Part 2: Trilaminar Embryo

Question 1

Does the entire spinal cord develop from the neural tube?

Answer 1

• no; the saccral region of the spinal cord develops from the mesoderm

Question 2

Each somite differentiates into what two structures?

Answer 2

• dorsolateral dermomyotome (low shh)
• ventromedial sclerotome (high shh)

Question 3

Dermomyotome develops into:

Answer 3

skin and muscle

• derived from somites

Question 4

Sclerotome develops into:

Answer 4

bone

• derived from somites
• forms the vertebrae around the neural tube
• interference with sclerotome migration leads to spina bifida

Question 5

The upper limb bud develops adjacent to what somites?

Answer 5

C5-T1

Question 6

The lower limb bud develops adjacent to what somites?

Answer 6

L2-S3

Question 7

Mesenchyme is:

Answer 7

• mesoderm that has been invaded by neural crest cells
• mix of mesoderm and neural crest cells

Question 8

Proximal to distal differential development in the limbs is controlled by:

Answer 8

• interactions between the apical ectodermal ridge and the underlying mesoderm

Question 9

Medial to lateral asymmetry in the limbs is controlled by:

Answer 9

• factors released from the zone of polarizing activity (ZPA) in the caudal limb bud.
  
  • ZPA releases shh

Question 10

Dorsal to ventral differential development in each limb is controlled by:

Answer 10

• different groups of transcription factors in each region
• controls development of extensors and flexors.

Question 11

Process of somite dermomyotome division:

Answer 11

1. somite dermomyotome divides into dermotome and myotome.
2. myotome divides into epimere and hypomere.
3. epimere becomes intrinsic back muscles.
4. hypomere divides into dorsal and ventral muscle masses in the limbs.
5. dorsal mass becomes extensor muscles.
6. ventral mass becomes flexor muscles.

Question 12

The epimere has a relationship with what spinal nerve?

Answer 12

• dorsal primary ramus
• becomes intrinsic back muscles

Question 13

The hypomere has a relationship with what spinal nerve?

Answer 13

• ventral primary ramus
• divides into dorsal and ventral muscle masses of the limbs.
  
  • dorsal = extensor
  • ventral = flexor

Question 14

What nerve innervates the limbs?

Answer 14

ventral primary ramus

Question 15

Process of bone development:

Answer 15

1. lateral plate mesoderm migrates into the limb to form bones and connnective tissue.
2. bone cartilage model formed.
3. bone cartilage model becomes ossified at diaphysis.
4. bone cartilage model becomes ossified at epiphysis.
5. diaphysis and epiphysis separated by epiphyseal (growth) plate.
6. diaphysis and epiphysis fuse.

Question 16

Primary ossification center is called:

Answer 16

• diaphysis
• located in the shaft

Question 17

Secondary ossification center is called:

Answer 17

• epiphysis
• one at each end of growing bones

Question 18

The primary (diaphysis) and secondary (epiphysis) ossification centers are separated by:

Answer 18

• epiphyseal (growth) plates.
• primary and secondary centers have different blood supplies.

Question 19

Relationship between skin overlying limb bud and spinal nerves for sensory innervation:

Answer 19

Linear/nearest dermatome

Question 20

Effect of lower limb development on the relationship between skin overlying limb bud and spinal nerves for sensory innervation:

Answer 20

• lower limbs rotate 180 degrees medially
• dorsal (extensor) muscle mass becomes ventral
• ventral (flexor) muscle mass becomes dorsal