470seaurchinspecificationgastrulation

470seaurchinspecificationgastrulation - Axis specification...

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Axis specification in sea urchins 1. Cytoplasmic polarity: What classic experiments show that the four cells of the four-cell zygote are totipotent (this is review)? -Dreisch sea urchin embryos -remove and isolate each of the four cells from the fertilization envelope and one another -result – 4 normal, but smaller embryos What classic experiments demonstrate that there is something in the vegetal region of the egg that is required for mesoderm and endoderm formation? -Driesch recombination experiment – changed direction of third cleavage by compressing early embryos b/n glass plates, causing the third division to be meridional (like first two) instead of equatorial -result – reshuffled nuclei; placing nuclei into presumptive ectoderm that would normally form endoderm -normal embryos obtained – nuclear determinants do not segregate Is there a localized molecule, normally associated with the Wnt pathway, that is found in the vegetal region that might contribute to this? -beta-catenin specifies micromeres and their ability to induce neighboring cells 2. Axis specification : What experiments show that the animal and vegetal tiers of cells in the 8-cell embryo are not equivalent, and not totipotent? -vegetal and animal tiers separate during 3 rd cleavage What comes about from cleavage to form 16-cell embryo? -vegetal tier undergoes unequal equatorial cleavage producing 4 large cells (macromeres) and 4 small cells (micromeres) at the vegetal pole -animal tier divides equatorially to produce two tiers an1 above an2 What role do micromeres
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