Congress Abstracts - Society for Developmental Biology

facilitan cambio en patrón morfogenetico del mesénquima de crestas conales hacia la proliferación y maduración del tejido. Let7 no
participa.
Program/Abstract # 240
The Cellular Basis of Limb bud Initiation
Gros, Jerome, (Institut Pasteur, France), Tabin, Cliff (Harvard Medical School, USA)
Vertebrate limbs first emerge as small buds at specific locations along the trunk. While a fair amount is known about the molecular
regulation of limb initiation and outgrowth, surprisingly, the cellular events underlying these processes have remained less clear. Here
we show that the mesenchymal limb progenitors arise through localized Epithelial-to-Mesenchymal Transition (EMT) of the coelomic
epithelium specifically within the presumptive limb fields, and that this process is the initiating event of limb bud formation. This
EMT is absolutely necessary to initiate limb buds and is regulated at least in part by Tbx5 and Fgf10 (key players in the molecular
circuitry known to genetically control limb initiation). The finding that limb buds initiate earlier than previously anticipated through
an EMT event and not through differential proliferation rates, as previously proposed, represents a paradigm shift that changes the
way one views the earliest events of limb formation and redefines the question of how the limb buds are placed at the proper axial
location.
Program/Abstract # 241
Gene regulation that initiates Sonic hedgehog expression in the limb bud
Tamura, Koji, (Tohoku University, Japan), Matsubara, Haruka; Yokoyama, Hitoshi (Tohoku University, Japan)
Sonic hedgehog (Shh) is a key molecule that plays a central role in limb morphogenesis. Shh expression in the early limb bud begins at
a restricted small area of the lateral mesoderm as a very small dot at the posterior margin of the bud. Genetic and embryological
evidences in the mouse and chick have suggested that positive regulators, such as dHAND, Hoxd11/d12 and Tbx2/3, and negative
regulators, including Gli3R and Alx4 , restrict the domain of Shh expression, determining the position of posterior-localized Shh in
early limb bud. However, available data in published reports and our own detailed observations indicate that these genes are expressed
in a region larger than the epression domain of Shh, suggesting that the posterior-restricted initiation of Shh expression cannot be
completely explained by any combinations of expression of these genes. To overcome this incompleteness, we further focused on
Tbx2/3, Hoxd12 and Fgf8 and examined the expression pattern of these genes in detail in sections. Tbx2, an upstream inducer of Shh ,
is expressed at the posterior (and anterior) periphery of the limb bud mesenchyme, and Tbx2 expression is negatively regulated by
Fgf8. Hoxd12, which can induce Shh, is expressed in posterior margin of the limb bud mesenchyme simultaneously Shh. FGFs are
known to have a function in maintenance of Shh expression with a feedback loop. Fgf8 is expressed in a special epithelial tissue, the
apical ectodermal ridge (AER). We found that Shh expression begins at a small area within Tbx2/3 and Hoxd12 positive domain
underneath the posterior margin of Fgf8 expression. These results suggest that the initial position of the Shh expression domain
corresponds to the Tbx2/3 and Hoxd12-positive region underneath the Fgf8-positive ectoderm at the posterior edge of the AER.
Program/Abstract # 242
Expression and functional analysis of transcription factor AP-2ß in limb development
Seki, Ryohei (MRC National Institute for Medical Research, UK); Suzuki, Takayuki (Nagoya University, Japan); Yokoyama, Hitoshi;
Tamura, Koji (Graduate School of Life Sciences, Japan)
Tetrapods have two pairs of limbs, which exhibit diverse morphology in digits. The length of a digit depends on the number and
length of phalanges. To elucidate the mechanisms that regulate these two parameters, we focused on a human familial syndrome
called Char syndrome causing digit malformation, including shortness and disappearance of the skeleton. It has been reported that the
responsible gene is the transcription factor AP-2β. Thus, we hypothesized that in normal development, AP-2β regulates the length and
number of skeletal elements in digits. We first investigated using chick embryos whether AP-2β carrying the same mutation as that in
Char syndrome gave rise to skeletal malformation of digits. The resultant limbs showed shortness and disappearance of the skeleton.
The expression analysis of AP-2β revealed a good correlation between the digit length and duration of AP-2β expression. Further
investigations suggested that AP-2β functions downstream of FGF signals from the apical ectodermal ridge, an epithelial structure
essential for limb outgrowth. Gain of function of AP-2β, however, resulted in no alteration in limb skeletal pattern. Co-overexpression
of AP-2β with AP-2α, which is another member of the AP-2 family and is also expressed in the distal limb bud, resulted in limb
skeletal malformations. To further elucidate the function of AP-2β, we examined alterations in cell proliferation, cell death and
expression pattern of some key genes involved in limb outgrowth or morphogenesis when the dominant-negative AP-2β was
introduced. Based on the results, we will discuss the mechanism of digit morphogenesis from the viewpoint of AP-2β function.
Program/Abstract # 243
Role of transcription factor EVI-1 in chondrogenesis
Cela, Petra; Balkova, Simona (Institute of Animal Physiology and Genetics, Czech Republic); Horakova, Dana; Buchtova, Marcela
(University of Veterinary and Pharmaceutical Sciences, Czech Republic); Richman, Joy M. (Life Sciences Institute, University of
British Columbia, Canada)
Ecotropical viral integration site 1 (EVI-1) is a transcription factor essential for vascularisation and cell proliferation during
embryogenesis. In our previous experiment, we analysed its expression during chicken embryonic development. Since strong
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