Intentional cranial vault deformation and induced changes of the cranial base and face

Three morphologically distinct populations of Peruvian crania (n = 130) were metrically analysed to quantify changes resulting from intentional artificial vault deformation. Two of these samples are artificially deformed (anteroposterior [AP] and circumferential [C] types). Measurements taken from lateral radiographs demonstrated that alternative forms of the cranial base angle (N-S-Ba, planum angle, planum sphenoidale to plane of the clivus and PANG angle, planum sphenoidale to basion-sella plane) and the orbital and OANG angles (orbital roof to plane of the clivus and basion-sella plane, respectively) of both deformed groups increased while the angle S-Ba-O decreased significantly with respect to the undeformed (N) sample. Changes in the AP group are largely due to anteroinferior displacement of the basion-sella plane. Similar changes in group C are amplified by this group's posterosuperior frontal migration. This migration results in a relatively shallow orbit at the orbital plate/frontal squama interface. Unlike the deformation experienced by the external vault plates, the basion-sella plane orientation remains stable with respect to the Frankfort Horizontal. Additionally, nasal region measurements such as maximum nasal aperture breadth and nasal height were largely stable between each deformed group and the undeformed group. However, facial (bimaxillary and bizygomatic), basicranial, cranial, and frontal breadths decreased significantly from group AP to group N to group C. Thus, gross morphological facial changes between each undeformed group and the control group are largely accounted for by dimensional changes in peripheral structures. These results stress the importance of the dynamic interrelationship between the cranial vault and base in the development of the craniofacial complex.     

Independent axes of genetic variation and parallel evolutionary divergence of opercle bone shape in threespine stickleback

Evolution of similar phenotypes in independent populations is often taken as evidence of adaptation to the same fitness optimum. However, the genetic architecture of traits might cause evolution to proceed more often toward particular phenotypes, and less often toward others, independently of the adaptive value of the traits. Freshwater populations of Alaskan threespine stickleback have repeatedly evolved the same distinctive opercle shape after divergence from an oceanic ancestor. Here we demonstrate that this pattern of parallel evolution is widespread, distinguishing oceanic and freshwater populations across the Pacific Coast of North America and Iceland. We test whether this parallel evolution reflects genetic bias by estimating the additive genetic variance-covariance matrix (G) of opercle shape in an Alaskan oceanic (putative ancestral) population. We find significant additive genetic variance for opercle shape and that G has the potential to be biasing, because of the existence of regions of phenotypic space with low additive genetic variation. However, evolution did not occur along major eigenvectors of G, rather it occurred repeatedly in the same directions of high evolvability. We conclude that the parallel opercle evolution is most likely due to selection during adaptation to freshwater habitats, rather than due to biasing effects of opercle genetic architecture.     

Combined function of HoxA and HoxB clusters in neural crest cells

The evolution of chordates was accompanied by critical anatomical innovations in craniofacial development, along with the emergence of neural crest cells. The potential of these cells to implement a craniofacial program in part depends upon the (non-)expression of Hox genes. For instance, the development of jaws requires the inhibition of Hox genes function in the first pharyngeal arch. In contrast, Hox gene products induce craniofacial structures in more caudal territories. To further investigate which Hox gene clusters are involved in this latter role, we generated HoxA;HoxB cluster double mutant animals in cranial neural crest cells. We observed the appearance of a supernumerary dentary-like bone with an endochondral ossification around a neo-Meckel's cartilage matrix and an attachment of neo-muscle demonstrating that HoxB genes enhance the phenotype induced by the deletion of the HoxA cluster alone. In addition, a cervical and hypertrophic thymus was associated with the supernumerary dentary-like bone, which may reflect its ancestral position near the filtrating system. Altogether these results show that the HoxA and HoxB clusters cooperated during evolution to lead to present craniofacial diversity.     

Molecular and therapeutic characterization of anti-ectodysplasin A receptor (EDAR) agonist monoclonal antibodies

The TNF family ligand ectodysplasin A (EDA) and its receptor EDAR are required for proper development of skin appendages such as hair, teeth, and eccrine sweat glands. Loss of function mutations in the Eda gene cause X-linked hypohidrotic ectodermal dysplasia (XLHED), a condition that can be ameliorated in mice and dogs by timely administration of recombinant EDA. In this study, several agonist anti-EDAR monoclonal antibodies were generated that cross-react with the extracellular domains of human, dog, rat, mouse, and chicken EDAR. Their half-life in adult mice was about 11 days. They induced tail hair and sweat gland formation when administered to newborn EDA-deficient Tabby mice, with an EC(50) of 0.1 to 0.7 mg/kg. Divalency was necessary and sufficient for this therapeutic activity. Only some antibodies were also agonists in an in vitro surrogate activity assay based on the activation of the apoptotic Fas pathway. Activity in this assay correlated with small dissociation constants. When administered in utero in mice or at birth in dogs, agonist antibodies reverted several ectodermal dysplasia features, including tooth morphology. These antibodies are therefore predicted to efficiently trigger EDAR signaling in many vertebrate species and will be particularly suited for long term treatments.     

Visualization of Chondrocyte Intercalation and Directional Proliferation via Zebrabow Clonal Cell Analysis in the Embryonic Meckel’s Cartilage

Development of the vertebrate craniofacial structures requires precise coordination of cell migration, proliferation, adhesion and differentiation. Patterning of the Meckel''s cartilage, a first pharyngeal arch derivative, involves the migration of cranial neural crest (CNC) cells and the progressive partitioning, proliferation and organization of differentiated chondrocytes. Several studies have described CNC migration during lower jaw morphogenesis, but the details of how the chondrocytes achieve organization in the growth and extension of Meckel’s cartilage remains unclear. The sox10 restricted and chemically induced Cre recombinase-mediated recombination generates permutations of distinct fluorescent proteins (RFP, YFP and CFP), thereby creating a multi-spectral labeling of progenitor cells and their progeny, reflecting distinct clonal populations. Using confocal time-lapse photography, it is possible to observe the chondrocytes behavior during the development of the zebrafish Meckel’s cartilage.Multispectral cell labeling enables scientists to demonstrate extension of the Meckel’s chondrocytes. During extension phase of the Meckel’s cartilage, which prefigures the mandible, chondrocytes intercalate to effect extension as they stack in an organized single-cell layered row. Failure of this organized intercalating process to mediate cell extension provides the cellular mechanistic explanation for hypoplastic mandible that we observe in mandibular malformations.     

Fgf8 haploinsufficiency results in distinct craniofacial defects in adult zebrafish

Significant progress has been made toward understanding the role of fgf8 in directing early embryonic patterning of the pharyngeal skeleton. Considerably less is known about the role this growth factor plays in the coordinated development, growth, and remodeling of the craniofacial skeleton beyond embryonic stages. To better understand the contributions of fgf8 in the formation of adult craniofacial architecture, we analyzed the skeletal anatomy of adult ace(ti282a)/fgf8 heterozygous zebrafish. Our results revealed distinct skeletal defects including facial asymmetries, aberrant craniofacial geometry, irregular patterns of cranial suturing, and ectopic bone formation. These defects are similar in presentation to several human craniofacial disorders (e.g., craniosynostosis, hemifacial microsomia), and may be related to increased levels of bone metabolism observed in ace(ti282a)/fgf8 heterozygotes. Moreover, skeletal defects observed in ace(ti282a)/fgf8 heterozygotes are consistent with expression patterns of fgf8 in the mature craniofacial skeleton. These data reveal previously unrecognized roles for fgf8 during skeletogenesis, and provide a basis for future investigations into the mechanisms that regulate craniofacial development beyond the embryo.