The genesis of cartilage size and shape during development and evolution

Specificities in the Structure of the Cartilage of Patients with Advanced Stages of Developmental Dysplasia of the Hip

Developmental dysplasia of the hip (DDH) presents varying degrees of femoral head dislocation, with severe cases leading to the formation of a new articular surface on the external side of the iliac bone-the neoacetabulum. Despite conventional understanding suggesting otherwise, a tissue resembling hyaline cartilage is found in the neoacetabulum and acetabulum of Crowe III and IV patients, indicating a potential for hyaline cartilage development without mechanical pressure. To test this theory, acetabular and femoral head cartilage obtained from patients with DDH was stained with hematoxylin-eosin and toluidine blue. The immunohistochemical analysis for collagen types II and VI and aggrecan was performed, as well as delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) analysis on a 7.0 T micro-MRI machine. The results obtained from DDH patients were compared to those of the control groups. Hyaline cartilage was found in the neoacetabulum and the acetabulum of patients with DDH. The nature of the tissue was confirmed with both the histological and the MRI analyses. The results of this study proved the presence of hyaline cartilage in patients with DDH at anatomical regions genetically predisposed to be bone tissue and at regions that are not subjected to mechanical stress. This is the first time that the neoacetabular cartilage of patients with advanced stages of DDH has been characterized in detail.

Species-specific sensitivity to TGFβ signaling and changes to the Mmp13 promoter underlie avian jaw development and evolution

Precise developmental control of jaw length is critical for survival, but underlying molecular mechanisms remain poorly understood. The jaw skeleton arises from neural crest mesenchyme (NCM), and we previously demonstrated that these progenitor cells express more bone-resorbing enzymes including Matrix metalloproteinase 13 (Mmp13) when they generate shorter jaws in quail embryos versus longer jaws in duck. Moreover, if we inhibit bone resorption or Mmp13, we can increase jaw length. In the current study, we uncover mechanisms establishing species-specific levels of Mmp13 and bone resorption. Quail show greater activation of and sensitivity to transforming growth factor beta (TGFβ) signaling than duck; where intracellular mediators like SMADs and targets like Runt-related transcription factor 2 (Runx2), which bind Mmp13, become elevated. Inhibiting TGFβ signaling decreases bone resorption, and overexpressing Mmp13 in NCM shortens the duck lower jaw. To elucidate the basis for this differential regulation, we examine the Mmp13 promoter. We discover a SMAD-binding element and single nucleotide polymorphisms (SNPs) near a RUNX2-binding element that distinguish quail from duck. Altering the SMAD site and switching the SNPs abolish TGFβ sensitivity in the quail Mmp13 promoter but make the duck promoter responsive. Thus, differential regulation of TGFβ signaling and Mmp13 promoter structure underlie avian jaw development and evolution.

Chondroitin/dermatan sulfate glycosyltransferase genes are essential for craniofacial development

Chondroitin/dermatan sulfate (CS/DS) proteoglycans are indispensable for animal development and homeostasis but the large number of enzymes involved in their biosynthesis have made CS/DS function a challenging problem to study genetically. In our study, we generated loss-of-function alleles in zebrafish genes encoding CS/DS biosynthetic enzymes and characterized the effect on development in single and double mutants. Homozygous mutants in chsy1, csgalnact1a, csgalnat2, chpfa, ust and chst7, respectively, develop to adults. However, csgalnact1a^-/- fish develop distinct craniofacial defects while the chsy1^-/- skeletal phenotype is milder and the remaining mutants display no gross morphological abnormalities. These results suggest a high redundancy for the CS/DS biosynthetic enzymes and to further reduce CS/DS biosynthesis we combined mutant alleles. The craniofacial phenotype is further enhanced in csgalnact1a^-/-;chsy1^-/- adults and csgalnact1a^-/-;csgalnact2^-/- larvae. While csgalnact1a^-/-;csgalnact2^-/- was the most affected allele combination in our study, CS/DS is still not completely abolished. Transcriptome analysis of chsy1^-/-, csgalnact1a^-/-and csgalnact1a^-/-;csgalnact2^-/- larvae revealed that the expression had changed in a similar way in the three mutant lines but no differential expression was found in any of fifty GAG biosynthesis enzymes identified. Thus, zebrafish larvae do not increase transcription of GAG biosynthesis genes as a consequence of decreased CS/DS biosynthesis. The new zebrafish lines develop phenotypes similar to clinical characteristics of several human congenital disorders making the mutants potentially useful to study disease mechanisms and treatment.

The components of the extracellular matrix are crucial for interactions and communication between cells during animal development and disease progression. One major component of the extracellular matrix is chondroitin sulfate/dermatan sulfate (CS/DS) proteoglycans, which support and modify cell functions and tissue homeostasis. The biosynthesis of CS/DS is complex and no genetic models have been developed to specifically reduce CS/DS in the zebrafish model organism. We have used CRISPR/Cas9 technology to knock out key CS/DS biosynthesis genes. We find that knocking out single genes rarely causes major effects on zebrafish morphology and viability, but by combining several knockout alleles we could observe malformations in the zebrafish craniofacial skeleton. In addition, one combination of alleles was embryonic lethal. Our findings describe the role of CS/DS in the development of the head skeleton and give insights in the regulation of genes involved in CS/DS biosynthesis. The zebrafish mutants generated in this study can be used as tools to further study human diseases caused by mutations in CS/DS biosynthesis enzymes.

Early shape divergence of developmental trajectories in the jaw of galeomorph sharks

Background

The onset of morphological differences between related groups can be tracked at early stages during embryological development. This is expressed in functional traits that start with minor variations, but eventually diverge to defined specific morphologies. Several processes during this period, like proliferation, remodelling, and apoptosis for instance, can account for the variability observed between related groups. Morphological divergence through development is often associated with the hourglass model, in which early stages display higher variability and reach a conserved point with reduced variability from which divergence occurs again to the final phenotype.

Results

Here we explored the patterns of developmental shape changes in the lower jaw of two shark species, the bamboo shark (Chiloscyllium punctatum) and the catshark (Scyliorhinus canicula). These two species present marked differences in their foraging behaviour, which is reflected in their adult jaw morphology. By tracing the developmental sequence of the cartilage condensation, we identified the onset of cartilage for both species at around stage 31. Other structures that developed later without a noticeable anlage were the labial cartilages, which appear at around stage 33. We observed that the lower jaw displays striking differences in shape from the earliest moments, without any overlap in shape through the compared stages.

Conclusions

The differences observed are also reflected in the functional variation in feeding mechanism between both species. Likewise, the trajectory analysis shows that the main differences are in the magnitude of the shape change through time. Both species follow a unique trajectory, which is explained by the timing between stages.

Exceptional Changes in Skeletal Anatomy under Domestication: The Case of Brachycephaly

"Brachycephaly" is generally considered a phenotype in which the facial part of the head is pronouncedly shortened. While brachycephaly is characteristic for some domestic varieties and breeds (e.g., Bulldog, Persian cat, Niata cattle, Anglo-Nubian goat, Middle White pig), this phenotype can also be considered pathological. Despite the superficially similar appearance of "brachycephaly" in such varieties and breeds, closer examination reveals that "brachycephaly" includes a variety of different cranial modifications with likely different genetic and developmental underpinnings and related with specific breed histories. We review the various definitions and characteristics associated with brachycephaly in different domesticated species. We discern different types of brachycephaly ("bulldog-type," "katantognathic," and "allometric" brachycephaly) and discuss morphological conditions related to brachycephaly, including diseases (e.g., brachycephalic airway obstructive syndrome). Further, we examine the complex underlying genetic and developmental processes and the culturally and developmentally related reasons why brachycephalic varieties may or may not be prevalent in certain domesticated species. Knowledge on patterns and mechanisms associated with brachycephaly is relevant for domestication research, veterinary and human medicine, as well as evolutionary biology, and highlights the profound influence of artificial selection by humans on animal morphology, evolution, and welfare.

