Locally released retinoic acid repatterns the first branchial arch cartilages in vivo

Disconnect between the developing eye and craniofacial prominences in the avian embryo

In the amniote embryo, the upper jaw and nasal cavities form through coordinated outgrowth and fusion of craniofacial prominences. Adjacent to the embryonic prominences are the developing eyes, which abut the maxillary and lateral nasal prominences. The embryos of extant sauropsids (birds and nonavian reptiles) develop particularly large eyes in comparison to mammals, leading researchers to propose that the developing eye may facilitate outgrowth of prominences towards the midline in order to aid prominence fusion. To test this hypothesis, we performed unilateral and bilateral ablation of the developing eyes in chicken embryos, with the aim of evaluating subsequent prominence formation and fusion. Our analyses revealed minor interaction between the developing craniofacial prominences and the eyes, inconsequential to the fusion of the upper beak. At later developmental stages, the skull exhibited only localized effects from missing eyes, while geometric morphometrics revealed minimal effect on overall shape of the upper jaw when it develops without eyes. Our results indicate that the substantial size of the developing eyes in the chicken embryo exert little influence over the fusion of the craniofacial prominences, despite their effect on the size and shape of maxillary prominences and components of the skull.

What's retinoic acid got to do with it? Retinoic acid regulation of the neural crest in craniofacial and ocular development

Retinoic acid (RA), the active derivative of vitamin A (retinol), is an essential morphogen signaling molecule and major regulator of embryonic development. The dysregulation of RA levels during embryogenesis has been associated with numerous congenital anomalies, including craniofacial, auditory, and ocular defects. These anomalies result from disruptions in the cranial neural crest, a vertebrate-specific transient population of stem cells that contribute to the formation of diverse cell lineages and embryonic structures during development. In this review, we summarize our current knowledge of the RA-mediated regulation of cranial neural crest induction at the edge of the neural tube and the migration of these cells into the craniofacial region. Further, we discuss the role of RA in the regulation of cranial neural crest cells found within the frontonasal process, periocular mesenchyme, and pharyngeal arches, which eventually form the bones and connective tissues of the head and neck and contribute to structures in the anterior segment of the eye. We then review our understanding of the mechanisms underlying congenital craniofacial and ocular diseases caused by either the genetic or toxic disruption of RA signaling. Finally, we discuss the role of RA in maintaining neural crest-derived structures in postembryonic tissues and the implications of these studies in creating new treatments for degenerative craniofacial and ocular diseases.

ARQ 087 inhibits FGFR signaling and rescues aberrant cell proliferation and differentiation in experimental models of craniosynostoses and chondrodysplasias caused by activating mutations in FGFR1, FGFR2 and FGFR3

Tyrosine kinase inhibitors are being developed for therapy of malignancies caused by oncogenic FGFR signaling but little is known about their effect in congenital chondrodysplasias or craniosynostoses that associate with activating FGFR mutations. Here, we investigated the effects of novel FGFR inhibitor, ARQ 087, in experimental models of aberrant FGFR3 signaling in cartilage. In cultured chondrocytes, ARQ 087 efficiently rescued all major effects of pathological FGFR3 activation, i.e. inhibition of chondrocyte proliferation, loss of extracellular matrix and induction of premature senescence. In ex vivo tibia organ cultures, ARQ 087 restored normal growth plate architecture and eliminated the suppressing FGFR3 effect on chondrocyte hypertrophic differentiation, suggesting that it targets the FGFR3 pathway specifically, i.e. without interference with other pro-growth pathways. Moreover, ARQ 087 inhibited activity of FGFR1 and FGFR2 mutants associated with Pfeiffer, Apert and Beare-Stevenson craniosynostoses, and rescued FGFR-driven excessive osteogenic differentiation in mouse mesenchymal micromass cultures or in ex vivo calvarial organ cultures. Our data warrant further development of ARQ 087 for clinical use in skeletal disorders caused by activating FGFR mutations.

