sox4b morphants specifically in the pituitary. Finally, we show that gata2 expression colocalizes with tshb, gsua and sox4b mRNAs in the pituitary of 48 hpf embryos. In conclusion, Sox4b is expressed in zebrafish during pituitary development and plays a crucial role in the differentiation of thyrotrope cells through induction of gata2 expression in the developing pituitary. Acknowledgments: F.N.R.S.; PAI. doi:10.1016/j.ydbio.2007.03.486 Program/Abstract # 415 Eph- and efn-dependent adhesion in the zebrafish hindbrain Hilary A. Kemp, Julie E. Cooke, Cecilia B. Moens HHMI, Basic Sciences Division, FHCRC, Seattle, WA, USA During vertebrate development, the hindbrain is transiently segmented into 7 rhombomeres(r) which later give rise to segment-specific neurons. The family of Eph RTKs and their efn ligands are expressed in complementary patterns in the early hindbrain. Mutual repulsive interactions between domains of Eph- and efn-expressing cells are believed to drive formation of rhombomere boundaries. We have shown that at least one receptor–ligand pair also have independent functions within the rhombomere interior. Specifically, in r3 and r5, cells depleted of EphA4 are unable to interact normally with their wild type EphA4+ counterparts. The efnB2a ligand expressed in r1, 4 and 7 is likewise required for normal cell adhesion in these rhombomeres. We generated mutant EphA4 and efnB2a alleles in order to determine which domains are required for their putative adhesive roles. Preliminary work suggests that efnB2a- mediated cell–cell adhesion does not require the efnB2a cytoplasmic domain; therefore adhesion is likely mediated by molecules that interact with efnB2a at the cell surface rather than by its intracellular effectors. To better understand intra- rhombomere cell interactions, we performed confocal time- lapse imaging of genetic mosaics in which EphA4+ and EphA4 - cells sort into distinct domains within r3 and r5, a process that mimics events at forming rhombomere boundaries. We find that cells with unequal levels of EphA4 are unable to intercalate with one another during a characteristic cross-midline cell division that occurs during the neural keel stage of neurulation. Thus EphA4 may function to maintain bilateral symmetry within the hindbrain neuroepithelium during convergent extension. doi:10.1016/j.ydbio.2007.03.487 Program/Abstract # 416 Zebrafish lacking a functional Dispatched 1 display variable pharyngeal arch defects in part due to defects in neural crest cell morphogenesis Tyler Schwend, Sara C. Ahlgren Dept. of Peds, Northwestern Univ School of Medicine, Chicago, IL, USA The vertebrate head skeleton is primarily derived from cranial neural crest cells (CNCC). The loss of Hedgehog (Hh) signaling in the vertebrate embryo causes severe craniofacial defects, in part due to defects in CNCC-derived tissues. Inactivation of the zebrafish Hh family members ShhA and Twhh (ShhB) leads to variable midline defects and reduced chondrogenesis in the developing cranium. Such variation in craniofacial midline defects is characteristic of the Hh- associated spectral disorder holoprosencephaly in humans. Dispatched 1 is a sterol sensing domain (SSD) protein involved in the release of ShhA and ShhB from their site of synthesis. We have demonstrated that homozygous zebrafish chameleon (con) mutant embryos, lacking functional Dis- patched 1 protein, display defective patterning in the hyoidal arch and an overall reduction in posterior pharyngeal arches. Treatments with cyclopamine to block Hh signaling at different stages reveal a requirement during CNCC chondro- genesis in the pharyngeal arches. These studies reveal previously uncharacterized roles for Hh signaling in jaw and branchial arch development. Further, we plan to determine if the Disp1-SSD motif is functionally required for normal craniofacial development in con embryos. SSD containing proteins are involved in trafficking of cholesterol and cholesterol-linked proteins, a role befitting Disp1 in the secretion of cholesterol-modified Shh ligands. We have created SSD-specific mutant versions of Disp1 and will inject these variants in con mutant embryos to attempt phenotypic rescue of cranial structures. doi:10.1016/j.ydbio.2007.03.488 Program/Abstract # 417 The function of the transcription factor Egr1 in zebrafish cartilage development Julia Dalcq, Vincent Pasque, Sonia Davila Ramos, Marc Muller Laboratory for Molecular Biology and Genetic Engineering, Tour GIGA, Bât. B34, Université of Liège, B-4000 Liège (Sart-Tilman), Belgium The cartilaginous elements that form the pharyngeal arches of the zebrafish derive from the cranial neural crest cells. The proper patterning and morphogenesis of neural crest-derived cartilages are regulated by reciprocal interac- tions between neural crest and other tissue types such as ectoderm, endoderm and mesoderm. We study the zebrafish transcription factor Egr1 (early growth response 1) involved, among others, in pituitary development, angiogenesis, wound healing and cancer. Functional studies (knock-down and rescue experiments) revealed that Egr1 is absolutely required for the formation of cranial cartilages. To examine more in detail the mechanisms by which Egr1 acts on the formation of cranial cartilages, we analyzed the expression of different marker genes involved in pharyngeal development in morpholino-injected embryos using in situ hybridization and transgenic specifically fluorescent embryos. Our results 439 ABSTRACTS / Developmental Biology 306 (2007) 436–449