MicroRNA function in craniofacial bone formation, regeneration and repair

Bone formation in the craniofacial complex is regulated by cranial neural crest (CNC) and mesoderm-derived cells. Different elements of the developing skull, face, mandible, maxilla (jaws) and nasal bones are regulated by an array of transcription factors, signaling molecules and microRNAs (miRs). miRs are molecular modulators of these factors and act to restrict their expression in a temporal-spatial mechanism. miRs control the different genetic pathways that form the craniofacial complex. By understanding how miRs function in vivo during development they can be adapted to regenerate and repair craniofacial genetic anomalies as well as bone diseases and defects due to traumatic injuries. This review will highlight some of the new miR technologies and functions that form new bone or inhibit bone regeneration.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

Growth trajectories of prenatal embryos of the deep-sea shark Chlamydoselachus anguineus (Chondrichthyes)

Chlamydoselachus anguineus, Garman 1884, commonly called the frilled shark, is a deep-sea shark species occurring up to depths of 1300 m. It is assumed to represent an ancient morphotype of sharks (e.g., terminal mouth opening, more than five gill slits) and thus is often considered to represent plesiomorphic traits for sharks. Therefore, its early ontogenetic developmental traits are important for understanding the evolution of its particular phenotype. Here, we established six stages for prenatal embryos and used linear measurements and geometric morphometrics to analyse changes in shape and size as well as their timing during different embryonic stages. Our results show a change in head shape and a relocation of the mouth opening at a late stage of development. We also detected a negative allometric growth of the head and especially the eye compared to the rest of the body and a sexual dimorphism in total body length, which differs from the known data for adults. A multivariate analysis of covariance shows a significant interaction of shape related to the logarithm of centroid size and developmental stage. Geometric morphometrics results indicate that the head shape changes as a covariate of body size while not accounting for differences between sexes. The growth pattern of stages 32 and 33 indicates a shift in head shape, thus highlighting the moment in development when the jaws start to elongate anteriorly to finally achieve the adult condition of terminal mouth opening rather than retaining the early embryonic subterminal position as is typical for sharks. Thus, the antero-terminal mouth opening of the frilled shark has to be considered a derived feature.

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.

How the diversity of the faces arises

BACKGROUND

The evolution of the face is crucial for each species to adapt to different diets, environments, and in some species, to promote social interaction. The diversity in the shapes of the face results from divergence in the process of facial development that begins during early embryonic development.

HIGHLIGHTS

Here we review the recent advancements in the understanding of the genetic, epigenetic, molecular, and cellular basis of facial diversity. We also review the robustness of facial development and how it relates to the evolution of the face. Finally, we discuss the current challenges in achieving a deeper understanding of facial diversity.

CONCLUSION

We have gained much knowledge with respect to cis-regulatory elements, gene expression, cellular behavior, and the physical forces in facial development in the past two decades. Significant interdisciplinary work is needed to integrate these varied pieces of information into a complete picture of how the diversity of faces arises.

Origin of the avian predentary and evidence of a unique form of cranial kinesis in Cretaceous ornithuromorphs

The avian predentary is a small skeletal structure located rostral to the paired dentaries found only in Mesozoic ornithuromorphs. The evolution and function of this enigmatic element is unknown. Skeletal tissues forming the predentary and the lower jaws in the basal ornithuromorph Yanornis martini are identified using computed-tomography, scanning electron microscopy, and histology. On the basis of these data, we propose hypotheses for the development, structure, and function of this element. The predentary is composed of trabecular bone. The convex caudal surface articulates with rostromedial concavities on the dentaries. These articular surfaces are covered by cartilage, which on the dentaries is divided into 3 discrete patches: 1 rostral articular cartilage and 2 symphyseal cartilages. The mechanobiology of avian cartilage suggests both compression and kinesis were present at the predentary-dentary joint, therefore suggesting a yet unknown form of avian cranial kinesis. Ontogenetic processes of skeletal formation occurring within extant taxa do not suggest the predentary originates within the dentaries, nor Meckel's cartilage. We hypothesize that the predentary is a biomechanically induced sesamoid that arose within the soft connective tissues located rostral to the dentaries. The mandibular canal hosting the alveolar nerve suggests that the dentary teeth and predentary of Yanornis were proprioceptive. This whole system may have increased foraging efficiency. The Mesozoic avian predentary apparently coevolved with an edentulous portion of the premaxilla, representing a unique kinetic morphotype that combined teeth with a small functional beak and persisted successfully for ∼60 million years.

Molecular and cellular mechanisms underlying the evolution of form and function in the amniote jaw

The amniote jaw complex is a remarkable amalgamation of derivatives from distinct embryonic cell lineages. During development, the cells in these lineages experience concerted movements, migrations, and signaling interactions that take them from their initial origins to their final destinations and imbue their derivatives with aspects of form including their axial orientation, anatomical identity, size, and shape. Perturbations along the way can produce defects and disease, but also generate the variation necessary for jaw evolution and adaptation. We focus on molecular and cellular mechanisms that regulate form in the amniote jaw complex, and that enable structural and functional integration. Special emphasis is placed on the role of cranial neural crest mesenchyme (NCM) during the species-specific patterning of bone, cartilage, tendon, muscle, and other jaw tissues. We also address the effects of biomechanical forces during jaw development and discuss ways in which certain molecular and cellular responses add adaptive and evolutionary plasticity to jaw morphology. Overall, we highlight how variation in molecular and cellular programs can promote the phenomenal diversity and functional morphology achieved during amniote jaw evolution or lead to the range of jaw defects and disease that affect the human condition.

Role of Spreader Flaps in Rhinoplasty: Analysis of Patients Undergoing Correction for Severe Septal Deviation with Long-Term Follow-Up

INTRODUCTION

The aim of this randomized controlled study was to analyze the long-term results of patients undergoing rhinoplasty because of severe septal deviation and to evaluate the stability of results.

MATERIALS AND METHODS

The study was performed with a randomized design. Patients were randomly divided into four groups: group 1, spreader flaps were used in combination with spreader grafts; group 2, spreader flaps were used alone; group 3, spreader grafts were used alone; and group 4, neither spreader flaps nor grafts flaps were used. Patients answered the Italian version of the FACE-Q rhinoplasty module. Anthropometric measurements were performed by AutoCAD for MAC. We determined the angle of deviation, and we compared the pre- and postoperative angles and compared patient satisfaction in the four groups using the Chi-squared test for unpaired data. Two plastic surgeons reviewed all the postoperative photographs of the study patients and rated the photographs on a scale of 1 to 5.

RESULTS

A total of 264 patients who underwent primary rhinoplasty between January 2010 and September 2016 satisfied the inclusion criteria and were finally enrolled in this study. Anthropometric measurements revealed statistically significant differences (P < 0.01) between the preoperative and postoperative values for the angle of septal deviation in group 1 versus the other groups. Over the long-term follow-up, group 1 maintained an angle close to 180 degrees (P < 0.01). Group 1 and group 3 were more satisfied compared with groups 2 and 4 (P < 0.01). According to evaluations by the 2 reviewers, group 1 and group 3 were the most satisfactory outcomes (P < 0.01).

CONCLUSIONS

This was the first randomized study to show that the combined use of the spreader flap and spreader graft is the best choice for a good long-term outcome and durable correction of septal deviation.

LEVEL OF EVIDENCE IV

This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .

Anatomy and Ontogeny of the Mandibular Symphysis in Alligator mississippiensis

Crocodylians evolved some of the most characteristic skulls of the animal kingdom with specializations for semiaquatic and ambush lifestyles, resulting in a feeding apparatus capable of tolerating high biomechanical loads and bite forces and a head with a derived sense of trigeminal-nerve-mediated touch. The mandibular symphysis accommodates these specializations being both at the end of a biomechanical lever and an antenna for sensation. Little is known about the anatomy of the crocodylian mandibular symphysis, hampering our understanding of form, function, and evolution of the joint in extant and extinct lineages. We explore mandibular symphysis anatomy of an ontogenetic series of Alligator mississippiensis using imaging, histology, and whole mount methods. Complex sutural ligaments emanating about a midline-fused Meckel's cartilage bridge the symphysis. These tissues organize during days 37-42 of in ovo development. However, interdigitations do not manifest until after hatching. These soft tissues leave a hub and spoke-like bony morphology of the symphyseal plate, which never fuses. Interdigitation morphology varies within the symphysis suggesting differential loading about the joint. Neurovascular canals extend throughout the mandibles to alveoli, integument, and bone adjacent to the symphysis. These features suggest the Alligator mandibular symphysis offers compliance in an otherwise rigid skull. We hypothesize a fused Meckel's cartilage offers stiffness in hatchling mandibles prior to the development of organized sutural ligaments and mineralized bone while offering a scaffold for somatic growth. The porosity of the dentaries due to neurovascular tissues likely allows transmission of sensory and proprioceptive information from the surroundings and the loaded symphysis. Anat Rec, 302:1696-1708, 2019. © 2019 American Association for Anatomy.