Transcriptome profiling reveals expression signatures of cranial neural crest cells arising from different axial levels

Background

Cranial neural crest cells (NCCs) are a unique embryonic cell type which give rise to a diverse array of derivatives extending from neurons and glia through to bone and cartilage. Depending on their point of origin along the antero-posterior axis cranial NCCs are rapidly sorted into distinct migratory streams that give rise to axial specific structures. These migratory streams mirror the underlying segmentation of the brain with NCCs exiting the diencephalon and midbrain following distinct paths compared to those exiting the hindbrain rhombomeres (r). The genetic landscape of cranial NCCs arising at different axial levels remains unknown.

Results

Here we have used RNA sequencing to uncover the transcriptional profiles of mouse cranial NCCs arising at different axial levels. Whole transcriptome analysis identified over 120 transcripts differentially expressed between NCCs arising anterior to r3 (referred to as r1-r2 migratory stream for simplicity) and the r4 migratory stream. Eight of the genes differentially expressed between these populations were validated by RT-PCR with 2 being further validated by in situ hybridisation. We also explored the expression of the Neuropilins (Nrp1 and Nrp2) and their co-receptors and show that the A-type Plexins are differentially expressed in different cranial NCC streams.

Conclusions

Our analyses identify a large number of genes differentially regulated between cranial NCCs arising at different axial levels. This data provides a comprehensive description of the genetic landscape driving diversity of distinct cranial NCC streams and provides novel insight into the regulatory networks controlling the formation of specific skeletal elements and the mechanisms promoting migration along different paths.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0147-z) contains supplementary material, which is available to authorized users.

BMP signaling regulates the fate of chondro-osteoprogenitor cells in facial mesenchyme in a stage-specific manner

BACKGROUND

Lineage tracing has shown that most of the facial skeleton is derived from cranial neural crest cells. However, the local signals that influence postmigratory, neural crest-derived mesenchyme also play a major role in patterning the skeleton. Here, we study the role of BMP signaling in regulating the fate of chondro-osteoprogenitor cells in the face.

RESULTS

A single Noggin-soaked bead inserted into stage 15 chicken embryos induced an ectopic cartilage resembling the interorbital septum within the palate and other midline structures. In contrast, the same treatment in stage 20 embryos caused a loss of bones. The molecular basis for the stage-specific response to Noggin lay in the simultaneous up-regulation of SOX9 and downregulation of RUNX2 in the maxillary mesenchyme, increased cell adhesiveness as shown by N-cadherin induction around the beads and increased RA pathway gene expression. None of these changes were observed in stage 20 embryos.

CONCLUSIONS

These experiments demonstrate how slight changes in expression of growth factors such as BMPs could lead to gain or loss of cartilage in the upper jaw during vertebrate evolution. In addition, BMPs have at least two roles: one in patterning the skull and another in regulating the skeletogenic fates of neural crest-derived mesenchyme. Developmental Dynamics 245:947-962, 2016. © 2016 Wiley Periodicals, Inc.

Pannexin 3 is required for late stage bone growth but not for initiation of ossification in avian embryos

BACKGROUND

Pannexin 3 (PANX3) is a channel-forming protein capable of stimulating osteogenesis in vitro. Here, we studied the in vivo roles of PANX3 in the chicken embryo using the RCAS retroviral system to over-express and knockdown expression during endochondral bone formation.

RESULTS

In the limbs, PANX3 RNA was first detected in the cartilage condensations and became restricted to the prehypertrophic cartilage of the epiphyses, diaphysis, and perichondrium. The increase in PANX3 was not sufficient to alter osteogenesis; however, knockdown with a virus containing an interference RNA construct caused a 20% reduction in bone volume. The control virus containing an shEGFP cassette did not affect development. Interestingly, the phenotype was restricted to later stages rather than to proliferation of the skeletogenic mesenchyme, formation of the cartilage condensation, or creation of the hypertrophic zones. In addition, there was also no change in readouts of Hedgehog, WNT, fibroblast growth factor, or bone morphogenetic protein signaling using either quantitative real-time polymerase chain reaction or radioactive in situ hybridization.

