MSK - Development Of Limbs Flashcards
What are limb buds?
Mass of mesenchyme covered by a layer of ectoderm
What week of development do limb buds appear?
4th
How are limb buds derived?
They come from the activation of mesenchyme within lateral mesoderm
Derived from the somatic layer of lateral plate mesoderm.
What are the ectoderm cells called at the tip of the bud called?
Apical ectoderm ridge
How does elongation of limbs occur?
Through the proliferation of mesenchyme core.
What is the apical ectodermal ridge needed for?
Critical for limb bud outgrowth
Orchestrates development proximal to distal
Final stage = appearance of paddles.
What does the AER exert on the immediate mesoderm?
Signals on underlying mesoderm - preventing differentiation.
What marks the boundary between dorsal and ventral limbs?
The AER (apical ectodermal ridge) exerts dorsalising or ventralising influences over core
What is the zone of polarising activity?
This is a small cluster of cells at the posterior base of the bud
It generates asymmetry in the limbs (anterior posterior axis)
Maintains AER
How are hands and feet sculpted?
Digital rays (cartilaginous models of distal bones)
Results in apoptosis between these
AER is broken up and only remains above the distal Rays,
What is syndactyly?
Fusion of digits (CT or bone)
What is polydactyly?
Extra digits
Genetic recessive trait
What is Amelia?
Complete absence of a limb
What is meromelia?
Partial absence of one or more limb structures
What causes phocomelia?
Thalidomide
How is the cartilaginous template of bone formed?
Condensation and differentiation of the lateral plate mesoderm
How is musclulature formed?
Myogenic precursors migrate into limbs from somites
How is muscle compartmentalised?
2 common masses form around the skeletal elements
Ventral = flexor
Dorsal = extensor
Which direction do limbs extend initially?
Ventrally
What is the position of limbs before rotation?
Thumbs up elbows out
Soles facing in knees out
Which direction to limbs rotate in?
Upper limb = laterally
Lower limb = medially
What is the limb position after rotation?
Thumbs out elbows in
Soles down knees up
(Anatomical position)
How are limbs innervated?
Spinal nerves enter the limb bud in early development
Upper limb bud = caudal spinal segment
Lower limb buds = lumbar and sacral segments
What is a dermotome?
Strip of skin supplied by a single nerve