FGF and TGFβ signaling link form and function during jaw development and evolution

How does form arise during development and change during evolution? How does form relate to function, and what enables embryonic structures to presage their later use in adults? To address these questions, we leverage the distinct functional morphology of the jaw in duck, chick, and quail. In connection with their specialized mode of feeding, duck develop a secondary cartilage at the tendon insertion of their jaw adductor muscle on the mandible. An equivalent cartilage is absent in chick and quail. We hypothesize that species-specific jaw architecture and mechanical forces promote secondary cartilage in duck through the differential regulation of FGF and TGFβ signaling. First, we perform transplants between chick and duck embryos and demonstrate that the ability of neural crest mesenchyme (NCM) to direct the species-specific insertion of muscle and the formation of secondary cartilage depends upon the amount and spatial distribution of NCM-derived connective tissues. Second, we quantify motility and build finite element models of the jaw complex in duck and quail, which reveals a link between species-specific jaw architecture and the predicted mechanical force environment. Third, we investigate the extent to which mechanical load mediates FGF and TGFβ signaling in the duck jaw adductor insertion, and discover that both pathways are mechano-responsive and required for secondary cartilage formation. Additionally, we find that FGF and TGFβ signaling can also induce secondary cartilage in the absence of mechanical force or in the adductor insertion of quail embryos. Thus, our results provide novel insights on molecular, cellular, and biomechanical mechanisms that couple musculoskeletal form and function during development and evolution.

FoxO6 regulates Hippo signaling and growth of the craniofacial complex

The mechanisms that regulate post-natal growth of the craniofacial complex and that ultimately determine the size and shape of our faces are not well understood. Hippo signaling is a general mechanism to control tissue growth and organ size, and although it is known that Hippo signaling functions in neural crest specification and patterning during embryogenesis and before birth, its specific role in postnatal craniofacial growth remains elusive. We have identified the transcription factor FoxO6 as an activator of Hippo signaling regulating neonatal growth of the face. During late stages of mouse development, FoxO6 is expressed specifically in craniofacial tissues and FoxO6-/- mice undergo expansion of the face, frontal cortex, olfactory component and skull. Enlargement of the mandible and maxilla and lengthening of the incisors in FoxO6-/- mice are associated with increases in cell proliferation. In vitro and in vivo studies demonstrated that FoxO6 activates Lats1 expression, thereby increasing Yap phosphorylation and activation of Hippo signaling. FoxO6-/- mice have significantly reduced Hippo Signaling caused by a decrease in Lats1 expression and decreases in Shh and Runx2 expression, suggesting that Shh and Runx2 are also linked to Hippo signaling. In vitro, FoxO6 activates Hippo reporter constructs and regulates cell proliferation. Furthermore PITX2, a regulator of Hippo signaling is associated with Axenfeld-Rieger Syndrome causing a flattened midface and we show that PITX2 activates FoxO6 expression. Craniofacial specific expression of FoxO6 postnatally regulates Hippo signaling and cell proliferation. Together, these results identify a FoxO6-Hippo regulatory pathway that controls skull growth, odontogenesis and face morphology.

The impact of dietary consistency on structural craniofacial components: Temporomandibular joint/condyle, condylar cartilage, alveolar bone and periodontal ligament. A systematic review and meta-analysis in experimental in vivo research

OBJECTIVE

The aim of this systematic review was to provide a comprehensive synthesis of available evidence evaluating the effect of dietary loading on temporomandibular joint/condyle, condylar cartilage, alveolar bone of the mandible and the periodontal ligament in healthy mice and rats.

DESIGN

Medline via PubMed, EMBASE and Open Grey databases were searched for published and unpublished literature. Search terms included "mandiblular condyle", "alveolar bone", "temporomandibular joint", "condylar cartilage", "periodontal ligament", "rat", "mice". After data extraction, risk of bias (SYRCLE) and reporting quality (ARRIVE) were assessed. Random effects meta-analyses were performed for the outcomes of interest where applicable.

RESULTS

A total of 33 relevant articles were considered in the systematic review, while only 6 studies were included in the quantitative synthesis. Risk of Bias in all studies was judged to be unclear to high overall, while reporting quality was suboptimal. Comparing soft to hard diet animals, significantly reduced anteroposterior condylar length (4 studies, weighted mean difference: -0.40 mm; 95% CI: -0.47, -0.32; p < 0.001) and width (4 studies, weighted mean difference: -0.043 mm; 95% CI: -0.51, -0.36; p < 0.001) were found in rats. Decreased anteroposterior condylar dimensions were detected for mice as well (2 studies, weighted mean difference: -0.049; 95% CI: -0.56, -0.43; p < 0.001).

CONCLUSIONS

Overall, there was strong evidence to suggest a significant effect of soft diet on reduced condylar dimensions in rodents; however, there is need for further high quality experimental studies to inform current knowledge on condylar cartilage, alveolar bone and periodontal ligament related outcomes.

Neural crest and the origin of species-specific pattern

For well over half of the 150 years since the discovery of the neural crest, the special ability of these cells to function as a source of species-specific pattern has been clearly recognized. Initially, this observation arose in association with chimeric transplant experiments among differentially pigmented amphibians, where the neural crest origin for melanocytes had been duly noted. Shortly thereafter, the role of cranial neural crest cells in transmitting species-specific information on size and shape to the pharyngeal arch skeleton as well as in regulating the timing of its differentiation became readily apparent. Since then, what has emerged is a deeper understanding of how the neural crest accomplishes such a presumably difficult mission, and this includes a more complete picture of the molecular and cellular programs whereby neural crest shapes the face of each species. This review covers studies on a broad range of vertebrates and describes neural-crest-mediated mechanisms that endow the craniofacial complex with species-specific pattern. A major focus is on experiments in quail and duck embryos that reveal a hierarchy of cell-autonomous and non-autonomous signaling interactions through which neural crest generates species-specific pattern in the craniofacial integument, skeleton, and musculature. By controlling size and shape throughout the development of these systems, the neural crest underlies the structural and functional integration of the craniofacial complex during evolution.

Evolution of beak morphology in the Ground Tit revealed by comparative transcriptomics

Background

Beak morphology exhibits considerable adaptive plasticity in birds, which results in highly varied or specialized forms in response to variations in ecology and life history. As the only parid species endemic to the Qinghai-Tibet Plateau, the Ground Tit (Parus humilis) has evolved a distinctly long and curved beak from other parids. An integration of morphometrics, phylogenetics, transcriptomics and embryology allows us to address the evolutionary and developmental mechanisms of the adaptive beak structure observed in the Ground Tit.

Results

A morphometric approach quantified that the Ground Tit has a comparatively longer and more decurved upper beaks than other parids. We estimated that the ancestor of the Ground Tit likely had a short straight upper beak similar to most current recognized parid species using an ancestral state reconstruction. This morphological specialization is considered an adaptation to its ground-oriented behavior on the high plateau. To identify genetic mechanisms behind this adaptive change, a comparative transcriptomic analysis was applied between the Ground Tit and its closely related species, the Great Tit (Parus major). We detected that 623 genes were significantly differentially expressed in embryonic upper beaks between the two species, 17 of which were functionally annotated to correlate with bone development and morphogenesis, although genes related to bone development were not found to undergo accelerated evolution in the Ground Tit. RT-qPCR validation confirmed differential expression of five out of eight genes that were selected from the 17 genes. Subsequent functional assays in chicken embryos demonstrated that two of these genes, FGF13 and ITGB3, may affect beak morphology by modulating levels of osteoblasts and osteoclasts.

