Embryology 2

Question 1

When does the embryonic period take place?

Answer 1

From week 3 until week 8.

Question 2

What is special about the embryonic period?

Answer 2

This is where congenital malformation is at greatest risk. Usually due to environmental exposure or drug therapy.

Question 3

Why is it important that nothing goes wrong during this period?

Answer 3

Because there are some narrow windows for some systems of the body. If this does not occur correctly some systems might be underdeveloped.

Question 4

So we have covered the pre-embryonic period from fertilisation to implantation. What are the steps of the embryonic period?

Answer 4

Gastrulation
Neurulation
Segmentation
Folding

Question 5

What are the primitive streak, pit and node?

Answer 5

A small invagination which runs along the dorsal surface of the epiblast.

Question 6

Why is the primitive streak important in bioethics?

Answer 6

A lot of countries regard the primitive streak as the formation of a unique organism. After this milestone growing a foetus outside of a uterus is no longer permitted in some countries.

Question 7

After the primitive streak is formed. What happens next?

Answer 7

The bilaminar disc becomes a trilaminar disc.

Question 8

What is the precursor of the three layers of the trilaminar disc?

Answer 8

The epiblast. The hypoblast is now redundant and no longer required so it fades away.

Question 9

What are the three layers of the trilaminar disc?

Answer 9

Ectoderm, mesoderm and endoderm.

Question 10

There are two nodes in the trilaminar disc where the ectoderm touches the endoderm (no mesoderm is present), why are those important?

Answer 10

These are the two main openings of the body, the future mouth and future anus.

Question 11

What will the ectoderm differentiate into?

Answer 11

Anything that has contact with the outside environment. Nervous system and epidermis.

Question 12

What will the mesoderm differentiate into?

Answer 12

Supporting tissues like muscle, cartilage, bone, vascular system.

Question 13

What will the endoderm differentiate into?

Answer 13

The epithelial lining of GI tract, resp. tract + parenchyma of glands.

Question 14

What is situs inversus? What is the cause? Are there any consequences?

Answer 14

A complete mirror image of the viscera. Commonly result from immotile cilia. This means that affected can be more susceptible to resp. tract infections.
No usual side effects of this condition.
Problems arise when there is no complete situs inversus. When some parts of the body are mirrored and others aren’t.

Question 15

How is left and right-sidedness defined in gastrulation?

Answer 15

Action of ciliated cells at the node and notochord will result in a left-ward flow of signalling molecules. The presence of these molecules define left-sidedness.
On the right side no signalling molecules are released. The absence of the molecules define right-sidedness.

Question 16

Briefly outline gastrulation.

Answer 16

A primitive streak, pit and node are formed.
Migration and invagination of the bilaminar disc occurs. The hypoblast becomes redundant and is no longer needed.
The epiblast differentiate into three distinct layers. This is called the trilaminar disc.
The notochord is also formed during this period.
The left side of the primitive node and notochord gets a flow of signalling molecules. This define left-sidedness.
There is an absence of signalling molecules on the right side of the notochord. This defines right-sidedness.

Question 17

What is the next step after gastrulation?

Answer 17

Neurulation.

Question 18

Where can the notochord be found?

Answer 18

In the middle of the trilaminar disc. Between the ectoderm and the endoderm. ‘inside’ the mesoderm.

Question 19

Outline what happens during neurulation.

Answer 19

The notochord signals the ectoderm to thicken.
A neural plate is formed.
The neural plates long sides then curl toward each other to form a tube called the neural tube.

Question 20

What is the next step after neurulation?

Answer 20

Segmentation.

Question 21

What happens to the mesoderm as the neural tube is formed?

Answer 21

The mesoderm differentiate into more specific types of mesoderm.

Question 22

What types of mesoderm is formed? Explain where they are located.

Answer 22

The paraxial mesoderm is most proximal to the neural tube.
Distal to the paraxial mesoderm is the intermediate mesoderm.
Distal to the intermediate mesoderm the intraembryonic coelom
Superior to the intraembryonic coelom is the somatic mesoderm
Inferior to the intraembryonic coelom and the somatic mesoderm is the splanchnic mesoderm.

Question 23

What does the paraxial mesoderm differentiate into?

Answer 23

During a period of around 15 days somites are formed. 42 to 44 pairs are formed in total. Some will disappear and leave 31 in total.

Question 24

What are the functions of the somites and where can they be found?

Answer 24

Since they are differentiated from paraxial mesoderm they can be found most proximal to the neural tube.
The somites will differentiate into sclerotomes, dermomyotomes and further dermatomes and myotomes.

Question 25

What will the dermatomes differentiate into?

Answer 25

Skin / dermis

Question 26

What will the myotomes differentiate into?

Answer 26

Muscles

Question 27

What will the sclerotomes differentiate into?

Answer 27

Bone

Question 28

After segmentation occur what is the next and final step of the embryonic period?

Answer 28

Folding.

Question 29

Why is folding required?

Answer 29

E.g. the buccopharyngeal area which will become the future mouth is below the cardiogenic area which will become the future heart.

Question 30

What are the two ways to imagine folding?

Answer 30

As folding a circle into an envelop. Head fold -> tail fold -> then lateral folding.
It can also be imagined as a drawstring purse.

Question 31

Check the drawings of folding again on page 37!

Answer 31

Yup