Distinct effects of Hoxa2 overexpression in cranial neural crest populations reveal that the mammalian hyomandibular-ceratohyal boundary maps within the styloid process

A multiple super-enhancer region establishes inter-TAD interactions and controls Hoxa function in cranial neural crest

Enhancer-promoter interactions preferentially occur within boundary-insulated topologically associating domains (TADs), limiting inter-TAD interactions. Enhancer clusters in linear proximity, termed super-enhancers (SEs), ensure high target gene expression levels. Little is known about SE topological regulatory impact during craniofacial development. Here, we identify 2232 genome-wide putative SEs in mouse cranial neural crest cells (CNCCs), 147 of which target genes establishing CNCC positional identity during face formation. In second pharyngeal arch (PA2) CNCCs, a multiple SE-containing region, partitioned into Hoxa Inter-TAD Regulatory Element 1 and 2 (HIRE1 and HIRE2), establishes long-range inter-TAD interactions selectively with Hoxa2, that is required for external and middle ear structures. HIRE2 deletion in a Hoxa2 haploinsufficient background results in microtia. HIRE1 deletion phenocopies the full homeotic Hoxa2 knockout phenotype and induces PA3 and PA4 CNCC abnormalities correlating with Hoxa2 and Hoxa3 transcriptional downregulation. Thus, SEs can overcome TAD insulation and regulate anterior Hoxa gene collinear expression in a CNCC subpopulation-specific manner during craniofacial development.

Hox proteins as regulators of extracellular matrix interactions during neural crest migration

Emerging during embryogenesis, the neural crest are a migratory, transient population of multipotent stem cell that differentiates into various cell types in vertebrates. Neural crest cells arise along the anterior-posterior extent of the neural tube, delaminate and migrate along routes to their final destinations. The factors that orchestrate how neural crest cells undergo delamination and their subsequent sustained migration is not fully understood. This review provides a primer about neural crest epithelial-to-mesenchymal transition (EMT), with a special emphasis on the role of the Extracellular matrix (ECM), cellular effector proteins of EMT, and subsequent migration. We also summarize published findings that link the expression of Hox transcription factors to EMT and ECM modification, thereby implicating Hox factors in regulation of EMT and ECM remodeling during neural crest cell ontogenesis.

Meis2 controls skeletal formation in the hyoid region

A vertebrate skull is composed of many skeletal elements which display enormous diversity of shapes. Cranial bone formation embodies a multitude of processes, i.e., epithelial-mesenchymal induction, mesenchymal condensation, and endochondral or intramembranous ossification. Molecular pathways determining complex architecture and growth of the cranial skeleton during embryogenesis are poorly understood. Here, we present a model of the hyoid apparatus development in Wnt1-Cre2-induced Meis2 conditional knock-out (cKO) mice. Meis2 cKO embryos develop an aberrant hyoid apparatus—a complete skeletal chain from the base of the neurocranium to lesser horns of the hyoid, resembling extreme human pathologies of the hyoid-larynx region. We examined key stages of hyoid skeletogenesis to obtain a complex image of the hyoid apparatus formation. Lack of Meis2 resulted in ectopic loci of mesenchymal condensations, ectopic cartilage and bone formation, disinhibition of skeletogenesis, and elevated proliferation of cartilage precursors. We presume that all these mechanisms contribute to formation of the aberrant skeletal chain in the hyoid region. Moreover, Meis2 cKO embryos exhibit severely reduced expression of PBX1 and HAND2 in the hyoid region. Altogether, MEIS2 in conjunction with PBX1 and HAND2 affects mesenchymal condensation, specification and proliferation of cartilage precursors to ensure development of the anatomically correct hyoid apparatus.

