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► Med Misc 25 > Development of the Limbs > Flashcards

Development of the Limbs Flashcards

1
Q

What does limb development begin with?

A

The activation of mesenchyme within the somatic layer of lateral mesoderm

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2
Q

What does the somatic mesoderm form?

A

The limb skeleton

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3
Q

What do somites form?

A

The limb musculature

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4
Q

When do limb buds appear?

A

Towards the end of week 4

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5
Q

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

A

It lags about two days behind

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6
Q

Where do limb buds appear?

A

On the ventero-lateral body wall

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7
Q

What happens to limb buds first?

A

They extend ventrally

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8
Q

What do limb buds consist of?

A

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

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9
Q

What is mesenchyme?

A

Flexible mesoderm

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10
Q

How does elongation of the limb bud occur?

A

Through proliferation of the mesenchyme core

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11
Q

What is the apical ectodermal ridge critical for?

A

Limb bud outgrowth

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12
Q

What does the AER orchestrate?

A

Limb development proximal to distal

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13
Q

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

A

It keeps it undifferentiated

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14
Q

What happens to undifferentiated mesenchyme in the limb buds?

A

It proliferates, leading to elongation

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15
Q

What happens to proximal mesenchyme?

A

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

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16
Q

What does the AER eventually do?

A

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

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17
Q

What is the zone of polarising activity (ZPA)?

A

A signally centre

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18
Q

Where is the ZPA located?

A

At the posterior base of the limb bud

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19
Q

What is the ZPA responsible for?

A

Generation of asymmetry in the limbs
Controls patterning
Maintains the AER

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20
Q

What does the AER mark?

A

The boundary between dorsal and ventral limb ectoderm

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21
Q

How does dorsoventral patterning occur?

A

The ectoderm exerts dorsalising and centralising influences over the mesenchyme core

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22
Q

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

A

ZPA

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23
Q

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

A

AER

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24
Q

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

A

Ectoderm

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25
Q

What are digital rays?

A

Mesenchyme condensations within plates forming cartilaginous models of the digital bones

26
Q

What happens to the AER in digital rays?

A

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

27
Q

What happens to the interdigital spaces of the digital rays?

A

They are progressively sculpted by apoptosis

28
Q

What are the stages in the formation of bones?

A

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
Q

How are muscles formed in the limbs?

A

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

30
Q

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

A

Ventral

Dorsal

31
Q

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

A

Flexors

32
Q

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

A

Extensors

33
Q

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

A

Individual muscles split from the common masses

34
Q

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

A

They are on opposite sides

35
Q

What happens as the limbs extend ventrally?

A

They rotate

36
Q

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

A

Laterally- thumb is lateral

37
Q

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

A

Medially- big toe is medial

38
Q

Where does the upper limb bud appear?

A

Opposite the caudal cervical spinal segments

39
Q

Where does the lower limb bud appear?

A

Opposite the lumbar and sacral spinal segments

40
Q

When do the spinal nerves enter the limb bud?

A

Early in its development

41
Q

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

A

Without the innervation, development stalls

42
Q

What happens to muscles in the developing limb?

A

They are compartmentalised

43
Q

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

A

Nerves grow into common muscle masses

44
Q

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

A

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

45
Q

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

A

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

46
Q

What do nails develop from?

A

Nail fields on the tips of the fingers

47
Q

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

A

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

48
Q

What is a dermatome?

A

Strip of skin supplied by single spinal nerve

49
Q

What is a myotome?

A

Muscle/group of muscles supplied by a single spinal nerve

50
Q

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

A

Upper limb

51
Q

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

A

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

52
Q

What is the incidence of limb defects?

A

6 in 10,000 live births

53
Q

What causes limb defects?

A

Usually hereditary

Teratogen defects have been described

54
Q

What are the common limb defects?

A

Amelia

Meromelia

55
Q

What is amelia?

A

Complete absence of a limb

56
Q

What is meromelia?

A

Partial absence of one or more limb structures

57
Q

Give an example of meromelia

A

Phocomelia (phalidomide)

58
Q

What are the digit defects?

A

Syndactole

Polydactyl

59
Q

What is syndactole?

A

Digits fused

60
Q

What causes syndactole?

A

Lack of apoptosis between digits

61
Q

What is polydactlyl?

A

Too many digits

► Med Misc 25 (69 decks)

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