MSK S3 - Limb Development

Question 0

When do limb buds begin to develop?

Answer 0

They appear towards the end of week 4

Lower limbs lag ~2 days behind the upper limbs

Question 1

How does limb development begin?

Answer 1

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

Question 2

Where do limb buds develop?

Answer 2

Appear on ventro-lateral body wall

Extend ventrally at first

Question 3

What are limb buds composed of?

Answer 3

Mesenchyme core

Thickened ectoderm at apex (apical ectoderm ridge/AER)

Question 4

How do limb buds extend?

Answer 4

Proliferation of the mesenchymal core

Question 5

How is the Apical Ectodermal Ridge important?

Answer 5

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

Question 6

What is the zone of polarising activity (ZPA)?

Answer 6

Signalling centre
Located at the posterior base of the limb bud
Responsible for asymmetry in the limbs
Controls patterning
Maintains AER

Question 7

What sets the anterior/posterior axis in limb development?

Answer 7

The ZPA

Question 8

What sets the proximal/distal axis in limb development?

Answer 8

The AER

Question 9

What sets the dorsal/ventral axis in limb development?

Answer 9

The ectoderm

Question 10

How are digits formed?

Answer 10

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

Question 11

How do bones form in limb development?

Answer 11

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

Question 12

How does musculature form?

Answer 12

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

Question 13

Describe the rotation of limbs during development

Answer 13

As limbs extend, they rotate
Upper limb rotates laterally so thumb is lateral
Lower limb rotates medially so big toe is medial

Question 14

Compare the upper and lower limb positions before and after rotation

Answer 14

Before:
Thumbs up, elbows out
Soles face in, knees face out
After:
Thumbs out, elbows down
Soles face down, knees up

Question 15

How does innervation of limbs develop?

Answer 15

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

Question 16

Describe brachial plexus development

Answer 16

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

Question 17

How do nails form?

Answer 17

From nail fields on the tips of fingers

Nail fields migrate onto the dorsal surface, bringing nerve supply with them

Question 18

What is a dermatome?

Answer 18

A strip of skin supplied by a single spinal nerve

Question 19

What is a myotome?

Answer 19

A muscle or group of muscles supplied by a single spinal nerve

Question 20

Are upper or lower limb defects more common?

Answer 20

Upper limb defects are more common

Question 21

What are some common limb defects?

Answer 21

Amelia

Meromelia

Question 22

Describe Amelia

Answer 22

The complete absence of a limb

Question 23

Describe meromelia

Answer 23

Partial absence of one or more limb structures
Eg Phocomelia (phalidomide)

Question 24

What are some common digit defects?

Answer 24

Syndactole

Polydactyly

Question 25

Describe syndactole

Answer 25

Failure of apoptosis between digits

Therefore digits fuse