limb development Flashcards
what is the proximal-distal axis of the limb
proximal = shoulder distal = tips of fingers
what is the anterior-posterior axis of the limb
anterior = thumb posterior = pinky
what is the dorsal-ventral axis of the limb
dorsal = back of hand ventral = palm
where is the limb field derived from?
lateral mesoderm
what experiments would you conduct to show that the determination of the limb field is an intrinsic property of the lateral mesoderm?
graft mesoderm from prospective limb into ectopic position —> limb formed
graft tissue not normally forming limb into a position fated to form limb —> limb not formed
which t-box TF is expressed in the forelimb
tbx5
what does the hindlimb give rise to?
leg bud
which t-box TF is expressed in the hindlimb
tbx4
what does the forelimb give rise to?
wing bud
which gene drives hindlimb identity
pitx1
which gene marks where the forelimb will form?
hoxc6
describe how limb formation is initiated
- hox proteins along AP axis determines position of future limb field —> hindlimb or forelimb?
- hox drives tbx expression via RA
- tbx TFs drive fgf10 expression in lateral mesoderm
- fgf10 drives fgf8 expression in ectoderm
- fgf8 secreted by AER drives fgf10 expression in lateral mesoderm and promotes cell proliferation of lateral mesoderm cells
- wnt signalling restricts size of fgf10 expression domain
what is the progress zone?
area of mesenchymal cells lying immediately behind the AER
what is the AER and what is it required for?
thickening of ectoderm overlying limb field
fgf rich region
needed for limb outgrowth
high FGF expression levels
what is the progress zone model?
AER drives cell proliferation of progress zone behind it
more cell divisions = more distal structures formed
why is the progress zone model incorrect?
in the early developing limb field there are already progenitors that can develop all skeletal elements
what is the two signal model?
- initially have all progenitors for all skeletal elements of limb
- the cells adopt cell fate based on their position along antagonising FGF and RA signalling gradients –> determines whether the cells will form proximal or distal structures
describe the antagonising signalling gradients involved in PD patterning of limb
AER = high FGF = most distal
lateral plate mesoderm + paraxial mesoderm = high RA = most proximal
cell fate depends on their position along the gradients
what genes does RA signalling activate during PD patterning of limb?
meis1
meis2
what genes does FGF signalling activate during PD patterning of limb?
hox13 = distal hox11 = proximal
what is the drosophila ortholog of hoxd13?
distalless
what is the drosophila ortholog of meis?
homothorax
what would be observed if the zpa or shh soaked bead is transplanted into the anterior limb?
mirror image duplication
what would be observed in limbs with Shh KO?
- loss of distal skeletal elements + severe truncation
- anterior most digit only observed
- loss of identity of zeugopod –> single bone formed
which gene is expressed in the dorsal limb?
wnt7a
which gene is expressed in the ventral limb and what drives its expression?
engrailed1 is expressed in the ventral limb
bmp signalling drives engrailed expression
where is lmx1b expressed in the limb?
dorsal half of the limb
what is the role of engrailed1 in patterning the limb?
inhibits wnt7a expression
restricts wnt7a expression to dorsal ectoderm
what does lateral mesoderm contribute to?
limb skeleton formation
