Abstracts - Society for Developmental Biology

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and electron microscopy studies of Xenopus neural tube closure. In these studies, Thomas Schroeder identified a cell
positioned at the interface between the converging epidermal layer and invaginating neural epithelium. Termed the “IS
cell”, for intermediate superficial cell, it has characteristics of both epidermal and neural epithelial cells, allowing a
transitional zone between these diverse cell types. We hypothesize that aqp3b is specifically expressed in IS cells and
suggest that IS cells and their expression of aqp3b are required for proper closure of the neural tube. IS cells do not
themselves undergo apical constriction, but, upon aqp3b inhibition, we observe a failure of apical constriction in cells that
normally form the dorsal-lateral hingepoints. The possibility of intercellular action of IS cells on the neighboring hinge
point cells has not been previously predicted. We are attempting to broaden these results to other species and to define the
mechanism by which IS cells and aqp3b participate in neural tube morphogenesis.
Program/Abstract # 39
Maintenance of axial patterning in the sea urchin embryo: A role of Wnt1 signaling
Angerer. Lynne M., National Institutes of Health, Bethesda, United States
The initial patterning along the anterior-posterior (AP) and dorsal-ventral (DV) axes of sea urchin embryos depends on
Wnt and Nodal signaling, respectively, and occurs before gastrulation. The secondary DV axial patterning relies on a Wntdependent
process that removes a suppressor of nodal expression from non-anterior ectoderm. Here we report that
unexpectedly, later, when gastrulation begins, Wnt signaling continues to affect Nodal signaling, not by supporting it, but
rather by preventing nodal expression in the ventral-posterior region of the embryo. This region normally gives rise to the
posterior-transverse ciliary band, the supra-anal ectoderm and ventral endoderm. When Wnt1 is knocked down, expression
of nodal and its target genes, gsc and bra, extends ectopically on the ventral side toward the blastopore. As a consequence,
initial fates of cells in this region are changed to oral ectoderm, as shown by lineage tracing. Strikingly, the ciliary band,
which forms adjacent to the nodal expression domain, is shifted significantly in the dorsal direction, toward and sometimes
beyond the position of the blastopore. This results in the blastopore and stomodeal regions being positioned in the same
ventral plane instead of approximately at a 90° angle. Before gastrulation, wnt1 expression is radial in posterior
blastomeres. But when gastrulation begins, it is lost from the dorsal side by a Nodal-dependent process and maintained
only on the ventral side, where it suppresses nodal. Thus, Wnt- and Nodal-dependent processes mutually antagonize each
other to maintain the body plan established at earlier stages by these same pathways.
Program/Abstract # 40
Membrane morphogenesis during tracheal tube development in Drosophila
Jayan Nandanan, N; Mathew, Renjith (EMBL, Heidelberg, Germany); Leptin, Maria, EMBO, Germany
The terminal cells of the Drosophila respiratory (tracheal) network contain seamless, membrane bounded intracellular
tubes through which air travels. The tube-containing extensions of these cells are elaborated during chemotactic growth
directed by developmental cues in embryos and subsequently ramify in response to signals from hypoxic tissues in larvae.
Extensive membrane traffic occurs during growth, as the cell rapidly and concomitantly elaborates two membrane domains
of opposite characteristics. The outer (basal) membrane migrates and grows with highly dynamic filopodial extensions,
while at the same time the inner tube of apical characteristics is constructed from as yet unknown membrane sources. We
find that the membranes forming the intracellular tubules contain lipids and proteins typical of apical plasma membranes in
polarized epithelial cells. The Drosophila synaptotagmin-like protein Bitesize (Btsz) and the activated form of its
interaction partner Moesin are also located at the growing luminal membrane. Our functional studies indicate that the actin
cytoskeleton, through its interaction with Btsz via Moesin directs apical membrane morphogenesis to create and maintain
distinct intracellular tubules. Using real-time in vivo imaging we have analysed the assembly of the intracellular tube. We
find that ER and Golgi rapidly distribute into the developing cellular extensions prior to the assembly of the membrane
bounded tube within. Redistribution of secretory machinery ahead of the growing tube seems to be a prerequisite for
proper tube extension.
Program/Abstract # 41
Asymmetric division of luminal cells produces low-polarity high-motility cells that collectively migrate to form
mammary ducts
Huebner, Robert J., Johns Hopkins Sch of Med, Baltimore, United States; Lechler, Terry (Duke, Durham, United States);
Ewald, Andrew (Johns Hopkins Sch of Med, Baltimore, United States)
We seek to understand the cellular and molecular mechanisms that underlie branching morphogenesis. To overcome the
limitations of optical imaging in the intact mouse, we rely on organotypic cultures of primary mammary epithelium. Using
3D time-lapse imaging and transgenic fluorescent reporters, we have resolved cell migration, proliferation, and tight
junction dynamics throughout branching morphogenesis. The initial stage of morphogenesis involves a growth factor