CONCLUSIONS

Based on the normal expression domains of PANX3 and the relatively late manifestation of the phenotype, it is possible that PANX3 hemichannels may be required to facilitate the transition of hypertrophic chondrocytes to osteoblasts, thereby achieving final bone size. Developmental Dynamics 245:913-924, 2016. © 2016 Wiley Periodicals, Inc.

Instability restricts signaling of multiple fibroblast growth factors

Fibroblast growth factors (FGFs) deliver extracellular signals that govern many developmental and regenerative processes, but the mechanisms regulating FGF signaling remain incompletely understood. Here, we explored the relationship between intrinsic stability of FGF proteins and their biological activity for all 18 members of the FGF family. We report that FGF1, FGF3, FGF4, FGF6, FGF8, FGF9, FGF10, FGF16, FGF17, FGF18, FGF20, and FGF22 exist as unstable proteins, which are rapidly degraded in cell cultivation media. Biological activity of FGF1, FGF3, FGF4, FGF6, FGF8, FGF10, FGF16, FGF17, and FGF20 is limited by their instability, manifesting as failure to activate FGF receptor signal transduction over long periods of time, and influence specific cell behavior in vitro and in vivo. Stabilization via exogenous heparin binding, introduction of stabilizing mutations or lowering the cell cultivation temperature rescues signaling of unstable FGFs. Thus, the intrinsic ligand instability is an important elementary level of regulation in the FGF signaling system.

Identification of genes related to beak deformity of chickens using digital gene expression profiling

Frequencies of up to 3% of beak deformity (normally a crossed beak) occur in some indigenous chickens in China, such as and Beijing-You. Chickens with deformed beaks have reduced feed intake, growth rate, and abnormal behaviors. Beak deformity represents an economic as well as an animal welfare problem in the poultry industry. Because the genetic basis of beak deformity remains incompletely understood, the present study sought to identify important genes and metabolic pathways involved in this phenotype. Digital gene expression analysis was performed on deformed and normal beaks collected from Beijing-You chickens to detect global gene expression differences. A total of >11 million cDNA tags were sequenced, and 5,864,499 and 5,648,877 clean tags were obtained in the libraries of deformed and normal beaks, respectively. In total, 1,156 differentially expressed genes (DEG) were identified in the deformed beak with 409 being up-regulated and 747 down-regulated in the deformed beaks. qRT-PCR using eight genes was performed to verify the results of DGE profiling. Gene ontology (GO) analysis highlighted that genes of the keratin family on GGA25 were abundant among the DEGs. Pathway analysis showed that many DEGs were linked to the biosynthesis of unsaturated fatty acids and glycerolipid metabolism. Combining the analyses, 11 genes (MUC, LOC426217, BMP4, ACAA1, LPL, ALDH7A1, GLA, RETSAT, SDR16C5, WWOX, and MOGAT1) were highlighted as potential candidate genes for beak deformity in chickens. Some of these genes have been identified previously, while others have unknown function with respect to thus phenotype. To the best of our knowledge, this is the first genome-wide study to investigate the transcriptome differences in the deformed and normal beaks of chickens. The DEGs identified here are worthy of further functional characterization.

Avian facial morphogenesis is regulated by c-Jun N-terminal kinase/planar cell polarity (JNK/PCP) wingless-related (WNT) signaling

