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Embryology > Development of Limbs > Flashcards

Development of Limbs Flashcards

1
Q

Which parts of the embryo specifically allow limb development?

A

The somites

The lateral plate mesoderm

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2
Q

What happens to the Notochord on limb formation?

A

It is marooned and a remnant forms - The nucleus pulpsis of the intervertebral disc.

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3
Q

What are the 3 degrees of asymmetry in the limb?

A

Proximal distal axis
Dorsal ventral asymmetry
Anterior posterior axis of asymmetry

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4
Q

What sequence does embryonic development occur in?

A

Head to tail

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5
Q

When do limb buds appear?

A

On the folded embryo. Upper limb buds first

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6
Q

What do the limb buds look like?

A

Flipper like structures.

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7
Q

What direction do the limb buds extend first?

A

Ventrally away from the trunk towards the belly.

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8
Q

What drives the significant elongation of the limb buds?

A

Cellular division

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9
Q

What happens before the limbs adopt their adult orientation?

A

Primordia of hands and feet begin to appear

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10
Q

What does limb bud development begin with?

A

Activation of mesenchyme in the somatic layer of the lateral plate mesoderm

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11
Q

What forms the limb buds?

A

Proliferation of somatopleuric lateral plate mesoderm

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12
Q

What are the limb buds made of?

A

An outer ectodermal cap

A core of mesoderm

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13
Q

What does the mesoderm differentiate into to allow lengthening of the limb bud?

A

Mesenchymal cells

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14
Q

What is the Apical Ectodermal Ridge?

A

Each limb bud forms the ectoderm along the apex of the limb bud, differentiates into a ridge like thickening (The AER)

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15
Q

Why is Apical Ectodermal ridge important?

A

It is essential for limb differentiation anf it induces this throughout the limb.

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16
Q

What does the Apical Ectodermal ridge express and what does this do?

A

FGF2, FGF4, FGF8.

They all induce limb outgrowth by maintaining proliferation in the progress zone

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17
Q

When does the Apical Ectodermal ridge regress?

A

When the limb flatterns into paddle appearance

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18
Q

What is the Zone of Polarising activity?

A

Responsible for the generation of mirror image limbs

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19
Q

What does the Zone of Polarising activity control?

A

The patterning of the limbs and maintenance of the AER

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20
Q

Where is the Zone of Polarising activity?

A

Small signalling centre found in the posterior limb bud margin

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21
Q

How are the hands and feet sculpted from the paddles?

A

There is development of a cartiaginous model of the bones and apoptosis occurs between these to sculpt.

22
Q

How do the digits proliferate?

A

The Apical Ectodermal ridge breaks up, but stays over the tips of the digitsso these regions can still proliferate.

23
Q

What does the Apical Ectodermal ridge signal to tissues?

A

To remain undifferentiated

24
Q

How does bone ossification occur?

A

The Apical Ectodermal ridge regresses so the cartilage model differentiates then undergoes endochondrial ossification

25
Q

Where is the primary ossification centre?

A

In the diaphysis

26
Q

Where is the secondary ossification centre?

A

In the epiphyses (Allows growth up to puberty)

27
Q

How does migration of myogenic precursers bring specific innervation with it?

A

They migrate from a somite that has its own spinal nerve so the specific innervation is brought with it during migration

28
Q

How are discrete muscle bellies formed?

A

Development of the precursers continues and the muscle masses split.

29
Q

What is the upper flexor compartment made from?

A

The ventral limb

30
Q

What is the upper extensor compartment made from?

A

The dorsal limb

31
Q

What is the lower extensor compartment made from?

A

Ventral limb

32
Q

What is the lower flexor compartment made from?

A

Dorsal limb.

33
Q

Why are the lower limb compartments opposite to the upper limb compartments?

A

Because the lower limb rotates during development

34
Q

Why is the rotation of the lower limb important?

A

It allows for the mirror image orientation of the upper and lower limbs.

35
Q

How is the proximal distal axis generated?

A

The AER exerts and inductive influence on the mesenchyme in the limb bud.
As the bud elongates the proximal tissue is too far from the AER
The mesenchyme differentiates

36
Q

How is dorsal ventral patterning generated?

A

The position of the AER marks the boundary between dorsal and central limb ectoderm. Both will release signals to induce relative differentiation

37
Q

How is the anterior posterior axis generated?

A

The ZPA secretes morphogen that diffuses into the tissue and creates a concentration gradient as it is broken down.

38
Q

What does pre axial refer to?

A

The thumb and big toe

39
Q

What does post axial refer to?

A

The little finger and toe side of the limb

40
Q

What is the significance of the pre and post axial ventral lines?

A

They represent a border in the limb where there is no overlap of dermatomes

41
Q

Which axial specification does the AER control?

A

Proximal distal asymmetry

Dorsal and ventral axis boundary

42
Q

Which axial specification does the ZPA control?

A

Maintains the AER

Anterior to posterior asymmetry

43
Q

Why is the upper limb effected more often by limb defects, compared to the lower limb?

A

Because of the lag period between their development.

44
Q

What are the common limb defects?

A 
Syndactyly
Polydactyly
Amelia
Meromelia
Phocomelia
45
Q

What is Syndactyly?

A

Fusion of the digits (can be skin or skin and bones)

46
Q

What is Polydactyly?

A

Extra digits

Genetic recessive

47
Q

What is Amelia?

A

Complete absence of a limb

Could be due to failure of the AER

48
Q

What is Meromelia?

A

Partial absence of one or more limb structures

49
Q

What is Phocomelia?

A

Varient of Meromelia where all of the long bones of the limbs are absent.

50
Q

Clinically, what is a dermatome?

A

A strip of skin that can be examined and is innervated by a single spinal nerve

51
Q

What is a myotome?

A

A muscle or group of muscles which are innervated by a single spinal nerve

Embryology (7 decks)

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