Mutation of foxl1 Results in Reduced Cartilage Markers in a Zebrafish Model of Otosclerosis

Bone diseases such as otosclerosis (conductive hearing loss) and osteoporosis (low bone mineral density) can result from the abnormal expression of genes that regulate cartilage and bone development. The forkhead box transcription factor FOXL1 has been identified as the causative gene in a family with autosomal dominant otosclerosis and has been reported as a candidate gene in GWAS meta-analyses for osteoporosis. This potentially indicates a novel role for foxl1 in chondrogenesis, osteogenesis, and bone remodelling. We created a foxl1 mutant zebrafish strain as a model for otosclerosis and osteoporosis and examined jaw bones that are homologous to the mammalian middle ear bones, and mineralization of the axial skeleton. We demonstrate that foxl1 regulates the expression of collagen genes such as collagen type 1 alpha 1a and collagen type 11 alpha 2, and results in a delay in jawbone mineralization, while the axial skeleton remains unchanged. foxl1 may also act with other forkhead genes such as foxc1a, as loss of foxl1 in a foxc1a mutant background increases the severity of jaw calcification phenotypes when compared to each mutant alone. Our zebrafish model demonstrates atypical cartilage formation and mineralization in the zebrafish craniofacial skeleton in foxl1 mutants and demonstrates that aberrant collagen expression may underlie the development of otosclerosis.

Different Ectopic Hoxa2 Expression Levels in Mouse Cranial Neural Crest Cells Result in Distinct Craniofacial Anomalies and Homeotic Phenotypes

Providing appropriate positional identity and patterning information to distinct rostrocaudal subpopulations of cranial neural crest cells (CNCCs) is central to vertebrate craniofacial morphogenesis. Hox genes are not expressed in frontonasal and first pharyngeal arch (PA1) CNCCs, whereas a single Hox gene, Hoxa2, is necessary to provide patterning information to second pharyngeal arch (PA2) CNCCs. In frog, chick and mouse embryos, ectopic expression of Hoxa2 in Hox-negative CNCCs induced hypoplastic phenotypes of CNCC derivatives of variable severity, associated or not with homeotic transformation of a subset of PA1 structures into a PA2-like identity. Whether these different morphological outcomes are directly related to distinct Hoxa2 overexpression levels is unknown. To address this issue, we selectively induced Hoxa2 overexpression in mouse CNCCs, using a panel of mouse lines expressing different Hoxa2 ectopic expression levels, including a newly generated Hoxa2 knocked-in mouse line. While ectopic Hoxa2 expression at only 60% of its physiological levels was sufficient for pinna duplication, ectopic Hoxa2 expression at 100% of its normal level was required for complete homeotic repatterning of a subset of PA1 skeletal elements into a duplicated set of PA2-like elements. On the other hand, ectopic Hoxa2 overexpression at non-physiological levels (200% of normal levels) led to an almost complete loss of craniofacial skeletal structures. Moreover, ectopic Hoxa5 overexpression in CNCCs, while also resulting in severe craniofacial defects, did not induce homeotic changes of PA1-derived CNCCs, indicating Hoxa2 specificity in repatterning a subset of Hox-negative CNCCs. These results reconcile some discrepancies in previously published experiments and indicate that distinct subpopulations of CNCCs are differentially sensitive to ectopic levels of Hox expression.

Whole-exome sequencing identified first homozygous frameshift variant in the COLEC10 gene in an Iranian patient causing 3MC syndrome type 3

BACKGROUND

3MC syndrome type 3 is an autosomal recessive disorder caused by mutations in the COLEC10 gene besides other genes like COLEC11 and MASP1. This disorder is characterized by facial dysmorphism, cleft lip and palate, postnatal growth deficiency, cognitive impairment, hearing loss, craniosynostosis, radioulnar synostosis, genital and vesicorenal anomalies, cardiac anomalies, caudal appendage, and umbilical hernia.

METHODS

In the present study, whole-exome sequencing was performed in order to identify disease causing variant in an Iranian 7-year-old affected girl with craniosynostosis, dolichocephaly, blepharoptosis, clinodactyly of the 5th finger, high myopia, long face, micrognathia, patent ductus arteriosus, downslanted palpebral fissures, telecanthus, and epicanthus inversus. Identified variant confirmation in the patient and segregation analysis in her family were performed using Sanger sequencing method.

