Somites and Embryonic Folding

Question 1

What are the three parts of the intraembryonic mesoderm?

Answer 1

paraxial mesoderm
intermediate mesoderm
lateral plate mesoderm

Question 2

What is the paraxial mesoderm?

Answer 2

mesoderm lying either side of the notochord. Forms gut, axial skeleton, voluntary musculature, dermis

Question 3

What is the intermediate mesoderm?

Answer 3

lateral to paraxial mesoderm. Forms urinary and genital

Question 4

What is the lateral plate mesoderm?

Answer 4

remainder of lateral mesoderm.

1. layer adjacent to ectoderm - somatopleuric mesoderm
2. layer adjacent to endoderm - splanchnopleuric mesoderm

Question 5

What does the somatopleuric mesoderm and splanchnopleuric mesoderm become?

Answer 5

1. striated voluntary muscle

2. smooth muscle of the gut

Question 6

What forms somites?

Answer 6

paraxial mesoderm

Question 7

What do the medial, intermediate and later parts of the somite form?

Answer 7

medial - sclerotome
intermediate - myotome
lateral - dermatome

Question 8

Derivatives of somites

Answer 8

3 occipital
8 cervical
12 thoraic
5 lumbar
5 sacral
1-5 coccygeal

Question 9

What do somites do?

Answer 9

precursor populations of cells which receive a segmental spinal nerve which follows the tissure wherever it migrates

Question 10

What is the intraembryonic coelom?

Answer 10

clefts in lateral plate mesoderm, infront of prochordal plate
fuses to form horse shape cavity

Question 11

What happens in week 4?

Answer 11

rapid growth (particularly lengthwise) which allows lateral and longitudinal folding to occur

Question 12

Which parts grow?

Answer 12

embryonic disc and amnion grow vigorously, but yolk sac hardly grows.
YS is attached ventrally to the embryo, lack of growth causes it to form a 3D cylinder

Question 13

Why does growth mainly occur in the thin flexible outer rim of the embryo?

Answer 13

developing neural tube and somites stiffen the dorsal axis of the embryo

Question 14

What occurs in longitudinal folding?

Answer 14

reversal
cranial end develops first 
rapid growth of neural tube
1,2,3,4,5 to 5,4,3,2,1
(brain, mouth, heart, diaphragm)
yolk sac pinched - endodermally lined tube (gut)
amniotic cavity enlarging lots

Question 15

Lateral folding

Answer 15

edges of amnion grow downwards, putting pressure on the yolk sac
gut tube cut off and suspended in mesoderm
reduced comms between intra and EEM

Question 16

What is the vitelline duct?

Answer 16

connection between gut tube and yolk sac

Question 17

What does ectoderm form?

Answer 17

skin

Question 18

What does mesoderm form?

Answer 18

dermatomes, own nerve supply