control embryos. However, analysis of neutral lipids by TLC showed that MBCD microinjection increased the fatty-acid content, while decreasing triacylglycerol levels. doi:10.1016/j.ydbio.2009.05.143 Program/Abstract # 123 Expression pattern of genes related to lipid rafts formation during Xenopus laevis development Amado G. Nathália, Débora M. Cerqueira, José G. Abreu Instituto de Ciências Biomédicas, UFRJ, Rio de Janeiro, RJ, Brazil Lipid rafts are membrane microdomains (MM) with high content of cholesterol and sphingolipids. It has been shown that these structures play a role in intracellular trafficking and can act as platforms for signaling complex. We showed recently that cholesterol depletion within plasma membrane disrupted MM assembly, affecting anterior development of Xenopus embryos, correlating MM and head formation. Two genes previously identified and involved with cholesterol biosynth- esis (Xen1) and organization of lipid rafts (Xen2) are here investigated. Since cholesterol and MM seems to play a role during Xenopus development, we decided to investigate the expression pattern of these genes during embryonic development of Xenopus laevis. The cDNAs containing ESTs of these genes were obtained at gene database. These sequences showed high degree of identity with human, zebra-fish, rat and mouse, and are therefore possible candidates for orthologues in Xenopus. In situ hybridization was performed in different stages of development and showed that these genes are expressed at the animal pole, branchial arches, optical vesicles, olfactory placodes, pronephric area and neural plate. At tailbud and tadpole stages, expression is detected at the dorsal region, along the anteroposterior axis and somites. At neurula stage Xen2 expression was restricted in neural crest tissue. RT-PCR analysis showed that both genes are expressed at early and latter stages. These results showed that Xen1 and 2 are expressed at specific areas and stages, suggesting that these orthologues can play a significant role during Xenopus development. doi:10.1016/j.ydbio.2009.05.144 Program/Abstract # 124 The effects of Wnt6 on somite patterning Eugenel B. Espiritu, Lisa M. Galli, Laura W. Burrus Department of Biol., SFS Univ., San Francisco, CA, USA In vertebrate development, somites are patterned by extrinsic signals, including Wnts. We are particularly interested in determining the role of Wnt6, which is expressed in the ectoderm overlying developing somites. Researchers have demonstrated that Wnt6 is important in the formation or maintenance of epithelial somite structures. Yet, the pathway used by Wnt6 in somite patterning has not yet been rigorously defined nor has the requirement for Wnt6. As a first step toward understanding the signaling pathway utilized by Wnt6, I tested Wnt6 activity in COS7 cells. I found that Wnt6 inhibits β-catenin dependent Wnt signaling, suggesting that Wnt6 acts through a β-catenin independent pathway in COS7 cells. As the signaling pathways used by Wnts are tissue dependent, I am now extending these studies into somites. To further investigate the role (s) of Wnt6 in somite patterning, I have examined the phenotypes of somites in embryos transfected with overexpression constructs for Wnt6. Interestingly, overexpression of Wnt6 causes an expansion in the size of the dermomyotome without affecting the size of the myotome. To test the requirement of Wnt6, I first generated a detailed map of the Wnt6 expression pattern using in situ hybridiza- tion. Wnt6 transcripts are expressed in the ectoderm overlying the segmental plate, epithelial somites, intermediate mesoderm, and lateral plate mesoderm. As the somites compartmentalize, Wnt6 expression decreases and becomes restricted to the ectoderm over- lying the lateral plate mesoderm and neural tube. With this information, I am currently knocking down Wnt6 expression using shRNA and analyzing the effects on the epithelial dermomyotome. doi:10.1016/j.ydbio.2009.05.145 Program/Abstract # 125 Fibroblast growth factors (FGFs) are required to maintain the presomitic mesoderm L.A. Naiche, Nakisha Holder, Mark B. Lewandoski National Cancer Institute, Frederick, MD 21702, USA The current model of somitogenesis postulates a Wavefront signal in the posterior of the embryo and that maintains the undiffer- entiated state of the presomitic mesoderm (PSM). Previous work in cultured embryos suggests that FGF activity is the Wavefront signal. However, it is unclear which (if any) of the six FGFs expressed in the PSM encode this activity, as our work and others' has shown that removal of any one ligand is insufficient to disrupt early somitogen- esis. Here we show that when both Fgf4 and Fgf8 are ablated in the PSM, only a few abnormal somites form and embryonic axis elongation stops at the 3-somite stage. Expression of most PSM genes is absent in mutants, including cycling genes, markers of undifferentiated PSM, and WNT genes. Significantly, somite markers are expanded into the PSM, suggesting that the loss of PSM markers is due to premature differentiation of the PSM into somitic tissue. Loss of WNT signaling also causes axis truncation, so we investigated whether the phenotype of the mutants was a secondary effect. When WNT signaling is restored by activation of a gain-of-function B- catenin allele in the mutant PSM, undifferentiated PSM markers such as T are partially restored, but axis elongation and somite formation are not rescued. This study demonstrates that loss of FGF signaling results in premature differentiation of the PSM, and provides the first genetic evidence identifying FGF ligands (Fgf4 and Fgf8) that encode Wavefront activity. Furthermore, these data suggest that FGF action is required to maintain WNT signaling, and that both signaling path- ways are required in parallel to maintain undifferentiated PSM tissue. doi:10.1016/j.ydbio.2009.05.146 Program/Abstract # 126 Alternative splicing of genes expressed during early embryonic development Timothée Revil b , Daniel Gaffney b , Christel Dias b,c , Jacek Majewski b,c , Loydie A. Jerome-Majewska a,b a Department of Pediatrics, Montreal Children's Hospital, Montreal, Canada, H3Z 2Z3 b Department of Human Genetics, McGill University, Montreal, Canada, H3A 1A4 c Genome Québec Innovation Centre, Montreal, Canada, H3A 1A4 A number of genes that are required for normal morphogenesis of developing embryos, including Fgf8, have been shown to have isoform- specific functions. To date, these genes have been identified in isolated experiments and by fortuitous splicing events associated with gene targeting. We predicted that alternative splicing, the process by which a single gene can produce several functionally distinct isoforms, is a common event required for establishment of the normal body plan. In this work, we present a whole genome approach, using exon Abstracts / Developmental Biology 331 (2009) 420–430 427