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Session Schedule & Abstracts

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Saturday 2nd July, 2016

Hard-Tissue Biology 1

Room: Salon G   9:30 am–11:00 am

Moderator(s): Boughner JC, Yaryhin O
HRD1-1  9:30 am  Development of the basal chondrocranial elements in lizards . Yaryhin O*, I. I. Schmalhausen institute of zoology NAS of Ukraine; Werneburg I, Senckenberg Center for Human Evolution and Palaeoenvironment (HEP) at Eberhard Karls Universität
Abstract: The neurocranium of vertebrates is mainly derived from early cartilaginous anlagen, the so-called chondrocranium. In general, two initial bar-shaped and paired chondrifications flank the notochord, the more rostral trabecles and the more caudal parachordals. In most reptiles, there is an additional component, the transverse acrochordal cartilage, which is placed between trabecles and parachordals. All these elements compose the base of the future chondrocranium. There are several theories concerning the development and interrelationship of these elements; i.e., the development of the basal plate, the formation of the basicranial fenestra, and the role of the acrochordal cartilage in the formation of crista sellaris. In the present study, we reexamined the basicranial development in one of the previously well-described skink species Chalcides ocellatus and compare it with that of Lacerta agilis. We found that C. ocellatus shows very similar conditions of early chondrocranial development when compared to L. agilis. The anterior most part of the notochord is not embedded into the basal plate as it was previously reported. It remains free. The medial edges of the parachordals form the lateral walls of the basicranial fenestra. Only the posterior portions of the parachordals fuse and form the basal plate. The space in-between the parachordals is fulfilled with a thin layer of cells, which, however, never chondrifies. The anterior most tips of the parachordals later fuse with the posterior edge of the acrochordal cartilage, which finally delimitates the basicranial fenestra anteriorly. Thus, crista sellaris does not form from the most anterior part of the basal plate, as it was previously thought, but from the acrochordal cartilage. We consider the observed processes a common development at least in lizards and discuss a variety of methodological approaches and differences in data interpretation as reasons for the anatomical differences reported in the literature.

HRD1-2  9:45 am  Body size and parafrontal bones in the Sphaerodactylidae (Reptilia: Squamata: Gekkota). Griffing AH*, Villanova University; Bauer AM, Villanova University
Abstract: Well-resolved phylogenetic hypotheses and ontogenetic data are often necessary to investigate the evolution of structural novelty. With recent increased resolution of gecko phylogenies, questions of homology and convergence can now be investigated. The gecko family Sphaerodactylidae comprises several genera of miniaturized geckos, including the smallest known amniote. The genera Aristelliger and Teratoscincus are exceptions, with taxa reaching snout-to-vent lengths of up to 150 mm. These genera possess enigmatic, supraorbital ossifications, parafrontal bones, which are not found in any other squamates. Originally believed to be a product of evolutionary convergence, these structures have remained uninvestigated since their discovery. Though relationships between other Old World sphaerodactylids remain unresolved, recent molecular and morphological data has supported a close relationship between Aristelliger and Teratoscincus. We investigated the ontogeny of these bones in both Aristelliger and Teratoscincus to better understand the relationship between body size and the presence of parafrontal bones in sphaerodactylids. We hypothesized that there is a threshold body size in sphaerodactylids, below which parafrontals do not develop, thus explaining their absence in miniaturized taxa. Cleared and stained, radiographed, and skeletonized adult and juvenile specimens were used to verify the presence of parafrontals, and if present, measure the total surface area they occupied in seven species of Aristelliger, six species of Teratoscincus, and their respective sister taxa. The relative surface area of parafrontal bones increases with increasing body size. However, body size in relation to the onset of parafrontal development, differs between species of Aristelliger and Teratoscincus. Our data suggest that the onset of parafrontal development is dependent on the ontogenetic stage, not a threshold size.

HRD1-3  10:00 am  Evolution, development, and function of the elaborate frontal sinuses of porcupines. Krentzel D*, University of Chicago; Angielczyk K, Field Museum
Abstract: Fronto-nasal sinuses are common features in the skulls of many mammals, although their function is unclear. We used micro-CT scanning to analyze the 3D internal anatomy of fronto-nasal sinuses in 12 species across the two independent lineages of Old and New World porcupines (Hystricidae and Erethizontidae). Both lineages have convergently evolved large fronto-nasal sinuses that create a prominent dome shape to the skull, with the sinuses sometimes being comparable in volume to the rest of the cranium. The integuments of these domes are covered in anteriorly projecting quills in erethizontids and highly elongated display quills that form a “crest” in hystricids. The sinuses in most erethizontids are small and maintain a flat shape to the dorsal skull roof. Within this family, two independent evolutions of domed sinuses have occurred in the largest taxa. We found that the hystricid Trichys completely lacks a frontonasal sinus, but the more derived Atherurus contains a small but well defined sinus. Ontogenetic data demonstrates that the sinus in Hystrix africaeaustralis invades the maxilla, parietals, and squamosal bones, creating near full coverage of the dorsolateral cranium with sinuses. Both families demonstrate an evolutionary relationship between fronto-nasal sinus volume and body size, as seen in other mammals. The data herein provide some support for the hypothesis that mammals utilize fronto-nasal sinuses to maintain cranial shape when evolving larger body size, and their existence allows for co-option into novel structural roles. The absence of a sinus in Trichys and the close association between sinus size and quill elaboration on the head argue against a necessary physiological role for the fronto-nasal sinuses in porcupines

