Unique organization of the frontonasal ectodermal zone in birds and mammals

Morphogens in the evolution of size, shape and patterning

Much of the striking diversity of life on Earth has arisen from variations in the way that the same molecules and networks operate during development to shape and pattern tissues and organs into different morphologies. However, we still understand very little about the potential for diversification exhibited by different, highly conserved mechanisms during evolution, or, conversely, the constraints that they place on evolution. With the aim of steering the field in new directions, we focus on morphogen-mediated patterning and growth as a case study to demonstrate how conserved developmental mechanisms can adapt during evolution to drive morphological diversification and optimise functionality, and to illustrate how evolution algorithms and computational tools can be used alongside experiments to provide insights into how these conserved mechanisms can evolve. We first introduce key conserved properties of morphogen-driven patterning mechanisms, before summarising comparative studies that exemplify how changes in the spatiotemporal expression and signalling levels of morphogens impact the diversification of organ size, shape and patterning in nature. Finally, we detail how theoretical frameworks can be used in conjunction with experiments to probe the role of morphogen-driven patterning mechanisms in evolution. We conclude that morphogen-mediated patterning is an excellent model system and offers a generally applicable framework to investigate the evolution of developmental mechanisms.

PBX1 and PBX3 transcription factors regulate SHH expression in the Frontonasal Ectodermal Zone through complementary mechanisms

Sonic hedgehog (SHH) signaling from the frontonasal ectodermal zone (FEZ) is a key regulator of craniofacial morphogenesis. Along with SHH, pre-B-cell leukemia homeobox (PBX) transcription factors regulate midfacial development. PBXs act in the epithelium during fusion of facial primordia, but their specific interactions with SHH have not been fully investigated. We hypothesized that PBX1/3 regulate SHH expression in the FEZ by activating or repressing transcription. The hypothesis was tested by manipulating PBX1/3 expression in chick embryos and profiling epigenomic landscapes at early developmental stages. PBX1/3 expression was perturbed in the chick face beginning at stage 10 (HH10) using RCAS viruses, and the resulting SHH expression was assessed at HH22. Overexpressing PBX1 expanded SHH expression, while overexpressing PBX3 decreased SHH expression. Conversely, reducing PBX1 expression decreased SHH expression, but reducing PBX3 induced ectopic SHH expression. We performed ATAC-seq and mapped binding of PBX1 and PBX3 with ChIP-seq on the FEZ at HH22 to assess direct interactions of PBX1/3 with the SHH locus. These multi-omics approaches uncovered a 400 bp PBX1-enriched element within intron 1 of SHH (chr2:8,173,222-8,173,621). Enhancer activity of this element was demonstrated by electroporation of reporter constructs in ovo and luciferase reporter assays in vitro . When bound by PBX1, this element upregulates transcription, while it downregulates transcription when bound by PBX3. The present study identifies a cis- regulatory element, named SFE1, that interacts with PBX1/3 to modulate SHH expression in the FEZ and establishes that PBX1 and PBX3 play complementary roles in SHH regulation during embryonic development.

Cytoneme signaling provides essential contributions to mammalian tissue patterning

During development, morphogens pattern tissues by instructing cell fate across long distances. Directly visualizing morphogen transport in situ has been inaccessible, so the molecular mechanisms ensuring successful morphogen delivery remain unclear. To tackle this longstanding problem, we developed a mouse model for compromised sonic hedgehog (SHH) morphogen delivery and discovered that endocytic recycling promotes SHH loading into signaling filopodia called cytonemes. We optimized methods to preserve in vivo cytonemes for advanced microscopy and show endogenous SHH localized to cytonemes in developing mouse neural tubes. Depletion of SHH from neural tube cytonemes alters neuronal cell fates and compromises neurodevelopment. Mutation of the filopodial motor myosin 10 (MYO10) reduces cytoneme length and density, which corrupts neuronal signaling activity of both SHH and WNT. Combined, these results demonstrate that cytoneme-based signal transport provides essential contributions to morphogen dispersion during mammalian tissue development and suggest MYO10 is a key regulator of cytoneme function.

Tissues and signals with true organizer properties in craniofacial development

Transplantation experiments have shown that a true organizer provides instructive signals that induce and pattern ectopic structures in the responding tissue. Here, we review craniofacial experiments to identify tissues with organizer properties and signals with organizer properties. In particular, we evaluate whether transformation of identity took place in the mesenchyme. Using these stringent criteria, we find the strongest evidence for the avian foregut ectoderm. Transplanting a piece of quail foregut endoderm to a host chicken embryo caused ectopic beaks to form derived from chicken mesenchyme. The beak identity, whether upper or lower as well as orientation, was controlled by the original anterior-posterior position of the donor endoderm. There is also good evidence that the nasal pit is necessary and sufficient for lateral nasal patterning. Finally, we review signals that have organizer properties on their own without the need for tissue transplants. Mouse germline knockouts of the endothelin pathway result in transformation of identity of the mandible into a maxilla. Application of noggin-soaked beads to post-migratory neural crest cells transforms maxillary identity. This suggests that endothelin or noggin rich ectoderm could be organizers (not tested). In conclusion, craniofacial, neural crest-derived mesenchyme is competent to respond to tissues with organizer properties, also originating in the head. In future, we can exploit such well defined systems to dissect the molecular changes that ultimately lead to patterning of the upper and lower jaw.

Sonic hedgehog signaling in craniofacial development

Mutations in SHH and several other genes encoding components of the Hedgehog signaling pathway have been associated with holoprosencephaly syndromes, with craniofacial anomalies ranging in severity from cyclopia to facial cleft to midfacial and mandibular hypoplasia. Studies in animal models have revealed that SHH signaling plays crucial roles at multiple stages of craniofacial morphogenesis, from cranial neural crest cell survival to growth and patterning of the facial primordia to organogenesis of the palate, mandible, tongue, tooth, and taste bud formation and homeostasis. This article provides a summary of the major findings in studies of the roles of SHH signaling in craniofacial development, with emphasis on recent advances in the understanding of the molecular and cellular mechanisms regulating the SHH signaling pathway activity and those involving SHH signaling in the formation and patterning of craniofacial structures.

Quantifying the relationship between cell proliferation and morphology during development of the face

Morphogenesis requires highly coordinated, complex interactions between cellular processes: proliferation, migration, and apoptosis, along with physical tissue interactions. How these cellular and tissue dynamics drive morphogenesis remains elusive. Three dimensional (3D) microscopic imaging poses great promise, and generates elegant images. However, generating even moderate through-put quantified images is challenging for many reasons. As a result, the association between morphogenesis and cellular processes in 3D developing tissues has not been fully explored. To address this critical gap, we have developed an imaging and image analysis pipeline to enable 3D quantification of cellular dynamics along with 3D morphology for the same individual embryo. Specifically, we focus on how 3D distribution of proliferation relates to morphogenesis during mouse facial development. Our method involves imaging with light-sheet microscopy, automated segmentation of cells and tissues using machine learning-based tools, and quantification of external morphology via geometric morphometrics. Applying this framework, we show that changes in proliferation are tightly correlated to changes in morphology over the course of facial morphogenesis. These analyses illustrate the potential of this pipeline to investigate mechanistic relationships between cellular dynamics and morphogenesis during embryonic development.