Conclusions

Our results provide preliminary evidence that development of the long decurved beak of the Ground Tit is likely regulated by transcriptional activities of multiple genes coordinating osteoblasts and osteoclasts. The integration of multiple approaches employed here sheds light on ecological and genetic mechanisms in the evolution of avian morphology.

Robo signaling regulates the production of cranial neural crest cells

Slit/Robo signaling plays an important role in the guidance of developing neurons in developing embryos. However, it remains obscure whether and how Slit/Robo signaling is involved in the production of cranial neural crest cells. In this study, we examined Robo1 deficient mice to reveal developmental defects of mouse cranial frontal and parietal bones, which are derivatives of cranial neural crest cells. Therefore, we determined the production of HNK1+ cranial neural crest cells in early chick embryo development after knock-down (KD) of Robo1 expression. Detection of markers for pre-migratory and migratory neural crest cells, PAX7 and AP-2α, showed that production of both was affected by Robo1 KD. In addition, we found that the transcription factor slug is responsible for the aberrant delamination/EMT of cranial neural crest cells induced by Robo1 KD, which also led to elevated expression of E- and N-Cadherin. N-Cadherin expression was enhanced when blocking FGF signaling with dominant-negative FGFR1 in half of the neural tube. Taken together, we show that Slit/Robo signaling influences the delamination/EMT of cranial neural crest cells, which is required for cranial bone development.

Abnormal WNT5A Signaling Causes Mandibular Hypoplasia in Robinow Syndrome

The study of rare genetic diseases provides valuable insights into human gene function. Here, we investigate dominant Robinow syndrome (RS), which affects the WNT5A signaling pathway. Autosomal dominant RS is caused by missense mutations in WNT5A or nonsense mutations in the adaptor protein DVL1 or DVL3. The recessive form of the disease is caused by loss-of-function mutations in the receptor ROR2. RS is characterized by hypertelorism, midface, and mandibular hypoplasia. Here, we focus on the missense mutations in WNT5A, since the impact on function is difficult to predict from in silico analysis. We used chicken embryo to express wild-type or 2 mutant versions of human WNT5A in the mandible and then examined the morphologic, cellular, and molecular effects. The 3 experimental viruses—wt WNT5A, WNT5AC83S, or WNT5AC182R—all caused shortening of the mandible on the injected side as compared with GFP controls. Although the phenotypes initially appeared similar, we uncovered specific disruption of chondrocyte polarity and shape, inhibition of cell migration, differences in target gene expression, and absence of JNK signaling only in the presence of mutant viruses. In addition, the missense mutations do not appear to block receptor binding, since in paracrine experiments, the mutant protein inhibits cell migration. In this study, we ruled out a straightforward gain or loss of function caused by the WNT5A missense mutations. Instead, the mutations are likely redirecting WNT signaling away from JNK-PCP toward other noncanonical pathways. We conclude that in RS, WNT5A missense mutations have dominant neomorphic effects that interfere with the function of the wild-type protein.

Effect of craniofacial genotype on the relationship between morphology and feeding performance in cichlid fishes

The relationship between morphology and performance is complex, but important for understanding the adaptive nature of morphological variation. Recent studies have sought to better understand this system by illuminating the interconnectedness of different functional systems; however, the role of genetics is often overlooked. In this study, we attempt to gain insights into this relationship by examining the effect of genotypic variation at putative craniofacial loci on the relationship between morphology and feeding performance in cichlids. We studied two morphologically disparate species, as well as a morphologically intermediate hybrid population. We assessed feeding performance, jaw protrusion, and general facial morphology for each fish. We also genotyped hybrid animals at six previously identified craniofacial loci. Cichlid species were found to differ in facial geometry, kinematic morphology, and performance. Significant correlations were also noted between these variables; however, the explanatory power of facial geometry in predicting performance was relatively poor. Notably, when hybrids were grouped by genotype, the relationship between shape and performance improved. This relationship was especially robust in animals with the specialist allele at sox9b, a well-characterized regulator of craniofacial development. These data suggest a novel role for genotype in influencing complex relationships between form and function.

Oriented clonal cell dynamics enables accurate growth and shaping of vertebrate cartilage

Cartilaginous structures are at the core of embryo growth and shaping before the bone forms. Here we report a novel principle of vertebrate cartilage growth that is based on introducing transversally-oriented clones into pre-existing cartilage. This mechanism of growth uncouples the lateral expansion of curved cartilaginous sheets from the control of cartilage thickness, a process which might be the evolutionary mechanism underlying adaptations of facial shape. In rod-shaped cartilage structures (Meckel, ribs and skeletal elements in developing limbs), the transverse integration of clonal columns determines the well-defined diameter and resulting rod-like morphology. We were able to alter cartilage shape by experimentally manipulating clonal geometries. Using in silico modeling, we discovered that anisotropic proliferation might explain cartilage bending and groove formation at the macro-scale.

DOI:http://dx.doi.org/10.7554/eLife.25902.001

ISLET1-Dependent β-Catenin/Hedgehog Signaling Is Required for Outgrowth of the Lower Jaw

Mandibular patterning information initially resides in the epithelium during development. However, how transcriptional regulation of epithelium-derived signaling controls morphogenesis of the mandible remains elusive. Using Shh^Cre to target the mandibular epithelium, we ablated transcription factor Islet1, resulting in a distally truncated mandible via unbalanced cell apoptosis and decreased cell proliferation in the distal mesenchyme. Loss of Islet1 caused a lack of cartilage at the distal tip, leading the fusion of two growing mandibular elements surrounding the rostral process of Meckel's cartilage. Loss of Islet1 results in dysregulation of mesenchymal genes important for morphogenesis of the mandibular arch. We revealed that Islet1 is required for the activation of epithelial β-catenin signaling via repression of Wnt antagonists. Reactivation of β-catenin in the epithelium of the Islet1 mutant rescued mandibular morphogenesis through sonic hedgehog (SHH) signaling to the mesenchyme. Furthermore, overexpression of a transgenic hedgehog ligand in the epithelium also partially restored outgrowth of the mandible. These data reveal functional roles for an ISLET1-dependent network integrating β-catenin/SHH signals in mesenchymal cell survival and outgrowth of the mandible during development.

Cranial joint histology in the mallard duck (Anas platyrhynchos): new insights on avian cranial kinesis

The evolution of avian cranial kinesis is a phenomenon in part responsible for the remarkable diversity of avian feeding adaptations observable today. Although osteological, developmental and behavioral features of the feeding system are frequently studied, comparatively little is known about cranial joint skeletal tissue composition and morphology from a microscopic perspective. These data are key to understanding the developmental, biomechanical and evolutionary underpinnings of kinesis. Therefore, here we investigated joint microstructure in juvenile and adult mallard ducks (Anas platyrhynchos; Anseriformes). Ducks belong to a diverse clade of galloanseriform birds, have derived adaptations for herbivory and kinesis, and are model organisms in developmental biology. Thus, new insights into their cranial functional morphology will refine our understanding of avian cranial evolution. A total of five specimens (two ducklings and three adults) were histologically sampled, and two additional specimens (a duckling and an adult) were subjected to micro-computed tomographic scanning. Five intracranial joints were sampled: the jaw joint (quadrate-articular); otic joint (quadrate-squamosal); palatobasal joint (parasphenoid-pterygoid); the mandibular symphysis (dentary-dentary); and the craniofacial hinge (a complex flexion zone involving four different pairs of skeletal elements). In both the ducklings and adults, the jaw, otic and palatobasal joints are all synovial, with a synovial cavity and articular cartilage on each surface (i.e. bichondral joints) ensheathed in a fibrous capsule. The craniofacial hinge begins as an ensemble of patent sutures in the duckling, but in the adult it becomes more complex: laterally it is synovial; whereas medially, it is synostosed by a bridge of chondroid bone. We hypothesize that it is chondroid bone that provides some of the flexible properties of this joint. The heavily innervated mandibular symphysis is already fused in the ducklings and remains as such in the adult. The results of this study will serve as reference for documenting avian cranial kinesis from a microanatomical perspective. The formation of: (i) secondary articular cartilage on the membrane bones of extant birds; and (ii) their unique ability to form movable synovial joints within two or more membrane bones (i.e. within their dermatocranium) might have played a role in the origin and evolution of modern avian cranial kinesis during dinosaur evolution.

