MSK Embryology Flashcards
When do the limb buds form?
End of the 4th week
Where does the musculoskeletal system originate from?
Somites (paraxial mesoderm)
Somatic lateral plate mesoderm
On what wall of the embryo do the limb buds form?
Ventro lateral
In what direction do limb buds initially extend?
Ventrally
As limb buds elongate what happens to them?
Upper limbs rotate 90 degrees laterally
Lower limbs rotate 90 degrees medially
Which limb buds develop first, the upper or lower limb bud?
The upper limb bud
Describe the growth of limb buds
Mesenchyme in somatic lateral mesoderm is activated, causing it to proliferate
Formation of AER at apex of limb bud, which induces an influence signalling further proliferation of adjacent mesenchyme and prevents differentiation
ZPA develops in base of limb bud and controls patterning and asymmetry as well as maintains AER
Limb buds grow longer and AER loses influence over the mesenchyme at the proximal region of the limb bud, hence proximal mesenchyme begins differentiating into different tissues (cartilage and bone)
Most distally, limb buds flatten forming paddles
AER breaks down allowing differentiation of mesenchyme to occur- formation of hyaline cartilage model
Some of the AER remains at the tips of the digital rays, so elongation continues in these regions
Apoptosis occurs between the digits, sculpting the shape of the digits
Endochondral ossification occurs of hyaline cartilage model
What is the AER and what does it do?
Apical ectodermal ridge
Thickened region of ectoderm which covers the entire apex of the limb bud (cuboidal and columnar epithelia)
-Exerts an inductive influence on adjacent mesenchyme causing elongation and preventing differentiation
-Marks the boundary between dorsal and ventral limb ectoderm
-Orchestrates proximal to distal limb development
What is ZPA and what does it do?
Zone of polarising activity
Signalling centre located at base of limb bud
- generates anterior posterior asymmetry in limb buds
- controls patterning and maintains AER
What controls the axes development of the limbs?
Anterior posterior - ZPA
Dorsal ventral - Ectoderm
Proximal distal - AER
How does muscle form in the limb buds?
Myogenic precursors migrate into limbs from the somites
They coalesce into 2 common muscle compartments around the newly formed skeletal elements
– ventral = flexor
– dorsal = extensor
Individual compartments then split to form muscle derivatives
Both the upper and lower limb have extensor and flexor compartments
- upper limb - anterior (flexor) posterior (extensor)
- lower limb - anterior (extensor) posterior (flexor)
How does innervation of the limb bud occur?
Upper limb bud appears opposite caudal cervical spinal segments
Lower limb bud appears opposite the lumbar and sacral spinal segments
Spinal nerves enter the limb bud early in its development
Brachial plexus- muscles are compartmentalised, nerves grow into common muscle masses
Upper limbs
Anterior divisions of brachial plexus
- medial and lateral cords
- supply anterior compartment
- flexor
Posterior divisions of brachial plexus
- posterior cord
- supply posterior compartment
- extensor
What is a dermotome?
Strip of skin supplied by a single spinal nerve
What is a myotome?
Muscle/ group of muscles supplied by a single spinal nerve
Which limb is generally more defected?
Upper limb affected more often than lower limb
