Post on 13-Apr-2017
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Vertebrate Embryonic Patterning 6
Limb Formation
Tetrapod Limbanterior
posterior
proximal distal
ventral
dorsal
Limb Bud Formation
Establishment of Limb Fields
Hox Gene Expression in Trunk During Limb Bud Development
Fig 23.21 Analysis of Biological Development 2nd ed, Kalthoff
FGF10 Induces FGF8
supernumerary limbs from implanting FGF10 beads
Positioning of the Limb Field• Forelimb position lies near anterior expression
boundary of hox b8, b9, c6, c9, & d9 • FGF8 expression in intermediate meso + hox
genes induces FGF10 in the lateral plate meso• Wnts in the lateral plate maintain FGF expression
in regions of hoxd9 expression• Between the fore & hindlimb buds, the
developing kidney (mesonephros) inhibits FGF expression
• Exogenous FGF can induce supernumerary limbs
Forelimb vs Hindlimb
Forelimb vs Hindlimb
Correlates with Tbx5 & Tbx4
Expression Respectively
Tbx5
Tbx4
Tbx4 Gain of Function Study
Tbx4 expressing retrovirus infected embryo
Proximal Distal Limb Axis
• AER– overlying ectoderm– responsible for maintaining outgrowth of limb
• Progress zone– underlying mesoderm– region of cell division– maintained in active mitotic state by AER
AER – Apical Ectodermal Ridge
AER Necessary for Limb Bud Outgrowth
Removal of AER at various times prematurely truncates limb
Reimplantation of older AER into younger bud causes loss of proximal structures
AER Necessary for Limb Bud Outgrowth
Reimplantation of younger AER into older bud causes duplication of distal structures
Interactions of AER and PZ
FGF8 in the AER
FGF in ectoderm AER3 day embryo
limb buds
Hox Code of the Limb
Proximal Distal
Hoxa & d = forelimb; Hoxc & d = hindlimb
Mammalian Hox Genes
Hox Genes Required for Proper Limb Development
Hoxa-11/Hoxd-11 double knockout
wild-type
human with synpolydactyly resulting from homozygous
HOXD-13 null mutation
Proximal-Distal Structures
Anterior-Posterior Limb Axis
Zone of Polarizing Activity
ZPA
Classic Experiments: Retinoic Acid Induced Limb Duplication
• Implanting RA soaked beads caused duplication of zeugopod/autopod structures
• Depletion of RA reduces the normal structures
Shh is Expressed in ZPA
Demonstration that Sonic Hedgehog is Primary ZPA Component
Dorsal-Ventral Gene Patterning in Limb Bud
Fig 23.23 Analysis of Biological Development 2nd ed, Kalthoff
Dorsal-Ventral Limb Axis
Lmx
1. Fgf-8 induces Shh2. Shh & Fgf-8 induce Fgf-43. All these induce Lmx-14. Wnt-7a in the ectoderm
represses Lmx-1 and induces Shh
BMPs
Molecular Interactions in Limb Bud
Dorsal-to-Ventral Transformations
WT mouse autopod
Wnt7a-/- mouse autopod
Homology of Process: A-P Axis
Homology of Process: D-V