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Cell Movement During Development

cell mov’t - changing patterns of cell adhesion  

  • cadherins - used in cell-to-cell interactions
    • transmembrane proteins that mediate Ca++ binding
    • cells w/ similar cadherins tend to go together
    • cells w/ most cadherins in interior, cells w/ least cadherins on outside
  • integrins - used in cell-to-substrate interactions (involving interactions w/ extracellular matrix)
    • used when most of tissue made up of spaces between cells (ECM)
    • connects cytoskeleton to the ECM
    • can change cytoskeleton growth, way cell secretes materials

induction - cell changes due to interactions w/ another cell 

  • mosaic development - in Drosophila, where determinants (developmental signals) guide cells on different development paths
  • regulative development - in mammals where cell-cell interactions determine development
  • proteins used as intercellular signals
  • organizers - groups of cells that produce signals for position to other cells
    • tells other cells of distance to organizer
    • morphogen - signal molecule for determining relative position

determination - cell’s commitment to a certain developmental path 

  • early cells totipotent, all capable expressing of their genes
  • chimera - organism w/ cells from different genetic lines
  • differentiation occurs at the end of developmental path, not the same thing as determination
  • positional labels - shows cell’s location in embryo, influences how body develops
  • cloning of Dolly the sheep on July 5, 1996 from fully-differentiated sheep shows that determination can be reversed

pattern formation - unfolding process that lays down the basic body plan 

  • currently only fully understood for Drosophila
  • sets up anterior/posterior, dorsal/ventral axis
  • bicoid protein gradient - determines anterior/posterior axis w/ aid of dynein (protein oskar also plays role for posterior determination)
  • accumulation of certain mRNA on 1 side of cell determines dorsal/ventral axis
  • polarity found by interactions between follicle cells and oocyte (egg)
  • gap genes - map out the subdivision of the embryo
    • hunchback mRNA - develops the thorax, cannot by blocked by nanos protein only in the anterior end
  • pair-rule genes - alters every other body segment when mutated
    • hairy - gene that divides embryo into 7 bands of proteins
  • segment polarity genes - subdivides the zones created by hairy

homeotic genes - gives identity to embryonic segments created in pattern formation 

  • bithorax complex - cluster of genes in 3rd chromosome in Drosophila that affect body parts of thoracic/abdominal segments
  • antennapedia complex - cluster of genes that affect body parts of anterior end
  • homeobox - sequence of 180 nucleotides that codes for the homeodomain (DNA-binding protein)
    • distinguishes portions of genome used for pattern formation
  • Hox genes - genes that contain homeoboxes
    • 4 copies in vertebrates
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