Gastrulation - Mammalian Development (Mouse) 1

Question 1

Three types of cells that form in the late blastocyst while implanting?

Answer 1

1. Trophoectoderm
2. Primitive Endoderm (Hypoblast)
3. Epiblast.

Question 2

What does Trophoectoderm give rise to?

Answer 2

Trophoblastic tissues:

1. Chorion
2. Ectoplacental Cone
3. Extra-embryonic ectoderm

Question 3

Hypoblast will give rise to?

Answer 3

endoderm of parietal and visceral yolk sacs

Question 4

Epiblast will give rise to?

Answer 4

Embryo that generates tissues of the adult mouse and the fetal portion of the placenta.

Question 5

Summary of the implanting blastocyst? How are these Epi, TE, and Hypoblast/PrE cells arranged topologically?

Answer 5

at the end of implantation embryo contains

1. TE
2. Hypoblast/PrE
3. Epiblast.
4. came from Trophoblast cells and 2. and 3. came from ICM.

Gene Expression: Lefty1

Question 6

After implantation stage Early Postimplantation Stage begins. What happens at this stage? and how does this happen?

Answer 6

— PD Polarity is created that leads to DVE cell specification.

1. embryo elongates and takes a cylinderical shape that gives rise to Proximal and Distal Axis.
2. The hypoblast gives rise to Visceral Endoderm (VE)
3. Then the VE surrounds the Epiblast on the distal end and the Extra embryonic ectoderm on the proximal end.
4. VE on the most distal end are called Distal Visceral Endoderm (DVE)

Question 7

What is the VE called at the distal end and proximal end and what cells is it made of?

Answer 7

emVE = distal end (squamous epithelium)
exVE = proximal end (cuboidal epithelium)

Question 8

Early Post implantation period gives rise to DVE cells and PD polarity. What happens next – First give the one line answer, then explain the sequence? [This is still pre-gastrulation, remember]

Answer 8

— cells within the emVE move, PD axis rotates, radial symmetry of the cylinder is broken and this gives rise to AP axis.

1. DVE cells migrate proximally embryonic - extraembryonic junction (EPI-ExE) and form the AVE.
2. This unilateral repositioning of the cells within emVE causes the PD axis to rotate.
3. This breaks the radial symmetry of the cylinder as AVE marks the future Anterior side of the embryo.

Gene expression: AVE represses NODAL and Wnt Signaling to restrict the formation of Primitive streak to the proximal posterior epiblast next to ExE.

Question 9

Once the AP Axis is set up during early gastrulation, AVE represses Nodal and Wnt on the anterior side causing the PS to form next to ExE. How is the PS formed after that?

Answer 9

– PS forms in the Epiblast at the extra-embryonic boundary. The PS defines the future posterior of the embryo and the site of gastrulation. Polonaise movements and medio-lateral cell intercalation movements begin to happen at the site of PS.

Gene expression:

1. Nodal/Smad and Wnt/Beta Catenin from extra embryonic, poster emVE regions induce Primitive Streak formation
2. Polonaise movements and medio-lateral cell intercalation is regulated by Fgf and Wnt/PCP signaling.

Question 10

After AP axis is set up, PS is formed. What type of cell movements occur at PS?

Answer 10

— Cell Ingression and EMT

1. cells close to streak form rosette like structures, which are thermodynamically unstable and lead to cell ingression.
2. cells ingress by elongating and acquiring a bottle shape at the streak as they lose their PD polarity, downregulate junctional and basement membrane proteins transitioning to Epithelium and mesenchyme (mesoderm).
3. Mesenchymal/Mesodermal cells migrate away from the streak and form a new layer between EPI and VE.

Gene Expression: Fgf, Wnt, downregulation of adhesion molecules like Ecadherin, Eomes at the Streak for EMT

Question 11

After the primitive streak forms and cell movements have led to EM Transition giving rise to the mesoderm layer between EPI and VE, what type of mesodermal cells are formed?

Answer 11

— extraembryonic mesoderm, paraxial, axial and DE arise from PS.

1. most posterior end of the streak gives rise to extra-embryonic mesoderm.
2. middle and anterior portion of the streak gives rise to cardiac, lateral plate and paraxial mesoderm.
3. anterior part of the streak gives rise to axial mesoderm and definitive endoderm (DE)

Question 12

Once extraembryonic mesoderm, paraxial, axial and DE is formed, what happens next?

Answer 12

– cellular intercalation and endoderm morphogenesis

1. cells