RESULTS

A novel homozygous frameshift deletion variant [NM_006438.5: c.128_129delCA; p.(Thr43AsnfsTer9)] was identified within the COLEC10 gene. Up to now, only three 3MC syndrome patients with mutations in the COLEC10 gene have been reported, and here, we report the fourth patient and the first homozygous frameshift variant.

CONCLUSION

Other genes and factors responsible for 3MC syndrome occurrence are remained to be discovered. We believe further investigation of the genes in the lectin complement pathway is needed to be done for the identification of other causes of this disease.

Anterior Hox Genes and the Process of Cephalization

During evolution, bilateral animals have experienced a progressive process of cephalization with the anterior concentration of nervous tissue, sensory organs and the appearance of dedicated feeding structures surrounding the mouth. Cephalization has been achieved by the specialization of the unsegmented anterior end of the body (the acron) and the sequential recruitment to the head of adjacent anterior segments. Here we review the key developmental contribution of Hox1-5 genes to the formation of cephalic structures in vertebrates and arthropods and discuss how this evolved. The appearance of Hox cephalic genes preceded the evolution of a highly specialized head in both groups, indicating that Hox gene involvement in the control of cephalic structures was acquired independently during the evolution of vertebrates and invertebrates to regulate the genes required for head innovation.

Evolutionary divergence of a Hoxa2b hindbrain enhancer in syngnathids mimics results of functional assays

Hoxa2 genes provide critical patterning signals during development, and their regulation and function have been extensively studied. We report a previously uncharacterized significant sequence divergence of a highly conserved hindbrain hoxa2b enhancer element in the family syngnathidae (pipefishes, seahorses, pipehorses, seadragons). We compared the hox cis-regulatory element variation in the Gulf pipefish and two species of seahorse against eight other species of fish, as well as human and mouse. We annotated the hoxa2b enhancer element binding sites across three species of seahorse, four species of pipefish, and one species of ghost pipefish. Finally, we performed in situ hybridization analysis of hoxa2b expression in Gulf pipefish embryos. We found that all syngnathid fish examined share a modified rhombomere 4 hoxa2b enhancer element, despite the fact that this element has been found to be highly conserved across all vertebrates examined previously. Binding element sequence motifs and spacing between binding elements have been modified for the hoxa2b enhancer in several species of pipefish and seahorse, and that the loss of the Prep/Meis binding site and further space shortening happened after ghost pipefish split from the rest of the syngnathid clade. We showed that expression of this gene in rhombomere 4 is lower relative to the surrounding rhombomeres in developing Gulf pipefish embryos, reflecting previously published functional tests for this enhancer. Our findings highlight the benefits of studying highly derived, diverse taxa for understanding of gene regulatory evolution and support the hypothesis that natural mutations can occur in deeply conserved pathways in ways potentially related to phenotypic diversity.

Uncovering the secreted signals and transcription factors regulating the development of mammalian middle ear ossicles

The mammalian middle ear comprises a chain of ossicles, the malleus, incus, and stapes that act as an impedance matching device during the transmission of sound from the tympanic membrane to the inner ear. These ossicles are derived from cranial neural crest cells that undergo endochondral ossification and subsequently differentiate into their final functional forms. Defects that occur during middle ear development can result in conductive hearing loss. In this review, we summarize studies describing the crucial roles played by signaling molecules such as sonic hedgehog, bone morphogenetic proteins, fibroblast growth factors, notch ligands, and chemokines during the differentiation of neural crest into the middle ear ossicles. In addition to these cell-extrinsic signals, we also discuss studies on the function of transcription factor genes such as Foxi3, Tbx1, Bapx1, Pou3f4, and Gsc in regulating the development and morphology of the middle ear ossicles.

