Congress Abstracts - Society for Developmental Biology

animal pole depending on signals released by the deep embryonic blastoderm. Cell manipulation and computational simulations
support an instructive role of these opposing cues in the control of early events of KV morphogenesis. We provide novel insights into
the role of mechanochemical control of progenitor cells, during the coordination of vertebrate organogenesis. Grant sponsors:
FONDECYT (1120558 and 1120579), the Scientific Millennium Initiative (P09-015-F)
Program/Abstract # 222
Cell cycle synchrony is lost before midblastula transition in zebrafish embryos.
Mendieta Serrano, Mario; Schnabel, Denhi; Lomelí, Hilda; Salas-Vidal, Enrique (Instituto de Biotecnología, Universidad Nacional
Autónoma de México, Mexico)
In zebrafish, classical studies indicate that after fertilization rapid and synchronous cleavages occur until midblastula transition
(MBT), when the cell cycle lengthens, cell division becomes asynchronous, cells start showing motile features and zygotic
transcription begins (Kane and Kimmel, 1993). More recent studies demonstrated that cell divisions start to drift out of synchrony at
the 4- to 8-cell stage transition, a drift that increases in subsequent cell cycles (Olivier et al., 2010). The aim of the present study was
to further characterize cell cycle synchrony during the cleavage– and early blastula-period in zebrafish. By immunolocalization of the
well-established mitosis marker, histone H3 phosphorylated at serine 10, we found evidence of mitotic asynchrony among blastomeres
at the 2- to 4-cell stages transition. In order to obtain further evidence of the synchrony shift, we visualized the nuclear dynamics in
living embryos during the first two cell cycles by injection of the DNA stain SYTOX Green. We found differences in mitosis
progression among blastomeres in most of the 2- and 4-cell stage embryos analysed. Interestingly, from the 16-cell to 512-cell stage
we demonstrate that nuclei number and mitotic indexes differ from those predicted by the classical synchronization model and the
difference increased as development advanced. In addition we observed a novel pattern of mitotic clusters that coincided in time with
the mitotic pseudo “waves” described to occur before the midblastula transition. Altogether, our findings indicate that early
development is less synchronic than previously reported and that synchrony is not a requirement for proper development in zebrafish.
Supported by IX201110 and IN205612.
Program/Abstract # 223
Loss of Dchs1b and Dchs2 leads to early developmental and cytoskeleton defects in the zebrafish embryo
Li, Nanbing (Jade) (Washington University, USA), Kim, Seok-hyung (Vanderbilt University, USA); Ma, Taylor; Helde, Kathryn;
Moens, Cecilia (Fred Hutchinson Cancer Res Ctr, USA); Solnica-Krezel, Lilianna (Washington University, USA)
Dachsous (Dchs), an atypical cadherin with a large extracellular domain, has been shown to regulate planar cell polarity, tissue size
and morphogenesis, and cell-cell adhesion in Drosophila and mammalian cell culture. Loss of Dchs1 function in mice leads to
postnatal multi-organ defects and lethality. Using zebrafish as a model system, we characterize embryonic phenotypes of loss-offunction
dchs mutants and begin to elucidate its function in vertebrates. In zebrafish, as in humans there are two dchs homologs (dchs1
and dchs2), with duplication of dchs1 (dchs1a and dchs1b). We have identified two nonsense mutations in dchs1b and one nonsense
mutation in dchs2. All three mutations occur in the extracellular cadherin repeats domain that likely cause strong/complete loss of
function. Whereas zygotic dchs1b and dchs2 mutants show no obvious phenotype, maternal-zygotic (MZ) dchs1b-/- embryos present
early defects in cytoplasmic segregation, cortical granule exocytosis, maternal mRNA translocation, and cell division. Both
MZdchsb1-/- and MZdchs2-/- mutants show defects in epiboly as well as convergence and extension gastrulation movements.
Moreover, MZdchs1b-/- mutants present altered expression of the Spemann-Mangold gastrula organizer and mesodermal genes. Our
studies indicate that these pleiotropic phenotypes may be caused by defects in the dynamics of the actin and microtubule cytoskeleton.
We are using small molecules targeted to specific components of the cytoskeleton in order to identify the primary cytoskeletal defect.
Additionally, we are using candidate gene approach to identify proteins that interact with Dchs. This work establishes a novel function
for Dchs in early vertebrate embryogenesis.
Program/Abstract # 224
Cell and Tissue Interactions Organise Apico-basal Polarity During Lumen Formation in vivo
Ward, Laura, (King's College London, UK), Buckley, Clare; Clarke, Jon (King's College London, UK)
Much of our current knowledge of cell polarisation is based on cultured cellular aggregates polarising in a stable environment, which
does not fully recapitulate the complex dynamics of embryonic development. To overcome this, our work uses live imaging of the
transparent zebrafish embryo as a model in which to examine polarisation strategies during morphogenesis in vivo . Zebrafish
neurulation involves the transformation of an initially solid primordium into an epithelial tube. We have recently shown that apical
proteins localise to the point where a neural cell intersects the tissue midline, rather than at the cell’s anti-basal extremity. We are now
investigating the mechanisms by which neural cells are able to sense the tissue architecture, assemble apical complexes at the midline
and form a lumen at this point. During convergence, cells from each side of the neural rod interdigitate across the tissue midline and
we have evidence to suggest that cadherin-based nascent adhesions are formed in the region where they meet. We have shown that this
interdigitation is necessary for localisation of apical junctional proteins to the midline. We have additionally shown that basally
located ECM components act to orientate apico-basal polarity, through examination of embryos lacking functional laminin. Strikingly,
in the absence of a laminin-rich basal lamina, discrete regions of the neural tube show inverted polarity, with proteins normally present
at the apical surface being mislocalised basally. In conclusion, we have shown that cellular interdigitation across the neural midline
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