Development of Limbs

Question 1

Which parts of the embryo specifically allow limb development?

Answer 1

The somites

The lateral plate mesoderm

Question 2

What happens to the Notochord on limb formation?

Answer 2

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

Question 3

What are the 3 degrees of asymmetry in the limb?

Answer 3

Proximal distal axis
Dorsal ventral asymmetry
Anterior posterior axis of asymmetry

Question 4

What sequence does embryonic development occur in?

Answer 4

Head to tail

Question 5

When do limb buds appear?

Answer 5

On the folded embryo. Upper limb buds first

Question 6

What do the limb buds look like?

Answer 6

Flipper like structures.

Question 7

What direction do the limb buds extend first?

Answer 7

Ventrally away from the trunk towards the belly.

Question 8

What drives the significant elongation of the limb buds?

Answer 8

Cellular division

Question 9

What happens before the limbs adopt their adult orientation?

Answer 9

Primordia of hands and feet begin to appear

Question 10

What does limb bud development begin with?

Answer 10

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

Question 11

What forms the limb buds?

Answer 11

Proliferation of somatopleuric lateral plate mesoderm

Question 12

What are the limb buds made of?

Answer 12

An outer ectodermal cap

A core of mesoderm

Question 13

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

Answer 13

Mesenchymal cells

Question 14

What is the Apical Ectodermal Ridge?

Answer 14

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

Question 15

Why is Apical Ectodermal ridge important?

Answer 15

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

Question 16

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

Answer 16

FGF2, FGF4, FGF8.

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

Question 17

When does the Apical Ectodermal ridge regress?

Answer 17

When the limb flatterns into paddle appearance

Question 18

What is the Zone of Polarising activity?

Answer 18

Responsible for the generation of mirror image limbs

Question 19

What does the Zone of Polarising activity control?

Answer 19

The patterning of the limbs and maintenance of the AER

Question 20

Where is the Zone of Polarising activity?

Answer 20

Small signalling centre found in the posterior limb bud margin

Question 21

How are the hands and feet sculpted from the paddles?

Answer 21

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

Question 22

How do the digits proliferate?

Answer 22

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

Question 23

What does the Apical Ectodermal ridge signal to tissues?

Answer 23

To remain undifferentiated

Question 24

How does bone ossification occur?

Answer 24

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

Question 25

Where is the primary ossification centre?

Answer 25

In the diaphysis

Question 26

Where is the secondary ossification centre?

Answer 26

In the epiphyses (Allows growth up to puberty)

Question 27

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

Answer 27

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

Question 28

How are discrete muscle bellies formed?

Answer 28

Development of the precursers continues and the muscle masses split.

Question 29

What is the upper flexor compartment made from?

Answer 29

The ventral limb

Question 30

What is the upper extensor compartment made from?

Answer 30

The dorsal limb

Question 31

What is the lower extensor compartment made from?

Answer 31

Ventral limb

Question 32

What is the lower flexor compartment made from?

Answer 32

Dorsal limb.

Question 33

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

Answer 33

Because the lower limb rotates during development

Question 34

Why is the rotation of the lower limb important?

Answer 34

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

Question 35

How is the proximal distal axis generated?

Answer 35

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

Question 36

How is dorsal ventral patterning generated?

Answer 36

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

Question 37

How is the anterior posterior axis generated?

Answer 37

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

Question 38

What does pre axial refer to?

Answer 38

The thumb and big toe

Question 39

What does post axial refer to?

Answer 39

The little finger and toe side of the limb

Question 40

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

Answer 40

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

Question 41

Which axial specification does the AER control?

Answer 41

Proximal distal asymmetry | Dorsal and ventral axis boundary

Question 42

Which axial specification does the ZPA control?

Answer 42

Maintains the AER | Anterior to posterior asymmetry

Question 43

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

Answer 43

Because of the lag period between their development.

Question 44

What are the common limb defects?

Answer 44

``` Syndactyly Polydactyly Amelia Meromelia Phocomelia ```

Question 45

What is Syndactyly?

Answer 45

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

Question 46

What is Polydactyly?

Answer 46

Extra digits | Genetic recessive

Question 47

What is Amelia?

Answer 47

Complete absence of a limb | Could be due to failure of the AER

Question 48

What is Meromelia?

Answer 48

Partial absence of one or more limb structures

Question 49

What is Phocomelia?

Answer 49

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

Question 50

Clinically, what is a dermatome?

Answer 50

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

Question 51

What is a myotome?

Answer 51

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