Flashcards in Devo Lect 18 - Vertebrate Limb Deck (17)
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Asymmetry
Ant, post, proximal, distal parts of limbs very different
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Comparing limbs across species
Same bones and organization, just different proportions based on function
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Disorders in limb development
Polydactyly: extra digits; Syndactyly: fused digits; Polymelia: extra limbs; phocomelia: shortened limbs
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Formation of limb bud
Mesenchyme cells migrate to limb field, proliferate, form AER on limb buds (essential); limbs formed by stage 23 (day 56)
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AER
Apical ectodermal ridge; forms on top of the the limb bud mesenchyme cells. Needed for development of distal limb structures. Removal results in truncated limb, length depends on when removed
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FGF-10 and FGF-8
Mesenchyme cells secrete FGF-10 which cause epithelium to form AER; AER secretes FGF-8 to maintain mesenchyme cells just underneath; Mesoderm-ectoderm interactoin
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Saunders’ experiment (Fig 16.8)
Formation of limb axis - proximal-distal; mesoderm proliferation under AER; removal and addition of AER and mesoderm; AER allows proliferation, induced mesoderm tells the limb what to be, FGF responsible for signaling
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Figure 16.8
Summarize this
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Anterior/Posterior formation of limb bud
Defined before limb bud present; smal block of mesoderm tissue = ZPA; transplanting it caused posterior development of digits on both sides of limb
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ZPA
Zone of polarizing activity; determines ant/post axis; releases Shh, forms a gradient (high = post., low = ant.)
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Shh
Development of vertebrae; Somite to bone; from Hensen’s node to determine R/L symmetry; controls polarity of limbs
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Role of Hox genes in limb
TFs, genes arranged in series on chromosome; expressed in order along limb axis; knockouts result in lack of only one structure (ie Hox 11 -/- lack forearm, but the rest is normal)
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HoxA/D KO experiments
Stops shortly after bud; if they then add HoxD, hand developed on there
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Hox genes and snakes
Expression is different; Hox genes expressed in certain regions in chick embryo, but the boundaries of those genes in snakes is different; some snakes form vestigial limbs, but lack Shh
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Hox genes in fins vs mammals
Early Hox expression similar, later it is much higher in AER in mammal; important step in evolution
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Cell death zones
apoptosis, esp. in the digits; triggered by BMPs, although these are expressed in ducks and chickens; BMPs are blocked by gremlin in the duck
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