2011_Questions_Week_11_Answers

2011_Questions_Week_11_Answers - 1. Python embryos...

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1. Python embryos completely lack the expression of forelimb buds. Why? -Pythons have expression of HoxC8 all the way to the anterior boundary of the spinal cord. As a result of 'posterior prevalence' there is likely no boundary of functional HoxB5-HoxC6 (forelimbs form at the anterior boundary of HoxC6 in almost all vertebrates). Indeed, there is likely no functional activity at all for B5 and C6. This means that the territory that normally forms forelimbs is converted to a thoracic identity. This is likely why no forelimbs form. 2. Limb outgrowth is a complex set of signals between the budding mesoderm and the adjacent ectoderm. A) Which key mesodermal component induces the formation of the apical ectodermal ridge (AER)? -FGF10 B) What molecule is sufficient to substitute for the AER in directing limb outgrowth? -FGF8 C) Which molecule(s) are necessary for AER-directed limb outgrowth? -FGF4 and FGF8 must both be knocked out. 3. Transgenic mice containing Prrx1Cre drive expression of Cre throughout limb mesenchyme, while another line, Msx2Cre, drives expression specifically in the AER. If you were to generate a conditional double knockout of FGF4 and FGF8 in the specifically in the limb mesenchyme, what would be the predicted phenotype? -There would not be a predicted phenotype since these genes are expressed only in the AER (which is in the overlying ectoderm). 4) The Progress Zone Model states that the longer mesenchymal cells spend in the progress zone, the more distal they will become. However, this long-standing model is not compatible with data from Sun et al. What was their result, and why is it not compatible? -Sun et al., 2002 showed that removing both FGF4/FGF8 in the AER resulted in a complete loss of hindlimb formation and severe forelimb defects. This is because Msx2Cre only deletes FGF4/FGF8 in the forelimbs after it has been very briefly expressed. This initial burst of expression is responsible for the limited, defective growth
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2011_Questions_Week_11_Answers - 1. Python embryos...

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