A proposed extension to the elongated styloid process definition: A morphological study with high-resolution tomography computer

The objective of the present study was to evaluate the morphology and angulation of the styloid process (SP), analyzing results stratified by gender, side and age on asymptomatic patients. We retrospectively analyzed 99 high-resolution computed tomography study images from asymptomatic patients. The images were assessed intra-observatory by a head and neck expert radiologist. Data for length, morphology, and angulation in a coronal and sagittal plane were recorded and stratified by age and gender. Morphology was classified according to the Langlais modified by Guimares classification. The mean lengths were 31.67±0.97mm and 31.64±0.90mm for the right and left sides respectively. Using the Langlais modified by Guimares classification, the normal type was the most prevalent and without a statistically significant difference when comparing between genders. A total of 27.6% presented a morphologically elongated SP (>25mm of continuous process), 49.5% presented a radiologically elongated SP (>30mm), and a total of 9.59% had a longer length than the proposed≤45mm. The right transverse angle was greater in men than women, and a statistically significant difference was found (P=0.010). We show the morphological variability of the SP. Although our results are reported with the traditional definition, the high prevalence supports the need for a new definition of elongated SP and a normal angulation ranges.

Severe head dysgenesis resulting from imbalance between anterior and posterior ontogenetic programs

Head dysgenesis is a major cause of fetal demise and craniofacial malformation. Although mutations in genes of the head ontogenetic program have been reported, many cases remain unexplained. Head dysgenesis has also been related to trisomy or amplification of the chromosomal region overlapping the CDX2 homeobox gene, a master element of the trunk ontogenetic program. Hence, we investigated the repercussion on head morphogenesis of the imbalance between the head and trunk ontogenetic programs, by means of ectopic rostral expression of CDX2 at gastrulation. This caused severe malformations affecting the forebrain and optic structures, and also the frontonasal process associated with defects in neural crest cells colonization. These malformations are the result of the downregulation of genes of the head program together with the abnormal induction of trunk program genes. Together, these data indicate that the imbalance between the anterior and posterior ontogenetic programs in embryos is a new possible cause of head dysgenesis during human development, linked to defects in setting up anterior neuroectodermal structures.

An atlas of anterior hox gene expression in the embryonic sea lamprey head: Hox-code evolution in vertebrates

In the hindbrain and the adjacent cranial neural crest (NC) cells of jawed vertebrates (gnathostomes), nested and segmentally-restricted domains of Hox gene expression provide a combinatorial Hox-code for specifying regional properties during head development. Extant jawless vertebrates, such as the sea lamprey (Petromyzon marinus), can provide insights into the evolution and diversification of this Hox-code in vertebrates. There is evidence for gnathostome-like spatial patterns of Hox expression in lamprey; however, the expression domains of the majority of lamprey hox genes from paralogy groups (PG) 1-4 are yet to be characterized, so it is unknown whether they are coupled to hindbrain segments (rhombomeres) and NC. In this study, we systematically describe the spatiotemporal expression of all 14 sea lamprey hox genes from PG1-PG4 in the developing hindbrain and pharynx to investigate the extent to which their expression conforms to the archetypal gnathostome hindbrain and pharyngeal hox-codes. We find many similarities in Hox expression between lamprey and gnathostome species, particularly in rhombomeric domains during hindbrain segmentation and in the cranial neural crest, enabling inference of aspects of Hox expression in the ancestral vertebrate embryonic head. These data are consistent with the idea that a Hox regulatory network underlying hindbrain segmentation is a pan vertebrate trait. We also reveal differences in hindbrain domains at later stages, as well as expression in the endostyle and in pharyngeal arch (PA) 1 mesoderm. Our analysis suggests that many Hox expression domains that are observed in extant gnathostomes were present in ancestral vertebrates but have been partitioned differently across Hox clusters in gnathostome and cyclostome lineages after duplication.

A Hox-TALE regulatory circuit for neural crest patterning is conserved across vertebrates

In jawed vertebrates (gnathostomes), Hox genes play an important role in patterning head and jaw formation, but mechanisms coupling Hox genes to neural crest (NC) are unknown. Here we use cross-species regulatory comparisons between gnathostomes and lamprey, a jawless extant vertebrate, to investigate conserved ancestral mechanisms regulating Hox2 genes in NC. Gnathostome Hoxa2 and Hoxb2 NC enhancers mediate equivalent NC expression in lamprey and gnathostomes, revealing ancient conservation of Hox upstream regulatory components in NC. In characterizing a lamprey hoxα2 NC/hindbrain enhancer, we identify essential Meis, Pbx, and Hox binding sites that are functionally conserved within Hoxa2/Hoxb2 NC enhancers. This suggests that the lamprey hoxα2 enhancer retains ancestral activity and that Hoxa2/Hoxb2 NC enhancers are ancient paralogues, which diverged in hindbrain and NC activities. This identifies an ancestral mechanism for Hox2 NC regulation involving a Hox-TALE regulatory circuit, potentiated by inputs from Meis and Pbx proteins and Hox auto-/cross-regulatory interactions.

