Physical forces drive the movement of tissues within the earlyembryo. Classical and modern approaches have been used to inferand in rare cases measure mechanical properties and the locationand magnitude of forces within embryos. Elongation of the dorsalaxis is a critical event in early vertebrate development yet themechanics of dorsal tissues in driving embryonic elongation thatlater support neural tube closure and formation of the centralnervous system are not known. Among vertebrates, amphibianembryos allow complex physical manipulation of embryonic tissuesthat are required to measure the mechanical properties of tissues.We measure the stiffness of dorsal isolate explants of frog (Xenopuslaevis) from gastrulation to neurulation. We find that tissues stiffenfrom less than 20 Pa to over 80 Pa in less than 8 h. Furthermore, byisolating specific axial and paraxial tissues we find that paraxialsomitic mesoderm is nearly twice as stiff as either the notochord orneural plate and at least 20-fold stiffer than the endoderm. Dorsalisolates treated with acute acting cytoskeletal inhibitors or preparedfrom fibronectin knock-down embryos reveal that stiffness is theproduct of a contractile actomyosin cortex rather than a fibrillarfibronectin extracellular matrix. Increasing dorsal stiffness andspatial variation between germ layers during neurulation suggestsa common vertebrate dorsal tissue architecture that may insurerobust neural tube closure and provide positional cell identity priorto organogenesis.

doi:10.1016/j.ydbio.2008.05.168

Program/Abstract # 157Regulation of cytoarchitecture in development: The roles ofIQGAP2, C-cadherin and Cdc42 in the early Xenopus embryosQinghua Tao, Sumeda Nandadasa, Stephanie Lang, Janet Heasman,Chris WylieDivision of Developmental Biology, Children's Hospital Medical Center,USA

The control of cytoarchitecture is fundamentally important inmulticellular organisms, both for tissue assembly during embryonicdevelopment and for adult tissue remodeling. The Xenopus blastulahas proven to be a useful model system for studying the mechanismsof cytoarchitecture control during early development. Becausecortical actin assembly at late blastula stage is controlled by mRNAsmaternally transcribed and stored in the egg, depletion of individualstored mRNAs allows rapid loss of function, as well as epistaticexperiments. We have previously shown that the cortical actinnetwork is essential for rigidity and shape in the early embryos, andthat the level of expression of C-cadherin on the cell surface is ratelimiting for cortical actin assembly at the blastula stage. We have alsoidentified two G protein-coupled receptors (GPCRs) that controlcortical actin assembly by controlling the expression level of C-cadherin on the cell surface in the late Xenopus blastulae. However, itis unclear how the activity of C-cadherin is controlled at the cellsurface to nucleate cortical actin assembly. Here, we provide evidencethat cdc42, a member of small Rho GTPases, is essential to mediate C-cadherin controlled cortical actin assembly. IQGAP2, a multi-domainscaffold protein is identified as a major effector protein of Cdc42 thatis necessary for cortical actin assembly controlled by C-cadherin atthe cell surface. We also show that actin nucleation factor Arp2/3complex is required for C-cadherin to assemble cortical actin at thecell surface.

doi:10.1016/j.ydbio.2008.05.169

Program/Abstract # 158A tale of two tails: Multiple pathways regulate cell adhesion andmorphogenesis in the zebrafish tailChris Thorpe, Yi YangDivision of Biology, Kansas State University, Manhattan, KS, USA

Multiple signaling pathways regulate development of the posteriorvertebrate body, which is derived from a population of progenitor cellscalled the tailbud. Fate specification and differentiation are closelylinked with cell migration to ensure that, as some cells exit the tailbudand differentiate, other cells are retained in the tailbud as undiffer-entiated precursors to support later tail outgrowth.

doi:10.1016/j.ydbio.2008.05.170

Program/Abstract # 159Neuregulin-mediated ErbB3 signaling is required for DRG neuronformationYasuko Honjo, Judith S. EisenInstitute of Neuroscience, University of Oregon, Eugene 97403-1254, USA

ErbB3 is a receptor-type tyrosine kinase that has been shown tohave important roles in neural and glial development. We found thaterbb3 mutant zebrafish have a defect in dorsal root ganglion (DRG)formation, as revealed early in development by absence of neuro-genin1-positive nascent DRG neurons and later by absence of HuC/D-positive DRG neurons. To learn whether neural crest cell migration isaffected in erbb3 mutants, we followed neural crest migration in livetransgenic embryos inwhich GFP expression is driven by the zebrafishsox10 promoter. In erbb3mutants, overall motility of neural crest cellsappeared unaffected. However, migrating neural crest cells failed topause near the positionwhere DRGs normally form. This is in contrasttowild types, inwhich a subset of migrating neural crest cells pause inthis location for many hours. Transient blockade of ErbB receptorsrevealed that ErbB receptor signaling is required for DRG neuronformation around the time migrating neural crest cells pause.Together these results suggest that ErbB3 signaling is required forDRG progenitors to recognize their target position during migration.We isolated genes encoding the ErbB3 ligands Neuregulin 1 (Nrg1)and Neuregulin 2 (Nrg2) and found that zebrafish has one nrg1 andtwo nrg2 (nrg2a and nrg2b) genes. To learn whether any of the Nrgsare involved in DRG neuron formation, we knock them downindividually and in combination using morpholinos and found thatNrg1 and Nrg2a act together during neural crest migration and DRGformation.

doi:10.1016/j.ydbio.2008.05.171

Program/Abstract # 160A role for zic genes during neural tube morphogenesis in zebrafishYevgenya Grinblat a, Molly Nyholm a, Aaron Taylor a, Shawn Burgess b

a Zoology and Anatomy, University of Wisconsin, Madison, WI, USAb NIH, Bethesda, MD, USA

Our studies are aimed at understanding the conserved roles of Zictranscription factors (TFs) during vertebrate brain development. Zicsare required for correct neurulation inmammals via amechanism thatis poorly understood. We have uncovered an essential role for zic2aand zic5 in hinge point (HP) formation during cranial neurulation inzebrafish. HP formation is an essential step in primary neurulation,

514 ABSTRACTS / Developmental Biology 319 (2008) 502–520