MSK - Development Of The Limbs

Question 1

What do somites develop into? (3)

Answer 1

Dermatone - skin
Myotome - muscles
Sclerotome - bones

Question 2

What is a somite?

Answer 2

Organisation of paraxial mesoderm into segments.
Somites are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo.
1st appear at day 20 in the occipital region.
31 in total

Question 3

When do limb buds become visible?

Answer 3

End of fourth week of development.

Question 4

Where do they become visible?

Answer 4

Ventro-lateral body wall

Question 5

Which limb bud appears first? Upper or lower limb bud?

Answer 5

Upper limb bud appears first followed by the lower limb bud 1 to 2 days later.

Question 6

What do the limb buds consist of?

Answer 6

Mescenchymal core covered by layer of cuboidal ectoderm

Question 7

Where is the anterior and posterior axis in the embryo?

Answer 7

Anterior towards the head
Posterior towards the tail

Remember in the adult
Superior = head
Anterior = Tail

Question 8

What is the proxio-distal axis?

Answer 8

Base of the limb to tips of the digits

Question 9

Where does the limb bud come from?

Answer 9

Activation of mesenchyme within lateral mesoderm.

Derived from the somatic later of lateral plate mesoderm

Question 10

What is the difference between Somatic and Splanchic

Answer 10

Somatic - to do with the body

Splanchic - to do with viscera

Question 11

What contributions to the limb skeleton?

Answer 11

Lateral plate mesoderm

Question 12

How does the limb bud elongate?

Answer 12

Proliferation of the mescenchyme core. Lots of tissue created not well differentiated

Question 13

What is the name of the thickened ectoderm at the apex of the limb bud?

Answer 13

Apical ectodermal ridge (AER)

Question 14

What is the importance of the apical ectodermal ridge?

Answer 14

Critical for limb bud outgrowth
Orchestrates limb development proximal to distal
Final stage is appearance of paddles
AER then regresses - gradually at first then almost entirely.

Question 15

What does the apical ectodermal ridge do?

Answer 15

Exert an inductive influence on the adjacent mesenchyme, causing it to remain undifferentiated

Question 16

As the limb grows, what happens to the cells furthest from the apical ectodermal ridge (AER)?

Answer 16

Proximal mesenchyme begins to differentiate into cartilage and muscle.

Molecular signals from the AER can only communicate to cells that are closest to it so the furthest part differentiates.

Question 17

What would happen in the absence of the apical ectodermal ridge (AER)?

Answer 17

The limb bud would differentiate

Question 18

What induces the development of the digits within the hand/foot plates?

Answer 18

Apical Ectodermal Ridge (AER)

Question 19

How are hand and foot plates formed?

Answer 19

Limb buds become flattened

Question 20

How is the boundary between the dorsal and ventral limb ectoderm marked?

Answer 20

Apical ectodermal ridge

Question 21

What regulates the patterning of the anteroposterior axis of the limb?

Answer 21

Zone of polarising activity

Question 22

What is the zone of polarising activity (ZPA) and role?

Answer 22

A cluster of mesenchyme all cells at the posterior border of the limb near the AER.
The cells have an important signalling role at the posterior base of the limb bud.
Controls patterning.
It ensures the AER is maintained until it is no longer needed
Generates asymmetry in the limbs

Question 23

What happens to mesenchyme to form the ‘digital rays’ hand and foot plates?

Answer 23

Mesenchyme condensations within plates.

Cartilaginous models of the digital bones

Question 24

How are fingers and toes formed?

Answer 24

Apoptosis (cell death) in the AER seperates the ridge into five parts.
Interdigital spaces progressively sculpted by programmed cell death
Further formation of the digits depends on their continued out growth under the influence of the five segments of ridge ectoderm, condensation of the mesenchyme to form cartilaginous digital Ray’s and the death of intervening tissue between the rays

Question 25

Which limb, upper or lower, is effected by limb defects more often?

Answer 25

Upper limb

Question 26

What causes limb defects?

Answer 26

Hereditary but teratogens induced defects have been described

Question 27

Name four common limb defects. What happens in each defect?

Answer 27

Syndactyly - fusion of the digits, don't get sculpting of digital rays. Degree of fusion varies. Polydactyly - extra digits on either hand or foot, genetic recessive trait Amelia - complete absence of a limb Meromelia - partial absence of a limb Phocomelia - exposure to thalidomide, lost extension of limb bud

Question 28

When does endochondral ossification begin?

Answer 28

During the end of the embryonic period

Question 29

What happens during ossification? | What mesoderm is involved?

Answer 29

Lateral plate mesoderm condenses and differentiates. | Cartilage model forms

Question 30

What type of ossification occurs in long bones?

Answer 30

Endochondral ossification

Question 31

Describe primary ossification. When are the centres present?

Answer 31

Primary ossification centres are present by the 12th week of development. From the primary centre in the shaft/diaphysis of the bone, endochondral ossification gradually progresses towards the ends of the cartilaginous model.

Question 32

Where do secondary ossification centres appear?

Answer 32

Epiphyses At birth, diaphysis is usually completely ossified but the two ends (epiphyses) are still cartilaginous. Shortly after ossification centres arise.

Question 33

Why are the epiphyseal growth plates important?

Answer 33

Epiphyseal growth plates are important in the growth in length of bones. Allows bone to grow after birth

Question 34

How do muscles begin to form?

Answer 34

Myogenic precursors migrate into limb buds from somites, they go either side of the developing limb bud

Question 35

What are the two common muscle masses around the newly formed skeletal elements?

Answer 35

``` Ventral = flexor Dorsal = extensor ```

Question 36

How are individual muscles formed? Explain segmental innervation of nerves.

Answer 36

They split from the common muscle masses. | They bring the nerve corresponding to that body segment which explains segmental innervation of nerves.

Question 37

What week of development do limbs rotate?

Answer 37

7th week

Question 38

What way do the upper and lower limbs rotate?

Answer 38

Limbs rotate in opposite directions. Upper limb - laterally. Lower limb - medially Anatomical position = thumb lateral, big toe medial

Question 39

What gives rise to the spinal nerves?

Answer 39

The functional segments of the spinal cord

Question 40

When do spinal nerves enter the limb bud?

Answer 40

Early during development, without innervation development stalls.

Question 41

Where does the upper and lower limb bud appear for innervation?

Answer 41

Upper limb bud - Opposite the caudal cervical spinal segment Lower limb bud - Opposite the lumbar and sacral spinal segment

Question 42

What cords do anterior divisions compartment?

Answer 42

Medial and lateral cords. Flexors are supplied by medial and lateral cords.

Question 43

What cords to posterior divisions compartment?

Answer 43

Posterior cords | All extensors are supplied by posterior cord

Question 44

What is the effect of a spinal nerve innervating a somite?

Answer 44

The somite and all of its derivates will be supplied by that one spinal nerve.