Session 3

Question 1

What is the significance of the notochord in neuralation?

Answer 1

1) It directs the conversion of overlying ectoderm to neurectoderm.
2) The notochord signals overlying ectoderm to thicken. The edges elevate out of the plane of the disk and curl towards each other, creating the neural tube.

Question 2

What are somites?

Answer 2

-bilaterally paired segments of the paraxial mesoderm.

Question 3

How does the coelom form?

Answer 3

lateral folding of the lateral branches of mesoderm which connect to form the coelom

Question 4

List the derivatives of paraxial mesoderm

Answer 4

• Axial skeleton (vertebral column and ribs)
• Dermis
• Muscles of the axial body wall
• Some limb muscles

Question 5

List the derivatives of somatic mesoderm

Answer 5

• Connective Tissue of limbs

- Contributes to the axial body wall

Question 6

List the derivatives of splanchnic mesoderm

Answer 6

• Smooth musculature

- Connective tissue and vasculature of gut

Question 7

List the derivatives of immediate mesoderm

Answer 7

Urogenital system e.g. kidneys, Ureters, gonads

Question 8

Describe the differentiation of somites into dermatome, myotome and sclerotome (4 steps)

Answer 8

1) “organised degeneration” as the ventral wall of the somite breaks down, leading to the formation of the sclerotome.
2) Further organisation of the dorsal portion forms the combined dermomyotome.
3) Myotome proliferates and migrates
4) The dermatome disperses.

Question 9

Discuss the fates of the dermatome, myotome and sclerotome

Answer 9

Dermatome- skin and dermis
Myotome- muscle
Sclerotome- bones

Question 10

Describe the usage of the words myotome and dermatome clinically

Answer 10

• The dermatome is a strip of skin supplied by a single spinal nerve
• The myotome is a muscle/group of muscles supplied by a single spinal nerve

Question 11

What is the role of the neural tube in nerve innovation?

Answer 11

Neural tube produces the nerves that innovates the dermomyotome.

Question 12

Describe the importance of embryonic folding

Answer 12

• Achieves 3D structure
• Creates the ventral body wall
• Pulls the amniotic membrane around the disk so that the embryo becomes suspended in the amniotic sac.
• Pulls the connective stalk ventrally
• Creation of a new cavity within the embryo (the coelom)

Question 13

What do somites give rise to?

Answer 13

somites gives rise to repeating structures (e.g. vertebrae, ribs, intercostal muscles) and this guides innervation.

Question 14

When do somites arise?

Answer 14

1) First pair appears at day 20 in the occipital region
2) More appear in the craniocaudal sequence at 3 pairs/day until there are 42-44 pairs by the end of week 5.
3) Some then disappear, leaving 31 pairs of spinal nerves in total.