Vertebrate Limb Bud Formation Flashcards
What are the chick equivalent of the following bones?
a) humerus
b) ulna/radius
c) carpals/digits
a) Stylopod
b) Zeugopod
c) Autopod
What digits do chicks have?
2, 3 and 4
Where do limb buds first appear?
From the flank of the embryo along the AP axis in precise locations
How do we know that the limb field is molecularly defined to form the limb?
Through transplantation studies eg take mesoderm from the prospective limb field and transplant ectopically means a limb will form
What are the two transcription factors which specify limb identity?
tbx 5 and tbx 4
Where is tbx 4 expressed?
The hind limb
Where is tbx 5 expressed?
The fore limb
What does tbx 5 inhibit through negative feedback?
Tbx 4
What happens if you express Tbx 4 in the fore limb?
The fore limb will become a leg
The position of the limb along the AP axis is dictated by what?
Hox genes
Which Hox gene determines the position of the forelimb?
hox 6
How are the hox genes and tbx genes related?
Hox gene expression provides a permissive enviroment that allows the synthesis of retinoic acid in the lateral mesoderm where the limb buds form
Retinoic acid induces the expression of Tbx
What occurs if beads soaked in FGF4 are placed along the AP axis in lateral mesoderm?
Placing beads in a flank region not destined for limb bud formation would develop an ectopic limb
The identity of the extra limb would depend on which limb the bead was closer to
What initiates FGF expression?
Tbx
What does Wnt3a specify?
The apical ectodermal ridge
What is the AER?
A thickening of ectoderm which lies over the limb bud which has a role in proximo-distal development of the limb bud
What are the parts of the limb bud?
Apical ectodermal ridge
Progress Zone
Polarising Region
What happens when the AER is removed at:
a) 3 days
b) 3.5 days
c) 4 days
a) no distal elements
b) no digits
c) nearly full compliment (digits slightly truncated)
What are the two models for the proximo-distal patterning of the vertebrae limb?
The progress zone model
Two Signal model
What is the progress zone model?
States that if a cell spends little time dividing in the progress zone it will be more proximal compared to if it spends a long time in the progress zone
This model was in place for a number of years but is not correct - fate mapping did not match these predictions
What is the two signal model?
Proposes there are two precursor cells for all proximal, intermediate and distal elements and cells will differentiate according to the signals they are exposed to
What are the two signals in the two signal model?
Proximal signal = retinoic acid
Distal signal = FGF
Retinoic acid drives the expression of which homeobox transcription factor?
Meis
What is Meis?
Specifies proximal elements