The taming of the neural crest: a developmental perspective on the origins of morphological covariation in domesticated mammals

Studies on domestication are blooming, but the developmental bases for the generation of domestication traits and breed diversity remain largely unexplored. Some phenotypic patterns of human neurocristopathies are suggestive of those reported for domesticated mammals and disrupting neural crest developmental programmes have been argued to be the source of traits deemed the 'domestication syndrome'. These character changes span multiple organ systems and morphological structures. But an in-depth examination within the phylogenetic framework of mammals including domesticated forms reveals that the distribution of such traits is not universal, with canids being the only group showing a large set of predicted features. Modularity of traits tied to phylogeny characterizes domesticated mammals: through selective breeding, individual behavioural and morphological traits can be reordered, truncated, augmented or deleted. Similarly, mammalian evolution on islands has resulted in suites of phenotypic changes like those of some domesticated forms. Many domesticated mammals can serve as valuable models for conducting comparative studies on the evolutionary developmental biology of the neural crest, given that series of their embryos are readily available and that their phylogenetic histories and genomes are well characterized.

Competition between Jagged-Notch and Endothelin1 Signaling Selectively Restricts Cartilage Formation in the Zebrafish Upper Face

The intricate shaping of the facial skeleton is essential for function of the vertebrate jaw and middle ear. While much has been learned about the signaling pathways and transcription factors that control facial patterning, the downstream cellular mechanisms dictating skeletal shapes have remained unclear. Here we present genetic evidence in zebrafish that three major signaling pathways − Jagged-Notch, Endothelin1 (Edn1), and Bmp − regulate the pattern of facial cartilage and bone formation by controlling the timing of cartilage differentiation along the dorsoventral axis of the pharyngeal arches. A genomic analysis of purified facial skeletal precursors in mutant and overexpression embryos revealed a core set of differentiation genes that were commonly repressed by Jagged-Notch and induced by Edn1. Further analysis of the pre-cartilage condensation gene barx1, as well as in vivo imaging of cartilage differentiation, revealed that cartilage forms first in regions of high Edn1 and low Jagged-Notch activity. Consistent with a role of Jagged-Notch signaling in restricting cartilage differentiation, loss of Notch pathway components resulted in expanded barx1 expression in the dorsal arches, with mutation of barx1 rescuing some aspects of dorsal skeletal patterning in jag1b mutants. We also identified prrx1a and prrx1b as negative Edn1 and positive Bmp targets that function in parallel to Jagged-Notch signaling to restrict the formation of dorsal barx1+ pre-cartilage condensations. Simultaneous loss of jag1b and prrx1a/b better rescued lower facial defects of edn1 mutants than loss of either pathway alone, showing that combined overactivation of Jagged-Notch and Bmp/Prrx1 pathways contribute to the absence of cartilage differentiation in the edn1 mutant lower face. These findings support a model in which Notch-mediated restriction of cartilage differentiation, particularly in the second pharyngeal arch, helps to establish a distinct skeletal pattern in the upper face.

The exquisite functions of the vertebrate face require the precise formation of its underlying bones. Remarkably, many of the genes required to shape the facial skeleton are the same from fish to man. In this study, we use the powerful zebrafish system to understand how the skeletal components of the face acquire different shapes during development. To do so, we analyze a series of mutants that disrupt patterning of the facial skeleton, and then assess how the genes affected in these mutants control cell fate in skeletal progenitor cells. From these genetic studies, we found that several pathways converge to control when and where progenitor cells commit to a cartilage fate, thus controlling the size and shape of cartilage templates for the later-arising bones. Our work thus reveals how regulating the timing of when progenitor cells make skeleton helps to shape the bones of the zebrafish face. As mutations in many of the genes studied are implicated in human craniofacial defects, differences in the timing of progenitor cell differentiation may also explain the wonderful diversity of human faces.

Neural crest-mediated bone resorption is a determinant of species-specific jaw length

Precise control of jaw length during development is crucial for proper form and function. Previously we have shown that in birds, neural crest mesenchyme (NCM) confers species-specific size and shape to the beak by regulating molecular and histological programs for the induction and deposition of cartilage and bone. Here we reveal that a hitherto unrecognized but similarly essential mechanism for establishing jaw length is the ability of NCM to mediate bone resorption. Osteoclasts are considered the predominant cells that resorb bone, although osteocytes have also been shown to participate in this process. In adults, bone resorption is tightly coupled to bone deposition as a means to maintain skeletal homeostasis. Yet, the role and regulation of bone resorption during growth of the embryonic skeleton have remained relatively unexplored. We compare jaw development in short-beaked quail versus long-billed duck and find that quail have substantially higher levels of enzymes expressed by bone-resorbing cells including tartrate-resistant acid phosphatase (TRAP), Matrix metalloproteinase 13 (Mmp13), and Mmp9. Then, we transplant NCM destined to form the jaw skeleton from quail to duck and generate chimeras in which osteocytes arise from quail donor NCM and osteoclasts come exclusively from the duck host. Chimeras develop quail-like jaw skeletons coincident with dramatically elevated expression of TRAP, Mmp13, and Mmp9. To test for a link between bone resorption and jaw length, we block resorption using a bisphosphonate, osteoprotegerin protein, or an MMP13 inhibitor, and this significantly lengthens the jaw. Conversely, activating resorption with RANKL protein shortens the jaw. Finally, we find that higher resorption in quail presages their relatively lower adult jaw bone mineral density (BMD) and that BMD is also NCM-mediated. Thus, our experiments suggest that NCM not only controls bone resorption by its own derivatives but also modulates the activity of mesoderm-derived osteoclasts, and in so doing enlists bone resorption as a key patterning mechanism underlying the functional morphology and evolution of the jaw.

Regulation of Jaw Length During Development, Disease, and Evolution

Molecular and cellular mechanisms that control jaw length are becoming better understood. This is significant since the jaws are not only critical for species-specific adaptation and survival, but they are often affected by a variety of size-related anomalies including mandibular hypoplasia, retrognathia, asymmetry, and clefting. This chapter overviews how jaw length is established during the allocation, proliferation, differentiation, and growth of jaw precursor cells, which originate from neural crest mesenchyme (NCM). The focus is mainly on results from experiments transplanting NCM between quail and duck embryos. Quail have short jaws whereas those of duck are relatively long. Quail-duck chimeras reveal that the determinants of jaw length are NCM mediated throughout development and include species-specific differences in jaw progenitor number, differential regulation of various signaling pathways, and the autonomous activation of programs for skeletal matrix deposition and resorption. Such insights help make the goal of devising new therapies for birth defects, diseases, and injuries to the jaw skeleton seem ever more likely.

Diversity in primary palate ontogeny of amniotes revealed with 3D imaging

The amniote primary palate encompasses the upper lip and the nasal cavities. During embryonic development, the primary palate forms from the fusion of the maxillary, medial nasal and lateral nasal prominences. In mammals, as the primary palate fuses, the nasal and oral cavities become completely separated. Subsequently, the tissue demarcating the future internal nares (choanae) thins and becomes the bucconasal membrane, which eventually ruptures and allows for the essential connection of the oral and nasal cavities to form. In reptiles (including birds), the other major amniote group, primary palate ontogeny is poorly studied with respect to prominence fusion, especially the formation of a bucconasal membrane. Using 3D optical projection tomography, we found that the prominences that initiate primary palate formation are similar between mammals and crocodilians but distinct from turtles and lizards, which are in turn similar to each other. Chickens are distinct from all non-avian lineages and instead resemble human embryos in this aspect. The majority of reptiles maintain a communication between the oral and nasal cavities via the choanae during primary palate formation. However, crocodiles appear to have a transient separation between the oral and nasal cavities. Furthermore, the three lizard species examined here, exhibit temporary closure of their external nares via fusion of the lateral nasal prominences with the frontonasal mass, subsequently reopening them just before hatching. The mechanism of the persistent choanal opening was examined in chicken embryos. The mesenchyme posterior/dorsal to the choana had a significant decline in proliferation index, whereas the mesenchyme of the facial processes remained high. This differential proliferation allows the choana to form a channel between the oral and nasal cavities as the facial prominences grow and fuse around it. Our data show that primary palate ontogeny has been modified extensively to support the array of morphological diversity that has evolved among amniotes.