AP-2α and AP-2β cooperatively function in the craniofacial surface ectoderm to regulate chromatin and gene expression dynamics during facial development

The facial surface ectoderm is essential for normal development of the underlying cranial neural crest cell populations, providing signals that direct appropriate growth, patterning, and morphogenesis. Despite the importance of the ectoderm as a signaling center, the molecular cues and genetic programs implemented within this tissue are understudied. Here, we show that removal of two members of the AP-2 transcription factor family, AP-2α and AP-2ß, within the early embryonic ectoderm of the mouse leads to major alterations in the craniofacial complex. Significantly, there are clefts in both the upper face and mandible, accompanied by fusion of the upper and lower jaws in the hinge region. Comparison of ATAC-seq and RNA-seq analyses between controls and mutants revealed significant changes in chromatin accessibility and gene expression centered on multiple AP-2 binding motifs associated with enhancer elements within these ectodermal lineages. In particular, loss of these AP-2 proteins affects both skin differentiation as well as multiple signaling pathways, most notably the WNT pathway. We also determined that the mutant clefting phenotypes that correlated with reduced WNT signaling could be rescued by Wnt1 ligand overexpression in the ectoderm. Collectively, these findings highlight a conserved ancestral function for AP-2 transcription factors in ectodermal development and signaling, and provide a framework from which to understand the gene regulatory network operating within this tissue that directs vertebrate craniofacial development.

Time to synchronize our clocks: Connecting developmental mechanisms and evolutionary consequences of heterochrony

Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970-80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.

The Pleiotropic Intricacies of Hedgehog Signaling: From Craniofacial Patterning to Carcinogenesis

Hedgehog signaling was discovered more than 40 years ago in experiments demonstrating that it is a fundamental mediator of limb development. Since that time, it has been shown to be important in development, homeostasis, and disease. The hedgehog pathway proceeds through a pathway highly conserved throughout animals beginning with the extracellular diffusion of hedgehog ligands, proceeding through an intracellular signaling cascade, and ending with the activation of specific target genes. A vast amount of research has been done elucidating hedgehog signaling mechanisms and regulation. This research has found a complex system of genetics and signaling that helps determine how organisms develop and function. This review provides an overview of what is known about hedgehog genetics and signaling, followed by an in-depth discussion of the role of hedgehog signaling in craniofacial development and carcinogenesis.

Ciliary and extraciliary Gpr161 pools repress hedgehog signaling in a tissue-specific manner

The role of compartmentalized signaling in primary cilia during tissue morphogenesis is not well understood. The cilia localized G protein-coupled receptor, Gpr161, represses hedgehog pathway via cAMP signaling. We engineered a knock-in at the Gpr161 locus in mice to generate a variant (Gpr161^mut1), which was ciliary localization defective but cAMP signaling competent. Tissue phenotypes from hedgehog signaling depend on downstream bifunctional Gli transcriptional factors functioning as activators or repressors. Compared to knockout (ko), Gpr161^mut1/ko had delayed embryonic lethality, moderately increased hedgehog targets, and partially down-regulated Gli3 repressor. Unlike ko, the Gpr161^mut1/ko neural tube did not show Gli2 activator-dependent expansion of ventral-most progenitors. Instead, the intermediate neural tube showed progenitor expansion that depends on loss of Gli3 repressor. Increased extraciliary receptor levels in Gpr161^mut1/mut1 prevented ventralization. Morphogenesis in limb buds and midface requires Gli repressor; these tissues in Gpr161^mut1/mut1 manifested hedgehog hyperactivation phenotypes—polydactyly and midfacial widening. Thus, ciliary and extraciliary Gpr161 pools likely establish tissue-specific Gli repressor thresholds in determining morpho-phenotypic outcomes.

Symmetry and fluctuation of cell movements in neural crest-derived facial mesenchyme

In the face, symmetry is established when bilateral streams of neural crest cells leave the neural tube at the same time, follow identical migration routes and then give rise to the facial prominences. However, developmental instability exists, particularly surrounding the steps of lip fusion. The causes of instability are unknown but inability to cope with developmental fluctuations are a likely cause of congenital malformations, such as non-syndromic orofacial clefts. Here, we tracked cell movements over time in the frontonasal mass, which forms the facial midline and participates in lip fusion, using live-cell imaging of chick embryos. Our mathematical examination of cell velocity vectors uncovered temporal fluctuations in several parameters, including order/disorder, symmetry/asymmetry and divergence/convergence. We found that treatment with a Rho GTPase inhibitor completely disrupted the temporal fluctuations in all measures and blocked morphogenesis. Thus, we discovered that genetic control of symmetry extends to mesenchymal cell movements and that these movements are of the type that could be perturbed in asymmetrical malformations, such as non-syndromic cleft lip.

This article has an associated ‘The people behind the papers’ interview.

Highlighted Article: Live imaging of the chick embryo face followed by mathematical analysis of mesenchymal cell tracks captures novel fluctuations between states of order/disorder as well as symmetry/asymmetry, revealing developmental instabilities that are part of normal morphogenesis.

Wnt Signaling Drives Correlated Changes in Facial Morphology and Brain Shape

Canonical Wnt signaling plays multiple roles critical to normal craniofacial development while its dysregulation is known to be involved in structural birth defects of the face. However, when and how Wnt signaling influences phenotypic variation, including those associated with disease, remains unclear. One potential mechanism is via Wnt signaling’s role in the patterning of an early facial signaling center, the frontonasal ectodermal zone (FEZ), and its subsequent regulation of early facial morphogenesis. For example, Wnt signaling may directly alter the shape and/or magnitude of expression of the sonic hedgehog (SHH) domain in the FEZ. To test this idea, we used a replication-competent avian sarcoma retrovirus (RCAS) encoding Wnt3a to modulate its expression in the facial mesenchyme. We then quantified and compared ontogenetic changes in treated to untreated embryos in the three-dimensional (3D) shape of both the SHH expression domain of the FEZ, and the morphology of the facial primordia and brain using iodine-contrast microcomputed tomography imaging and 3D geometric morphometrics (3DGM). We found that increased Wnt3a expression in early stages of head development produces correlated variation in shape between both structural and signaling levels of analysis. In addition, altered Wnt3a activation disrupted the integration between the forebrain and other neural tube derivatives. These results show that activation of Wnt signaling influences facial shape through its impact on the forebrain and SHH expression in the FEZ, and highlights the close relationship between morphogenesis of the forebrain and midface.

