the Sox9-CreERT2;R26R ^tdTomato mouse line followed by single-cell RNA sequencing (scRNA-seq) analysis using a mouse model of calvarial bone repair. To identify a therapeutic target for bone regeneration, further computational analysis was performed focusing on the identification of the cell-cell interactions, followed by pharmacological assessments with a critical-size calvarial bone defect mouse model.

Results

Lineage tracing analysis showed that skeletal progenitors marked by Sox9 were activated upon bone injury and contributed to bone repair by differentiating into osteoblasts. The scRNA-seq analysis characterized heterogeneous cell populations at the bone defect sites; the computational analysis predicted a bifurcated lineage from skeletal progenitors toward osteogenic and adipogenic lineages. Chemokine C-C motif ligand 9 (Ccl9) was identified as a signaling molecule that regulates bone regeneration in the mouse model, possibly through the regulation of adipogenic differentiation at the bone defect site.

Conclusion

Multipotential skeletal progenitors and the direction of the cell differentiation were characterized at single cell resolution in a mouse bone repair model. The Ccl9 signaling pathway may be a key factor directing osteogenesis from the progenitors in the model and may be a therapeutic target for bone regeneration.

Single-cell RNA-sequencing of mammalian brain development: insights and future directions

Understanding human brain development is of fundamental interest but is also very challenging. Single-cell RNA-sequencing studies in mammals have revealed that brain development is a highly dynamic process with tremendous, previously concealed, cellular heterogeneity. This Spotlight discusses key insights from these studies and their implications for experimental models. We survey published single-cell RNA-sequencing studies of mouse and human brain development, organized by anatomical regions and developmental time points. We highlight remaining gaps in the field, predominantly concerning human brain development. We propose future directions to fill the remaining gaps, and necessary complementary techniques to create an atlas integrated in space and time of human brain development.

Single-cell profiling of human dura and meningioma reveals cellular meningeal landscape and insights into meningioma immune response

BACKGROUND

Recent investigations of the meninges have highlighted the importance of the dura layer in central nervous system immune surveillance beyond a purely structural role. However, our understanding of the meninges largely stems from the use of pre-clinical models rather than human samples.

METHODS

Single-cell RNA sequencing of seven non-tumor-associated human dura samples and six primary meningioma tumor samples (4 matched and 2 non-matched) was performed. Cell type identities, gene expression profiles, and T cell receptor expression were analyzed. Copy number variant (CNV) analysis was performed to identify putative tumor cells and analyze intratumoral CNV heterogeneity. Immunohistochemistry and imaging mass cytometry was performed on selected samples to validate protein expression and reveal spatial localization of select protein markers.

RESULTS

In this study, we use single-cell RNA sequencing to perform the first characterization of both non-tumor-associated human dura and primary meningioma samples. First, we reveal a complex immune microenvironment in human dura that is transcriptionally distinct from that of meningioma. In addition, we characterize a functionally diverse and heterogenous landscape of non-immune cells including endothelial cells and fibroblasts. Through imaging mass cytometry, we highlight the spatial relationship among immune cell types and vasculature in non-tumor-associated dura. Utilizing T cell receptor sequencing, we show significant TCR overlap between matched dura and meningioma samples. Finally, we report copy number variant heterogeneity within our meningioma samples.

CONCLUSIONS

Our comprehensive investigation of both the immune and non-immune cellular landscapes of human dura and meningioma at single-cell resolution builds upon previously published data in murine models and provides new insight into previously uncharacterized roles of human dura.

The clinical manifestations, molecular mechanisms and treatment of craniosynostosis

Craniosynostosis is a major congenital craniofacial disorder characterized by the premature fusion of cranial suture(s). Patients with severe craniosynostosis often have impairments in hearing, vision, intracranial pressure and/or neurocognitive functions. Craniosynostosis can result from mutations, chromosomal abnormalities or adverse environmental effects, and can occur in isolation or in association with numerous syndromes. To date, surgical correction remains the primary treatment for craniosynostosis, but it is associated with complications and with the potential for re-synostosis. There is, therefore, a strong unmet need for new therapies. Here, we provide a comprehensive review of our current understanding of craniosynostosis, including typical craniosynostosis types, their clinical manifestations, cranial suture development, and genetic and environmental causes. Based on studies from animal models, we present a framework for understanding the pathogenesis of craniosynostosis, with an emphasis on the loss of postnatal suture mesenchymal stem cells as an emerging disease-driving mechanism. We evaluate emerging treatment options and highlight the potential of mesenchymal stem cell-based suture regeneration as a therapeutic approach for craniosynostosis.