Systematic review of hormonal and genetic factors involved in the nonsyndromic disorders of the lower jaw

Mandibular disorders are among the most common birth defects in humans, yet the etiological factors are largely unknown. Most of the neonates affected by mandibular abnormalities have a sequence of secondary anomalies, including airway obstruction and feeding problems, that reduce the quality of life. In the event of lacking corrective surgeries, patients with mandibular congenital disorders suffer from additional lifelong problems such as sleep apnea and temporomandibular disorders, among others. The goal of this systematic review is to gather evidence on hormonal and genetic factors that are involved in signaling pathways and interactions that are potentially associated with the nonsyndromic mandibular disorders. We found that members of FGF and BMP pathways, including FGF8/10, FGFR2/3, BMP2/4/7, BMPR1A, ACVR1, and ACVR2A/B, have a prominent number of gene-gene interactions among all identified genes in this review. Gene ontology of the 154 genes showed that the functional gene sets are involved in all aspects of cellular processes and organogenesis. Some of the genes identified by the genome-wide association studies of common mandibular disorders are involved in skeletal formation and growth retardation based on animal models, suggesting a potential direct role as genetic risk factors in the common complex jaw disorders. Developmental Dynamics 248:162-172, 2019. © 2018 Wiley Periodicals, Inc.

Ectopic expression of Hoxb1 induces cardiac and craniofacial malformations

Members of the large family of Hox transcription factors are encoded by genes whose tightly regulated expression in development and in space within different embryonic tissues confer positional identity from the neck to the tips of the limbs. Many structures of the face, head, and heart develop from cell populations expressing few or no Hox genes. Hoxb1 is the member of its chromosomal cluster expressed in the most rostral domain during vertebrate development, but never by the multipotent neural crest cell population anterior to the cerebellum. We have developed a novel floxed transgenic mouse line, CAG-Hoxb1,-EGFP (CAG-Hoxb1), which upon recombination by Cre recombinase conditionally induces robust Hoxb1 and eGFP overexpression. When induced within the neural crest lineage, pups die at birth. A variable phenotype develops from E11.5 on, associating frontonasal hypoplasia/aplasia, micrognathia/agnathia, major ocular and forebrain anomalies, and cardiovascular malformations. Neural crest derivatives in the body appear unaffected. Transcription of effectors of developmental signaling pathways (Bmp, Shh, Vegfa) and transcription factors (Pax3, Sox9) is altered in mutants. These outcomes emphasize that repression of Hoxb1, along with other paralog group 1 and 2 Hox genes, is strictly necessary in anterior cephalic NC for craniofacial, visual, auditory, and cardiovascular development.

Evolution and development of the fish jaw skeleton

The evolution of the jaw represents a key innovation in driving the diversification of vertebrate body plans and behavior. The pharyngeal apparatus originated as gill bars separated by slits in chordate ancestors to vertebrates. Later, with the acquisition of neural crest, pharyngeal arches gave rise to branchial basket cartilages in jawless vertebrates (agnathans), and later bone and cartilage of the jaw, jaw support, and gills of jawed vertebrates (gnathostomes). Major events in the evolution of jaw structure from agnathans to gnathostomes include axial regionalization of pharyngeal elements and formation of a jaw joint. Hox genes specify the anterior-posterior identity of arches, and edn1, dlx, hand2, Jag1b-Notch2 signaling, and Nr2f factors specify dorsal-ventral identity. The formation of a jaw joint, an important step in the transition from an un-jointed pharynx in agnathans to a hinged jaw in gnathostomes involves interaction between nkx3.2, hand2, and barx1 factors. Major events in jaw patterning between fishes and reptiles include changes to elements of the second pharyngeal arch, including a loss of opercular and branchiostegal ray bones and transformation of the hyomandibula into the stapes. Further changes occurred between reptiles and mammals, including the transformation of the articular and quadrate elements of the jaw joint into the malleus and incus of the middle ear. Fossils of transitional jaw phenotypes can be analyzed from a developmental perspective, and there exists potential to use genetic manipulation techniques in extant taxa to test hypotheses about the evolution of jaw patterning in ancient vertebrates. This article is categorized under: Comparative Development and Evolution > Evolutionary Novelties Early Embryonic Development > Development to the Basic Body Plan Comparative Development and Evolution > Body Plan Evolution.