Insights Into the Complexity of Craniofacial Development From a Cellular Perspective

The head represents the most complex part of the body and a distinctive feature of the vertebrate body plan. This intricate structure is assembled during embryonic development in the four-dimensional process of morphogenesis. The head integrates components of the central and peripheral nervous system, sensory organs, muscles, joints, glands, and other specialized tissues in the framework of a complexly shaped skull. The anterior part of the head is referred to as the face, and a broad spectrum of facial shapes across vertebrate species enables different feeding strategies, communication styles, and diverse specialized functions. The face formation starts early during embryonic development and is an enormously complex, multi-step process regulated on a genomic, molecular, and cellular level. In this review, we will discuss recent discoveries that revealed new aspects of facial morphogenesis from the time of the neural crest cell emergence till the formation of the chondrocranium, the primary design of the individual facial shape. We will focus on molecular mechanisms of cell fate specification, the role of individual and collective cell migration, the importance of dynamic and continuous cellular interactions, responses of cells and tissues to generated physical forces, and their morphogenetic outcomes. In the end, we will examine the spatiotemporal activity of signaling centers tightly regulating the release of signals inducing the formation of craniofacial skeletal elements. The existence of these centers and their regulation by enhancers represent one of the core morphogenetic mechanisms and might lay the foundations for intra- and inter-species facial variability.

Evolutionary and ontogenetic changes of the anatomical organization and modularity in the skull of archosaurs

Comparative anatomy studies of the skull of archosaurs provide insights on the mechanisms of evolution for the morphologically and functionally diverse species of crocodiles and birds. One of the key attributes of skull evolution is the anatomical changes associated with the physical arrangement of cranial bones. Here, we compare the changes in anatomical organization and modularity of the skull of extinct and extant archosaurs using an Anatomical Network Analysis approach. We show that the number of bones, their topological arrangement, and modular organization can discriminate birds from non-avian dinosaurs, and crurotarsans. We could also discriminate extant taxa from extinct species when adult birds were included. By comparing within the same framework, juveniles and adults for crown birds and alligator (Alligator mississippiensis), we find that adult and juvenile alligator skulls are topologically similar, whereas juvenile bird skulls have a morphological complexity and anisomerism more similar to those of non-avian dinosaurs and crurotarsans than of their own adult forms. Clade-specific ontogenetic differences in skull organization, such as extensive postnatal fusion of cranial bones in crown birds, can explain this pattern. The fact that juvenile and adult skulls in birds do share a similar anatomical integration suggests the presence of a specific constraint to their ontogenetic growth.

miR-199 family contributes to regulation of sonic hedgehog expression during craniofacial development

BACKGROUND

The frontonasal ectodermal zone (FEZ) is a signaling center that regulates patterned development of the upper jaw, and Sonic hedgehog (SHH) mediates FEZ activity. Induction of SHH expression in the FEZ results from SHH-dependent signals from the brain and neural crest cells. Given the role of miRNAs in modulating gene expression, we investigated the extent to which miRNAs regulate SHH expression and FEZ signaling.

RESULTS

In the FEZ, the miR-199 family appears to be regulated by SHH-dependent signals from the brain; expression of this family increased from HH18 to HH22, and upon activation of SHH signaling in the brain. However, the miR-199 family is more broadly expressed in the mesenchyme of the frontonasal process and adjacent neuroepithelium. Downregulating the miR-199 genes expanded SHH expression in the FEZ, resulting in wider faces, while upregulating miR-199 genes resulted in decreased SHH expression and narrow faces. Hypoxia inducible factor 1 alpha (HIF1A) and mitogen-activated protein kinase kinase kinase 4 (MAP3K4) appear to be potential targets of miR-199b. Reduction of MAP3K4 altered beak development but increased apoptosis, while reducing HIF1A reduced expression of SHH in the FEZ and produced malformations independent of apoptosis.

CONCLUSIONS

Our results demonstrate that this miRNA family appears to participate in regulating SHH expression in the FEZ; however, specific molecular mechanisms remain unknown.

Comparative growth in the olfactory system of the developing chick with considerations for evolutionary studies

Despite the long-held assumption that olfaction plays a relatively minor role in the behavioral ecology of birds, crown-group avians exhibit marked phylogenetic variation in the size and form of the olfactory apparatus. As part of a larger effort to better understand the role of olfaction and olfactory tissues in the evolution and development of the avian skull, we present the first quantitative analysis of ontogenetic scaling between olfactory features [olfactory bulbs (OBs) and olfactory turbinates] and neighboring structures (cerebrum, total brain, respiratory turbinates) based on the model organism Gallus gallus. The OB develops under the predictions of a concerted evolutionary model with rapid early growth that is quickly overcome by the longer, sustained growth of the larger cerebrum. A similar pattern is found in the nasal cavity where the morphologically simple (non-scrolled) olfactory turbinates appear and mature early, with extended growth characterizing the larger and scrolled respiratory turbinates. Pairwise regressions largely recover allometric relationships among the examined structures, with a notable exception being the isometric trajectory of the OB and olfactory turbinate. Their parallel growth suggests a unique regulatory pathway that is likely driven by the morphogenesis of the olfactory nerve, which serves as a structural bridge between the two features. Still, isometry was not necessarily expected given that the olfactory epithelium covers more than just the turbinate. These data illuminate a number of evolutionary hypotheses that, moving forward, should inform tradeoffs and constraints between the olfactory and neighboring systems in the avian head.

Differential cellular proliferation underlies heterochronic generation of cranial diversity in phyllostomid bats

Background

Skull diversity in the neotropical leaf-nosed bats (Phyllostomidae) evolved through a heterochronic process called peramorphosis, with underlying causes varying by subfamily. The nectar-eating (subfamily Glossophaginae) and blood-eating (subfamily Desmondontinae) groups originate from insect-eating ancestors and generate their uniquely shaped faces and skulls by extending the ancestral ontogenetic program, appending new developmental stages and demonstrating peramorphosis by hypermorphosis. However, the fruit-eating phyllostomids (subfamilies Carollinae and Stenodermatinae) adjust their craniofacial development by speeding up certain developmental processes, displaying peramorphosis by acceleration. We hypothesized that these two forms of peramorphosis detected by our morphometric studies could be explained by differential growth and investigated cell proliferation during craniofacial morphogenesis.

