MSK: Development Of The Limbs Flashcards
When does the limb bud appear?
End of the 4th week
Which limb development lags, the upper or lower limbs?
The lower limbs: 2 days behind
What does the limb bud consist of?
A core of proliferating mesenchymal cells with a flexible ectoderm covering
Name the 3 controllers of axial specification.
- Anterior-Posterior axis: Zone of polarising activity (ZPA)
- Proximal- Distal axis: Apical Ectodermal Ridge (AER)
- Dorsal-Ventral axis: Ectoderm
What does the AER signal (inductive influence) do to the immediately underlying mesenchyme?
Ensures it remains undifferentiated
Where is the ZPA located?
The posterior base of the limb bud
How do digits form?
- Mesenchyme condensations within the plates
- Cartilaginous models of the digits form
- AER breaks up and is maintains only over the tips of the digital rays
- Apoptosis of the tissue between the digits removes webbing
Name 2 limb defects that affect the digits and explain what they are.
- Syndactyly: fusion of digits (may involve just connective tissue or bones)
- Polydactyl: extra digits (genetic recessive trait)
Name 3 underlying problems in morphogenesis.
Malformation: mutation in genome, or genetic abnormality
Deformation: contracting bands of mesenchymal tissue affects a healthy embryo
Disruption: due to an external agent (e.g. Thalidomide
Where does the upper limb bud appear?
Caudal cervical spinal segments
Where does the lower limb bud appear?
Opposite the lumbar and sacral spinal segments
How do muscles form in the limbs?
- Myogenic precursors migrate into the limbs from somites
- Form 2 common muscle masses around the newly formed skeletal elements
- Individual muscles then split from common masses
Which way do the upper limbs rotate during development?
Laterally
Which way do the lower limbs rotate during development?
Medially