Lizard Blastema Organoid Model Recapitulates Regenerated Tail Chondrogenesis

(1) Background: Lizard tail regeneration provides a unique model of blastema-based tissue regeneration for large-scale appendage replacement in amniotes. Green anole lizard (Anolis carolinensis) blastemas contain fibroblastic connective tissue cells (FCTCs), which respond to hedgehog signaling to create cartilage in vivo. However, an in vitro model of the blastema has not previously been achieved in culture. (2) Methods: By testing two adapted tissue dissociation protocols and two optimized media formulations, lizard tail FCTCs were pelleted in vitro and grown in a micromass blastema organoid culture. Pellets were analyzed by histology and in situ hybridization for FCTC and cartilage markers alongside staged original and regenerating lizard tails. (3) Results: Using an optimized serum-free media and a trypsin- and collagenase II-based dissociation protocol, micromass blastema organoids were formed. Organoid cultures expressed FCTC marker CDH11 and produced cartilage in response to hedgehog signaling in vitro, mimicking in vivo blastema and tail regeneration. (4) Conclusions: Lizard tail blastema regeneration can be modeled in vitro using micromass organoid culture, recapitulating in vivo FCTC marker expression patterns and chondrogenic potential.

Bone Marrow Mesenchymal Stromal Cells: Identification, Classification, and Differentiation

Bone marrow mesenchymal stromal cells (BMSCs), identified as pericytes comprising the hematopoietic niche, are a group of heterogeneous cells composed of multipotent stem cells, including osteochondral and adipocyte progenitors. Nevertheless, the identification and classification are still controversial, which limits their application. In recent years, by lineage tracing and single-cell sequencing, several new subgroups of BMSCs and their roles in normal physiological and pathological conditions have been clarified. Key regulators and mechanisms controlling the fate of BMSCs are being revealed. Cross-talk among subgroups of bone marrow mesenchymal cells has been demonstrated. In this review, we focus on recent advances in the identification and classification of BMSCs, which provides important implications for clinical applications.

Embryonic requirements for Tcf12 in the development of the mouse coronal suture

A major feature of Saethre-Chotzen syndrome is coronal craniosynostosis, the fusion of the frontal and parietal bones at the coronal suture. It is caused by heterozygous loss-of-function mutations in either of the bHLH transcription factors TWIST1 and TCF12. Although compound heterozygous Tcf12; Twist1 mice display severe coronal synostosis, the individual role of Tcf12 had remained unexplored. Here, we show that Tcf12 controls several key processes in calvarial development, including the rate of frontal and parietal bone growth, and the boundary between sutural and osteogenic cells. Genetic analysis supports an embryonic requirement for Tcf12 in suture formation, as combined deletion of Tcf12 in embryonic neural crest and mesoderm, but not in postnatal suture mesenchyme, disrupts the coronal suture. We also detected asymmetric distribution of mesenchymal cells on opposing sides of the wild-type frontal and parietal bones, which prefigures later bone overlap at the sutures. In Tcf12 mutants, reduced asymmetry is associated with bones meeting end-on-end, possibly contributing to synostosis. Our results support embryonic requirements of Tcf12 in proper formation of the overlapping coronal suture.

Single-cell analysis identifies a key role for Hhip in murine coronal suture development

Craniofacial development depends on formation and maintenance of sutures between bones of the skull. In sutures, growth occurs at osteogenic fronts along the edge of each bone, and suture mesenchyme separates adjacent bones. Here, we perform single-cell RNA-seq analysis of the embryonic, wild type murine coronal suture to define its population structure. Seven populations at E16.5 and nine at E18.5 comprise the suture mesenchyme, osteogenic cells, and associated populations. Expression of Hhip, an inhibitor of hedgehog signaling, marks a mesenchymal population distinct from those of other neurocranial sutures. Tracing of the neonatal Hhip-expressing population shows that descendant cells persist in the coronal suture and contribute to calvarial bone growth. In Hhip^-/- coronal sutures at E18.5, the osteogenic fronts are closely apposed and the suture mesenchyme is depleted with increased hedgehog signaling compared to those of the wild type. Collectively, these data demonstrate that Hhip is required for normal coronal suture development.