Results

We obtained cranial tissues from four wild-caught bat species representing a range of facial diversity and labeled mitotic cells using immunohistochemistry. During craniofacial development, all bats display a conserved spatiotemporal distribution of proliferative cells with distinguishable zones of elevated mitosis. These areas were identified as modules by the spatial distribution analysis. Ancestral state reconstruction of proliferation rates and patterns in the facial module between species provided support, and a degree of explanation, for the developmental mechanisms underlying the two models of peramorphosis. In the long-faced species, Glossophaga soricina, whose facial shape evolved by hypermorphosis, cell proliferation rate is maintained at lower levels and for a longer period of time compared to the outgroup species Miniopterus natalensis. In both species of studied short-faced fruit bats, Carollia perspicillata and Artibeus jamaicensis, which evolved under the acceleration model, cell proliferation rate is increased compared to the outgroup.

Conclusions

This is the first study which links differential cellular proliferation and developmental modularity with heterochronic developmental changes, leading to the evolution of adaptive cranial diversity in an important group of mammals.

SHH Signaling Pathway Drives Pediatric Bone Sarcoma Progression

Primary bone tumors can be divided into two classes, benign and malignant. Among the latter group, osteosarcoma and Ewing sarcoma are the most prevalent malignant primary bone tumors in children and adolescents. Despite intensive efforts to improve treatments, almost 40% of patients succumb to the disease. Specifically, the clinical outcome for metastatic osteosarcoma or Ewing sarcoma remains poor; less than 30% of patients who present metastases will survive 5 years after initial diagnosis. One common and specific point of these bone tumors is their ability to deregulate bone homeostasis and remodeling and divert them to their benefit. Over the past years, considerable interest in the Sonic Hedgehog (SHH) pathway has taken place within the cancer research community. The activation of this SHH cascade can be done through different ways and, schematically, two pathways can be described, the canonical and the non-canonical. This review discusses the current knowledge about the involvement of the SHH signaling pathway in skeletal development, pediatric bone sarcoma progression and the related therapeutic options that may be possible for these tumors.

Finding the Unicorn, a New Mouse Model of Midfacial Clefting

Human midfacial clefting is a rare subset of orofacial clefting and in severe cases, the cleft separates the nostrils splitting the nose into two independent structures. To begin to understand the morphological and genetic causes of midfacial clefting we recovered the Unicorn mouse line. Unicorn embryos develop a complete midfacial cleft through the lip, and snout closely modelling human midfacial clefting. The Unicorn mouse line has ethylnitrosourea (ENU)-induced missense mutations in Raldh2 and Leo1. The mutations segregate with the cleft face phenotype. Importantly, the nasal cartilages and surrounding bones are patterned and develop normal morphology, except for the lateral displacement because of the cleft. We conclude that the midfacial cleft arises from the failure of the medial convergence of the paired medial nasal prominences between E10.5 to E11.5 rather than defective cell proliferation and death. Our work uncovers a novel mouse model and mechanism for the etiology of midfacial clefting.

Heterochronic shifts and conserved embryonic shape underlie crocodylian craniofacial disparity and convergence

The distinctive anatomy of the crocodylian skull is intimately linked with dietary ecology, resulting in repeated convergence on blunt- and slender-snouted ecomorphs. These evolutionary shifts depend upon modifications of the developmental processes which direct growth and morphogenesis. Here we examine the evolution of cranial ontogenetic trajectories to shed light on the mechanisms underlying convergent snout evolution. We use geometric morphometrics to quantify skeletogenesis in an evolutionary context and reconstruct ancestral patterns of ontogenetic allometry to understand the developmental drivers of craniofacial diversity within Crocodylia. Our analyses uncovered a conserved embryonic region of morphospace (CER) shared by all non-gavialid crocodylians regardless of their eventual adult ecomorph. This observation suggests the presence of conserved developmental processes during early development (before Ferguson stage 20) across most of Crocodylia. Ancestral state reconstruction of ontogenetic trajectories revealed heterochrony, developmental constraint, and developmental systems drift have all played essential roles in the evolution of ecomorphs. Based on these observations, we conclude that two separate, but interconnected, developmental programmes controlling craniofacial morphogenesis and growth enabled the evolutionary plasticity of skull shape in crocodylians.

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.

Neural crest and the patterning of vertebrate craniofacial muscles

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

Evolvability of the vertebrate craniofacial skeleton

The skull is a vertebrate novelty. Morphological adaptations of the skull are associated with major evolutionary transitions, including the shift to a predatory lifestyle and the ability to masticate while breathing. These adaptations include the chondrocranium, dermatocranium, articulated jaws, primary and secondary palates, internal choanae, the middle ear, and temporomandibular joint. The incredible adaptive diversity of the vertebrate skull indicates an underlying bauplan that promotes evolvability. Comparative studies in craniofacial development suggest that the craniofacial bauplan includes three secondary organizers, two that are bilaterally placed at the Hinge of the developing jaw, and one situated in the midline of the developing face (the FEZ). These organizers regulate tissue interactions between the cranial neural crest, the neuroepithelium, and facial and pharyngeal epithelia that regulate the development and evolvability of the craniofacial skeleton.

Developmental nonlinearity drives phenotypic robustness

Robustness to perturbation is a fundamental feature of complex organisms. Mutations are the raw material for evolution, yet robustness to their effects is required for species survival. The mechanisms that produce robustness are poorly understood. Nonlinearities are a ubiquitous feature of development that may link variation in development to phenotypic robustness. Here, we manipulate the gene dosage of a signaling molecule, Fgf8, a critical regulator of vertebrate development. We demonstrate that variation in Fgf8 expression has a nonlinear relationship to phenotypic variation, predicting levels of robustness among genotypes. Differences in robustness are not due to gene expression variance or dysregulation, but emerge from the nonlinearity of the genotype–phenotype curve. In this instance, embedded features of development explain robustness differences. How such features vary in natural populations and relate to genetic variation are key questions for unraveling the origin and evolvability of this feature of organismal development.

Developmental processes often involve nonlinearities, but the consequences for translating genotype to phenotype are not well characterized. Here, Green et al. vary Fgf8 signaling across allelic series of mice and show that phenotypic robustness in craniofacial shape is explained by a nonlinear effect of Fgf8 expression.

Discovery, Diagnosis, and Etiology of Craniofacial Ciliopathies

Seventy-five percent of congenital disorders present with some form of craniofacial malformation. The frequency and severity of these malformations makes understanding the etiological basis crucial for diagnosis and treatment. A significant link between craniofacial malformations and primary cilia arose several years ago with the determination that ∼30% of ciliopathies could be primarily defined by their craniofacial phenotype. The link between the cilium and the face has proven significant, as several new "craniofacial ciliopathies" have recently been diagnosed. Herein, we reevaluate public disease databases, report several new craniofacial ciliopathies, and propose several "predicted" craniofacial ciliopathies. Furthermore, we discuss why the craniofacial complex is so sensitive to ciliopathic dysfunction, addressing tissue-specific functions of the cilium as well as its role in signal transduction relevant to craniofacial development. As a whole, these analyses suggest a characteristic facial phenotype associated with craniofacial ciliopathies that can perhaps be used for rapid discovery and diagnosis of similar disorders in the future.

