Lecture 3 - vertebrate limb Flashcards
Is the thumb considered anterior or posterior?
anterior
Is the pinky finger considered anterior or posterior?
posterior
What type of limbs share homologous skeletal features along the proximal distal axis?
Tetrapod limbs
Outline the stages of embryonic development
Early cleavage - gastrulation - somitogenesis - pharyngula - neurulation - limb development
What is the first sign of limb development?
Limb buds form on either side of the embryo
What occurs after the formation of a limb bud?
the limb bud elongates towards distal end.
What occurs simultaneously to the growth of limbs?
differentiation of the skeleton
What are the 3 distinct regions in the limb bud?
- ectoderm
- mesoderm
- endoderm
What is found in the ectoderm of the limb bud?
AER - apical ectodermal ridge (epithelium)
What is found in the mesoderm of the limb bud?
- progress zone (mesenchymal cells below AER) - lots of separation
- differentiating tissues
Where is the AER (apical ectodermal ridge) found in terms of P/A & D/V?
Posterior dorsal axis
When does growth occur in limbs?
When cells proliferate at the end of the limb bed.
What occurs to cells when they leave the proliferative zone?
They specialise
What is the consequence of the removal of the AER (apical ectodermal ridge)?
Tructation of the limb - meaning signals MAY BE instructive (telling them what to specialise) or external signals
What is needed in the progress zone?
something secreted by the AER
What can older AER be substituted for?
Younger AER
How does the signal from AER impact the progress zone?
The signal is not instructing the progress zone on what to make, but it is permissive to growth.
What is the name given to the molecules that permits growth?
FGFs - fibroblast growth factors
How many FGFs are found in mice?
22
What can FGF expression lead to?
Gravel proliferation (growth)
What 2 FGFs are expressed strongly, at the same time and in overlapping domains?
Fgf4 & Fgf8
What occurs as a result of a mutation in Fgf8?
Mild limb defect
What occurs as a result of a mutation in Fgf4?
No limb defect
What is redundancy?
The inclusion of extra components which are not strictly necessary to functioning
What word describes the level of requirement Fgf8 & Fgf4 hold for limb development?
Redundant
What does the fact that FGF8 soaked beads rescue AER ablation suggest about the use of AER?
suggests that AER is only required as a source of FGF
What does ectopic expression of FHF in the flank lead to?
ectopic limbs (using beads of genetic methods)
What can be said about FGF signalling as a result of ectopic limbs growing as a result of ectopic expression?
FGF signalling is sufficient for limb development
What scientist researched anterior to posterior patterning?
Saunders
What occurred when Saunders moved the limb bud part from the mesoderm from the posterior to anterior?
Mirror image was created, with most posterior digit formed in most anterior location
What complex is involved in the process of A/P patterning?
ZPA - zone of polarising activity (cluster of mesodermal cells)
What is the ZPA (zone of polarising activity) made of?
Cluster of mesodermal cells
What occurred when the limb bud was removed, along with the mesoderm, but the ectoderm was left?
Toes develop distally but not A/P axis
Hypothetically, how would you go about restoring an A/P axis?
addition of ZPA
What leads to A/P axis polarity?
A signal is secreted by the ZPA
What is the secreted morphogen proposed by Turing?
ZPA is secreting morphogen signal, which diffuses away from posterior or anterior, to establish a gradient.
How do the different digits attached to a limb differentiate?
They differ in level of morphogen expressed - one digit will have high concentration with another containing intermediate levels.
What other hormone expression correlates to the ZPA?
SHH (sonic hedgehog)
What is SHH (sonic hedgehog)?
Secreted ligand - identified in Dosophilia
How is SHH required for A/P?
SHH soaked beads induce posterior cell fates in a dosage dependent manner.
What is AER required for?
AER is required for outgrowth and surgical removal results in loss of distal structures.
What mimics the effect of AER?
FGF - and is a limb organiser
How does FGF mimic the effects of AER and act as a limb organiser?
- expressed in the right place
- beads rescue the loss of the ZPA
- beads can induce reversed polarity in limbs
- mutant looses posterior limb structures
