5. Recall the ZPA and AER, and their roles in limb growth and patterning. Anteri

Question

5. Recall the ZPA and AER, and their roles in limb growth and patterning. Anterior ALimb bud AER ZPA Limb buds Posterior Embryos with their AER removed show no limb outgrowth, and weak A-P digit patterning, Given these results, describe the concentration gradient and amount of SHH that you expect to find in these embryos, compared to embryos with intact AERs, and explain your answer a. b. Predict the limb morphology of embryos that have half of their AER transplanted to ectoderm on their ventral side, and explain your answer. You isolate a protein secreted by cells of the AER called Fibroblast Growth Factor (FGF), and hypothesize that it is the inductive signal that causes normal limb outgrowth. Describe an experiment that would test this hypothesis and explain the results that would confirm your prediction (note: FGF has a role many developmental sequences and so you cannot knock out the gene or eliminate its transcription). 4) c.

Explanation / Answer

a. In an embryo, for the proximodistal limb patterning,the AER (Apical Ectodermal Ridge) is the first signaling center to appear.Removal of AER results in proximal limb truncation.

The second signaling center to appear is the ZPA(zone of polarizing activity) along posterior limb bud.The signaling molecule is SHH. Expression of SHH Iin the posterior mesenchyme of the developing limb bud rgulates patterning and growth of the developing limb by activation of Hedgehog(Hh) pathway.In the embryo where AER is removed,it will result in loss of Hh signaling ,thereby resulting in disruption of normal digit pattern.

Exposure to SHH controls the number and identity of digits produced. The concentration gradient of SHH will be the same in embryos with AER and in those where AER is removed.But since AER is removed the SHH signaling pathway is disrupted, therefore resulting in abnormal digit patterning.

b.If half of the AER is transplanted on the ventral side of ectoderm ,the limbs will show a loss of ventral diferentiation distally and ectopic formation of dorsal structures ventrally.This is because the homeobox transcription factor En1 is expressed in the ventral ectoderm and ventral half of AER and loss of En1 function will result in the above mentioned defect.

c.The FGF secreted by the AER is responsible for limb outgrowth.When an experiment was conducted where AER was removed ,but FGF soaked beads were placed in AER's place normal limb develpoment was seen to proceed.However, proliferation might cease prematurely,unless additional beads are added to maintain appropriate levels of FGF.

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