Wingless-related proteins (WNTs) regulate extension of the central axis of the vertebrate embryo (convergent extension) as well as morphogenesis of organs such as limbs and kidneys. Here, we asked whether WNT signaling directs facial morphogenesis using a targeted approach in chicken embryos. WNT11 is thought to mainly act via β-catenin-independent pathways, and little is known about its role in craniofacial development. RCAS::WNT11 retrovirus was injected into the maxillary prominence, and the majority of embryos developed notches in the upper beak or the equivalent of cleft lip. Three-dimensional morphometric analysis revealed that WNT11 prevented lengthening of the maxillary prominence, which was due in part to decreased proliferation. We next determined, using a series of luciferase reporters, that WNT11 strongly induced JNK/planar cell polarity signaling while repressing the β-catenin-mediated pathway. The activation of the JNK-ATF2 reporter was mediated by the DEP domain of Dishevelled. The impacts of altered signaling on the mesenchyme were assessed by implanted Wnt11- or Wnt3a-expressing cells (activates β-catenin pathway) into the maxillary prominence or by knocking down endogenous WNT11 with RNAi. Host cells were attracted to Wnt11 donor cells. In contrast, cells exposed to Wnt3a or the control cells did not migrate. Cells in which endogenous WNT11 was knocked down were more oriented and shorter than those exposed to exogenous WNT11. The data suggest that JNK/planar cell polarity WNT signaling operates in the face to regulate several morphogenetic events leading to lip fusion.

Dual functions for WNT5A during cartilage development and in disease

Mouse and human genetic data suggests that Wnt5a is required for jaw development but the specific role in facial skeletogenesis is unknown. We mapped expression of WNT5A in the developing chicken skull and found that the highest expression was in early Meckel's cartilage but by stage 35 expression was decreased to background. We focused on chondrogenesis by targeting a retrovirus expressing WNT5A to the mandibular prominence prior to cell differentiation. Unexpectedly, there were no phenotypes in the first 6days following injection; however later the mandibular bones and Meckel's cartilage were reduced or missing on the treated side. To examine the effects on cartilage differentiation we treated micromass cultures from mandibular mesenchyme with Wnt5a-conditioned media (CM). Similar to in vivo viral data, cartilage differentiates normally, but, after 6days of culture, nearly all Alcian blue staining is lost. Collagen II and aggrecan were also decreased in treated cultures. The matrix loss was correlated with upregulation of metalloproteinases, MMP1, MMP13, and ADAMTS5 (codes for Aggrecanase). Moreover, Marimastat, an MMP and Aggrecanase inhibitor rescued cartilage matrix in Wnt5a-CM treated cultures. The pathways mediating these cartilage and RNA changes were investigated using luciferase assays. Wnt5a-CM was a potent inhibitor of the canonical pathway and strongly activated JNK/PCP signaling. To determine whether the matrix loss is mediated by repression of canonical signaling or activation of the JNK pathway we treated mandibular cultures with either DKK1, an antagonist of the canonical pathway, or a small molecule that antagonizes JNK signaling (TCS JNK 6o). DKK1 slightly increased cartilage formation and therefore suggested that the endogenous canonical signaling represses chondrogenesis. To test this further we added an excess of Wnt3a-CM and found that far fewer cartilage nodules differentiated. Since DKK1 did not mimic the effects of Wnt5a we excluded the canonical pathway from mediating the matrix loss phenotype. The JNK antagonist partially rescued the Wnt5a phenotype supporting this non-canonical pathway as the main mediator of the cartilage matrix degradation. Our study reveals two new roles for WNT5A in development and disease: 1) to repress canonical Wnt signaling in cartilage blastema in order to promote normal differentiation and 2) in conditions of excess to stimulate degradation of mature cartilage matrix via non-canonical pathways.

Can you hear me now? Understanding vertebrate middle ear development

The middle ear is a composite organ formed from all three germ layers and the neural crest. It provides the link between the outside world and the inner ear, where sound is transduced and routed to the brain for processing. Extensive classical and modern studies have described the complex morphology and origin of the middle ear. Non-mammalian vertebrates have a single ossicle, the columella. Mammals have three functionally equivalent ossicles, designated the malleus, incus and stapes. In this review, I focus on the role of genes known to function in the middle ear. Genetic studies are beginning to unravel the induction and patterning of the multiple middle ear elements including the tympanum, skeletal elements, the air-filled cavity, and the insertion point into the inner ear oval window. Future studies that elucidate the integrated spatio-temporal signaling mechanisms required to pattern the middle ear organ system are needed. The longer-term translational benefits of understanding normal and abnormal ear development will have a direct impact on human health outcomes.

