Lecture 19 Flashcards
How does the pathway of wing development in Drosophila differ from that of it vertebrate counterparts, particularly, in the way in which the Lmx1b homologue acts
Apterous acts upstream of the wnt7a homologue in Drosophila. Apterous acts to upregulate the expression of wingless leading to the adoption of a dorsal cell fate
From medial to lateral, recall the mesodermal tissues in the developing chick embryo
Axial mesoderm, Paraxial mesoderm, intermediate mesoderm and lateral mesoderm
As the limb grows its specifies the skeletal element that the cells adopt depending on how long they spend proliferating in the progress zone, T or F
T
Explain the interaction between tbx4 and 5 that act to restrict the limb fields
There is a negative regulation mechanism between the tbx 4 and 5 genes. Tbx5 can block the expansion of tbx4 expression which prevents the hindlimb field from progression anteriorly into the forelimb field
Which two genes show an exquisite complimentary expression pattern with one being uniquely expressed in the dorsal ectoderm whilst the other in the ventral ectoderm
Engrailed1 is only expressed in the dorsal ectoderm whilst wnt7a is expressed in the ventral ectoderm
Homothorax is the Drosophila homologue of Meis1/2. What is the importance of this gene in fruit fly development
Homothorax is important in controlling the patterning of the proximal wing
What is the result of lmx1b knockout
Lmx1b knockout results in a ventralised limb with no dorsal mesenchymal development
FGF was considered to have a role in formation of the limb bud. Describe an experiment that could be carried out in chicks to determine if indeed, FGFs are involved in limb formation
Implantation of FGF-soaked beads into the flank of chick embryos and observe if ectopic limbs develop
It was later found that ectopic grafting of the ZPA resulted in a mirror image duplication of autopod structures. What was seen in a similar experiment where grafting occurred earlier in limb development
There was also a duplication of the ulna
Describe how FGF10 expression in the lateral mesoderm at the level of the limb is induced by the intermediate mesoderm
The intermediate mesoderm produces large amounts of FGF8 which induces the FGF10 expression by the lateral mesoderm
Chick wings have three digits, describe the nomenclature of these digits
The chick wing has three digits named digit 2,3 and 4. There is no digit 1
Why does digit 1 often develop to some extent in the limbs of sonic hedgehog knockout mice
The anteriormost limb, or digit 1 forms independently of sonic hedgehog signalling. Ihh expression is seen in the most anterior region of the limb bud and acts independently of sonic hedgehog to dictate some autopod development. Ihh maintains the expression of sonic hedgehog target genes in the anterior region of the limb bud in the absence of sonic hedgehog hence leading to the digit development seen in sonic hedgehog -/- mice. Similarly Ihh -/- mice did not show development of the anteriormost digit
How does sonic hedgehog dictate digit formation in the chick wing
Each threshold of sonic hedgehog correlates with the formation of a specific digit. At the lowest sonic hedgehog concentration digit 2 forms. Intermediate sonic hedgehog concentrations correspond to digit 3 with the highest sonic hedgehog concentration dictating digit 4
Below is a list of limb bone structures and their embryonic precursors regions. Match up the corresponding bones and embryonic structures. A - humerus, B - Metacarpals, C - Radius, D - Carpals, E - Phalanges, F - Ulna. i - Stylopod, ii - Autopod, iii - Zuegopod
A - i, B - ii, C - iii, D - ii, E - ii, F - iii
The temporal model for patterning the vertebrate limb was determined to be incorrect, T or F
T
Which genes are mainily involved in proximo-distal patterning of the limb
RA and FGF
How does the chick forelimb differ to its hindlimb
The chick wing only has three digits whereas the hindlimb or leg has 4. The leg has digits 1,2,3 and 4
Recall the pathway of interaction between the genes involved in the d-v patterning of the vertebrate limb
BMP –> En1 –| Wnt7a –> Lmx1b
The key genes involved in specification of the limbs were determined to be, FGF8 and 10, Retinoic acid, Hox genes and tbx transcription factors. Put these genes in the sequence in which they act to dictate limb field development
Hox genes –> retinoic acid –> tbx4/5 –> FGF10 –> FGF8
What the name of the structure that forms in the posterior limb bud that expresses the sonic hedgehog morphogen and plays a key role in patterning of the limb
Zone of Polarising Activity (ZPA)
Which tbx gene is responsible for specifying the posterior or hindlimb bud
Tbx4
Explain how interactions between the two secreted signals in vertebrate limb patterning act to form the stylopod
The stylopod is the most proximal region of the limb. This is the regions that’s is exposed to the highest levels of RA. Retinoic acid acts to induce the expression of Meis1 and 2
Describe an experiment that could be carried out in chicks that would provide the evidence that the ZPA controls anterior-posterior patterning in the limb. What would you expect to see
Surgical removal of the ZPA from a donor chick embryo and implantation into the anterior limb bud of a donor embryo. This grafting would cause a mirror image duplication of distal limb structures
Describe the phenotype of a sonic hedgehog mutant with regards to the effects on limb development
Sonic hedgehog knockout mice exhibit a complete loss of the distal most skeletal elements as well as a loss of identity of the zeugopods
