Abstracts Organogenesis Program/Abstract # 443 An Enu screen reveals novel genes required for mammalian forebrain development Rolf W. Stottmann, Yujuan Yun, David Beier Division of Genetics, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, USA In order to identify more genes required for mammalian forebrain development, we have conducted an ENU mutagenesis screen in the mouse. We used a traditional breeding strategy and observe for phenotypes at E18.5. This forward genetic approach is unbiased and designed to uncover novel genes or to identify a novel role in brain development for known genes. Our screen has three components: a traditional phenotypic screen complemented by histological analysis, use of a reporter allele to highlight distinct brain structures (RARE- lacZ), and use of a sensitizing allele to increase the incidence of defects in neural developmental (Pafah1b1). To date, we have identified and mapped twelve mutations of interest, eight of which affect CNS development, and we have identified the causal gene for four of these by positional cloning. Each of these appears to be a mutation in a gene with no previous evidence for the role we uncover with this experiment. The most remarkable phenotype uncovered to date shows severe cortical agenesis as well as defects in retinal develop- ment and the appendicular skeleton. The causal gene is in the cholesterol biosynthesis pathway, revealing a requirement for embryonic cholesterol metabolism in brain development. Other mutations show significant neurodevelopmental phenotypes such as cortical hypocellularity, hydrocephaly, ataxia and tremors, optic nerve guidance defects, anterior encephalocele. Thus, we demonstrate the utility of a forward genetic approach in studying neurodevelopment and describe our efforts to enrich our screen for mutations affecting forebrain development. doi:10.1016/j.ydbio.2008.05.424 Program/Abstract # 444 Evidence for cell sorting in the pituitary gland Shannon W. Davis, Amanda H. Mortensen, Mary A. Potok, Sally A. Camper Department of Human Genetics, University of Michigan, Ann Arbor, MI, USA Organogenesis of any organ requires a specific cellular organiza- tion that can result from specific cellular movements, such as neuron migration in the retina or the branching of lung epithelia. The anterior lobe of the pituitary gland is commonly thought of as a “mixed bag” of cells types, requiring little active movement of cells. While determin- ing the embryonic date of cell specification within the pituitary gland, we found evidence that supports the movement or sorting of cells within the embryonic pituitary gland. Pituitary cells appear somewhat stratified within the pituitary depending on the time with which cells leave the proliferative zone and begin to differentiate (Mol Endocrinol 19:698). Therefore, rostrally positioned gonadotropes should be specified before caudally positioned somatotropes. We hypothesized that the rostral/caudal position of cells within the pituitary predicts the date of cell specification. However, our preliminary data suggest that this is not true. In fact, gonadotropes born on the same embryonic day are not grouped together but are distributed along the rostral/ caudal axis of the pituitary, suggesting that pituitary cells may be moving or sorting themselves into networks with stereotypical locations. This is particularly intriguing given recent evidence that somatotropes form an interconnecting network that has physiological relevance (Proc Natl Acad Sci 46:16880). We hypothesize that cells within the anterior lobe of the pituitary are not constrained in their location, but are actively moving within the anterior lobe to locations of physiological significance. (NIH R37HD30428 and R01HD34283) doi:10.1016/j.ydbio.2008.05.425 Program/Abstract # 445 Reassessing Bmp’s role in early development of cranial placodes Bruce B. Riley a , Hye-Joo Kwon a , Neha Bhat a , Robert A. Cornell b a Biology Department, Texas A&M University, College Station, TX 77843, USA b Department of Anatomy and Cell Biology, University of Iowa, Iowa City, IA 52242, USA During vertebrate gastrulation, preplacodal ectoderm arises in a restricted domain surrounding the anterior neural plate and then gives rise to cranial placodes. Placodes later produce paired sensory structures of the head. Most current models posit that preplacodal identity is specified by a Bmp gradient. However, our studies in zebrafish support a strikingly different model of Bmp action. Initially, Bmp acts within its domain of expression to induce several transcription factors (Gata3, Foxi1, Ap2a and Ap2c) that autonomously specify preplacodal competence throughout the nonneural ectoderm. Once induced, these factors no longer require Bmp. Subsequently, short-range signals from neurectoderm act combinatorially with the competence factors to induce a new set of transcription factors (Dlx, Msx, Six, and Eya) along the neural-nonneural interface. These cooperate to specify and stabilize preplacodal identity. Finally, in late gastrulation several Bmp-antagonists (Chordin, Follistatin-1, and Cvl-2) are expressed in paraxial cephalic mesoderm beneath the Developmental Biology 319 (2008) 599–610 Contents lists available at ScienceDirect Developmental Biology journal homepage: www.elsevier.com/developmentalbiology 0012-1606/$ – see front matter