Gastrulation In Other Animals Flashcards
1
Q
What is gastrulation?
A
- formation of flat trilaminar embryo
- extensive cell rearrangement
- there are different mechanisms in different organisms for gastrulation but the end results are the same
2
Q
What do the 3 germ layers give rise to?
A
- Ectoderm = epidermis, skin appendages, CNS and PNS
- Mesoderm = muscles, bone and connective tissue
- Endoderm = epithelial lining of GIT and lungs
3
Q
What is blastocyst? What is blastoderm? What is morula?
A
- Blastocyst = 200-300 totipotent cells (called blastula in other animals)
- Blastoderm = blastula having the form of a disc of cells on top a yolk
- Morula = 16 totipotent cells
4
Q
What are the 3 mechanisms of gastrulation?
A
- Involution
- Ingression
- Invagination
5
Q
What is invagination?
A
-Occurs in Drosophila
- In the blastoderm, the columnar epithelial cells are morphologically similar
- The ones on the ventral portion are fated to be mesoderm
- The ones at the extreme ends (cranial and caudal ends) are fated to be endoderm
- Invagination occurs as the cells become internalised (folding along ventral portion)
- This is essentially folding of epithelium along ventral portion of embryo
- This is driven by changes in shapes of the cells in this portion - they go from columnar to bottled shaped
- Once the cells have been internalised, they undergo epithelial mesenchyme transition which allows them to spread inside the embryo to give rise to mesoderm and endoderm derivatives
- Once inside the embryo, the mesoderm primordium (cells fated to be mesoderm) disperses into single cells and attach to the ectoderm
6
Q
What is involution?
A
Occurs in zebra fish and xenopus
Zebrafish:
- blastoderm sits on top of yolk sac
- epiboly occurs where blastoderm cells spread around outside of the yolk sac
- when epiboly is completed, the yolk sac will be engulfed by the blastoderm
- halfway through epiboly, gastrulation occurs
- The cells at the edge of the blastoderm start to become internalised by involution
- They move in opposite direction to epiboly and spread underneath the epiplast cells (as the outercells are undergoing epiboly)
- By the end of this process, the cells have engulfed the yolk sac and are divided into 2 layers
- The outer epiblast underwent epiboly and the inner hypoblasts underwent involution
- The hypoblast give rise to mesoderm and endoderm whilst epiblast give rise to ectoderm
Xenopus:
- the animal pole cells of the blastula migrate down to engulf the cells in the vegetal pole, which are fated to become endoderm
- as they engulf them, a blastopore appears in the region destined to become the dorsal part of embryo
- In the blastopore, there is folding of a group of cells via involution which moves in the opposite direction to the animal pole cells
- By the end of this process, the ectoderm has engulfed the cells that will go on to form the mesoderm and endoderm
7
Q
How does involution occur in zebra fish?
A
- The blastoderm cells spread around the yolk - this movement is called epiboly
- Once epiboly is completed, the yolk will be fully engulfed by the blastoderm cells
- Half way through epiboly, gastrulation occurs
- Cells at the edge of the blastoderm become internalised in the embryo by involution (they fold inside and move in the opposite direction to epiboly)
- They will spread all underneath the cells that stay on the external side (which are called the epiblast cells)
- By the end of this whole process, embryo would have completely engulfed the yolk and are already subdivided into 2 layers (the internal layer will give rise to mesoderm and endoderm). External layer gives rise to ectoderm
8
Q
How does the involution occur in xenopus?
A
- Cells of the animal pole of the blastula will spread to engulf the cells in the vegetal pole
- As they engulf them, a structure called ‘blastopore’ (opening) forms on the dorsal past
- In the blastopore, there is folding of group of cells in a similar way to zebra fish, by involution
- The mesoderm primoridum cells move in the opposite direction to epiboly
- By the end of this process, we get mesoderm, endoderm and ectoderm
9
Q
What is ingression?
A
- Occurs in chick embryos
- Embryo sits on top of the yolk
- Blastula embryo is a bilaminar disc (external ones are epiblast which gives rise to embryo proper; internal ones are hypoblast which give rise to extra-embryonic tissue)
- The posterior part of the embryo gets a groove extending to the opposite side of the disc (thickening of the epiblast - primitive streak - extends from posterior to anterior) - it will form the midline of the developing embryo
- Cells of the primitive streak will undergo epithelial mesenchyme transition and will ingress (freely migrate) inside the embryo, underneath the epiblast, to form the mesoderm and endoderm
- By the end of gastrulation, the primitive streak starts to regress
- As it regresses, it lays down the body plan of the developing embryo. It sets up the anterior-posterior axis, medial-lateral axis and ventral-dorsal axis.
Formation of chick embryo body plan:
- Gastrulation occurs from posterior to anterior, which is the direction in which the primitive streak extends
- At the end of gastrulation, the primitive streak regresses, laying down the body plan for development as chick grows
- It will set up the anterior-posterior, medial-lateral and dorsal-ventral axes of the embryo
10
Q
What is mouse gastrulation?
A
-Ingression
- In the blastocyst, the 2nd cell fate decision is taking place as ICM will form epiblast or hypoblast
- This whole structure organises as a cylinder with the epiblast on the inside surrounded by the primitive endoderm (hypoblast)
- As this tube forms, gastrulation begins
- Gastrulation in mice occur through primitive streak (from posterior to anterior)
- cells in the epiblast undergo epithelial mesenchyme transition along the primitive streak
- This progresses anteriorly and cells move away from the primitive streak as they become internalised into the embryo
- By the end of gastrulation, the embryo still looks a bit like a cylinder but several cell types can be distinguished
- Several inductive signals that are released by different tissues eventually establish all 3 body axis
11
Q
What is fish and frogs gastrulation?
A
-Dorsal surfaces are towards the outside; ventral structures are on the yolk
12
Q
What is chick gastrulation?
A
- Flat structure as the yolk is so big
- dorsal part is on the outside; ventral portion is on the yolk
13
Q
What is mouse gastrulation?
A
- Embryo is bent dorsally
- Ventral surfaces are on the outside
- So as you can see, they are organised inside out
- So now they have to turn around so that the inside is actually the inside and the outside is actually the outside
- This happens by twisting - the posterior part of the embryo twists around and moves towards the anterior
14
Q
How would you compare the end process of gastrulation in fish, frog, chick and mouse embryos?
A
- Although the shape, organisation and morphology are different, they are still very similar
- They all have an anterior-posterior axis, dorsal-ventral axis and medial-lateral axis
- In fish and frog, the dorsal structures are bent outwards with ventral structures located internally
- The chick continues to be a very flat structure because the yolk underneath is very big. The dorsal part is oreintated towards the outside, whilst the ventral part in against the yolk
-The mouse embryo is bent dorsally so the dorsal structures are located in the middle and the ventral structures are facing the outside
15
Q
What is turning of mouse embryo?
A
- At the end of gastrulation, the embryo is essentially inside out
- As a result, the embryo has to turn to put the inside structures inside the embryo
- This occurs in a twisting motion, where the posterior part of embryo twists around and moves to the anterior end
- In this whole process, the embryo closes up with internal structures now located in the inside
-A similar process occurs in human embryo