The Cyprinodon variegatus genome reveals gene expression changes underlying differences in skull morphology among closely related species

BACKGROUND

Understanding the genetic and developmental origins of phenotypic novelty is central to the study of biological diversity. In this study we identify modifications to the expression of genes at four developmental stages that may underlie jaw morphological differences among three closely related species of pupfish (genus Cyprinodon) from San Salvador Island, Bahamas. Pupfishes on San Salvador Island are trophically differentiated and include two endemic species that have evolved jaw morphologies unlike that of any other species in the genus Cyprinodon.

RESULTS

We find that gene expression differs significantly across recently diverged species of pupfish. Genes such as Bmp4 and calmodulin, previously implicated in jaw diversification in African cichlid fishes and Galapagos finches, were not found to be differentially expressed among species of pupfish. Instead we find multiple growth factors and cytokine/chemokine genes to be differentially expressed among these pupfish taxa. These include both genes and pathways known to affect craniofacial development, such as Wnt signaling, as well as novel genes and pathways not previously implicated in craniofacial development. These data highlight both shared and potentially unique sources of jaw diversity in pupfish and those identified in other evolutionary model systems such as Galapagos finches and African cichlids.

CONCLUSIONS

We identify modifications to the expression of genes involved in Wnt signaling, Igf signaling, and the inflammation response as promising avenues for future research. Our project provides insight into the magnitude of gene expression changes contributing to the evolution of morphological novelties, such as jaw structure, in recently diverged pupfish species.

Mouth development

A mouth is present in all animals, and comprises an opening from the outside into the oral cavity and the beginnings of the digestive tract to allow eating. This review focuses on the earliest steps in mouth formation. In the first half, we conclude that the mouth arose once during evolution. In all animals, the mouth forms from ectoderm and endoderm. A direct association of oral ectoderm and digestive endoderm is present even in triploblastic animals, and in chordates, this region is known as the extreme anterior domain (EAD). Further support for a single origin of the mouth is a conserved set of genes that form a 'mouth gene program' including foxA and otx2. In the second half of this review, we discuss steps involved in vertebrate mouth formation, using the frog Xenopus as a model. The vertebrate mouth derives from oral ectoderm from the anterior neural ridge, pharyngeal endoderm and cranial neural crest (NC). Vertebrates form a mouth by breaking through the body covering in a precise sequence including specification of EAD ectoderm and endoderm as well as NC, formation of a 'pre-mouth array,' basement membrane dissolution, stomodeum formation, and buccopharyngeal membrane perforation. In Xenopus, the EAD is also a craniofacial organizer that guides NC, while reciprocally, the NC signals to the EAD to elicit its morphogenesis into a pre-mouth array. Human mouth anomalies are prevalent and are affected by genetic and environmental factors, with understanding guided in part by use of animal models. WIREs Dev Biol 2017, 6:e275. doi: 10.1002/wdev.275 For further resources related to this article, please visit the WIREs website.

A tissue-specific role for intraflagellar transport genes during craniofacial development

Primary cilia are nearly ubiquitous, cellular projections that function to transduce molecular signals during development. Loss of functional primary cilia has a particularly profound effect on the developing craniofacial complex, causing several anomalies including craniosynostosis, micrognathia, midfacial dysplasia, cleft lip/palate and oral/dental defects. Development of the craniofacial complex is an intricate process that requires interactions between several different tissues including neural crest cells, neuroectoderm and surface ectoderm. To understand the tissue-specific requirements for primary cilia during craniofacial development we conditionally deleted three separate intraflagellar transport genes, Kif3a, Ift88 and Ttc21b with three distinct drivers, Wnt1-Cre, Crect and AP2-Cre which drive recombination in neural crest, surface ectoderm alone, and neural crest, surface ectoderm and neuroectoderm, respectively. We found that tissue-specific conditional loss of ciliary genes with different functions produces profoundly different facial phenotypes. Furthermore, analysis of basic cellular behaviors in these mutants suggests that loss of primary cilia in a distinct tissue has unique effects on development of adjacent tissues. Together, these data suggest specific spatiotemporal roles for intraflagellar transport genes and the primary cilium during craniofacial development.

Implications of Vertebrate Craniodental Evo-Devo for Human Oral Health

Highly processed diets eaten by postindustrial modern human populations coincide with higher frequencies of third molar impaction, malocclusion, and temporomandibular joint disorders that affect millions of people worldwide each year. Current treatments address symptoms, not causes, because the multifactorial etiologies of these three concerns mask which factors incline certain people to malocclusion, impaction, and/or joint issues. Deep scientific curiosity about the origins of jaws and dentitions continues to yield rich insights about the developmental genetic mechanisms that underpin healthy craniodental morphogenesis and integration. Mounting evidence from evolution and development (Evo-Devo) studies suggests that function is another mechanism important to healthy craniodental integration and fit. Starting as early as weaning, softer diets and thus lower bite forces appear to relax or disrupt integration of oral tissues, alter development and growth, and catalyze impaction, malocclusion, and jaw joint disorders. How developing oral tissues respond to bite forces remains poorly understood, but biomechanical feedback seems to alter balances of local bone resorption and deposition at the tooth-bone interface as well as affect tempos and amounts of facial outgrowth. Also, behavioral changes in jaw function and parafunction contribute to degeneration and pain in joint articular cartilages and masticatory muscles. The developmental genetic contribution to craniodental misfits and disorders is undeniable but still unclear; however, at present, human diet and jaw function remain important and much more actionable clinical targets. New Evo-Devo studies are needed to explain how function interfaces with craniodental phenotypic plasticity, variation, and evolvability to yield a spectrum of healthy and mismatched dentitions and jaws.

Growth factor signaling alters the morphology of the zebrafish ethmoid plate

Craniofacial development relies on coordinated tissue interactions that allow for patterning and growth of the face. We know a priori that the Wingless, fibroblast growth factor, Hedgehog and transforming growth factor-beta growth factor signaling pathways are required for the development of the face, but how they contribute to the shape of the face is largely untested. Here, we test how each signaling pathway contributes to the overall morphology of the zebrafish anterior neurocranium. We tested the contribution of each signaling pathway to the development of the ethmoid plate during three distinct time periods: the time of neural crest migration [10 hour post fertilization (hpf)]; once the neural crest is resident in the face (20 hpf); and finally at the time at which the cartilaginous condensations are being initiated (48 hpf). Using geometric morphometric analysis, we conclude that each signaling pathway contributes to the shape, size and morphology of the ethmoid plate in a dose-, and time-dependent fashion.

