Limb development Flashcards
What are the 3 axes of limbs?
Proximal-distal
Anterior-posterior (thumb-pinkie)
Dorsal-ventral (palm-knuckle)
What are the 3 types of bones in a tetrapod limb?
Stylopod (1 bone), zeugopod (2 bones), autopods (many bones)
Is the limb anatomy of tetrapods a homologous or analogous feature?
Homologous, it existed in Tiktaalik
What genes specify limb regions?
Hox genes
What happens if a Hox gene that specifies parts of the limb are mutated?
Only affects that one region of the limb
Much of the limbs come out of which type of mesoderm?
Lateral plate
What are the 3 steps to form a limb bud?
- Trunk mesoderm is specified to form a limb
- Tbx transcription factors act as intermediate signals
- Limb bud initiation
What signals specify the trunk mesoderm to form a limb bud?
Retinoic acid, FGF, and a posterior to anterior Hox gene code
What do both Tbx transcription factors do?
Cause an EMT in future limb cells
Which transcription factor specifies the forelimb?
Tbx5
Which transcription factor specifies the hindlimb?
Tbx4
How does limb bud initiation happen?
Fgf10 is released from the lateral plate mesoderm, which signals the ectoderm to produce Wnt3a. Wnt3a causes the ectoderm to produce Fgf8. Fgf8 causes the mesoderm to produce more Fgf10
Which signal is released from the lateral plate mesoderm to cause initiation of the limb bud?
Fgf10
What does Fgf10 do?
Signals the mesoderm to produce Wnt3a
What does Wnt3a do?
Signals the ectoderm to produce Fgf8
What does Fgf8 do?
Signals the mesoderm to produce more Fgf10
What happens if we put an FGF bead between the forelimb bud and the hindlimb bud?
Forms a new chimeric limb with both forelimb and hindlimb traits
What does the apical ectodermal ridge do?
Maintains proximal-distal growth and expansion in the limb
What types of molecules are expressed in the apical ectodermal ridge?
Molecules that define the anterior-posterior axis in the limb
What happens if you remove the apical ectodermal ridge?
Limb growth stops
What happens if you add another apical ectodermal ridge?
The limb gets duplicated along the anterior-posterior axis
What happens if you transplant the apical ectodermal ridge from the hindlimb to the forelimb and replace the AER of the forelimb?
The forelimb develops as a hindlimb instead
What happens if you transplant generic mesenchyme tissue into the limb and replace the AER?
Limb growth stops
What determines the limb structure that mesenchyme cells will become?
Their location
What are the 3 domains along the proximal-distal axis of a developing limb?
Frozen domain, active domain, inhibitory domain
What happens if you replace the apical ectodermal ridge with a bead that’s releasing FGF?
Limb growth occurs as normal
What happens in the frozen domain of a limb?
The tissue is already determined and specified to become bone or muscle
What happens in the active domain of a limb?
The FGF signal is moving away, and TGF-beta is allowed to be expressed, causing cell specification
What happens in the inhibitory domain of a limb?
The AER and underlying mesoderm is driving growth and is producing FGF and stopping cell specification
How do cells get from the inhibitory domain to the active domain?
When the cells in the inhibitory domain are dividing, they push the AER away and cells can become specified once they’re out of range of the FGF being produced by the AER
Why is Turing’s Reaction Diffusion Model able to predict why the autopod bones branch like they do?
Because TGF-beta produces its own inhibitors and also promotes its own activity, leading to predictable patterns
What determines if a limb is long or short?
How long the AER is active and the concentration of signals
What does the zone of polarizing activity do?
Specifies the posterior limb bud mesoderm and creates the anterior-posterior axis
What happens if you transplant a second zone of polarizing activity onto a limb?
Creates a second anterior-posterior axis
What signal does the zone of polarizing activity produce?
Shh
How does digit specification happen?
Shh gradient from posterior to anterior
How do different digits get specified?
How much Shh is around and how long the cells are exposed
What digit gets specified when there is no Shh signalling?
Anterior phenotype - thumb
Where is BMP being produced? Why is it there?
Produced by mesoderm between the digits. Shh signalling allows it to be expressed
How does BMP signalling help establish digit identity?
Allows the right type of cartilage to be expressed in the correct place
What signal specifies dorsal tissue in a limb? What produces it? What does it signal to?
Wnt7a. Produced in the ectoderm and stays in the ectoderm
What transcription factor produces a dorsal phenotype?
Lmx1b
What is Nail-Patella syndrome?
Interruption of Wnt signalling that reduces the dorsal phenotype of the limb
What effect do growth hormones have on a limb?
Causes expansion of cartilage tissue in the bone growth plate and lengthens the limb
What effect does estrogen have on a limb?
Low levels stimulate bone growth, but high levels cause chondrocyte death and the bone growth plate to close