Mastication markedly affects mandibular condylar cartilage growth, gene expression, and morphology

INTRODUCTION

Mandibular growth is believed to be strongly related to mastication. Furthermore, mandibular condylar cartilage is known to be derived from neural crest cells. We examined whether the degree of chewing affects condylar cartilage growth of the mandible.

METHODS

Mice were fed diets with varying hardness. Genes specific to neural crest-derived cells were measured by real-time polymerase chain reaction to compare the expression changes between the mandibular and tibia cartilages. The mandibular condylar cartilage was then evaluated histologically, and proliferation was evaluated using proliferating cell nuclear antigen. Immunostaining was conducted for osteopontin, type X collagen, and Musashi1, and real-time polymerase chain reaction was used to assess the expression levels of osteopontin and type X collagen.

RESULTS

Markers including P75, Wnt-1, Musashi1, and Nestin were upregulated in the mandibular condylar cartilage as compared with the tibial cartilage. Histologic assessment of the mandibular cartilage showed that the hypertrophic chondrocyte zone was statistically significantly thicker in mice fed a hard diet. Chondrocyte proliferation and Musashi1 expression were lower in mice fed a hard diet. After 4 weeks, numerous osteopontin and type X collagen-positive cells were observed in mice fed a mixed diet.

CONCLUSIONS

Mastication affects the balance between differentiation and proliferation in the mandibular condylar cartilage. This phenomenon might be attributed to the presence of neural crest-derived cells.

Assessing species-specific contributions to craniofacial development using quail-duck chimeras

The generation of chimeric embryos is a widespread and powerful approach to study cell fates, tissue interactions, and species-specific contributions to the histological and morphological development of vertebrate embryos. In particular, the use of chimeric embryos has established the importance of neural crest in directing the species-specific morphology of the craniofacial complex. The method described herein utilizes two avian species, duck and quail, with remarkably different craniofacial morphology. This method greatly facilitates the investigation of molecular and cellular regulation of species-specific pattern in the craniofacial complex. Experiments in quail and duck chimeric embryos have already revealed neural crest-mediated tissue interactions and cell-autonomous behaviors that regulate species-specific pattern in the craniofacial skeleton, musculature, and integument. The great diversity of neural crest derivatives suggests significant potential for future applications of the quail-duck chimeric system to understanding vertebrate development, disease, and evolution.

Multiple developmental mechanisms regulate species-specific jaw size

Variation in jaw size during evolution has been crucial for the adaptive radiation of vertebrates, yet variation in jaw size during development is often associated with disease. To test the hypothesis that early developmental events regulating neural crest (NC) progenitors contribute to species-specific differences in size, we investigated mechanisms through which two avian species, duck and quail, achieve their remarkably different jaw size. At early stages, duck exhibit an anterior shift in brain regionalization yielding a shorter, broader, midbrain. We find no significant difference in the total number of pre-migratory NC; however, duck concentrate their pre-migratory NC in the midbrain, which contributes to an increase in size of the post-migratory NC population allocated to the mandibular arch. Subsequent differences in proliferation lead to a progressive increase in size of the duck mandibular arch relative to that of quail. To test the role of pre-migratory NC progenitor number in regulating jaw size, we reduced and augmented NC progenitors. In contrast to previous reports of regeneration by NC precursors, we find that neural fold extirpation results in a loss of NC precursors. Despite this reduction in their numbers, post-migratory NC progenitors compensate, producing a symmetric and normal-sized jaw. Our results suggest that evolutionary modification of multiple aspects of NC cell biology, including NC allocation within the jaw primordia and NC-mediated proliferation, have been important to the evolution of jaw size. Furthermore, our finding of NC post-migratory compensatory mechanisms potentially extends the developmental time frame for treatments of disease or injury associated with NC progenitor loss.

Embryonic craniofacial bone volume and bone mineral density in Fgfr2(+/P253R) and nonmutant mice

BACKGROUND

Quantifying multiple phenotypic aspects of individual craniofacial bones across early osteogenesis illustrates differences in typical bone growth and maturation and provides a basis for understanding the localized and overall influence of mutations associated with disease. We quantify the typical pattern of bone growth and maturation during early craniofacial osteogenesis and determine how this pattern is modified in Fgfr2(+/P253R) Apert syndrome mice.

RESULTS

Early differences in typical relative bone density increase are noted between intramembranous and endochondral bones, with endochondral bones normally maturing more quickly during the prenatal period. Several craniofacial bones, including the facial bones of Fgfr2(+/P253R) mice, display lower volumes during the earliest days of osteogenesis and lower relative densities until the perinatal period relative to unaffected littermates.

CONCLUSIONS

Estimates of bone volume and linear measures describing morphology do not necessarily covary, highlighting the value of quantifying multiple facets of gross osteological phenotypes when exploring the influence of a disease causing mutation. Differences in mechanisms of osteogenesis likely underlie differences in intramembranous and endochondral relative density increase. The influence of the FGFR2 P253R mutation on bone volume changes across the prenatal period and again after birth, while its influence on relative bone density is more stable.

Evolution of a developmental mechanism: Species-specific regulation of the cell cycle and the timing of events during craniofacial osteogenesis

Neural crest mesenchyme (NCM) controls species-specific pattern in the craniofacial skeleton but how this cell population accomplishes such a complex task remains unclear. To elucidate mechanisms through which NCM directs skeletal development and evolution, we made chimeras from quail and duck embryos, which differ markedly in their craniofacial morphology and maturation rates. We show that quail NCM, when transplanted into duck, maintains its faster timetable for development and autonomously executes molecular and cellular programs for the induction, differentiation, and mineralization of bone, including premature expression of osteogenic genes such as Runx2 and Col1a1. In contrast, the duck host systemic environment appears to be relatively permissive and supports osteogenesis independently by providing circulating minerals and a vascular network. Further experiments reveal that NCM establishes the timing of osteogenesis by regulating cell cycle progression in a stage- and species-specific manner. Altering the time-course of D-type cyclin expression mimics chimeras by accelerating expression of Runx2 and Col1a1. We also discover higher endogenous expression of Runx2 in quail coincident with their smaller craniofacial skeletons, and by prematurely over-expressing Runx2 in chick embryos we reduce the overall size of the craniofacial skeleton. Thus, our work indicates that NCM establishes species-specific size in the craniofacial skeleton by controlling cell cycle, Runx2 expression, and the timing of key events during osteogenesis.

A simple PCR-based strategy for estimating species-specific contributions in chimeras and xenografts

Many tissue-engineering approaches for repair and regeneration involve transplants between species. Yet a challenge is distinguishing donor versus host effects on gene expression. This study provides a simple molecular strategy to quantify species-specific contributions in chimeras and xenografts. Species-specific primers for reverse transcription quantitative real-time PCR (RT-qPCR) were designed by identifying silent mutations in quail, duck, chicken, mouse and human ribosomal protein L19 (RPL19). cDNA from different pairs of species was mixed in a dilution series and species-specific RPL19 primers were used to generate standard curves. Then quail cells were transplanted into transgenic-GFP chick and resulting chimeras were analyzed with species-specific primers. Fluorescence-activated cell sorting (FACS) confirmed that donor- and host-specific levels of RPL19 expression represent actual proportions of cells. To apply the RPL19 strategy, we measured Runx2 expression in quail-duck chimeras. Elevated Runx2 levels correlated with higher percentages of donor cells. Finally, RPL19 primers also discriminated mouse from human and chick. Thus, this strategy enables chimeras and/or xenografts to be screened rapidly at the molecular level.

FishFace: interactive atlas of zebrafish craniofacial development at cellular resolution

BACKGROUND

The vertebrate craniofacial skeleton may exhibit anatomical complexity and diversity, but its genesis and evolution can be understood through careful dissection of developmental programs at cellular resolution. Resources are lacking that include introductory overviews of skeletal anatomy coupled with descriptions of craniofacial development at cellular resolution. In addition to providing analytical guidelines for other studies, such an atlas would suggest cellular mechanisms underlying development.

