11. Embryology 2

Question 1

What happens in the embryonic period?

Answer 1

All major structures (limbs) and systems are formed

Question 2

When and where does the primitive streak appear?

Answer 2

In week 3

Appears on dorsal surface of epiblast

Question 3

What are the 2 main steps in gastrulation?

Answer 3

Migration and invagination

Displacement of hypoblast and creation of third layer

Question 4

What happens in migration and invagination?

Answer 4

Epiblast cells push through epithelial layer

Question 5

What happens in displacement of hypoblast?

Answer 5

3rd layer is formed
Becomes trilaminar disc
Holes from in mesoderm which will layer become mouth and anus

Question 6

What are the 3 layers that are created in gastrulation?

Answer 6

Ectoderm
Mesoderm
Endoderm

Question 7

What does the ectoderm form into?

Answer 7

Organs and structures that maintain contact with outside world
E.g. nervous system, epidermis

Question 8

What does the mesoderm form into?

Answer 8

Supporting tissues

E.g. muscle, cartilage, bone, vascular system

Question 9

What does the endoderm form into?

Answer 9

Internal structures

E.g. epithelial lining of GI tract, respiratory tract, parenchyma of glands

Question 10

When does gastrulation occur?

Answer 10

Week 3 of development

Start of embryonic period

Question 11

What happens in gastrulation?

Answer 11

Bilaminar disc converted to trilaminar disc

3 germ layers: ectoderm, mesoderm, endoderm

Question 12

How does gastrulation occur?

Answer 12

Primitive streak forms in caudal epiblast, leads to migration and invagination of epiblast cells

Question 13

Why does gastrulation occur?

Answer 13

Ensure correct placement of precursor tissues to allow subsequent morphogenesis

Question 14

What is the notochord and when is it formed?

Answer 14

Solid rod of cells running in midline with important signalling role
Formed in neurulation

Question 15

What does the notochord do?

Answer 15

Releases chemicals so ectoderm can respond, responds by thickening

Question 16

What are somites?

Answer 16

Organisation of parasail mesoderm into segments
First pair appear at day 20 in occipital region
Appear 3 pairs/day until 42-44 pairs present

Question 17

How many somites are there in total at the end?

Answer 17

31, therefore 31 pairs of spinal nerves

Question 18

What happens in organised degeneration of somites?

Answer 18

Ventral wall of somite breaks down leaving to formation of sclerotome
Further organisation of dorsal portion forms combined dermomyotome

Question 19

What happens to the myotome?

Answer 19

Proliferates and migrates

Dermatologist disperses

Question 20

What does the dermatome later form?

Answer 20

Skin section, dermis

Question 21

What does the myotome late form?

Answer 21

Muscle section, muscles

Question 22

What does the sclerotome later form?

Answer 22

Hard tissue section, bones

Question 23

What does the organisation of mesoderm into somites give rise to?

Answer 23

Repeating structures

E.g. vertebrae,ribs, intercostal muscles, spinal cord segments

Question 24

What is the order of folding?

Answer 24

Head fold
Tail fold
Lateral folding

Question 25

What does folding achieve?

Answer 25

Creates ventral body
Pulls amniotic membrane around disc
Embryo becomes suspended in amniotic sac
Pulls connecting stalk ventrally

Question 26

What has happened by the end of week 4?

Answer 26

Nervous system started to form
Segments appeared, assigning specific tasks to specific cells
Embryo folded, putting everything in right place