Unit 3: Limb development Flashcards
What are the 3 axis of the limb ?
1) Proximal-Distal
2) Anterior- Posterior
3) Dorsal- Ventral
What 3 components is the Proximal- Distal subdivided into ?
1) Stylopod
2) Zeugopod
3) Autopod
What is the stylopod ?
The humerus
What is the zeugopod ?
The ulna and radius
What is the autopod ?
The metacarpals and digits
What germ layer gives rise to the limbs ?
- Lateral plate mesoderm
- Ectoderm
What does inner limb tissue develop from ?
It develops from lateral plate mesoderm
What does the limb bud consist of ?
Loose mesenchyme from the lateral plate mesoderm covered by epithelial tissue from ectoderm
What is muscle formed from ?
Paraxial mesoderm (somites)
What are nerves formed from ?
Neuroectoderm
How is the apical ectodermal ridge formed (AER) ?
By a localised thickening (cuboidal to columnar shape cells)
What is the apical ectodermal ridge (AER) ?
A region of thickened ectoderm at the distal (tip) end of the limb bud that plays a crucial role in regulating the growth and patterning of the developing limb
What does removal of the AER lead to ?
Truncations
What is a truncation ?
The absence or incomplete development of a part of a limb
Removal of AER leads to truncations, what does this mean (2) ?
1) The AER regulates P-D outgrowth
2) Proximal tissues are laid down first
What can be substituted for AER ?
Fibroblast growth factors (FGFs)
What would you think the result of removing Fgf10 early during limb development would be ?
No signs of limb development
Does the apical ectodermal ridge control P-D patterning ?
No
What is the progress zone (PZ) ?
- A crucial region for the growth and patterning of limbs
- It consists of proliferating, undifferentiated mesodermal cells that move from the distal part of the limb bud towards the proximal region.
The first cells to leave the PZ, differentiate into what identity ?
Proximal identity
The last cells to leave the PZ, differentiate into what identity ?
Distal identity
What set of genes that are expressed along the P-D axis, specify the fate of cells ?
HOX A genes
What is the relationship between time a cell spends in PZ and HOXA gene expression ?
The longer a cell is in the PZ, the more HOXA genes it expresses
What do HOXA genes determine ?
Wether that cell is part of the stylopod, zeugopod or autopod
What is the zone of polarising activity (ZPA) ?
A specialized group of mesodermal cells located in the posterior region of the limb bud
What does the ZPA produce ?
A difffusable signalling molecule whose effect is concentration dependant (Morphogen)
What can ZPA activity be mimicked by ?
Sonic hedgehog (SHH)
What genes are often expressed in cells to determine their position along an axis ?
HOXD genes (Determine antero-posterior identity)
What does HOXA gene expression determine ?
Proximal- Distal identity
What is D-V patterning controlled by ?
Ectoderm (Wnt7a is the dorsal factor)
How is the development of the 3 axis coordinated (3) ?
1) AER removal leads to loss of ZPA
2) Loss of ZPA leads to loss of AER
3) Loss of dorsal ectoderm signal leads to loss of AER and therefore ZPA
Which is the correct statement about the progress zone ?
1) It is in the ectoderm
2) It resides under the apical ectodermal bridge
3) The whole of it expresses shh
4) It is derived from paraxial mesoderm
2) It resides under the apical ectodermal ridge
