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6 lecture ZOO NEW

6 lecture ZOO NEW - Cell Sorting and Reaggregations...

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Cell Sorting and Reaggregations Differing sponge species (species specificity) Dissociation of sponges and reaggregations Single sponge Reaggregations of sea urchin embryos Specific cell sorting
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Differential Cell Affinities Townes and Holtfreter (1955) Reaggregated cells become spatially separated Final positions reflect embryonic origins
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Selective Affinity - Holtfreter
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Selective affinities Inner surface of ectoderm (+) for mesoderm Inner surface of ectoderm (-) for endoderm Mesoderm (+) for both ectoderm and endoderm Thus… because of these affinities… The cells sort into their proper positional relationships Over time, these selective affinities can change – just because you start out for an affinity for one cell group, the affinity can change as you develop and become more adherent to another type cell.
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Conclusions of the Experiments 1) Reaggregated cells become spatially segregated 2) Final positions reflect embryonic positions 3) Selective affinities changes* - (as these change, it seems they will sort themselves along the lines of the most thermodynamic stable relationship
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Histotypic Aggregations Reconstruction of complex tissues from individual cells (trypsinized embryos) Reaggregation of 15 day mouse epidermal cells – mouse is very will differentiated. Epidermal – keratinized (ectoderm) Dermal layer (mesoderm) Can also see hair follicles If you put these into a Ca-deficient medium and add trypsin to them (cleaves proteins), the cells will separate apart (see figures 3.25 and 3.26 in your book)
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Differential Adhesion Hypothesis Cells interact to form an aggregate with the smallest interfacial free energy – thermodynamically stable pattern ( hence a thermodynamic model ) Sorting requires cell types differ in their strength of adhesion May be based on - Same glue molecules (differ in amounts of glue) - Different glue molecules with varying strengths
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Cell Cell Adhesions Steinberg’s Model Based on strength of adhesions
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