Flashcards in MSK Session 3 (Lecture 3.1) Deck (21):
On which side of the developing embryo do the limb buds appear? In what order?
1) Ventro-lateral body wall
2) Upper limb > Lower limb
When do limb buds appear?
End of 4th week.
What makes up the core and lining of the limb bud?
Core of proliferating mesenchymal cells with an ectoderm covering
Where does the limb bud come from?
Activation of mesenchyme within lateral mesoderm derived from the somatic layer of the lateral plate.
What does the somatic mesoderm and the somites give rise to?
1) Limb skeleton
What is the AER?
Apical ectodermal ridge
This is a thickened ectoderm covering at the apex of the limb bud
Describe the 3 degrees of asymmetry.
1) Proximal - Distal axis e.g. shoulder to fingertips
2) Dorso - Ventral axis e.g. front and back
3) Anterior - Posterior acis e.g. L&R hands mirror images
What is the AER important for?
1) Limb bud outgrowth
2) Limb development proximal to distal
3) Appearance of paddles
How is the P-D axis generated?
1) AER exerts an inductive influence on the immediately underlying mesenchyme to remain undifferentiated and for it to keep growing.
2) The proximal mesenchyme begins to differentiate into constituent tissues e.g. bone as no signal is inhibiting its differentiation
3) AER induces development of the digits within the hand/foot plates
How is the D-V axis generated?
AER marks boundary between dorsal & ventral limb ectoderm (which exerts dorsalising and ventralising influences over mesenchyme core)
What is the ZPA and its importance?
Zone of polarising acivity
Generation of asymmetry in the limbs (A-P axis). Has a signalling centre located at the posterior base of the limb bud which controls both patterning and maintains the AER.
What are digital rays and where are they found?
Mesenchyme condensations within the hand/foot plates. Form the cartilaginous models of digital bones.
Where is the AER maintained as it breaks up?
Maintained over tips of the digital rays with the interdigital spaces sculpted by programmed cell death.
Which limb is affected more by defects?
Describe syndactyly and polydactyly.
Syndactyly - Fusion of digits, may involve CT or bones
Polydactyly - extra digits, recessive trait
What is amelia and meromelia?
Amelia - Complete absence of a limb
Meromelia - partial absence of one or more limb structures
What migrates and where to start the development of the musculature in limbs?
What do they form?
Myogenic precursors migrate into limbs from somites
Coalesce into 2 common muscle masses around the newly formed skeletal elements (ventral=flexor, dorsal = extensor)
What differentiates (muscle related) the U/L from the L/L?
U/L : flexor is anterior, extensor is posterior
L/L : flexor is posterior, extensor is anterior
What does rotation of the limb do and how?
Limbs extend ventrally at first but rotate with the U/L rotating laterally and the L/L rotating medially. Brings right muscle compartments in the right place.
What is opposite the upper and lower limb buds?
Upper - caudal cervical spinal segments
Lower - Lumbar & sacral spinal segments