Congress Abstracts - Society for Developmental Biology

Program/Abstract # 218
Lfng regulates the synchronized oscillation of the mouse segmentation clock via trans-repression of Notch signalling
Okubo, Yusuke, (National Institute of Health Sciences, Japan); Sugawara, Takeshi; Abe-Koduka, Natsumi (National Institute of
Genetics, Mishima, Japan); Kanno, Jun (National Institute of Health Sciences, Japan); Kimura, Akatsuki; Saga, Yumiko (National
Institute of Genetics, Japan)
The metameric features of vertebrates are based on the structure of the somites, which are sequentially produced (one by one) as a
segmented cell mass from the anterior end of the presomitic mesoderm. The timing of this periodicity is controlled by the oscillation
of gene expression, so called segmentation clock. In mice, the core component of the segmentation clock is the negative feedback loop
that regulates Hes7 expression and incorporates another clock gene Lunatic fringe (Lfng), the product of which in turn represses Notch
activation and generates Notch signal activity oscillations. In addition, a synchronization mechanism is required to form a sharp
somite boundary. Although the intracellular mechanisms that underlie the activities of these oscillators are now well understood, the
regulation of the intercellular coupling among clock cells that enable synchronization is largely unknown in mice. Notch signalling is
required for the induction of several genes including clock genes, thus it has been difficult to analyze synchronization mechanisms
independent of gene expression regulation. To overcome this difficulty, we used both experimental and theoretical approaches. Here
we show, using chimeric embryos composed of wild-type cells and Delta like 1 (Dll1)-null cells, that Dll1-mediated Notch signalling
is responsible for the synchronization mechanism. By analyzing Lfng chimeric embryos and Notch signal reporter assays using a coculture
system, we further find that Lfng represses Notch activity in neighboring cells by modulating Dll1 function. Finally, numerical
simulations confirm that the repressive effect of Lfng against Notch activities in neighboring cells can sufficiently explain the
synchronization in vivo. Collectively, we provide a new model in which Lfng has a crucial role in intercellular coupling of the
segmentation clock through a trans-repression mechanism.
Program/Abstract # 219
Roles for Hoxa-5 in regulating chick cervical vertebral morphology
Mansfield, Jennifer; Chen, Jessica; Zahid, Soombal; Shilts, Meghan; Habbsa, Samima; Aronowitz, Danielle; Rokins, Karimah;
Weaver, Sara (Barnard College, Columbia University, USA)
The vertebrate axial skeleton and its associated muscles and connective tissue develop from somites. Although somites form in the
same way along the body axis, each vertebral segment develops with a unique morphology appropriate to its position. Hox
transcription factors specify segmental identities prior to somite segmentation, but also continue to be expressed in segmented somites.
Here, we examined the role of Hoxa-5 in chick cervical somites using gain and partial loss-of-function approaches. We show that after
somite segmentation, Hoxa-5 is expressed in a sub-domain of lateral sclerotome, and that this restricted expression pattern is
influenced by signals that pattern the somite medial-lateral axis (Shh and Fgf-8). Hoxa-5 knockdown after segmentation specifically
affects the morphology of ventral-lateral vertebral cartilage, which is derived from the Hoxa-5 expression domain. We hypothesize
that one role for chick Hoxa-5 in patterning cervical segments is to locally influence precursors of the ventral-lateral vertebral
cartilage, which develop differential morphologies across the cervical-thoracic transition.
Program/Abstract # 220
A Novel Mechanism Underlies Growth Plate Cartilage Column Formation
Romereim, Sarah M. (Northwestern University, USA)
Growth plate cartilage, the driving and shaping force of bone development, achieves directional growth by stacking proliferative zone
chondrocytes into clonal columns to form a specific tissue architecture. The way in which these columns are formed is poorly
understood and has been widely hypothesized to be similar to the migratory process of convergent extension. A novel application of
time lapse confocal microscopy of the growth plate shows that recently divided cells do not migrate to take their place in the column.
Instead, daughter cells rotate around the division interface. This process is regulated externally by signaling molecules and depends on
cell adhesion. The discovery of this cell behavior not only provides insight into the mechanism of bone elongation but also offers a
fresh perspective on directed growth in other systems.
Program/Abstract # 221
Opposing tensile forces and migratory behaviour drive tissue convergence during zebrafish laterality organ development
Pulgar, Eduardo; Santibañez, Felipe; Härtel, Steffen; Concha, Miguel (ICBM - BNI, University of Chile, Chile)
Changes in cell shape and tissue organisation involves the orchestrated integration of polarising cues from interdependent
biomechanical processes. Cells interact with their environment by direct cell-cell and cell-matrix contact and through sensing
diffusible factors such as migratory signals. Cellular responses to these cues enable cells to orient their polarity and develop a
stereotyped supra-cellular organisation. How mechanical and chemical cues interact to control this phenomenon remains unclear. In
our lab we are studying this issue during early development of the zebrafish laterality organ, known as the Kupffer’s vesicle (KV). KV
progenitor cells, the dorsal forerunner cells (DFCs), originate from ingression of dorsal marginal cells of the surface epithelium (EVL)
at the onset of epiboly, a mechanism dependent by Nodal. Time-lapse microscopy showed that as epiboly progresses DFCs becomes
increasingly elongated while converge to the midline. Detailed analysis of this early event revealed the presence of two main
morphogenetic forces that display a biased spatial distribution along the animal-vegetal axis: (i) a vegetally-directed pulling force
dependent on the attachment between DFCs apical membranes and the EVL-yolk cell, and (ii) a protrusive activity directed to the
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