Gastrulation

Question 1

What does the inner cell mass give rise to?

Answer 1

embryo proper

Question 2

What is an important feature of inner cell mass cells?

Answer 2

pluripotent (can from all cells of the mature organisms)

Question 3

When do the first signs of differentiation of the inner cell mass occur?

Answer 3

around 7 days after fertilisation

Question 4

What are the two layers that the ICM differentiates into?

Answer 4

• upper layer called the epiblast

- lower layer called the hypoblast

Question 5

What do the layers of the inner cell mass form?

Answer 5

bilaminar embryonic disk

Question 6

How is the amniotic cavity formed?

Answer 6

A cavity appears within the epiblast which enlarges to form the amniotic cavity

Question 7

What happens 10 days after fertilisation?

Answer 7

• hypoblast cells migrate moline the blastocoele next to the cytotrophoblast
• these cells become flattened and from the extracoelomic membrane
• cells differentiate between the cytotrophoblast and the extracoelomic membrane forming the extraembryonic mesoderm
• extraembryonic mesoderm expands to surround the embryo and amnion entirely

Question 8

How is the chorionic cavity formed?

Answer 8

• cavities appear within the extraembryonic mesoderm

- these enlarge forming the cavity

Question 9

What happens 13 days after fertilisation?

Answer 9

• by this time the chorionic cavity has expanded to surround the embryo which remain attached to the chorion via the connecting stalk
• embryo is now ready to start gastrulation

Question 10

What is gastrulation?

Answer 10

the formation of three germ layers

Question 11

When does gastrulation start and how does it start?

Answer 11

• 3rd week of development

- starts with formation of primitive streak on the surface of the epiblast at the cadual end

Question 12

What happens after the formation of the primitive streak?

Answer 12

• the primitive node develops at the cephalic end of the streak
• cells of the epiblast migrate towards the streak
• upon arrival at the primitive streak, cells change shape and detach from the epiblast layer migrating between the epiblast and hypoblast layers this is called invagination

Question 13

How are the three germ layers formed?

Answer 13

• first cells to enter groove displace the hypoblast and form the embryonic endoderm
• cells that invaginate next produce the embryonic mesoderm that lies between the endoderm and the epiblast
• the epiblast cells that do not invaginate form the embryonic ectoderm

Question 14

What are the three germ layers called?

Answer 14

• ectoderm
• mesoderm
• endoderm

Question 15

What happens to the cells when they migrate through the primitive streak?

Answer 15

• lose properties of the epiblast epithelial sheet

- cells adopt a characteristic morphology called bottle cells

Question 16

What happens to bottle cells once they pass through the streak?

Answer 16

they resume mesenchymal morphology and properties and migrate as individuals to their final destination

Question 17

What do cells invaginating through the primitive node become?

Answer 17

-notochord and paraxial mesoderm

Question 18

What do cells leaving through the primitive streak become?

Answer 18

paraxial mesoderm, lateral plate mesoderm or extraembryonic mesoderm, depending on the level at which they exit

Question 19

How is the notochord formed?

Answer 19

-the primitive streak expands cranially and then retreats caudally
-as it retreats the notochord is laid down in its wake
(cranial structures are more advanced than caudal structures)

Question 20

What is the dorsal-ventral axis established by?

Answer 20

separation of the inner cell mass into the epiblast and hypoblast

Question 21

How is the anterior-posterior axis established?

Answer 21

By the formation of the primitive streak

Question 22

What proves that the inner cell mass is pluripotent?

Answer 22

If you kill one blastomere with a needle and transfer it into another mother with a different coat colour, the baby has the original mother’s coat colour

Question 23

What are non-identical twins called when they are in separate chorionic sacs and how are they formed?

Answer 23

dizygotic (formed when two oocytes are fertilised and each one implants separately)

Question 24

What are identical twins called when they are in separate chorionic sacs and how are they formed?

Answer 24

monozygotic (a two-cell zygote splits and two blastocysts implant separately)

Question 25

What are the two ways that twins can be in a shared chorionic sac?

Answer 25

• monozygotic diamniotic= one morula but 2 ICMs and discs, two amnions but a shared chorion. Inner cell mass splits and forms two embryonic disks
• monozygotic, monoamniotic=one blastocyst and bilaminar disk, two streaks, amnion and chorion are shared this occurs when a single bilaminar disk forms two primitive streaks

Question 26

What are the three modes of monozygotic twinning?

Answer 26

A= cleavage of early embryo each half developing as a completely separate embryo
B= splitting of the ICM forming two embryos enclosed in a common trophoblast (most common)
C=the ICM does not separate completely forming an ICM