Study Questions - Lecture 17 & 18
1. What are the three general areas of the developing vertebrate limb?
The three general areas of the developing vertebrate limb are the proximal stylopod,
zeugopod, and the autopod.
2. What embryonic regions contribute to the developing limb bud?
The embryonic regions that contribute to the developing limb bud are the lateral plate
mesoderm and from the myotome responsible for limb muscle precursor cells.
3. What type of gene expression (gene family name) is associated with the A-P
positioning of developing limb buds?
The genes that are involved in the A-P positioning of the developing limb bud are Hox
genes along the axes.
4. Describe the expression patterns of FGF8 and FGF10 in the developing limb bud
and during limb outgrowth. Outline the model for the signalling pathway by which
these expression patterns are consolidated and maintained.
FGF10 is initially expressed throughout the lateral plate mesoderm. The expression
pattern is then restricted to the sites of limb bud formation by Wnts specific for the fore
limb and hind limb area. This localized expression in the lateral plate mesoderm then
regulates FGF8 and Wnt3a in the ectoderm of limb buds. These are all necessary for limb
outgrowth. 5. What genes are differentially expressed in the developing fore limb vs. hind limb?
The developing fore limb will express Tbx5 while the hind limb will express Tbx4.
6. What is the apical ectodermal ridge and what is its function?
The apical ectodermal ridge runs along the distal margin of the limb bud. It functions as a
major signaling center with 3 major roles:
-maintaining plasticity of underlying mesenchyme so that proliferation will lead to
proximal to distal limb growth
-maintaining expression of molecules that generate the A-P axis
-interacting with proteins specifying A-P and D-V axes so that each cell can differentiate
7. What group of cells in the developing limb bud specify the character (fore vs. hind)
of the developing limb?
The group of cells that specify the character of the developing limb is known as the
progress zone. 8. How was it determined that the specification of proximal vs. distal structure in the
developing limb is not generated by signals from the AER?
It was proven that the AER did not carry all the information as these were proven to be
-older AERs combined with younger mesoderm would make distal structures in proximal
-younger AER combined with older mesoderm would make repeated structures
9. What is the “progress zone model” of proximal/distal specification in the developing
limb? What evidence suggests that this model is not correct?
In the progress zone model, it was thought that the amount of time the mesodermal cell
spends dividing in the progress zone determined how distal it’s specification was. So if it
had more time, it would result in a more distal part. However, when FGF was knocked
out in mice, distal elements formed normally while proximal ones did not.
10. What is the “early allocation and progenitor expansion” model of the
proximal/distal specification in the developing limb? What experimental evidence
supports this model?
In the early allocation and progenitor expansion model, it was thought that early cells are
already specified and division just increases the number of discrete cell populations.
Support came from experimental removal of the AER, “zones” of distal structures could
be destroyed and lead to the formation of only proximal structures.
11. What is the developmental function of the “zone of polarizing activity”?
The developmental function of the zone of polarizing activity is to specify A-P axis of a
12. What important paracrine factor is expressed specifically in the ZPA? How was it
determined that this gene is sufficient and necessary for ZPA function?
The important paracrine factor expressed in the ZPA is Shh. It was determined that Shh
was sufficient and necessary for ZPA function by placing a cell pellet secreting Shh onto
the presumptive anterior region while normal ZPA was present at the posterior and
forming a limb with two axes.
13. What does my cat have extra toes?
The cat has extra toes because of a gain of function mutation in Shh.
14. What evidence suggests that interdigitary tissue is also involved in digit
specification? Experiments where interdigitary tissue was removed would lead to duplications of certain
digits. For example, if the tissue between digit 2 and 3 were removed, both of them
would now result in digit 2 identity.
15. What signalling molecule associated with specification of dorsal vs. ventral in the
vertebrate limb? How is this related to nail-patella syndrome in humans?
The signalling molecule associated with specification of D-V axis in the vertebrate limb
is Wnt which is expressed in the dorsal ectoderm of limb buds. This signalling activates
the transcription factor Lim1 in dorsal mesenchyme which is fuccient and necessary for
In humans, mutants of Lim1 will lack dorsal limb structures. This causes nail-patella
syndrome as dorsal structures will not form correctly.
16. What type of signalling operates at the junction of dorsal and ventral mesoderm in
the developing limb to establish the position of AER?
Notch signalling is important at the junction of dorsal and ventral mesdoderm in
developing limb to establish the position of AER. It will relay the dorsal ventral signals