The function and regulation of TBX22 in avian frontonasal morphogenesis

The frontonasal mass gives rise to the facial midline and fuses with the maxillary prominence to form the upper lip. Here we focus on the regulation and function of TBX22, a repressor dynamically expressed in the frontonasal mass. Both FGF and Noggin (a BMP antagonist) strongly induce gTBX22, however, each has opposite effects on morphogenesis - Noggin inhibits whereas FGF stimulates growth. To determine whether TBX22 mediates these effects, we used retroviruses to locally increase expression levels. RCAS::hTBX22 decreased proliferation, reduced expression of MSX2 and DLX5 and caused cleft lip. Decreased levels of endogenous gTBX22 were also observed but were not the primary cause of the phenotype as determined in rescue experiments. Our data suggest that genetic or environmental insults such as those affecting the BMP pathway could lead to a gain-of-function of TBX22 and predispose an individual to cleft lip.

Novel skeletogenic patterning roles for the olfactory pit

The position of the olfactory placodes suggests that these epithelial thickenings might provide morphogenetic information to the adjacent facial mesenchyme. To test this, we performed in ovo manipulations of the nasal placode in the avian embryo. Extirpation of placodal epithelium or placement of barriers on the lateral side of the placode revealed that the main influence is on the lateral nasal, not the frontonasal, mesenchyme. These early effects were consistent with the subsequent deletion of lateral nasal skeletal derivatives. We then showed in rescue experiments that FGFs are required for nasal capsule morphogenesis. The instructive capacity of the nasal pit epithelium was tested in a series of grafts to the face and trunk. Here, we showed for the first time that nasal pits are capable of inducing bone, cartilage and ectopic PAX7 expression, but these effects were only observed in the facial grafts. Facial mesenchyme also supported the initial projection of the olfactory nerve and differentiation of the olfactory epithelium. Thus, the nasal placode has two roles: as a signaling center for the lateral nasal skeleton and as a source of olfactory neurons and sensory epithelium.

Retinoic acid induces prostatic bud formation

Formation of prostatic buds from the urogenital sinus (UGS) to initiate prostate development requires localized action of several morphogenetic factors. This report reveals all-trans-retinoic acid (RA) to be a powerful inducer of mouse prostatic budding that is associated with reciprocal changes in expression of two regulators of budding: sonic hedgehog (Shh) and bone morphogenetic protein 4 (Bmp4). Localization of retinoid signaling and expression of RA synthesis, metabolism, and receptor genes in the UGS on embryonic days 14.5-17.5 implicate RA in the mechanism of bud initiation. In UGS organ culture, RA increased prostatic budding, increased Shh expression, and decreased Bmp4. Prostatic budding was stimulated in the absence of RA by recombinant SHH, by blocking BMP4 signaling with NOGGIN, or by combined treatment with SHH and NOGGIN in UGS organ culture media. These observations suggest that reciprocal changes in hedgehog and BMP signaling by RA may regulate bud initiation.

FGF signals from the nasal pit are necessary for normal facial morphogenesis

Fibroblast growth factors (FGFs) are required for brain, pharyngeal arch, suture and neural crest cell development and mutations in the FGF receptors have been linked to human craniofacial malformations. To study the functions of FGF during facial morphogenesis we locally perturb FGF signalling in the avian facial prominences with FGFR antagonists, foil barriers and FGF2 protein. We tested 4 positions with antagonist-soaked beads but only one of these induced a facial defect. Embryos treated in the lateral frontonasal mass, adjacent to the nasal slit developed cleft beaks. The main mechanisms were a block in proliferation and an increase in apoptosis in those areas that were most dependent on FGF signaling. We inserted foil barriers with the goal of blocking diffusion of FGF ligands out of the lateral edge of the frontonasal mass. The barriers induced an upregulation of the FGF target gene, SPRY2 compared to the control side. Moreover, these changes in expression were associated with deletions of the lateral edge of the premaxillary bone. To determine whether we could replicate the effects of the foil by increasing FGF levels, beads soaked in FGF2 were placed into the lateral edge of the frontonasal mass. There was a significant increase in proliferation and an expansion of the frontonasal mass but the skeletal defects were minor and not the same as those produced by the foil. Instead it is more likely that the foil repressed FGF signaling perhaps mediated by the increase in SPRY2 expression. In summary, we have found that the nasal slit is a source of FGF signals and the function of FGF is to stimulate proliferation in the cranial frontonasal mass. The FGF independent regions correlate with those previously determined to be dependent on BMP signaling. We propose a new model whereby, FGF-dependent microenvironments exist in the cranial frontonasal mass and caudal maxillary prominence and these flank BMP-dependent regions. Coordination of the proliferation in these regions leads ultimately to normal facial morphogenesis.