Why the short face? Developmental disintegration of the neurocranium drives convergent evolution in neotropical electric fishes

Convergent evolution is widely viewed as strong evidence for the influence of natural selection on the origin of phenotypic design. However, the emerging evo‐devo synthesis has highlighted other processes that may bias and direct phenotypic evolution in the presence of environmental and genetic variation. Developmental biases on the production of phenotypic variation may channel the evolution of convergent forms by limiting the range of phenotypes produced during ontogeny. Here, we study the evolution and convergence of brachycephalic and dolichocephalic skull shapes among 133 species of Neotropical electric fishes (Gymnotiformes: Teleostei) and identify potential developmental biases on phenotypic evolution. We plot the ontogenetic trajectories of neurocranial phenotypes in 17 species and document developmental modularity between the face and braincase regions of the skull. We recover a significant relationship between developmental covariation and relative skull length and a significant relationship between developmental covariation and ontogenetic disparity. We demonstrate that modularity and integration bias the production of phenotypes along the brachycephalic and dolichocephalic skull axis and contribute to multiple, independent evolutionary transformations to highly brachycephalic and dolichocephalic skull morphologies.

Fluctuations in Evolutionary Integration Allow for Big Brains and Disparate Faces

In theory, evolutionary modularity allows anatomical structures to respond differently to selective regimes, thus promoting morphological diversification. These differences can then influence the rate and direction of phenotypic evolution among structures. Here we use geometric morphometrics and phenotypic matrix statistics to compare rates of craniofacial evolution and estimate evolvability in the face and braincase modules of a clade of teleost fishes (Gymnotiformes) and a clade of mammals (Carnivora), both of which exhibit substantial craniofacial diversity. We find that the face and braincase regions of both clades display different degrees of integration. We find that the face and braincase evolve at similar rates in Gymnotiformes and the reverse in Carnivora with the braincase evolving twice as fast as the face. Estimates of evolvability and constraints in these modules suggest differential responses to selection arising from fluctuations in phylogenetic integration, thus influencing differential rates of skull-shape evolution in these two clades.

Craniofacial Ciliopathies Reveal Specific Requirements for GLI Proteins during Development of the Facial Midline

Ciliopathies represent a broad class of disorders that affect multiple organ systems. The craniofacial complex is among those most severely affected when primary cilia are not functional. We previously reported that loss of primary cilia on cranial neural crest cells, via a conditional knockout of the intraflagellar transport protein KIF3a, resulted in midfacial widening due to a gain of Hedgehog (HH) activity. Here, we examine the molecular mechanism of how a loss of primary cilia can produce facial phenotypes associated with a gain of HH function. We show that loss of intraflagellar transport proteins (KIF3a or IFT88) caused aberrant GLI processing such that the amount of GLI3FL and GLI2FL was increased, thus skewing the ratio of GLIFL to GLIR in favor of the FL isoform. Genetic addition of GLI3R partially rescued the ciliopathic midfacial widening. Interestingly, despite several previous studies suggesting midfacial development relies heavily on GLI3R activity, the conditional loss of GLI3 alone did not reproduce the ciliopathic phenotype. Only the combined loss of both GLI2 and GLI3 was able to phenocopy the ciliopathic midfacial appearance. Our findings suggest that ciliopathic facial phenotypes are generated via loss of both GLI3R and GLI2R and that this pathology occurs via a de-repression mechanism. Furthermore, these studies suggest a novel role for GLI2R in craniofacial development.

Primary cilia are ubiquitous organelles that serve to transduce molecular signals within a cell. Loss of functional primary cilia results in a disease class called ciliopathies. Ciliopathies have a broad range of phenotypes; however, severe facial anomalies are commonly associated with this disease class. The facial midline is particularly sensitive to loss of primary cilia, frequently undergoing a significant widening. This phenotype is similar to that which occurs when there are gain-of-function defects in the Sonic Hedgehog pathway. This manuscript addresses the molecular basis for midfacial widening in ciliopathies. Importantly, we determine mechanisms to both rescue and phenocopy the ciliopathic midfacial phenotype. In sum, this work provides novel insight into the molecular mechanisms of midfacial patterning and the extent to which loss of cilia impact that process.

Frontonasal Dysplasia: Towards an Understanding of Molecular and Developmental Aetiology

The complex anatomy of the skull and face arises from the requirement to support multiple sensory and structural functions. During embryonic development, the diverse component elements of the neuro- and viscerocranium must be generated independently and subsequently united in a manner that sustains and promotes the growth of the brain and sensory organs, while achieving a level of structural integrity necessary for the individual to become a free-living organism. While each of these individual craniofacial components is essential, the cranial and facial midline lies at a structural nexus that unites these disparately derived elements, fusing them into a whole. Defects of the craniofacial midline can have a profound impact on both form and function, manifesting in a diverse array of phenotypes and clinical entities that can be broadly defined as frontonasal dysplasias (FNDs). Recent advances in the identification of the genetic basis of FNDs along with the analysis of developmental mechanisms impacted by these mutations have dramatically altered our understanding of this complex group of conditions.

The Nervous System Orchestrates and Integrates Craniofacial Development: A Review

Development of a head is a dazzlingly complex process: a number of distinct cellular sources including cranial ecto- and endoderm, mesoderm and neural crest contribute to facial and other structures. In the head, an extremely fine-tuned developmental coordination of CNS, peripheral neural components, sensory organs and a musculo-skeletal apparatus occurs, which provides protection and functional integration. The face can to a large extent be considered as an assembly of sensory systems encased and functionally fused with appendages represented by jaws. Here we review how the developing brain, neurogenic placodes and peripheral nerves influence the morphogenesis of surrounding tissues as a part of various general integrative processes in the head. The mechanisms of this impact, as we understand it now, span from the targeted release of the morphogens necessary for shaping to providing a niche for cellular sources required in later development. In this review we also discuss the most recent findings and ideas related to how peripheral nerves and nerve-associated cells contribute to craniofacial development, including teeth, during the post- neural crest period and potentially in regeneration.

Hedgehog receptor function during craniofacial development

The Hedgehog signalling pathway plays a fundamental role in orchestrating normal craniofacial development in vertebrates. In particular, Sonic hedgehog (Shh) is produced in three key domains during the early formation of the head; neuroectoderm of the ventral forebrain, facial ectoderm and the pharyngeal endoderm; with signal transduction evident in both ectodermal and mesenchymal tissue compartments. Shh signalling from the prechordal plate and ventral midline of the diencephalon is required for appropriate division of the eyefield and forebrain, with mutation in a number of pathway components associated with Holoprosencephaly, a clinically heterogeneous developmental defect characterized by a failure of the early forebrain vesicle to divide into distinct halves. In addition, signalling from the pharyngeal endoderm and facial ectoderm plays an essential role during development of the face, influencing cranial neural crest cells that migrate into the early facial processes. In recent years, the complexity of Shh signalling has been highlighted by the identification of multiple novel proteins that are involved in regulating both the release and reception of this protein. Here, we review the contributions of Shh signalling during early craniofacial development, focusing on Hedgehog receptor function and describing the consequences of disruption for inherited anomalies of this region in both mouse models and human populations.

