Limb Formation3 Flashcards
Role of FGFs in P/D outgrowth
FGF10 in the limb bud mesenchyme induces the ectoderm to form the apical ectodermal ridge at the D/V junction
- FGF8 / FGF4 in the AER are required for limb P/D outgrowth
Conditional KO
Providing a tissue specific source of cre
(Tissue specific promoter + specific enhancer) + CRE
- Removes only from cells of interest
- Added with CRISPR
Genetic Redundancy
One FGF can compensate for the loss of another
Ectrodactyly
The absence or malformation of one or more central digits due to failure to maintain medical apical ectodermal ridge signaling.
Essentials of the AER
- Controls P/D outgrowth
- FGF4/8
- First morphological distinction of the limb but
- Ectodermal
- Maintains progress zone
- promotes proliferation
- maintains undifferentiated state
- prevents cartilage forming
- induced by FGF10 with Wnt3 through the underlying mesenchyme
- Maintains expression of molecules in ZPA
Progress Zone
Region of undifferentiated mesenchyme located beneath the apical ectodermal ridge. Provides pool of progenitor cells that proliferate and progressively differentiate to form the different segments of the limb
- P/D specification correlates with the age of the PZ mesenchyme
HOX genes
- set limb but positions
- pattern the P/D axis
- Set location of ZPA
- Pattern A/P of digits
How various surgical manipulations/transplantation experiments with the AER, PZ, and ZPA have helped reveal their functions
- Removing AER results in no limb outgrowth
- Replacing the AER with beads soaked with FGFs (FGF8) can rescue limb outgrowth
- Transplanting a limb but with PZ and AER to a naive site leads to a complete limb. Meaning that the PZ possess the positional information necessary for P/D specification
- Transplanting PZ mesenchyme of different ages to the distal tip of the limb but results in limb segments corresponding to the age of the PZ
- Transplanting ZPA to the anterior leads to mirrored duplication of digits
- Implanting beads soaked in SHH mimics ZPA transplantation
Opposing Gradient/Two signal model for P/D outgrowth
Retinoic Acid and FGFs/Wnt create a P/D gradient. Influence the fate of PZ cells
Basic idea behind Reaction-Diffusion Mechanisms
- Activator: activates itself and inhibitor
- Inhibitor: inhibits the activator
- Diffusion rates: the activator diffuses more slowly than the inhibitor.
Zone of Polarizing Activity (ZPA)
Specifies A/P axis
- ZRS - activates SHH
- SHH - sufficient for posterior activity
- Digit 5 has longes time of expression
Hx mutation
SHH mis-expression in Posterior and Anterior
Polydactyly
Triphalangeal thumb syndrome
Important Roles for gene regulatory sequences in development