lec 25- limb development part 2 Flashcards
what accomplishes the patterning of the digits along the AP axis?
the ZPA
how can one detect the ZPA?
by its expression of sonic hedgehog mRNA
explain the Hedgehog signaling pathway when no hedgehog is there:
-The Patched receptor inhibits smoothened
-when Smoothened is inhibited, the transcription factor (Ci) is held in cytoplasm by two protein complexes
-Ci in the complex associated with smoothened is phosphorylated by several protein kinass
-phosphorylated Ci cleaved forming truncated protein that enters the nucleus and represses Hedgehog target genes
explain the hedgehog signaling pathway when hedgehog is present:
The hedgehog receptor Patched no longer inhibits Smoothened, it is phosphorylated
-the transcription factor Ci is released from both of its complexes, allowing it to enter the nucleus and activate gene transcription
what produces and secretes sonic hedgehog?
The polarizing region (ZPA)
how are different digits specified by sonic hedgehog?
sonic hedgehog is a morphogen, resulting in different threshold concentrations of the sonic hedgehog signals specifying different digits
what is the evidence behind ZPA producing morphogen?
- Grafting another polarizing region onto the anterior margin of a limb buds results in a mirror-image duplication of the digits, extra digits form in response to the gradient of signal from the ZPA
- if a very small piece of another polarizing region is grafted onto the anterior, a weak gradient is produced and only an extra digit 2 is produced
what evidence is there behind sonic hedgehog patterning the digits?
- it is expressed in ZPA
- when beads that release sonic hedgehog are inserted under the anterior ectoderm of limb buds, mirror image digits are produced
- in mouse, mutants that don’t express sonic hedgehog lose posterior patterning of limbs
how does sonic hedgehog pattern the digits according to the temporal gradient model?
-Shh (sonic hedgehog) is expressed by some of the cells that give rise to digits while these cells are still in the polarizing region
-when they are displaced out of the ZPA, they no longer express Shh but they are still exposed to the gradient of Shh signal
-digit 1 develops in an Shh-independant manner, cells aren’t exposed to Shh and their progenitors never made it
-cells that form digit 2 are exposed to lower concentrations of the Shh signals but their progenitors never made any Shh signal
-cells that give rise to digits 4, 5, and part of 3 are exposed to the Shh gradient and they arose from progenitors that made the Shh signals when they were inside the ZPA
-other half of digit 3 is exposed to the Shh signals but progenitors never made it
what causes pattern defects?
mis-expression of sonic hedgehog
e.g. polydactyly
what evidence is there that BMP signals from the interdigital region affect digit identity?
-removal of the interdigital space causes digit transformation, inhibition of BMP signalling mimics removal of interdigital space
e.g. removal causes digit 2 –> 1
how do digits separate?
from programmed cell death of the regions between the cartilaginous elements, cell death is activated by BMP
what stops programmed cell death of the regions between the cartilaginous elements?
blocking of BMP signalling
where are the two different Hox genes expressed in the chick wing bud?
-Hoxd is expressed on the posterior tip of the wing bud
-Hoxa is expressed in a pattern distributed along the proximal distal axis
where are the Hoxa13 and Hoxd13 genes expressed?
where the wrist and digits form
where are the Hoxa11 and Hoxd11 genes expressed?
where the radius and ulna form
where are Hoxa9 and Hoxd9 genes expressed?
where the humerus forms
do the loss of Hox genes result in homeotic transformations in limbs?
no, rather they are missing