The chick embryo as a model for the effects of prenatal exposure to alcohol on craniofacial development

Prenatal exposure to ethanol results in fetal alcohol spectrum disorder (FASD), a syndrome characterised by a broad range of clinical manifestations including craniofacial dysmorphologies and neurological defects. The characterisation of the mechanisms by which ethanol exerts its teratogenic effects is difficult due to the pleiotropic nature of its actions. Different experimental model systems have been employed to investigate the aetiology of FASD. Here, I will review studies using these different model organisms that have helped to elucidate how ethanol causes the craniofacial abnormalities characteristic of FASD. In these studies, ethanol was found to impair the prechordal plate-an important embryonic signalling centre-during gastrulation and to negatively affect the induction, migration and survival of the neural crest, a cell population that generates the cartilage and most of the bones of the skull. At the cellular level, ethanol appears to inhibit Sonic hedgehog signalling, alter levels of retionoic acid activity, trigger a Ca(2+)-CamKII-dependent pathway that antagonises WNT signalling, affect cytoskeletal dynamics and increase oxidative stress. Embryos of the domestic chick Gallus gallus domesticus have played a central role in developing a working model for the effects of ethanol on craniofacial development because they are easily accessible and because key steps in craniofacial development are particularly well established in the avian embryo. I will finish this review by highlighting some potential future avenues of fetal alcohol research.

Ectodermal Wnt controls nasal pit morphogenesis through modulation of the BMP/FGF/JNK signaling axis

BACKGROUND

Mutations of WNT3, WNT5A, WNT9B, and WNT11 genes are associated with orofacial birth defects, including nonsyndromic cleft lip with cleft palate in humans. However, the source of Wnt ligands and their signaling effects on the orofacial morphogenetic process remain elusive.

RESULTS

Using Foxg1-Cre to impair Wnt secretion through the inactivation of Gpr177/mWls, we investigate the relevant regulation of Wnt production and signaling in nasal-facial development. Ectodermal ablation of Gpr177 leads to severe facial deformities resulting from dramatically reduced cell proliferation and increased cell death due to a combined loss of WNT, FGF and BMP signaling in the developing facial prominence. In the invaginating nasal pit, the Gpr177 disruption also causes a detrimental effect on migration of the olfactory epithelial cells into the mesenchymal region. The blockage of Wnt secretion apparently impairs the olfactory epithelial cells through modulation of JNK signaling.

CONCLUSIONS

Our study thus suggests the head ectoderm, including the facial ectoderm and the neuroectoderm, as the source of canonical as well as noncanonical Wnt ligands during early development of the nasal-facial prominence. Both β-catenin-dependent and -independent signaling pathways are required for proper development of these morphogenetic processes.

Developmental mechanisms underlying variation in craniofacial disease and evolution

Craniofacial disease phenotypes exhibit significant variation in penetrance and severity. Although many genetic contributions to phenotypic variation have been identified, genotype-phenotype correlations remain imprecise. Recent work in evolutionary developmental biology has exposed intriguing developmental mechanisms that potentially explain incongruities in genotype-phenotype relationships. This review focuses on two observations from work in comparative and experimental animal model systems that highlight how development structures variation. First, multiple genetic inputs converge on relatively few developmental processes. Investigation of when and how variation in developmental processes occurs may therefore help predict potential genetic interactions and phenotypic outcomes. Second, genetic mutation is typically associated with an increase in phenotypic variance. Several models outlining developmental mechanisms underlying mutational increases in phenotypic variance are discussed using Satb2-mediated variation in jaw size as an example. These data highlight development as a critical mediator of genotype-phenotype correlations. Future research in evolutionary developmental biology focusing on tissue-level processes may help elucidate the "black box" between genotype and phenotype, potentially leading to novel treatment, earlier diagnoses, and better clinical consultations for individuals affected by craniofacial anomalies.

Chronic up-regulation of sonic hedgehog has little effect on postnatal craniofacial morphology of euploid and trisomic mice

BACKGROUND

In Ts65Dn, a mouse model of Down syndrome (DS), brain and craniofacial abnormalities that parallel those in people with DS are linked to an attenuated cellular response to sonic hedgehog (SHH) signaling. If a similarly reduced response to SHH occurs in all trisomic cells, then chronic up-regulation of the pathway might have a positive effect on development in trisomic mice, resulting in amelioration of the craniofacial anomalies.

RESULTS

We crossed Ts65Dn with Ptch1(tm1Mps/+) mice and quantified the craniofacial morphology of Ts65Dn;Ptch(+/-) offspring to assess whether a chronic up-regulation of the SHH pathway rescued DS-related anomalies. Ts65Dn;Ptch1(+/-) mice experience a chronic increase in SHH in SHH-receptive cells due to haploinsufficiency of the pathway suppressor, Ptch1. Chronic up-regulation had minimal effect on craniofacial shape and did not correct facial abnormalities in Ts65Dn;Ptch(+/-) mice. We further compared effects of this chronic up-regulation of SHH with acute pathway stimulation in mice treated on the day of birth with a SHH pathway agonist, SAG. We found that SHH affects facial morphology differently based on chronic vs. acute postnatal pathway up-regulation.

CONCLUSIONS

Our findings have implications for understanding the function of SHH in craniofacial development and for the potential use of SHH-based agonists to treat DS-related abnormalities.

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.

Utilizing the chicken as an animal model for human craniofacial ciliopathies

The chicken has been a particularly useful model for the study of craniofacial development and disease for over a century due to their relatively large size, accessibility, and amenability for classical bead implantation and transplant experiments. Several naturally occurring mutant lines with craniofacial anomalies also exist and have been heavily utilized by developmental biologist for several decades. Two of the most well known lines, talpid(2) (ta(2)) and talpid(3) (ta(3)), represent the first spontaneous mutants to have the causative genes identified. Despite having distinct genetic causes, both mutants have recently been identified as ciliopathic. Excitingly, both of these mutants have been classified as models for human craniofacial ciliopathies: Oral-facial-digital syndrome (ta(2)) and Joubert syndrome (ta(3)). Herein, we review and compare these two models of craniofacial disease and highlight what they have revealed about the molecular and cellular etiology of ciliopathies. Furthermore, we outline how applying classical avian experiments and new technological advances (transgenics and genome editing) with naturally occurring avian mutants can add a tremendous amount to what we currently know about craniofacial ciliopathies.

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.