DESCRIPTION

We present the Fish Face Atlas, an online, 3D-interactive atlas of craniofacial development in the zebrafish Danio rerio. Alizarin red-stained skulls scanned by fluorescent optical projection tomography and segmented into individual elements provide a resource for understanding the 3D structure of the zebrafish craniofacial skeleton. These data provide the user an anatomical entry point to confocal images of Alizarin red-stained zebrafish with transgenically-labelled pharyngeal arch ectomesenchyme, chondrocytes, and osteoblasts, which illustrate the appearance, morphogenesis, and growth of the mandibular and hyoid cartilages and bones, as viewed in live, anesthetized zebrafish during embryonic and larval development. Confocal image stacks at high magnification during the same stages provide cellular detail and suggest developmental and evolutionary hypotheses.

CONCLUSION

The FishFace Atlas is a novel learning tool for understanding craniofacial skeletal development, and can serve as a reference for a variety of studies, including comparative and mutational analyses.

Morphology and diversity of the mandibular symphysis of archosauriforms

Archosauromorphs radiated into numerous trophic niches during the Mesozoic, many of which were accommodated by particular suites of cranial adaptations and feeding behaviours. The mandibular symphysis, the joint linking the mandibles, is a poorly understood craniomandibular joint that may offer significant insight into skull function and feeding ecology. Using comparative data from extant amniotes, we investigated the skeletal anatomy and osteological correlates of relevant soft tissues in a survey of archosauromorph mandibular symphyses. Characters were identified and their evolution mapped using a current phylogeny of archosauriforms with the addition of non-archosauriform archosauromorphs. Extinct taxa with the simple Class I condition (e.g. proterochampsids, ‘rauisuchians’), rugose Class II (aetosaurs, protosuchians, silesaurids) and interdigitating Class III symphyses (e.g. phytosaurs, crocodyliforms) and finally fused Class IV (avians) build the joints in expected ways, although they differ in the contributions of bony elements and Meckel’s cartilage. Optimization of the different classes of symphyses across archosauromorph clades indicates that major iterative transitions from plesiomorphic Class I to derived, rigid Class II–IV symphyses occurred along the lines to phytosaurs, aetosaurs, a subset of poposauroids, crocodyliformes, pterosaurs and birds. These transitions in symphyseal morphology also appear to track changes in dentition and potentially diet.

Fibroblast growth factor receptor 1 (FGFR1) variants and craniofacial variation in Amerindians and related populations

OBJECTIVES

The polymorphic site rs4647905 of the FGFR1 gene was previously associated with a decrease in cephalic index (CI). Here, we evaluate the relationships between genotypes and cephalometric measurements and indices in one Mexican Native and two mestizo Mexican populations using two haplotype-tag SNPs (rs4647905 and rs3213849) that represent >85% of the FGFR1 variability, plus three other SNPs (rs2293971, rs2304000, and rs930828) situated nearby. In addition, we genotyped five South American natives, two European, one African, and one Siberian populations to evaluate their intra and intercontinental population diversity.

METHODS

The five SNPs were tested and the craniofacial measurements and indices were collected using standardized procedures. Principal Component Analysis was used to verify individual/population comparisons. Associations were performed through the generalized linear model (GLM), coefficient of determination R(2) and linear regression tests.

RESULTS

We found a tendency for a decrease in CI in individuals homozygous for allele rs4647905C, regardless of the population to which they belong, though the effect is more pronounced in mestizo. When the GLM analyses were performed using the absolute/linear cephalometric measurements, a statistically significant association was found between four SNPs and head length in the mestizo population.

CONCLUSIONS

FGFR1 polymorphisms, especially rs4647905, can have an important role in the normal human skull variation, primarily due to their influence in head length, which would affect other cephalometric absolute/linear measures as well as indices like CI as a result of the pervasive nature of the morphological integration that characterizes the human skull.

Evolution of the osteoblast: skeletogenesis in gar and zebrafish

BACKGROUND

Although the vertebrate skeleton arose in the sea 500 million years ago, our understanding of the molecular fingerprints of chondrocytes and osteoblasts may be biased because it is informed mainly by research on land animals. In fact, the molecular fingerprint of teleost osteoblasts differs in key ways from that of tetrapods, but we do not know the origin of these novel gene functions. They either arose as neofunctionalization events after the teleost genome duplication (TGD), or they represent preserved ancestral functions that pre-date the TGD. Here, we provide evolutionary perspective to the molecular fingerprints of skeletal cells and assess the role of genome duplication in generating novel gene functions. We compared the molecular fingerprints of skeletogenic cells in two ray-finned fish: zebrafish (Danio rerio)--a teleost--and the spotted gar (Lepisosteus oculatus)--a "living fossil" representative of a lineage that diverged from the teleost lineage prior to the TGD (i.e., the teleost sister group). We analyzed developing embryos for expression of the structural collagen genes col1a2, col2a1, col10a1, and col11a2 in well-formed cartilage and bone, and studied expression of skeletal regulators, including the transcription factor genes sox9 and runx2, during mesenchymal condensation.

RESULTS

Results provided no evidence for the evolution of novel functions among gene duplicates in zebrafish compared to the gar outgroup, but our findings shed light on the evolution of the osteoblast. Zebrafish and gar chondrocytes both expressed col10a1 as they matured, but both species' osteoblasts also expressed col10a1, which tetrapod osteoblasts do not express. This novel finding, along with sox9 and col2a1 expression in developing osteoblasts of both zebrafish and gar, demonstrates that osteoblasts of both a teleost and a basally diverging ray-fin fish express components of the supposed chondrocyte molecular fingerprint.

CONCLUSIONS

Our surprising finding that the "chondrogenic" transcription factor sox9 is expressed in developing osteoblasts of both zebrafish and gar can help explain the expression of chondrocyte genes in osteoblasts of ray-finned fish. More broadly, our data suggest that the molecular fingerprint of the osteoblast, which largely is constrained among land animals, was not fixed during early vertebrate evolution.

Structured coculture of mesenchymal stem cells and disc cells enhances differentiation and proliferation

PURPOSE

During in vivo stem cell differentiation, mature cells often induce the differentiation of nearby stem cells. Accordingly, prior studies indicate that a randomly mixed coculture can help transform mesenchymal stem cells (MSC) into nucleus pulposus cells (NPC). However, because in vivo signaling typically occurs heterotopically between adjacent cell layers, we hypothesized that a structurally organized coculture between MSC and NPC will result in greater cell differentiation and proliferation over single cell-type controls and cocultures with random organization.

METHODS

We developed a novel bilaminar cell pellet (BCP) system where a sphere of MSC is enclosed in a shell of NPC by successive centrifugation. Controls were made using single cell-type pellets and coculture pellets with random organization. The pellets were evaluated for DNA content, gene expression, and histology.

RESULTS

A bilaminar 3D organization enhanced cell proliferation and differentiation. BCP showed significantly more cell proliferation than pellets with one cell type and those with random organization. Enhanced differentiation of MSC within the BCP pellet relative to single cell-type pellets was demonstrated by quantitative RT-PCR, histology, and in situ hybridization.

CONCLUSIONS

The BCP culture system increases MSC proliferation and differentiation as compared to single cell type or randomly mixed coculture controls.

Developmental Patterns in Mesozoic Evolution of Mammal Ears

Complex structures with significant biological function can arise multiple times in evolution by common gene patterning and developmental pathways. The mammalian middle ear, with its significant hearing function, is such a complex structure and a key evolutionary innovation. Newly discovered fossils have now shown that the detachment of the ear from the jaw, an important transformation of the middle ear in early mammals, has major homoplasies; the morphogenesis of these homoplasies is also illuminated by new genetic studies of ear development in extant mammals. By extrapolating the developmental morphogenesis of genetic studies into the early mammal fossil record, evolution of the middle ear in early mammals provides an integrated case study of how development has impacted, mechanistically, the transformation of a major structural complex in evolution.

