7. Somites and embryonic folding

Question 1

the intraembryonic mesoderm

Answer 1

becomes divided into three columns- paraxial, intermediate and lateral plate mesoderm

Question 2

how does the intraembryonic coelom arise

Answer 2

cavities develop in the mesoderm of the lateral plate and the cardiogenic area (region of septum transversum) to form intraembryonic coelom- this is initially isolated from extraembryonic coelom

Question 3

What happens to the intraembryonic coelom eventually?

Answer 3

a horse shoe shaped cavity is formed which becomes continuous with the extraembryonic coelom

Question 4

What will the intrambryonic coelom form?

Answer 4

pericardial cavity

pleural cavity

peritoneal cavity

Question 5

What does the paraxial column form?

Answer 5

somites:

the medial part forms sclerotome (vertebral column, ribs, sternum)

the intermediate part forms myotome (muscles)

the lateral part forms dermatome (dermis)

Question 6

What does the intermediate column form

Answer 6

nephrotome, genitourinary system

Question 7

what does the lateral plate mesoderm become and what is it continuous with

Answer 7

muscles of the parietal (ventral body wall) or muscles of visceral wall (wall of gut)

it is continuous with extra-embryonic mesoderm

Question 8

What happens at the beginning of the fourth week

Answer 8

paraxial mesoderm forms paired cuboidal bodies in a cranio-caudal sequence- SOMITES each receiving spinal nerves that follows tissue wherever it migrates

eventually there are:

3 occipital

8 cercival

12 thoracic

5 lumbar

1-5 coccygeal

occipital-> tongue musculature

Question 9

where does the neural tube close from

Answer 9

the middle

Question 10

How can somites be used to tell the age of embryo

Answer 10

at 4 weeks all of the somites would have formed

Question 11

when is risk of the first congential abnormality

Answer 11

when the neural tube starts to close

Question 12

Answer 12

Question 13

What are the types of mesoderm the lateral plate will split to

Answer 13

somatopleuric mesoderm: continuos with mesoderm of the amniotic cavity- forms striated muscle

• somatic and parietal

splanchnopleuric mesoderm (continuous with mesoderm of yolk sac) forms smooth muscle of gut

• splanchnic (relating to viscera) and visceral (gut wall)

Question 14

what is unique regarding the lower limb nerve supply?

Answer 14

there is twisting of the lower limb nerve supply

Question 15

Longitudinal folding 1

Answer 15

reversal folding

1. initially the embryo forms a flat oval disc

Question 16

Longitudinal folding 2

Answer 16

Expansion of neural tube starts to change the shape of the embryo

the prospective heart and diaphragm (1,2,3) lie in front of the prospective mouth (3-4) and the prospective brain (5) lies behind it

Question 17

Longitudinal folding 3

Answer 17

Differential growth of the embryo causes it to fold both cranio-caudally and laterally.

The previously flat trilaminar disc comes up to take the general body shape.

Rapid growth of neural tube is important in producing reversal.

Question 18

Longitudinal folding 5

Answer 18

After reversal the amnion is only attached to the embru over a small region- the UMBILICUS

A constriction of produced at the junction of embryo and yolk sac.

part of the yolk sac has become incorporated into the gut tube.

yolk sac is pinched off with connecting stalk forming the umbilical cord

Question 19

Lateral folding 1

Answer 19

the edges of amnion grow towards yolk sac

Question 20

Lateral folding 2

Answer 20

The folding is greatly reducing the communication between intra and extra embryonic coelom.

In the region of the umbilicus a connection between the gut tube and yolk sac remains- the VITELLO INTESTINAL DUCT

Question 21

Lateral folding 3

Answer 21