Probing the origin of matching functional jaws: roles of Dlx5/6 in cranial neural crest cells

Gnathostome jaws derive from the first pharyngeal arch (PA1), a complex structure constituted by Neural Crest Cells (NCCs), mesodermal, ectodermal and endodermal cells. Here, to determine the regionalized morphogenetic impact of Dlx5/6 expression, we specifically target their inactivation or overexpression to NCCs. NCC-specific Dlx5/6 inactivation (NCC^∆Dlx5/6) generates severely hypomorphic lower jaws that present typical maxillary traits. Therefore, differently from Dlx5/6 null-embryos, the upper and the lower jaws of NCC^∆Dlx5/6 mice present a different size. Reciprocally, forced Dlx5 expression in maxillary NCCs provokes the appearance of distinct mandibular characters in the upper jaw. We conclude that: (1) Dlx5/6 activation in NCCs invariably determines lower jaw identity; (2) the morphogenetic processes that generate functional matching jaws depend on the harmonization of Dlx5/6 expression in NCCs and in distinct ectodermal territories. The co-evolution of synergistic opposing jaws requires the coordination of distinct regulatory pathways involving the same transcription factors in distant embryonic territories.

Nonbifurcating Carotid Artery: A Case Report with a Review of Embryogenesis

Patient: Male, 66

Final Diagnosis: Nonbifurcating carotid artery

Symptoms: No symptoms

Medication: —

Clinical Procedure: —

Specialty: Neurology

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Coupling the roles of Hox genes to regulatory networks patterning cranial neural crest

The neural crest is a transient population of cells that forms within the developing central nervous system and migrates away to generate a wide range of derivatives throughout the body during vertebrate embryogenesis. These cells are of evolutionary and clinical interest, constituting a key defining trait in the evolution of vertebrates and alterations in their development are implicated in a high proportion of birth defects and craniofacial abnormalities. In the hindbrain and the adjacent cranial neural crest cells (cNCCs), nested domains of Hox gene expression provide a combinatorial'Hox-code' for specifying regional properties in the developing head. Hox genes have been shown to play important roles at multiple stages in cNCC development, including specification, migration, and differentiation. However, relatively little is known about the underlying gene-regulatory mechanisms involved, both upstream and downstream of Hox genes. Furthermore, it is still an open question as to how the genes of the neural crest GRN are linked to Hox-dependent pathways. In this review, we describe Hox gene expression, function and regulation in cNCCs with a view to integrating these genes within the emerging gene regulatory network for cNCC development. We highlight early roles for Hox1 genes in cNCC specification, proposing that this may be achieved, in part, by regulation of the balance between pluripotency and differentiation in precursor cells within the neuro-epithelium. We then describe what is known about the regulation of Hox gene expression in cNCCs and discuss this from the perspective of early vertebrate evolution.

Gene bivalency at Polycomb domains regulates cranial neural crest positional identity

The cranial neural crest cells are multipotent cells that provide head skeletogenic mesenchyme and are crucial for craniofacial patterning. We analyzed the chromatin landscapes of mouse cranial neural crest subpopulations in vivo. Early postmigratory subpopulations contributing to distinct mouse craniofacial structures displayed similar chromatin accessibility patterns yet differed transcriptionally. Accessible promoters and enhancers of differentially silenced genes carried H3K27me3/H3K4me2 bivalent chromatin marks embedded in large enhancer of zeste homolog 2-dependent Polycomb domains, indicating transcriptional poising. These postmigratory bivalent chromatin regions were already present in premigratory progenitors. At Polycomb domains, H3K27me3 antagonized H3K4me2 deposition, which was restricted to accessible sites. Thus, bivalent Polycomb domains provide a chromatin template for the regulation of cranial neural crest cell positional identity in vivo, contributing insights into the epigenetic regulation of face morphogenesis.

