07Chemotaxis-PresentationFormat

07Chemotaxis-PresentationFormat - Important Things Cells Do...

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Important Things Cells Do Grow ……… will do a lot of this soon Differentiate Move Produce Degrade/Breakdown Die ……………………
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Cell Migration Bacteria, Sperm …. Swim …. Flagella, Cilia Mammalian Cells …. Migrate ….. Adhesion + Cytoskeletal Forces and Rearrangments. http://cellix.imba.oeaw.ac.at/ Introduction Cytoskeleton Moving w/Actin Adhesion Polarization - Microtubules
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Introduction An essential property of eukaryotic cells is their ability to orient in response to spatial cues . Only by correctly interpreting spatial changes in external stimuli can yeast cells mate, soil amoebae form spores, progeny of a fertilized egg form an organism, or neutrophils crawl toward their prey. Cells are known to respond to gradients of external stimuli such as chemoattractants, but how they sense and interpret gradients remains mysterious. The mystery goes beyond our ignorance of the biochemical basis of gradient sensing. More fundamentally, we have not even definitively identified the external cues sensed and interpreted by the cells and the respective roles of these cues in determining their responses to gradients.
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Generally accepted theory Bacteria migrate up gradients of chemoattractants, in a process called chemotaxis. Chemotaxing bacteria assess gradients temporally , by moving through the attractant concentration field, sensing the local ambient concentration, comparing the concentration at a given moment with concentrations at previous times, and changing swimming behavior accordingly (1). It has been proposed that the larger cells of eukaryotes , in contrast, sense gradients at a given moment by comparing attractant concentrations at different positions on their surfaces and thus orient themselves to crawl in the up- gradient direction by interpreting the spatial cues present in their location at that moment. In other words, such cells assess the gradient spatially and respond to purely spatial cues by directed chemotactic migration .
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Chemotaxis Eukaryotes Prokaryotes Timing of tumbling changes depending on time dependent changes in conc. Bacteria so small can’t detect spatial gradients Swim fast, can detect spatial conc change by timing. Polarization, Protrusion/retraction Large enough to sense spatial gradient Too slow to detect spatial conc change by moving
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But is the common theory true? It has been shown that neutrophils can sense relatively steep gradients of chemoattractant, supplied by a micropipette, without moving and therefore without comparing ambient concentrations in different locations. In the micropipette experiments, however, the side of a cell that is first exposed to the attractant is also the side that will eventually be exposed to a higher attractant concentration . Thus, the spatial pattern of PIP3 accumulation might be influenced by this temporal pattern of application of attractant . Moreover, it remains unclear whether actual migration of these cells up a gradient can
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This note was uploaded on 02/23/2010 for the course BME 321 taught by Professor Shuichitakayama during the Winter '10 term at University of Michigan.

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07Chemotaxis-PresentationFormat - Important Things Cells Do...

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