Embryonic development of Python sebae - I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs

This study explores the post-ovipositional craniofacial development of the African Rock Python (Python sebae). We first describe a staging system based on external characteristics and next use whole-mount skeletal staining supplemented with Computed tomography (CT) scanning to examine skeletal development. Our results show that python embryos are in early stages of organogenesis at the time of laying, with separate facial prominences and pharyngeal clefts still visible. Limb buds are also visible. By 11 days (stage 3), the chondrocranium is nearly fully formed; however, few intramembranous bones can be detected. One week later (stage 4), many of the intramembranous upper and lower jaw bones are visible but the calvaria are not present. Skeletal elements in the limbs also begin to form. Between stages 4 (day 18) and 7 (day 44), the complete set of intramembranous bones in the jaws and calvaria develops. Hindlimb development does not progress beyond stage 6 (33 days) and remains rudimentary throughout adult life. In contrast to other reptiles, there are two rows of teeth in the upper jaw. The outer tooth row is attached to the maxillary and premaxillary bones, whereas the inner row is attached to the pterygoid and palatine bones. Erupted teeth can be seen in whole-mount stage 10 specimens and are present in an unerupted, mineralized state at stage 7. Micro-CT analysis reveals that all the young membranous bones can be recognized even out of the context of the skull. These data demonstrate intrinsic patterning of the intramembranous bones, even though they form without a cartilaginous template. In addition, intramembranous bone morphology is established prior to muscle function, which can influence bone shape through differential force application. After careful staging, we conclude that python skeletal development occurs slowly enough to observe in good detail the early stages of craniofacial skeletogenesis. Thus, reptilian animal models will offer unique opportunities for understanding the early influences that contribute to perinatal bone shape.

Regulation of integrin α10 expression in chondrocytes by the transcription factors AP-2ε and Ets-1

Expression of integrin alpha10 is initiated at the beginning of chondrogenesis and continues throughout cartilage development in adult cartilage. In our study, we aim to identify regulatory sequences that control the cell-type specific expression of the human integrin alpha10 gene. Therefore, promoter constructs harboring 1139bp 5' of the transcriptional start site of the human integrin alpha10 gene were analyzed. Our experiments localized a promoter region that directs high levels of expression specifically in chondrocytes. A sequence analysis detected three consensus AP-2 binding sites within this functional domain. Functionality of these sites was tested and confirmed by cotransfection of AP-2 in a luciferase reporter assay. Interestingly, EMSA identified AP-2epsilon as the major AP-2 protein binding to the AP-2 consensus sequences. Additionally, Ets-1 was shown to be a positive regulator of the integrin alpha10 expression whereas Sox9 was irrelevant. Taken together, these results suggest that AP-2epsilon and Ets-1 are involved in the regulation of integrin alpha10 transcription in chondrocytes.