Facial Morphogenesis: Physical and Molecular Interactions Between the Brain and the Face

Morphogenesis of the brain and face is intrinsically linked by a number of factors. These include: origins of tissues, adjacency allowing their physical interactions, and molecular cross talk controlling growth. Neural crest cells that form the facial primordia originate on the dorsal neural tube. In the caudal pharyngeal arches, a Homeobox code regulates arch identity. In anterior regions, positional information is acquired locally. Second, the brain is a structural platform that influences positioning of the facial primordia, and brain growth influences the timing of primordia fusion. Third, the brain helps induce a signaling center, the frontonasal ectodermal zone, in the ectoderm, which participates in patterned growth of the upper jaw. Similarly, signals from neural crest cells regulate expression of fibroblast growth factor 8 in the anterior neural ridge, which controls growth of the anterior forebrain. Disruptions to these interactions have significant consequences for normal development of the craniofacial complex, leading to structural malformations and birth defects.

The Roles of Hedgehog Signaling in Upper Lip Formation

Craniofacial development consists of a highly complex sequence of the orchestrated growth and fusion of facial processes. It is also known that craniofacial abnormalities can be detected in 1/3 of all patients with congenital diseases. Within the various craniofacial abnormalities, orofacial clefting is one of the most common phenotypic outcomes associated with retarded facial growth or fusion. Cleft lip is one of the representative and frequently encountered conditions in the spectrum of orofacial clefting. Despite various mechanisms or signaling pathways that have been proposed to be the cause of cleft lip, a detailed mechanism that bridges individual signaling pathways to the cleft lip is still elusive. Shh signaling is indispensable for normal embryonic development, and disruption can result in a wide spectrum of craniofacial disorders, including cleft lip. This review focuses on the current knowledge about the mechanisms of facial development and the etiology of cleft lip that are related to Shh signaling.

Signals from the brain induce variation in avian facial shape

BACKGROUND

How developmental mechanisms generate the phenotypic variation that is the raw material for evolution is largely unknown. Here, we explore whether variation in a conserved signaling axis between the brain and face contributes to differences in morphogenesis of the avian upper jaw. In amniotes, including both mice and avians, signals from the brain establish a signaling center in the ectoderm (the Frontonasal ectodermal zone or "FEZ") that directs outgrowth of the facial primordia.

RESULTS

Here we show that the spatial organization of this signaling center differs among avians, and these correspond to Sonic hedgehog (Shh) expression in the basal forebrain and embryonic facial shape. In ducks this basal forebrain domain is present almost the entire width, while in chickens it is restricted to the midline. When the duck forebrain is unilaterally transplanted into stage matched chicken embryos the face on the treated side resembles that of the donor.

CONCLUSIONS

Combined with previous findings, these results demonstrate that variation in a highly conserved developmental pathway has the potential to contribute to evolutionary differences in avian upper jaw morphology. Developmental Dynamics 244:1133-1143, 2015. © 2015 Wiley Periodicals, Inc.

A molecular mechanism for the origin of a key evolutionary innovation, the bird beak and palate, revealed by an integrative approach to major transitions in vertebrate history

The avian beak is a key evolutionary innovation whose flexibility has permitted birds to diversify into a range of disparate ecological niches. We approached the problem of the mechanism behind this innovation using an approach bridging paleontology, comparative anatomy, and experimental developmental biology. First, we used fossil and extant data to show the beak is distinctive in consisting of fused premaxillae that are geometrically distinct from those of ancestral archosaurs. To elucidate underlying developmental mechanisms, we examined candidate gene expression domains in the embryonic face: the earlier frontonasal ectodermal zone (FEZ) and the later midfacial WNT-responsive region, in birds and several reptiles. This permitted the identification of an autapomorphic median gene expression region in Aves. To test the mechanism, we used inhibitors of both pathways to replicate in chicken the ancestral amniote expression. Altering the FEZ altered later WNT responsiveness to the ancestral pattern. Skeletal phenotypes from both types of experiments had premaxillae that clustered geometrically with ancestral fossil forms instead of beaked birds. The palatal region was also altered to a more ancestral phenotype. This is consistent with the fossil record and with the tight functional association of avian premaxillae and palate in forming a kinetic beak.

A dynamic Shh expression pattern, regulated by SHH and BMP signaling, coordinates fusion of primordia in the amniote face

The mechanisms of morphogenesis are not well understood, yet shaping structures during development is essential for establishing correct organismal form and function. Here, we examine mechanisms that help to shape the developing face during the crucial period of facial primordia fusion. This period of development is a time when the faces of amniote embryos exhibit the greatest degree of similarity, and it probably results from the necessity for fusion to occur to establish the primary palate. Our results show that hierarchical induction mechanisms, consisting of iterative signaling by Sonic hedgehog (SHH) followed by Bone morphogenetic proteins (BMPs), regulate a dynamic expression pattern of Shh in the ectoderm covering the frontonasal (FNP) and maxillary (MxP) processes. Furthermore, this Shh expression domain contributes to the morphogenetic processes that drive the directional growth of the globular process of the FNP toward the lateral nasal process and MxP, in part by regulating cell proliferation in the facial mesenchyme. The nature of the induction mechanism that we discovered suggests that the process of fusion of the facial primordia is intrinsically buffered against producing maladaptive morphologies, such as clefts of the primary palate, because there appears to be little opportunity for variation to occur during expansion of the Shh expression domain in the ectoderm of the facial primordia. Ultimately, these results might explain why this period of development constitutes a phylotypic stage of facial development among amniotes.

Acute upregulation of hedgehog signaling in mice causes differential effects on cranial morphology

Hedgehog (HH) signaling, and particularly signaling by sonic hedgehog (SHH), is implicated in several essential activities during morphogenesis, and its misexpression causes a number of developmental disorders in humans. In particular, a reduced mitogenic response of cerebellar granule cell precursors to SHH signaling in a mouse model for Down syndrome (DS), Ts65Dn, is substantially responsible for reduced cerebellar size. A single treatment of newborn trisomic mice with an agonist of the SHH pathway (SAG) normalizes cerebellar morphology and restores some cognitive deficits, suggesting a possible therapeutic application of SAG for treating the cognitive impairments of DS. Although the beneficial effects on the cerebellum are compelling, inappropriate activation of the HH pathway causes anomalies elsewhere in the head, particularly in the formation and patterning of the craniofacial skeleton. To determine whether an acute treatment of SAG has an effect on craniofacial morphology, we quantitatively analyzed the cranial form of adult euploid and Ts65Dn mice that were injected with either SAG or vehicle at birth. We found significant deformation of adult craniofacial shape in some animals that had received SAG at birth. The most pronounced differences between the treated and untreated mice were in the midline structures of the facial skeleton. The SAG-driven craniofacial dysmorphogenesis was dose-dependent and possibly incompletely penetrant at lower concentrations. Our findings illustrate that activation of HH signaling, even with an acute postnatal stimulation, can lead to localized dysmorphology of the skull by generating modular shape changes in the facial skeleton. These observations have important implications for translating HH-agonist-based treatments for DS.