Abstracts - Society for Developmental Biology

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dependent transition from a bilayered to a stratified epithelium. Using inducible genetic systems, we have shown that
activation of Rac was sufficient for stratification but not branching. However, constitutively active MEK was sufficient to
induce initiation of new ducts in the absence of growth factors. We have shown that stratification involves the creation of
unpolarized, motile interior cells between the apical and basal cell layers. We observe generation of interior cells through
polarity breaking divisions of luminal epithelial cells. New duct initiation involves coordinated migration of these interior
cells, but can occur in the absence of proliferation. A major conceptual challenge for this collective migration is the
presence of intercellular junctions. Imaging of ZO1-GFP revealed the lumen is maintained throughout morphogenesis, but
that this luminal space is dynamic. We observed fusion and fission of ZO1-GFP domains along with de novo formation of
secondary lumens. Our data suggest that mammary morphogenesis begins with a proliferation-driven establishment of
unpolarized migratory cells, which then migrate to elaborate the epithelial network, and repolarize through the de novo
formation of tight junction lined luminal spaces.
Program/Abstract # 42
Regulation of early lineages in the mouse embryo
Manzanares, Miguel, Centro Nacional de Investigaciones Cardiovasculares, Spain
To better understand how gene regulatory networks act in the mammalian blastocyst to define the first embryonic lineages,
we are searching for cis-regulatory sequences that control the spatial and temporal expression of genes in the network. In
this way we aim to identify the mechanisms by which different stem cell populations at the blastocyst stage are specified
and maintained, and which is the regulatory link between early stochastic expression and late maintenance of core
transcription factors in the network. To do so, we are using a comparative genomic approach combined with transient
transgenesis to find regulatory elements important in the transcriptional network responsible for lineage determination.
Once putative elements are identified, their capacities are tested in vivo in mouse embryos by examining their ability to
drive lineage-restricted expression of a reporter gene. We are also taking advantage of blastocyst derived stem cells –both
trophoblast (TS) and embryonic (ES) stem cells– to find the specific signaling inputs and roles of the regulatory elements
characterized in a tissue culture assay. Using this approach, we are studying the regulation of the key trophectoderm
transcription factor Cdx2, and have found that specific cis-regulatory elements are involved in different aspects of its
expression in the trophectoderm of the blastocyst and in trophoblast-derived stem cells.
Program/Abstract # 43
Investigating body axis extension in the mouse embryo using a single-cell resolution fluorescent Wnt reporter
Ferrer Vaquer, Anna; Tian, Guangnan; Hadjantonakis, Anna-Katerina, Sloan-Kettering Institute, New York, United
States
Elongation of the mouse anterior-posterior axis depends on the sequential addition of cells to the caudal end of the embryo.
Progenitors residing within the tailbud region fuel the production of cells contributing to the developing somites, neural
tube and notochord. Detailed lineage analysis and fate-mapping studies have identified several subdomains of progenitors
residing within the node, primitive streak and caudal lateral epiblast (CLE) each of which exhibit different developmental
potentials. Notably, progenitors residing at the node-streak border (NSB) represent true multipotent stem cells, since they
are able to give rise to neural ectoderm as well as axial and paraxial mesoderm. Interestingly, this NSB subdomain displays
high levels of TCF/Lef:H2B-GFP, a fluorescent canonical Wnt signaling reporter we recently generated. Taking advantage
of the brightness and single-cell resolution afforded by this reporter we microdissected the various regions of the caudal
part of the embryo at E8.5 (8-10 somite stage). Including the ventral part of the NSB, which has been proposed to act as a
niche for neuromesodermal progenitors located at and around the dorsal NSB. Gene expression profiling of the ventral
NSB region and comparison with the neighboring CLE and paraxial mesoderm will help decipher the mechanisms that
regulate behavior of these multipotent axial progenitors. We have also generated a TCF/Lef:CreERT2 strain, representing
a new inducible Cre recombinase driver line under the regulation of canonical Wnt-responsive elements. The
TCF/Lef:CreERT2 strain should allow us to fate map and genetically ablate the NSB region, as well as conditionally
modulate the levels of ß-Catenin to further characterize the role of the NSB in axial elongation.
Program/Abstract # 44
Claudin-10 functions on the right side of Hensen’s node to direct left-right patterning
Collins, Michelle M.; Ryan, Aimee (McGill Unversity, Montreal, Canada
In vertebrates, organs must be asymmetrically positioned within the body cavity to allow for normal physiological
function. The origin of this asymmetry is initiated during gastrulation in an evolutionarily conserved molecular cascade.
We have identified a novel molecule, Claudin-10, that plays a role in directing asymmetric organ positioning in the chick.
Claudins are integral components of tight junctions, which act to restrict movement of ions and small molecular within the