Control of retinoic acid synthesis and FGF expression in the nasal pit is required to pattern the craniofacial skeleton

Endogenous retinoids are important for patterning many aspects of the embryo including the branchial arches and frontonasal region of the embryonic face. The nasal placodes express retinaldehyde dehydrogenase-3 (RALDH3) and thus retinoids from the placode are a potential patterning influence on the developing face. We have carried out experiments that have used Citral, a RALDH antagonist, to address the function of retinoid signaling from the nasal pit in a whole embryo model. When Citral-soaked beads were implanted into the nasal pit of stage 20 chicken embryos, the result was a specific loss of derivatives from the lateral nasal prominences. Providing exogenous retinoic acid residue development of the beak demonstrating that most Citral-induced defects were produced by the specific blocking of RA synthesis. The mechanism of Citral effects was a specific increase in programmed cell death on the lateral (lateral nasal prominence) but not the medial side (frontonasal mass) of the nasal pit. Gene expression studies were focused on the Bone Morphogenetic Protein (BMP) pathway, which has a well-established role in programmed cell death. Unexpectedly, blocking RA synthesis decreased rather than increased Msx1, Msx2, and Bmp4 expression. We also examined cell survival genes, the most relevant of which was Fgf8, which is expressed around the nasal pit and in the frontonasal mass. We found that Fgf8 was not initially expressed along the lateral side of the nasal pit at the start of our experiments, whereas it was expressed on the medial side. Citral prevented upregulation of Fgf8 along the lateral edge and this may have contributed to the specific increase in programmed cell death in the lateral nasal prominence. Consistent with this idea, exogenous FGF8 was able to prevent cell death, rescue most of the morphological defects and was able to prevent a decrease in retinoic acid receptorbeta (Rarbeta) expression caused by Citral. Together, our results demonstrate that endogenous retinoids act upstream of FGF8 and the balance of these two factors is critical for regulating programmed cell death and morphogenesis in the face. In addition, our data suggest a novel role for endogenous retinoids from the nasal pit in controlling the precise downregulation of FGF in the center of the frontonasal mass observed during normal vertebrate development.

A new origin for the maxillary jaw

One conserved feature of craniofacial development is that the first pharyngeal arch has two components, the maxillary and mandibular, which then form the upper and lower jaws, respectively. However, until now, there have been no tests of whether the maxillary cells originate entirely within the first pharyngeal arch or whether they originate in a separate condensation, cranial to the first arch. We therefore constructed a fate map of the pharyngeal arches and environs with a series of dye injections into stage 13-17 chicken embryos. We found that from the earliest stage examined, the major contribution to the maxillary bud is from post-optic mesenchyme with a relatively minor contribution from the maxillo-mandibular cleft. Cells labeled within the first pharyngeal arch contributed exclusively to the mandibular prominence. Gene expression data showed that there were different molecular codes for the cranial and caudal maxillary prominence. Two of the genes examined, Rarbeta (retinoic acid receptor beta) and Bmp4 (bone morphogenetic protein) were expressed in the post-optic mesenchyme and epithelium prior to formation of the maxillary prominence and then were restricted to the cranial half of the maxillary prominence. In order to determine the derivatives of the maxillary prominence, we performed focal injections of CM-DiI into the stage 24 maxillary prominence. Labeled cells contributed to the maxillary, palatine, and jugal bones, but not the other elements of the upper beak, the premaxilla and prenasal cartilage. We also determined that the cranial cells give rise to more distal parts of the upper beak, whereas caudal cells form proximal structures. Grafts of stage 24 maxillary prominences were also analyzed to determine skeletal derivatives and these results concurred with the DiI maps. These early and later fate maps indicate that the maxillary prominence and its skeletal derivatives are not derived from the first pharyngeal arch but rather from a separate maxillary condensation that occurs between the eye and the maxillo-mandibular cleft. These data also suggest that during evolution, recession of the first pharyngeal arch-derived palatoquadrate cartilage to a more proximal position gave way to the bony upper jaw of amniotes.

