MSK Development of the limbs Flashcards
What structures of the embryo form the limb skeleton and limb musculature?
Limb development is formed by activation of mesoderm in the somatic layer of lateral mesoderm:
- limb skeleton formed by somatic mesoderm
- limb musculature formed by somites
What week do limb buds appear?
Where do the limb buds form from?
What do the limb buds consist of?
End of week 4 (lower limb buds lag by 2 days) and the limbs are well differentiated by week 8
They appear on the ventro-lateral body and extend ventrally at first
They are made of a mesenchyme core with thickened ectoderm at the apex (apical ectodermal ridge)
What is the function of the apical ectodermal ridge of the limb bud?
It is critical for growth because it keeps the mesenchyme underlying it undifferentiated allowing it to keep proliferating and elongating.
The proximal mesenchyme that is far from the AER differentiates into bone
What is the zone of polarising activity in the limb bud?
It is located at the posterior base of the limb bud and is responsible for generating the asymmetry of the limbs. It also maintains the apical ectodermal ridge
How do the hand and foot plates get formed into hands and feet?
Mesenchyme condensations in the plates make cartilage models of the digital bones
The apical ectoderm ridge is maintained only over the tips of the digits and the interdigital spaces are sculpted by apoptosis.
Describe the formation of bones in the hands and feet
Signals from the apical ectoderm region to remain undifferentiated stop, so the lateral plate mesoderm differentiates into a cartilage bone model.
There is endochondral ossification at primary and secondary ossification centres
What is the difference between a dermatome and myotome?
A dermatome is a strip of skin supplied by a single spinal nerve
A myotome is a muscle/ group of muscles supplied by a single spinal nerve
What are some common limb defects?
Upper limbs affected more commonly than lower limbs - te 2 day lag period means they have different critical periods
- amelia, absence of a limb
- meromelia, partial absence of a limb
What are some common digit defects?
Syndactole - fused digits due to lack of apoptosis
Polydactyly - too many digits
How do the myogenic precursors migrate to form muscles in the correct places?
Migrate into the limbs from somites and form 2 common muscle masses around the new skeletal elements
- flexors are ventral
- extensors are dorsal
As the limbs extend ventrally they rotate
The upper limb rotates laterally to make the thumb lateral
The lower limb rotates medially to make the big toe medial
What is the innervation of the limb buds?
Upper limb bud is opposite caudal cervical spinal segments
Lower limb is opposite lumbar and sacral spinal segments
Spinal nerves enter the buds early, otherwise development stalls
What are the movements most strongly associated with each myotome in the cervical region?
C5 Elbow flexion
C6 Wrist extension
C7 Elbow extension
C8 Finger flexion
What are the movements most strongly associated with each myotome in the thoracic region?
T1 Finger abduction
What are the movements most strongly associated with each myotome in the lumbar region?
L2 Hip flexion
L3 Knee extension
L4 Ankle dorsiflexion
L5 Great toe extension
What are the movements most strongly associated with each myotome in the sacral region?
S1 Ankle plantarflexion
