Gastrulation Flashcards
What does the inner cell mass give rise to?
embryo proper
What is an important feature of inner cell mass cells?
pluripotent (can from all cells of the mature organisms)
When do the first signs of differentiation of the inner cell mass occur?
around 7 days after fertilisation
What are the two layers that the ICM differentiates into?
- upper layer called the epiblast
- lower layer called the hypoblast
What do the layers of the inner cell mass form?
bilaminar embryonic disk
How is the amniotic cavity formed?
A cavity appears within the epiblast which enlarges to form the amniotic cavity
What happens 10 days after fertilisation?
- 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
How is the chorionic cavity formed?
- cavities appear within the extraembryonic mesoderm
- these enlarge forming the cavity
What happens 13 days after fertilisation?
- 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
What is gastrulation?
the formation of three germ layers
When does gastrulation start and how does it start?
- 3rd week of development
- starts with formation of primitive streak on the surface of the epiblast at the cadual end
What happens after the formation of the primitive streak?
- 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
How are the three germ layers formed?
- 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
What are the three germ layers called?
- ectoderm
- mesoderm
- endoderm
What happens to the cells when they migrate through the primitive streak?
- lose properties of the epiblast epithelial sheet
- cells adopt a characteristic morphology called bottle cells
What happens to bottle cells once they pass through the streak?
they resume mesenchymal morphology and properties and migrate as individuals to their final destination
What do cells invaginating through the primitive node become?
-notochord and paraxial mesoderm
What do cells leaving through the primitive streak become?
paraxial mesoderm, lateral plate mesoderm or extraembryonic mesoderm, depending on the level at which they exit
How is the notochord formed?
-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)
What is the dorsal-ventral axis established by?
separation of the inner cell mass into the epiblast and hypoblast
How is the anterior-posterior axis established?
By the formation of the primitive streak
What proves that the inner cell mass is pluripotent?
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
What are non-identical twins called when they are in separate chorionic sacs and how are they formed?
dizygotic (formed when two oocytes are fertilised and each one implants separately)
What are identical twins called when they are in separate chorionic sacs and how are they formed?
monozygotic (a two-cell zygote splits and two blastocysts implant separately)
What are the two ways that twins can be in a shared chorionic sac?
- 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
What are the three modes of monozygotic twinning?
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
