Embryology Weeks 3-8

Question 1

When is the biggest period of change?

Answer 1

Weeks 3-8

Question 2

What indicates that gastrulation will occur?

Answer 2

Formation of the primitive streak

Question 3

In short, what is gastrulation?

Answer 3

The formation of a trilaminar disk from a bilaminar disk. There are 3 layers of the trilaminar disk- the endoderm, ectoderm and mesoderm.

Question 4

Describe in more detail the process of gastrulation

Answer 4

Cells in epiblast layer build up and squeeze through the junctions in the ectoderm. They spread out and form a new layer between the epiblast and hypoblast. The hypoblast is displaced and disintegrates and the formation of 3 layers- endo,meso and ectoderm form the trilaminar disk.

Question 5

How is left and right established in the embryo?

Answer 5

The primitive node (at the end of the primitive streak) has cilia which are responsible for ‘wafting’ leftward signals to the left and the absence of these signals indicates the right side.

Question 6

What does the Ectoderm go on to form?

Answer 6

Organs and structures that maintain contact with the outside world eg skin, nervous system

Question 7

What does the Mesoderm go on to form?

Answer 7

Mesoderm goes on to form supporting tissue eg muscles, bones, vascular system and cartilage

Question 8

What does the Endooderm go on to form?

Answer 8

Internal structures eg epithelial lining of GI tract and respiratory tract

Question 9

When does gastrulation take place?

Answer 9

3rd week of development, marking the start of the embryonic period

Question 10

What is the notochord?

Answer 10

A solid rod of cells running down the midline with important signalling roles.

Question 11

Describe the role of the notochord

Answer 11

It releases signalling molecules that diffuse out and act on the ectoderm stimulating it to thicken and convert into neuroectoderm

Question 12

What. Does neuroectoderm go on to form?

Answer 12

The central nervous system

Question 13

Describe the process of neurulation

Answer 13

Signalling molecules from the notochord stimulate thickening of the ectoderm converting it to neuroectoderm. The edges of the neural plate curl up towards each other forming a hollow tube- the neural tube.

Question 14

What happens to the mesoderm in neurulation?

Answer 14

It splits into 2 types of mesoderm somatic mesoderm and splanchnic mesoderm. The gap between these layers is the intra e,Byronic coelom.

Question 15

What are somites?

Answer 15

Embryonic body segments that go on to develop into the basis for the MSK system

Question 16

Give 3 examples of tissue that somites go on to form.

Answer 16

Dermatome- skin section
Myotome- muscle section
Sclerotome- Bone

Question 17

What is segmentation?

Answer 17

The formation of somites form the paraxial mesoderm.

Question 18

What is the purpose of segmentation?

Answer 18

Segmentation gives rise to repeating structures such as vertebrae, rubs, intercostal muscles and spinal cord segments. It also guides innervation.

Question 19

How many somites are there?

Answer 19

31

Question 20

Describe what happens and the result of ‘head and tail’ and lateral folding?

Answer 20

Head and tail-
Massive growth of neural tube forces downward pressure on embryonic disk. It results in only the ectoderm being presented to the outside. The amniotic sac is wrapped around everything.
Lateral-
Massive growth of the somites pushes down laterally. Space between somatic and splanchnic layer increases. Amniotic sac covers everything and the yolk sac and cavity is now inside the embryo.