Lecture 4: Early embryology pt. 2

Question 1

Describe mesoderm differentiation into…

• notochord
• somites

Answer 1

• central mesoderm cells migrate & ingress through the primitive streak (cranial most part of the node) then differentiate into the notochord
• paraxial mesoderm forms a segmented series of “beads” along both sides of the neural tube in a craniocaudal sequence.

Question 2

Describe mesoderm differentiation into…

• intermediate mesoderm
• lateral plate

Answer 2

• part of the mesoderm that connects the paraxial and lateral plate mesoderm
• part of the mesoderm layer that remains thinner on the outer edges of the mesoderm

Question 3

Describe the formation of the intraembryonic coelom.

Answer 3

When lateral plate opens into paraxial and splanchnic layer then the intraembryonic cavity is formed.

Question 4

Name the 5 regions of the mesoderm.

Answer 4

• paraxial mesoderm
• intermediate mesoderm
• splanchnic mesoderm
• somatic
• intraembryonic coelom

Question 5

List the derivatives of each region of the mesoderm:

1. Paraxial mesoderm
2. Intermediate mesoderm
3. Splanchnic mesoderm
4. Somatic mesoderm
5. Intraembryonic coelom

Answer 5

1. (somites–>) vertebrae, ribs, muscle, dermis
2. urogenital tract
3. viscera
4. body muscles
5. abdominal & thoracic cavities

Question 6

Describe the neural tube formation and its fate.

Answer 6

1. Notochord converts ectoderm –> neuroectoderm
2. Notochord continues neuroectoderm thickening –> neural plate
3. Thickening causes lateral edges to rise and form neural folds
4. Folds curl towards each other at midline & form neural tube.
5. Fusion of neural tube starts at 5th somite in craniocaudal direction
6. Neural tube will become the future spinal cord.

Question 7

Describe how embryonic folding happens.

Answer 7

1. Cranial side folds towards caudal side
2. Caudal side folds towards cranial side
3. Trilaminar disc makes 2 lateral fold..
4. Endoderm is mostly encased within ectoderm except for exposed centre.

Question 8

Describe somite differentiation into sclerotome

Answer 8

In the 4th week somites become mesenchymal and move to both sides of the neural tube.

Somites will later develop into sclerotomes which will later become vertebrae, ribs and bone.

Question 9

Explain the use of dermatome & myotome when describing the supply of nerves & muscles in an adult.

Answer 9

Dermomyotome (D+M) retains nerves from the spinal nerve root of the neural tube. When limbs are growing, nerves can therefore still grow into them as myotomes will develop into muscles for the abdominal/thoracic cavity.

Question 10

Describe somite differentiation into dermatomes.

Answer 10

Dorsomedial & ventrolateral regions of somite cells will differentiate into precursor muscle cells.
The cells in between become the dermatome: develops into the dermis.

Question 11

Describe somite differentiation into myotomes.

Answer 11

Both precursor muscle cell groups move under the dermatome to make the dermomyotome. The myotome lies under the dermatome: develops into the muscles.