Making a Limb Flashcards
Limbs are _________ to the body plan
Appendages
Which organism is a good model for studying limb development?
Chick
What are limb buds?
The site for limb growth
Limb buds have what kind of information?
Positional and axes info
What processes occur at the limb bud?
Cell differentiation, growth, morphogenesis
What are Hox genes?
A group of related genes that control the body plan of an embryo
Which gene can be knocked out of mouse making forelimbs develop at a more anterior level?
Hoxb5
What does the Hoxb5 gene do and to what organism?
If knocked out of mice it causes the forelimbs to develop at a more anterior level
How does knocking out a Hoxb5 gene cause the forelimbs to develop at a more anterior level?
Restricts expression of fibroblast growth factors (FGFs) to limb-forming regionss
In chicks is FGF required for limb bud formation?
Yes
FGF signalling is ________ for limb bud formation
Required/Sufficient
In chicks is FGF sufficient for limb bud formation?
Yes
What does the homozygous knockout of FGF10 result in?
Embryos lacking limb buds
Which gene can be homozygously knocked out to cause a lack of limb buds in chicks?
FGF10
Local application of FGF4 induces what?
Ectopic (abnormally located) limb buds
Which gene can induce ectopic limb buds in chicks?
FGF4
Limb bud structure
Apical Ectodermal Ridge (AER)
Progress Zone (PZ
Zone of Polarising Activity (ZPA)
What is the AER?
Apical ectodermal ridge
- Ridge of thickened epithelium around the tip of the limb bud
The AER rigid, why?
Keeps the limb bud flattened
What does the AER maintain?
Cell proliferation in the PZ
What does removal of the AER result in?
Truncation of the limb
Does the time of removal of the AER affect the truncation of the limb?
If removed at 3 days, only proximal elements of the arm formed. At days, more distal elements formed
Can you induce normal development after removing the AER?
Yes, replaced the AER with beads soaked in FGF4
What happens if you remove the AER and replace with beads soaked in FGF4?
Normal development of the limb
What is the PZ?
Progress Zone
- Distal region of limb bud mesenchyme (mesodermal cells)
Is the PZ the distal or proximal region of the limb bud mesenchyme?
Distal
What are the cells like in the PZ doing?
Undifferentiated and rapidly dividing
What does the length of time in the PZ determine?
Cell fate
How is cell fate in the limb bud determined?
Length of time in the PZ
Along which axis does the length of time in the progress zone determine cell fate?
Proximal-distal axis
What is the ZPA?
Zone of Polarising Activity
- Region of limb bud mesenchyme which is an organising centre to pattern the anterior-posterior axis
Which axis does the ZPA pattern?
Anterior-posterior axis
Where can the ZPA be transplanted to induce symmetrical duplications of normal limb elects about the anterior-posterior axis?
Underneath the anterior AER
What happens when the ZPA is transplanted underneath the anterior AER?
The ZPA induces symmetrical duplications of normal limb elements about the anterior-posterior axis
Which gene is expressed in a very discrete zone of the limb bud?
Sonic Hedgehog (SHH)
What acts a morphogen in the limb bud?
SHH
What does SHH pattern by acting as a morphogen?
The anterior-posterior axis
What can alternatively be done instead of transplanting the ZPA underneath the anterior AER to induce symmetrical duplications of limb elements about the anterior-posterior axis?
Implant an SHH-soaked bead as it mimics the action of ZPA
Which class of genes are also expressed in overlapping zones of the limb bud?
Hox genes
What roles do Hox genes have in the limb bud?
- May specify the ZPA
- Deletion of Hox genes –> loss of SHH expression in ZPA (lead to limb truncation)
- Transplantation of ZPA establishes new domains of the gene expression in the limb bum
What does the deletion of Hox genes lead to?
Loss of SHH expression in the ZPA
What do transplant experiments demonstrate about the limb bud?
That the mesoderm of the limb bud determines the pattern of cell death
What regulates the pattern of cell death in limb buds?
Hox genes
