MSK Session 3- Development Of The Limbs Flashcards
What forms limb musculature?
Somites
What forms the limb skeleton?
Mesenchyme within the somatic layer of the lateral mesoderm.
When do limb buds begin to appear?
End of week 4
Which are slightly developmentally ahead, and by how many days?
Upper limb buds
Approx 2 days
Which way do the limb buds grow?
Ventrally
What is the structure of a limb bud?
Core- mesenchyme
Apex- apical ectodermal ridge (thickened ectoderm)
What are the roles of the apical ectodermal ridge (AER)? (3)
Signalling role by exerting an influence over mesenchymal cells directly beneath it to read undifferentiated and divide, therefore causing limb growth in a proximal to distal direction.
Induces hand and foot plates to develop digits, before it regresses.
Marks the boundary between the dorsal and ventral limb ectoderm.
What happens to mesenchyme that is too proximal compared to the AER?
It is no longer influenced by the AER and therefore differentiates into tissues.
What is the role of the zone of polarising activity (ZPA) (3) and where is it found?
It is a signalling centre located in the posterior base of the limb bud.
It is responsible for generating asymmetry in the limbs, controlling patterning and maintaining the AER.
How do the hand and foot plates develop so that digits appear?
- digital Rays appear; which are condensations of mesenchyme within the hand and foot plates. They form cartilaginous models of digital bones.
- the AER regresses, but remains over the digital Rays to allow digital growth.
- apoptosis of the interdigital spaces sculpts the hand and foot plates.
How do muscles arise?
- Myogenic precursors from somites migrate into the limb buds along with their innervation, and form 2 common muscle masses (ventral/flexor compartment and dorsal/extensor compartment) around the new skeletal elements.
- individual muscles then arise from these two compartments.
Where are flexors located in the upper and lower limb respectively?
Upper- anteriorly, lower-posteriorly.
Describe rotation of the upper limb.
The upper limb rotates laterally so that the thumbs face out and the elbows move down.
Describe rotation of the lower limb.
The lower limb rotates medially, so the soles move down and the knees move up.
What happens to development if the limb buds aren’t innervated early on?
Development stalls
