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Session Schedule & Abstracts




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Thursday 30th June, 2016

PHA1
Symposium : The vertebrate pharynx: crossroads in evolution and development 1

Room: Salon H   9:30 am–11:00 am

Moderator(s): A. Huysseune & A. Tucker
PHA1-1  9:30 am  Pharyngeal remodelling in development and evolution. Graham A*, King's College London   anthony.graham@kcl.ac.uk
Abstract: Pharyngeal segmentation is a characteristic of all vertebrates and during development it is first seen in the appearance of a series of bulges on the lateral surface of the head, the pharyngeal arches. In all gnathostomes, the first two pharyngeal arches form the jaw and hyoid while the more posterior arches will generate different components in different vertebrate clades. In many chondrichthyans, the embryonic segmental organisation of the pharynx is maintained in the adult arrangement of the gills slits. Conversely, in many osteichthyans, the posterior gill bearing arches are covered by the operculum. However, with the evolution of the tetrapods there has been a loss of segmentation in the adult form as a result of the remodelling of the pharynx that occurs during metamorphosis in amphibia and embryogenesis in amniotes. This process involves the expansion of the second arch such that it comes to cover and enclose the more posterior arches. This remodelling event is crucial in the organisation of the mature pharynx, and we have begun to identify the signalling pathways that direct this process and how these have been modified during evolution. The emergence of the tetrapods also involved a reduction in the number of pharyngeal segments. We will further discuss the mode through which this has been achieved and suggest that this can give us insights into how the number of pharyngeal segments can be controlled.

PHA1-2  10:00 am  Molecular basis of the lamprey pharyngeal development. Jandzik D*, University of Colorado Boulder; Romasek M, University of Colorado Boulder; Square TA, University of Colorado Boulder; Cattell MV, University of Colorado Boulder; Medeiros DM, University of Colorado Boulder   
Abstract: Lamprey, a member of the jawless vertebrate clade (cyclostomes), a sister group to jawed vertebrates (gnathostomes), is a fantastic model for the study of evolution and development of the vertebrate pharynx. Using comparative approach between these two groups has a great potential to reveal the core vertebrate patterns and mechanisms of pharyngeal evo-devo. Simple morphology of the pharyngeal apparatus, serial organization, relatively slow development of lamprey embryos, and good amenability to genetic manipulations make it possible to study the morphogenetic and developmental processes in great detail. Similar to gnathostomes, morphogenesis of the lamprey pharynx is driven by the successive formation of bilateral outpocketings of endoderm, called pharyngeal pouches. Endoderm of these pouches fuses with overlying ectoderm and encloses a mesodermal core to form the pharyngeal arches. The arches are populated by ventrally migrating neural crest cells, which give rise to the pharyngeal skeleton and musculature. Formation of the lamprey pharynx and its derivatives involves expression of several members of the vertebrate pharyngeal gene regulatory network, including transcription factors Pax1/9, Tbx 1/10, and Ripply3 and signaling molecules of FGF, Hedgehog, and WNT pathways. In addition, Retinoic acid signaling is critical for pouch formation and establishing anteroposterior pouch polarity, and together with FGFs and Endothelin signaling plays a role in later pharyngeal skeletogenesis.

PHA1-3  10:30 am  Tightly orchestrated epithelial transitions drive pharyngeal pouch formation in zebrafish. Choe CP, University of Southern California; Crump GD*, University of Southern California   gcrump@usc.edu
Abstract: Transitions in cell morphology are essential for the remodeling of epithelia during development. In the vertebrate head, remodeling of the pharyngeal endodermal epithelium generates a series of outpocketings, or pouches, which divide the developing face into its basic segmental units, the pharyngeal arches. Pouches are ancestral forms of head segmentation, likely common to all deuterostomes. In vertebrates, pouches generate a number of important organs, such as the thymus and parathyroid, and promote development of the posterior facial skeleton. Using time-lapse recordings and transgenic and mutant analysis in zebrafish, we have found that pouch formation involves a series of epithelial transitions controlled by signaling factors in the neighboring mesoderm and ectoderm. The formation of pouches is initiated by localized Wnt11r signals from the mesoderm, guided to the periphery by Fgf8a signaling, and finally reshaped into mature pouch bilayers by combined Wnt4a and Eph-ephrin signaling. I will discuss how these multiple signaling pathways are integrated to generate the precise stereotypical branching pattern of the pharyngeal endoderm.



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