HRD1-4  10:15 am  Predicting calvarial growth in normal and craniosynostotic mice using finite element analysis. Marghoub A*, University College London; Libby J, University of Hull; Babbs C, University of Oxford; Wilkie A,OM, University of Oxford; Fagan MJ, University of Hull; Moazen M, University College London
Abstract: At birth, the cranium consists of multiple bones joining at their edges by soft tissues called sutures. Early fusion of sutures is a medical condition, known as craniosynostosis. The mutant (Fgfr2C342Y/+) Crouzon mouse is a well-established animal model displaying premature bicoronal suture fusion and an invaluable model to understand the biomechanics of skull growth and craniosynostosis. The aim of this study was to develop a computer model that can predict calvarial growth in both wild type (WT) and mutant (MT) mice. Two ontogenetic series of WT and MT mice were scanned using micro-CT. A 3D finite element model of a WT mouse skull at day 3 postnatal development age (P3) was created and used to predict WT and MT calvarial growth at P7, P10 and P20 where intracranial volume in mouse plateaus. Input parameters to the model were estimated based on a series of parallel experimental studies, and sensitivity studies carried out to determine their effect on the model predictions through comparison of overall calvarial shape and suture ossification to ex vivo specimens. By appropriate selection of the input and remodelling parameters the model could predict the radial expansion of the calvarial bones and bone formation at sutures at P7 and P10 in the WT mouse. For example, the difference of calvarial length between the ex vivo and FE prediction was 5%. Further, the model predicted the overall shape of the MT skull at P10, which has a slightly taller, wider and shorter profile compared to the equivalent WT skull at P10. The developed models are the first models of mouse calvarial growth. The close match between the predicted shape of models and ex vivo data build confidence in the modelling approach. However, further studies are required to refine the models. The aim is to use such models in the long term for human individual-specific modelling of craniosynostosis.

HRD1-5  10:30 am  The developmental genetics of mammalian tooth and jaw morphological integration and evolution. Boughner JC*, University of Saskatchewan; Raj MT, University of Saskatchewan; Phen A, University of Saskatchewan; Uppal J, University of Saskatchewan; Greer J, University of Saskatchewan; Paradis MR, University of Saskatchewan
Abstract: Phenotypic diversity of vertebrate mouths is nothing if not impressive. Yet the developmental genetic mechanisms that allow dental and jaw morphologies to evolve, sometimes dramatically, but still fit and function together properly, remain enigmatic. Using an edentulous p63 mouse mutant with normal mandible morphogenesis despite the early and complete arrest of tooth development, we tested the hypothesis that a tooth-specific gene regulatory network (GRN) exists that has no significant impact on jaw formation. Using microarray and RT-QPCR analyses of p63-/- and wildtype mice aged embryonic days (E) 10-13, in complement with micro-CT scanning and 3D geometric morphometric studies of prenatal mouse heads, we identified for the first time in tooth organs a subset of genes (e.g., Fermt1, Cbln1, Krt8) acting in a putative GRN that regulates the development of the mandibular dentition with virtually no impact on mandible morphogenesis. This GRN thus provides a mechanism via which dental phenotype can vary and evolve without deleteriously affecting the lower face. Conversely, our findings suggest that this GRN is important to tooth and bone morphogenesis of the upper jaw. Thus our work aligns with the current consensus that different sets of genes pattern and drive upper jaw vs. lower jaw skeletal formation; however, for the first time, our work extends this framework to apply to the integration of upper and lower dentitions with their respective jaw skeletons. Further, our results suggest that functional integration in the absence of pleiotropy is the mechanism enforcing coordinated evolutionary change between the jaw and its dentition. This work was funded by an NSERC Discovery Grant to JCB, and supported by a CIHR-THRUST MSc Fellowship to MTR and College of Medicine Summer Research Scholarships to JG, JU and MRP.

HRD1-6  10:45 am  Aberrant amelogenesis and osteogenesis in DSPP mutant mice. Cusack BJ, University of Pittsburgh; Kang R, University of Pittsburgh; Chong R, University of Pittsburgh; Yang Xu, University of Pittsburgh; Beniash E, University of Pittsburgh; Verdelis K, University of Pittsburgh; Szabo-Rogers HL*, University of Pittsburgh
Abstract: Dentin sialophosphoprotein (DSPP) is one of the major non-collagenous proteins present in dentin, cementum and alveolar bone; it is also transiently expressed by ameloblasts. In humans, many mutations have been found in DSPP, and are associated with two autosomal-dominant genetic diseases - dentinogenesis imperfecta II (DGI-II) and dentin dysplasia. Both disorders result in the development of hypomineralized and mechanically compromised teeth. Since dentin and enamel formation are interdependent, we decided to investigate the process of the onset of enamel mineralization in young Dspp-/- animals. We focused our analysis on the constantly erupting mouse incisor to capture all of the stages of odontogenesis in one place. Using high-resolution Micro-CT, we revealed that the onset of enamel matrix deposition occurs closer to the cervical loop and both secretion and maturation of enamel are accelerated in Dspp-/- incisors compared to the Dspp+/- control. Finally, for the first time we demonstrate expression of Dspp mRNA in secretory ameloblasts from embryonic day 16.5 in the mouse. These data led to the hypothesis that Dspp protein is required for normal development of the alveolar bone and tooth even earlier during embryogenesis. To test this hypothesis, we have initiated the analysis of the fetal stages of development and have found that the alveolar bone is defective in the Dspp-/- animals. We are currently testing if Hedgehog (HH) signaling, fibroblast growth factor (FGF) and bone morphogenetic protein (BMP) signaling pathways are changed in the Dspp-/- animals. We will determine if the loss of Dspp protein has an effect on cell physiology or the extracellular matrix during embryogenesis. In summary, our data show that DSPP is required for craniofacial development.

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