1.4 Limb development Flashcards
What are the three axis of the limb?
Proximal-distal - divided into stylopod, zuegopod and autopod (formed as one with joints added later)
Anterior-posterior
Dorsal-ventral
How does limb tissue develop from the lateral plate mesoderm?
Limb origin = LPM covered by ectoderm
How is the limb bud formed?
Inter-limb stops growing - signs of limb bud
Consists of loose mesenchyme from LPM covered by ectoderm
Inner tissues (bone + connective tissue) are LPM
Paraxial mesoderm = muscles
Neuroectoderm = nerves
What is limb bud outgrowth?
Limb bud mesenchyme includes a morphogenic change in distal limb ectoderm Localised thickening (cubiodal - columnar) forms Apical Ectodermal Ridge (AER) Removal AER = truncations
What does truncation due to AER removal tell us?
AER regulates P-D outgrowth
Proximal tissues laid down first
Fibroblast growth factors can substitute AER
FGF’s in ectopic process = ectopic limbs
What is the progress zone (PZ) and what are models of PZ controlling PD patterning?
PZ = rapidly dividing mesenchymal cells under AER
Rapid cell division means cells have to leave PZ, 1st to leave, differentiate to have a proximal identity, 2nd to leave have a distal identity
Cells in PZ already partly committed to a fate along P-D axis dependant on position
There are zones within PZ - each proliferates and expands, initial positioning of a PZ cell determines fate
What controls P-D patterning?
HOX A genes patterning P-D axis
Longer a cell is in PZ, more HOX A gene it expresses, this determines whether a cell is part of the stylopod, zeugopod or autopod
What is A-P patterning?
Formation of digits
Removal of posterior distal limb mesenchyme disturbs A-P patterning (tissue is called zone of polarising activity ZPA) - always located at the posterior-distal position + adjacent to the AER
What is the mechanism of zone of polarising activity?
ZPA transplanted anterior to limb bid produces extra digits
Works in dose dependant manner to specify posterior fates
Produces diffusible signalling molecule affect is concentration dependant
ZPA activity mimicked by shh, shh expressed in ZPA
Where does the dorsal-ventral patterning information reside?
If you rotate ectoderm 180º D-V patterning also rotated 180º, controlled by ectoderm
Dorsal ectoderm patterns mesoderm - dorsal mesoderm
Ventral ectoderm patterns mesoderm - ventral mesoderm
If D-V ectoderm identity lost AER doesn’t form in the correct place
Dorsal signal is Wnt7a
What is the evidence for integration of signalling cascades?
AER removal = loss of ZPA and vice versa
Loss of dorsal ectoderm = loss of AER and therefore ZPA
Establishment of ZPA (shh) by AER (Fgf8) and dHAND
Induction of Fgf8 by shh
shh maintained by Wnt7a
Determination of AER size by Wnt7a