Differing contributions of the first and second pharyngeal arches to tympanic membrane formation in the mouse and chick

We have proposed that independent origins of the tympanic membrane (TM), consisting of the external auditory meatus (EAM) and first pharyngeal pouch, are linked with distinctive middle ear structures in terms of dorsal-ventral patterning of the pharyngeal arches during amniote evolution. However, previous studies have suggested that the first pharyngeal arch (PA1) is crucial for TM formation in both mouse and chick. In this study, we compare TM formation along the anterior-posterior axis in these animals using Hoxa2 expression as a marker of the second pharyngeal arch (PA2). In chick, the EAM begins to invaginate at the surface ectoderm of PA2, not at the first pharyngeal cleft, and the entire TM forms in PA2. Chick-quail chimera that have lost PA2 and duplicated PA1 suggest that TM formation is achieved by developmental interaction between a portion of the EAM and the columella auris in PA2, and that PA1 also contributes to formation of the remaining part of the EAM. By contrast, in mouse, TM formation is highly associated with an interdependent relationship between the EAM and tympanic ring in PA1.

COLEC10 is mutated in 3MC patients and regulates early craniofacial development

3MC syndrome is an autosomal recessive heterogeneous disorder with features linked to developmental abnormalities. The main features include facial dysmorphism, craniosynostosis and cleft lip/palate; skeletal structures derived from cranial neural crest cells (cNCC). We previously reported that lectin complement pathway genes COLEC11 and MASP1/3 are mutated in 3MC syndrome patients. Here we define a new gene, COLEC10, also mutated in 3MC families and present novel mutations in COLEC11 and MASP1/3 genes in a further five families. The protein products of COLEC11 and COLEC10, CL-K1 and CL-L1 respectively, form heteromeric complexes. We show COLEC10 is expressed in the base membrane of the palate during murine embryo development. We demonstrate how mutations in COLEC10 (c.25C>T; p.Arg9Ter, c.226delA; p.Gly77Glufs*66 and c.528C>G p.Cys176Trp) impair the expression and/or secretion of CL-L1 highlighting their pathogenicity. Together, these findings provide further evidence linking the lectin complement pathway and complement factors COLEC11 and COLEC10 to morphogenesis of craniofacial structures and 3MC etiology.

The 3MC syndrome is a unifying term amalgamating four rare recessive genetic disorders with overlapping features namely; Mingarelli, Malpuech, Michels and Carnevale syndromes. It is characterised by facial malformations including, high-arched eyebrows, cleft lip/palate, hypertelorism, developmental delay and hearing loss. We previously reported that lectin complement pathway genes COLEC11 and MASP1/3 were mutated in 3MC syndrome patients. Here we describe a new gene from the same pathway, COLEC10, mutated in 3MC patients. Our results show that COLEC10 is expressed in craniofacial tissues during development. We demonstrate how CL-L1, the protein expressed by COLEC10, can act as a cellular chemoattractant in vitro, controlling cell movement and migration. We overexpressed constructs carrying COLEC10 non-sense mutations found in our patients, CL-L1 failed to be expressed and secreted. Moreover, when we expressed a missense COLEC10 construct, CL-L1 was expressed but failed to be secreted. In sum, we discovered a new gene, COLEC10, mutated in 3MC syndrome and we propose a pathogenic mechanism for 3MC relating to the failure of CL-L1 function and its craniofacial developmental consequences.

