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Flashcards in MSK S3 - Limb Development Deck (26)
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0

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

1

When do limb buds begin to develop?

They appear towards the end of week 4
Lower limbs lag ~2 days behind the upper limbs

2

Where do limb buds develop?

Appear on ventro-lateral body wall
Extend ventrally at first

3

What are limb buds composed of?

Mesenchyme core
Thickened ectoderm at apex (apical ectoderm ridge/AER)

4

How do limb buds extend?

Proliferation of the mesenchymal core

5

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

6

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

7

What sets the anterior/posterior axis in limb development?

The ZPA

8

What sets the proximal/distal axis in limb development?

The AER

9

What sets the dorsal/ventral axis in limb development?

The ectoderm

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

How do nails form?

From nail fields on the tips of fingers
Nail fields migrate onto the dorsal surface, bringing nerve supply with them

18

What is a dermatome?

A strip of skin supplied by a single spinal nerve

19

What is a myotome?

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

20

Are upper or lower limb defects more common?

Upper limb defects are more common

21

What are some common limb defects?

Amelia
Meromelia

22

Describe Amelia

The complete absence of a limb

23

Describe meromelia

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

24

What are some common digit defects?

Syndactole
Polydactyly

25

Describe syndactole

Failure of apoptosis between digits
Therefore digits fuse