Development of the Limbs Flashcards Preview

ESA 2- Musculoskeletal System > Development of the Limbs > Flashcards

Flashcards in Development of the Limbs Deck (61):
1

What does limb development begin with?

The activation of mesenchyme within the somatic layer of lateral mesoderm

2

What does the somatic mesoderm form?

The limb skeleton

3

What do somites form?

The limb musculature

4

When do limb buds appear?

Towards the end of week 4

5

How does development of the lower limbs differ from that of the upper limbs?

It lags about two days behind

6

Where do limb buds appear?

On the ventero-lateral body wall

7

What happens to limb buds first?

They extend ventrally

8

What do limb buds consist of?

A mesenchyme core with a thickened ectoderm at the apex, the apical ectodermal ridge (AER)

9

What is mesenchyme?

Flexible mesoderm

10

How does elongation of the limb bud occur?

Through proliferation of the mesenchyme core

11

What is the apical ectodermal ridge critical for?

Limb bud outgrowth

12

What does the AER orchestrate?

Limb development proximal to distal

13

What is the effect of AER on the mesenchyme immediately underlying it?

It keeps it undifferentiated

14

What happens to undifferentiated mesenchyme in the limb buds?

It proliferates, leading to elongation

15

What happens to proximal mesenchyme?

It begins to differentiate into its constituent tissues as it is too far away from the AER to receive the signals to stay undifferentiated

16

What does the AER eventually do?

Induces the development of the digits within the hand/foot plates and regresses

17

What is the zone of polarising activity (ZPA)?

A signally centre

18

Where is the ZPA located?

At the posterior base of the limb bud

19

What is the ZPA responsible for?

Generation of asymmetry in the limbs
Controls patterning
Maintains the AER

20

What does the AER mark?

The boundary between dorsal and ventral limb ectoderm

21

How does dorsoventral patterning occur?

The ectoderm exerts dorsalising and centralising influences over the mesenchyme core

22

What controls the anterior-posterior specification in development of the limbs?

ZPA

23

What controls the proximal-distal specification in development of the limbs?

AER

24

What controls dorsal-ventral specification in development of the limbs?

Ectoderm

25

What are digital rays?

Mesenchyme condensations within plates forming cartilaginous models of the digital bones

26

What happens to the AER in digital rays?

It breaks up, and is maintained only over the tips of the digital rays

27

What happens to the interdigital spaces of the digital rays?

They are progressively sculpted by apoptosis

28

What are the stages in the formation of bones?

Signals from the AER to remain undifferentiated stop
Lateral plate mesoderm condenses and differentiates
Cartilage model forms
Endocondral ossification
Primary and secondary ossification centres form

29

How are muscles formed in the limbs?

Myogenic precursors migrate into the limbs from the somites and coalesce into two common muscle masses around the newly formed skeletal elements

30

What are the two common muscle masses in the developing limb?

Ventral
Dorsal

31

What does the ventral muscle mass in the developing limb form?

Flexors

32

What does the dorsal muscle mass in the developing limb form?

Extensors

33

What happens to the common muscle masses in the developing limbs?

Individual muscles split from the common masses

34

How do the extensor and flexor compartments differ in the upper and lower limb?

They are on opposite sides

35

What happens as the limbs extend ventrally?

They rotate

36

In what direction does the upper limb rotate as it extends?

Laterally- thumb is lateral

37

In what direction does the lower limb rotate as it extends?

Medially- big toe is medial

38

Where does the upper limb bud appear?

Opposite the caudal cervical spinal segments

39

Where does the lower limb bud appear?

Opposite the lumbar and sacral spinal segments

40

When do the spinal nerves enter the limb bud?

Early in its development

41

What happens if the spinal nerves don't enter the limb bud early in its development?

Without the innervation, development stalls

42

What happens to muscles in the developing limb?

They are compartmentalised

43

What happens to the compartments of muscles in the developing limb?

Nerves grow into common muscle masses

44

What happens to the anterior divisions of nerves for the anterior (ventral) compartment of muscles?

They regroup to form medial and lateral cords of the brachial plexus, and therefore medial and lateral cords supply flexors

45

What happens to the posterior divisions of nerves for the posterior (dorsal) compartment of muscles?

They regroup to form the posterior cord, therefore the posterior cord branches supply extensors

46

What do nails develop from?

Nail fields on the tips of the fingers

47

How do nail fields get to where they need to be?

They migrate onto the dorsal surface, bringing their nerve supply with them

48

What is a dermatome?

Strip of skin supplied by single spinal nerve

49

What is a myotome?

Muscle/group of muscles supplied by a single spinal nerve

50

What is more commonly affected by limb defects, upper or lower?

Upper limb

51

Why is the incidence of upper and lower limb defects different?

Because the lag period of two days mean they have different critical periods

52

What is the incidence of limb defects?

6 in 10,000 live births

53

What causes limb defects?

Usually hereditary
Teratogen defects have been described

54

What are the common limb defects?

Amelia
Meromelia

55

What is amelia?

Complete absence of a limb

56

What is meromelia?

Partial absence of one or more limb structures

57

Give an example of meromelia

Phocomelia (phalidomide)

58

What are the digit defects?

Syndactole
Polydactyl

59

What is syndactole?

Digits fused

60

What causes syndactole?

Lack of apoptosis between digits

61

What is polydactlyl?

Too many digits