MSK S3 - Limb Development Flashcards
When do limb buds begin to develop?
They appear towards the end of week 4
Lower limbs lag ~2 days behind the upper limbs
How does limb development begin?
With the activation of mesenchyme within the somatic layer of the lateral mesoderm
This somatic mesoderm forms the limb skeleton
Somites form the limb musculature
Where do limb buds develop?
Appear on ventro-lateral body wall
Extend ventrally at first
What are limb buds composed of?
Mesenchyme core
Thickened ectoderm at apex (apical ectoderm ridge/AER)
How do limb buds extend?
Proliferation of the mesenchymal core
How is the Apical Ectodermal Ridge important?
Keeps the mesenchyme directly beneath it undifferentiated
Orchestrates limb development proximally to distally
Proximal mesenchyme begins to differentiate as the AER moves too far away to receive signals to remain undifferentiated
Induces digit formation then regresses
What is the zone of polarising activity (ZPA)?
Signalling centre Located at the posterior base of the limb bud Responsible for asymmetry in the limbs Controls patterning Maintains AER
What sets the anterior/posterior axis in limb development?
The ZPA
What sets the proximal/distal axis in limb development?
The AER
What sets the dorsal/ventral axis in limb development?
The ectoderm
How are digits formed?
Mesenchyme condenses into cartilaginous models of bones of digits
AER breaks up & is only maintained over the tips of the digital rays
Interdigital spaces are formed by apoptosis
How do bones form in limb development?
Signals from the AER to remain undifferentiated stop
Lateral plate mesoderm condenses
Cartilaginous models of bones form
Endochondral ossification occurs from primary and secondary ossification centres
How does musculature form?
Myogenic precursors migrate into limbs from somites
These coalesce into two common muscle masses around the skeletal components
Ventral - flexors
Dorsal - extensors
Individual muscles later split from these masses
Describe the rotation of limbs during development
As limbs extend, they rotate
Upper limb rotates laterally so thumb is lateral
Lower limb rotates medially so big toe is medial
Compare the upper and lower limb positions before and after rotation
Before: Thumbs up, elbows out Soles face in, knees face out After: Thumbs out, elbows down Soles face down, knees up
How does innervation of limbs develop?
Limb buds form caudal cervical spinal segments (upper limb) and lumbar and sacral spinal segments (lower limb)
Nerves enter limb bud early in development
Without this, development stalls
Describe brachial plexus development
Muscles are compartmentalised and nerves grow into them
The anterior divisions for the anterior compartment regroup to form the medial and lateral cords
Therefore the medial and lateral cord branches supply flexors
The posterior divisions for the posterior compartment regroup to form the posterior cord
Therefore the posterior cord branches supply extensors
How do nails form?
From nail fields on the tips of fingers
Nail fields migrate onto the dorsal surface, bringing nerve supply with them
What is a dermatome?
A strip of skin supplied by a single spinal nerve
What is a myotome?
A muscle or group of muscles supplied by a single spinal nerve
Are upper or lower limb defects more common?
Upper limb defects are more common
What are some common limb defects?
Amelia
Meromelia
Describe Amelia
The complete absence of a limb
Describe meromelia
Partial absence of one or more limb structures Eg Phocomelia (phalidomide)
What are some common digit defects?
Syndactole
Polydactyly
Describe syndactole
Failure of apoptosis between digits
Therefore digits fuse