Upper beak truncation in chicken embryos with the cleft primary palate mutation is due to an epithelial defect in the frontonasal mass

In this study, we used the chicken mutant strain known as cleft primary palate (cpp) to study the mechanisms of beak outgrowth. cpp mutants have complete truncation of the upper beak with normal development of the lower beak. Based on structural analysis and grafts of facial prominences, we localized the defect to the frontonasal mass and its derivatives. Several explanations that would account for the outgrowth defect were investigated, including abnormal expression of genes in the frontonasal epithelium, intrinsic defects in epithelium and/or mesenchyme defects in epithelial-mesenchymal signalling, a localized decrease in cell proliferation or a localized increase in programmed cell death. One of the genes expressed in the frontonasal epithelial growth zone, Fgf8, failed to down-regulate and was maintained for at least 48 hr beyond the time when down-regulation normally occurs. Recombination experiments further illustrated that the frontonasal mass epithelium was abnormal in the cpp mutants, whereas mutant mesenchyme was capable of normal outgrowth when combined with wild-type epithelium. Cell proliferation was not decreased in mutant embryos nor was cell death initially increased. Later, at stages 31-32, when the prenasal cartilage begins directed outgrowth, there was an increase in cell death within the mutant upper but not lower beak cartilage. The cpp beak truncation, therefore, is due to an epithelial defect in the frontonasal mass that is coincident with a failure to down-regulate expression of Fgf8.

Protective Effects of Prenatal Administration of Folic Acid on Retinoic Acid-Induced Cellular Damages of Meckel's Cartilage in Rats

Craniofacial malformations are among the most common congenital deformities. Meckel's cartilage plays a major role in the development of the mandible and is highly susceptible to maternal teratogenic drug use. We therefore investigated possible protective effects of prenatal administration of folic acid on a retinoic-acid induced maxillofacial defect model. Sprague-Dawley pregnant female rats (n=36) were used in this study. Retinoic acid was administered orally at the dose of 40, 60, or 80 mg/kg respectively on gestational day 8. Folic acid of 4.0 mg/kg was injected intraperitoneally on 7th, 8th and 9th days of pregnancy. Animals were sacrificed on the day 17th. Administration of retinoic acid at all doses resulted in statistically significant decreases in mean fetal weight and mean fetal height and the increase in mortality rate, and caused severe ultrastructural damages in Meckel's cartilage. Folic acid administration prevented the decrease in mean fetal weight and height of the embryos treated with retinoic acid of 40 mg/kg. In addition, there was a marked decrease in the number of degenerated chondrocytes and an improvement in the structure of granular endoplasmic reticulum along with intact nuclei. We conclude that folic acid has protective effects on retinoic acid-induced intracellular damages in Meckel's cartilage.

Endogenous bone morphogenetic proteins regulate outgrowth and epithelial survival during avian lip fusion

Our expression studies of bone morphogenetic proteins (BMPs) and Noggin (a BMP antagonist) in the embryonic chicken face suggested that BMP signals were important for closure of the upper lip or primary palate. We noted that Noggin expression was restricted to the frontonasal mass epithelium but was reduced at the corners of the frontonasal mass (globular processes) just prior to fusion with the adjacent maxillary prominences. We therefore performed gain- and loss-of-function experiments to determine the role of BMPs in lip formation. Noggin treatment led to reduced proliferation and outgrowth of the frontonasal mass and maxillary prominences and ultimately to the deletion of the maxillary and palatine bones. The temporary block in BMP signalling in the mesenchyme also promoted epithelial survival. Noggin treatment also upregulated expression of endogenous BMPs, therefore we investigated whether increasing BMP levels would lead to the same phenotype. A BMP2 bead was implanted into the globular process and a similar phenotype to that produced by Noggin resulted. However, instead of a decrease in proliferation, defects were caused by increased programmed cell death, first in the epithelium and then in the mesenchyme. Programmed cell death was induced primarily in the lateral frontonasal mass with very little cell death medial to the bead. The asymmetric cell death pattern was correlated with a rapid induction of Noggin in the same embryos, with transcripts complementary to the regions with increased cell death. We have demonstrated a requirement for endogenous BMP in the proliferation of facial mesenchyme and that mesenchymal signals promote either survival or thinning of the epithelium. We furthermore demonstrated in vivo that BMP homeostasis is regulated by increasing expression of ligand or antagonist and that such mechanisms may help to protect the embryo from changes in growth factor levels during development or after exposure to teratogens.