Dissection of the Auditory Bulla in Postnatal Mice: Isolation of the Middle Ear Bones and Histological Analysis

In most mammals, auditory ossicles in the middle ear, including the malleus, incus and stapes, are the smallest bones. In mice, a bony structure called the auditory bulla houses the ossicles, whereas the auditory capsule encloses the inner ear, namely the cochlea and semicircular canals. Murine ossicles are essential for hearing and thus of great interest to researchers in the field of otolaryngology, but their metabolism, development, and evolution are highly relevant to other fields. Altered bone metabolism can affect hearing function in adult mice, and various gene-deficient mice show changes in morphogenesis of auditory ossicles in utero. Although murine auditory ossicles are tiny, their manipulation is feasible if one understands their anatomical orientation and 3D structure. Here, we describe how to dissect the auditory bulla and capsule of postnatal mice and then isolate individual ossicles by removing part of the bulla. We also discuss how to embed the bulla and capsule in different orientations to generate paraffin or frozen sections suitable for preparation of longitudinal, horizontal, or frontal sections of the malleus. Finally, we enumerate anatomical differences between mouse and human auditory ossicles. These methods would be useful in analyzing pathological, developmental and evolutionary aspects of auditory ossicles and the middle ear in mice.

Early development of the malleus and incus in humans

It is widely accepted by developmental biologists that the malleus and incus of the mammalian middle ear are first pharyngeal arch derivatives, a contention based originally on classical embryology that has now been backed up by molecular evidence from rodent models. However, it has been claimed in several studies of human ossicular development that the manubrium of the malleus and long process of the incus are actually derived from the second arch. This 'dual-arch' interpretation is commonly presented in otolaryngology textbooks, and it has been used by clinicians to explain the aetiology of certain congenital abnormalities of the human middle ear. In order to re-examine the origins of the human malleus and incus, we made three-dimensional reconstructions of the pharyngeal region of human embryos from 7 to 28 mm crown-rump length, based on serial histological sections from the Boyd Collection. We considered the positions of the developing ossicles relative to the pharyngeal pouches and clefts, and the facial and chorda tympani nerves. Confirming observations from previous studies, the primary union between first pharyngeal pouch and first cleft found in our youngest specimens was later lost, the external meatus developing rostroventral to this position. The mesenchyme of the first and second arches in these early embryos seemed to be continuous, but the boundaries of the developing ossicles proved to be very hard to determine at this stage. When first distinguishable, the indications were that both the manubrium of the malleus and the long process of the incus were emerging within the first pharyngeal arch. We therefore conclude that the histological evidence, on balance, favours the 'classical' notion that the human malleus and incus are first-arch structures. The embryological basis of congenital ossicular abnormalities should be reconsidered in this light.

Functional and Comparative Genomics of Hoxa2 Gene cis-Regulatory Elements: Evidence for Evolutionary Modification of Ancestral Core Element Activity

Hoxa2 is an evolutionarily conserved developmental regulatory gene that functions to specify rhombomere (r) and pharyngeal arch (PA) identities throughout the Osteichthyes. Japanese medaka (Oryzias latipes) hoxa2a, like orthologous Hoxa2 genes from other osteichthyans, is expressed during embryogenesis in r2–7 and PA2-7, whereas the paralogous medaka pseudogene, ψhoxa2b, is expressed in noncanonical Hoxa2 domains, including the pectoral fin buds. To understand the evolution of cis-regulatory element (CRE) control of gene expression, we conducted eGFP reporter gene expression studies with extensive functional mapping of several conserved CREs upstream of medaka hoxa2a and ψhoxa2b in transient and stable-line transgenic medaka embryos. The CREs tested were previously shown to contribute to directing mouse Hoxa2 gene expression in r3, r5, and PA2-4. Our results reveal the presence of sequence elements embedded in the medaka hoxa2a and ψhoxa2b upstream enhancer regions (UERs) that mediate expression in r4 and the PAs (hoxa2a r4/CNCC element) or in r3–7 and the PAs ψhoxa2b r3–7/CNCC element), respectively. Further, these elements were shown to be highly conserved among osteichthyans, which suggests that the r4 specifying element embedded in the UER of Hoxa2 is a deeply rooted rhombomere specifying element and the activity of this element has been modified by the evolution of flanking sequences that redirect its activity to alternative developmental compartments.