Timing of ossification in duck, quail, and zebra finch: intraspecific variation, heterochronies, and life history evolution

Skeletogenic heterochronies have gained much attention in comparative developmental biology. The temporal appearance of mineralized individual bones in a species - the species ossification sequence - is an excellent marker in this kind of study. Several publications describe interspecific variation, but only very few detail intraspecific variation. In this study, we describe and analyze the temporal order of ossification of skeletal elements in the zebra finch, Taeniopygia guttata, the Japanese quail, Coturnix coturnix japonica, and the White Pekin duck, a domestic race of the mallard Anas platyrhynchos, and explore patterns of intraspecific variation in these events. The overall sequences were found to be conserved. In the duck, variability is present in the relative timing of ossification in the occipital, the basisphenoid and the otic regions of the skull and the phalanges in the postcranium. This variation appears generally in close temporal proximity. Comparison with previously published data shows differences in ossification sequence in the skull, the feet, and the pelvis in the duck, and especially the pelvis in the quail. This clearly documents variability among different breeds.

Overview of the transcriptome profiles identified in hagfish, shark, and bichir: current issues arising from some nonmodel vertebrate taxa

Because of their crucial phylogenetic positions, hagfishes, sharks, and bichirs are recognized as key taxa in our understanding of vertebrate evolution. The expression patterns of the regulatory genes involved in developmental patterning have been analyzed in the context of evolutionary developmental studies. However, in a survey of public sequence databases, we found that the large-scale sequence data for these taxa are still limited. To address this deficit, we used conventional Sanger DNA sequencing and a next-generation sequencing technology based on 454 GS FLX sequencing to obtain expressed sequence tags (ESTs) of the Japanese inshore hagfish (Eptatretus burgeri; 161,482 ESTs), cloudy catshark (Scyliorhinus torazame; 165,819 ESTs), and gray bichir (Polypterus senegalus; 34,336 ESTs). We deposited the ESTs in a newly constructed database, designated the "Vertebrate TimeCapsule." The ESTs include sequences from genes that can be effectively used in evolutionary developmental studies; for instance, several encode cartilaginous extracellular matrix proteins, which are central to an understanding of the ways in which evolutionary processes affected the skeletal elements, whereas others encode regulatory genes involved in craniofacial development and early embryogenesis. Here, we discuss how hagfishes, sharks, and bichirs contribute to our understanding of vertebrate evolution, we review the current status of the publicly available sequence data for these three taxa, and we introduce our EST projects and newly developed database.

Mesenchymal and mechanical mechanisms of secondary cartilage induction

Secondary cartilage occurs at articulations, sutures, and muscle attachments, and facilitates proper kinetic movement of the skeleton. Secondary cartilage requires mechanical stimulation for its induction and maintenance, and accordingly, its evolutionary presence or absence reflects species-specific variation in functional anatomy. Avians illustrate this point well. In conjunction with their distinct adult mode of feeding via levered straining, duck develop a pronounced secondary cartilage at the insertion (i.e., enthesis) of the mandibular adductor muscles on the lower jaw skeleton. An equivalent cartilage is absent in quail, which peck at their food. We hypothesized that species-specific pattern and a concomitant dissimilarity in the local mechanical environment promote secondary chondrogenesis in the mandibular adductor enthesis of duck versus quail. To test our hypothesis we employed two experimental approaches. First, we transplanted neural crest mesenchyme (NCM) from quail into duck, which produced chimeric "quck" with a jaw complex resembling that of quail, including an absence of enthesis secondary cartilage. Second, we modified the mechanical environment in embryonic duck by paralyzing skeletal muscles, and by blocking the ability of NCM to support mechanotransduction through stretch-activated ion channels. Paralysis inhibited secondary cartilage, as evidenced by changes in histology and expression of genes that affect chondrogenesis, including members of the FGF and BMP pathways. Ion channel inhibition did not alter enthesis secondary cartilage but caused bone to form in place of secondary cartilage at articulations. Thus, our study reveals that enthesis secondary cartilage forms through mechanisms that are distinct from those regulating other secondary cartilage. We conclude that by directing the musculoskeletal patterning and integration of the jaw complex, NCM modulates the mechanical forces and molecular signals necessary to control secondary cartilage formation during development and evolution.

Role of Dlx genes in craniofacial morphogenesis: Dlx2 influences skeletal patterning by inducing ectomesenchymal aggregation in ovo

Dlx homeodomain transcription factors are expressed in neural crest-derived mesenchyme of the pharyngeal arches and are required for patterning of the craniofacial skeleton. However, the cellular and molecular mechanisms by which Dlx factors control skeletogenesis in the facial primordia are unclear. We have investigated the function of Dlx2 and Dlx5 by sustained misexpression in ovo. We find that RCAS-Dlx2- and RCAS-Dlx5-infected avian embryos exhibit very similar patterns of local, stereotypical changes in skeletal development in the upper jaw. The changes include ectopic dermal bone along the jugal arch, and ectopic cartilages that develop between the quadrate and the trabecula. The ectopic cartilage associated with the trabecula is reminiscent of a normally occurring element in this region in some bird taxa. Analysis of the distribution of RCAS-Dlx2-infected cells suggests that Dlx2 induces aggregation of undifferentiated mesenchyme, which subsequently develops into the ectopic skeletal elements. Comparison of infected embryos with restricted or widespread misexpression, and of embryos in which Dlx genes were delivered to migratory or postmigratory neural crest, indicate that there are limited regions of competence in which the ectopic elements can arise. The site-specific differentiation program that the aggregates undergo may be dependent on local environmental signals. Our results suggest that Dlx factors mediate localization of ectomesenchymal subpopulations within the pharyngeal arches and in doing so define where skeletogenic condensations will arise. Consequently, variation in Dlx expression or activity may have influenced the morphology of jaw elements during vertebrate evolution.

Neural control of gene recruitment in metazoans

Gene recruitment played a critical role in metazoan evolution. Yet, there is no consensus on whether it is an accidental event or a result of an inherent "gene recruiting" mechanism. The prevailing opinion among biologists is that gene recruitment results from random changes in genes or their regulatory regions, but the supporting evidence is poor and controversial. Herein, I present a mechanism in which gene recruitment is a neurally determined event, an adaptive response to changes in environmental conditions. In support of the hypothesis, I present evidence on the manipulative expression of genes in the central nervous system, as well as neurally determined examples of gene recruitment in transgenerational developmental plasticity and in evolution of metazoans.

Molecular mechanisms of cranial neural crest cell migration and patterning in craniofacial development

During vertebrate craniofacial development, neural crest cells (NCCs) contribute much of the cartilage, bone and connective tissue that make up the developing head. Although the initial patterns of NCC segmentation and migration are conserved between species, the variety of vertebrate facial morphologies that exist indicates that a complex interplay occurs between intrinsic genetic NCC programs and extrinsic environmental signals during morphogenesis. Here, we review recent work that has begun to shed light on the molecular mechanisms that govern the spatiotemporal patterning of NCC-derived skeletal structures – advances that are central to understanding craniofacial development and its evolution.

The beak of the other finch: coevolution of genetic covariance structure and developmental modularity during adaptive evolution

The link between adaptation and evolutionary change remains the most central and least understood evolutionary problem. Rapid evolution and diversification of avian beaks is a textbook example of such a link, yet the mechanisms that enable beak's precise adaptation and extensive adaptability are poorly understood. Often observed rapid evolutionary change in beaks is particularly puzzling in light of the neo-Darwinian model that necessitates coordinated changes in developmentally distinct precursors and correspondence between functional and genetic modularity, which should preclude evolutionary diversification. I show that during first 19 generations after colonization of a novel environment, house finches (Carpodacus mexicanus) express an array of distinct, but adaptively equivalent beak morphologies-a result of compensatory developmental interactions between beak length and width in accommodating microevolutionary change in beak depth. Directional selection was largely confined to the elimination of extremes formed by these developmental interactions, while long-term stabilizing selection along a single axis-beak depth-was mirrored in the structure of beak's additive genetic covariance. These results emphasize three principal points. First, additive genetic covariance structure may represent a historical record of the most recurrent developmental and functional interactions. Second, adaptive equivalence of beak configurations shields genetic and developmental variation in individual components from depletion by natural selection. Third, compensatory developmental interactions among beak components can generate rapid reorganization of beak morphology under novel conditions and thus greatly facilitate both the evolution of precise adaptation and extensive diversification, thereby linking adaptation and adaptability in this classic example of Darwinian evolution.