L3 Vertebrate limbs Flashcards
Why does tetrapod limbs all share homologous skeletal features?
Tetrapod limbs all share
homologous skeletal features along
the proximal distal axis and
therefor probably evolved from
a common ancestor
What are the processes that leads to limb development?
- Early cleavage
- Gastrulation
- Somitogenesis/Neurulation
- Limb development.
What is a tetrapod limb?
A tetrapod limb is a limb with a skeletal structure found in tetrapods, which are four-limbed vertebrates.
What does it mean if a tetrapod limb is homologous?
Means that it shares a common evolutionary origin and fundamental layout, even though the limbs may be modified for different functions like walking, swimming, or flying
What are the three distinct regions in the limb bud?
- Ectoderm: AER (Apical ectodermal ridge) (Epithelium)
- Mesoderm: Progress zone (mesenchymal cells that are below the AER)
- Differentiating tissues
What does AER stand for?
Stands for apical ectodermal ridge.
What is apical ectodermal ridge?
It’s a specialised group of cells that forms on the tip of a developing limb bud
Where is the apical Ectodermal ridge located?
It’s located at the distal end (farthest point from the body) of the limb bud
How did early experimental biologists examine the AER?
They used surgical methods
What happens if we remove the AER?
The removal of the AER truncates the limb
What is the progress zone?
Progress zone is an area where rapid cell division occurs, driven by the signals from the AER
What is the progress zone in context of tetrapod limb development?
The progress zone refers to the region of mesenchyme cells located just beneath the Apical Ectodermal Ridge (AER) at the tip of a developing limb bud
What genes were found that were responsible for AER activity?
Signal molecules called Fibroblast Growth Factors (FGFs)
What two FGFs are strongly expressed within the AER?
Fgf8 and fgf4 are expressed strongly, at the right time and in overlapping expression domains
What were the effect of FGF8 and FGF4 mutations?
FGF8 mutation has a mild limb defect
FGF4 mutant has a no limb defect
What does redundancy mean?
The inclusion of extra components which are not strictly necessary to functioning
Refers to the presence of multiple, overlapping mechanisms or pathways that ensure proper limb deveopment
Are FGF8 and FGF4 redundant?
Yes
Is genetic redundandy common in vertebrates?
Yes
FGF signalling is required for…?
Limb development
What are FGF8 soaked beads?
The beads are typically small agarose beads that are soaked in a solution containing Fibroblast Growth Factor (FGF8)
Why are FGF8 soaked beads used?
The Apical Ectodermal Ridge (AER) is a crucial signaling center in the developing limb bud, and it secretes FGFs, including FGF8.
The beads are used to mimic the function of the AER, allowing researchers to study the effects of FGF8 signaling in a controlled manner
What happens if we remove the AER from a limb bud and replaced it with an FGF8 soaked bead?
The limb will continue to grow
What is AER ablation?
Experimental removal of the Apical Ectodermal Ridge (AER) during limb bud development
What does ectopic expression mean?
Refers to the expression of a gene in a location or at a time where it is not normally expressed
What happens when ectopic expression of FGF occurs?
It leads to ectopic limbs which proves that FGF signalling is sufficient for limb development
What does ZPA stand for?
Zone of polarising activity
What is zone of polarising activity?
A special region of cells in the developing limb bud that plays a crucial role in determining the anterior - posterior (front to back) axis of the limb
Required for posterior fates and polarises the limb
What was the hypothesis regarding ZPA?
Hypothesis: a signal is secreted by the ZPA to give the A/P axis polarity
How did the hypothesis: a signal is secreted by the XPA to give the A/P axis polarity come about?
1) Gentle centrifugation of trypsin - dissociated leg bud mesoderm is placed into an Ectodermal jacket of leg bud.
2) Without ZPA - toes develop dismally but no A/P axis
3) Without ZPA placed in the jacket, normal limb with reversed A/P polarity
What is ZPA?
The Zone of Polarizing Activity (ZPA) is a crucial signaling center in the developing limb bud, responsible for establishing the anterior-posterior axis of the limb.
What is morphogen?
Morphogens establish concentration gradients within developing tissues. These gradients provide positional information to cells, influencing their fate and differentiation
What is Shh?
Shh - Sonic Hedgehog is a secreted ligand
Where were Hedgehog signals first identified?
They were first identified in Drosophila
