What two parts make up the blastocyst?
trophoblast and inner cell mass
What is the trophoblast and what does it give rise to?
Outer epithelial layer of the blastocyst which give rise to extraembryonic structures like placenta.
What is the inner cell mass and what does it give rise to?
Embyronic stem cells of the blastocyst which give rise to the embryo.
Which part of the blastocyst differentiates?
Inner cell mass
What are the layers the inner cell mass splits into and what is in the middle of those layers?
The ICM splits into the epiblast and the hypoblast.
The middle of these 2 layers forms the embrypnic disc.
Which part of the late blastocyst give rise to the embryo?
Only the embryonic disc
What does the epiblast become?
Give rise to the 3 primary germ layers, primitive streak and distinct tissue in adults.
What does the hypoblast become?
Yolk sac
What is the bilaminar disc?
Refers to the embryonic disc.
How does the primitive streak form and what is so important about it?
- On the upper surface (caudal end) of the bilaminar disc, a line of thickened cell appears (primitive streak).
- It defines the major body axis of the embryo.
What does gastrulation involve?
Process whereby the bilaminar embryonic disc undergoes reorganisation to form a trilaminar disc.
How is the primitive node and primitive groove formed?
- At the cranial end of the embryonic disc, the primitive streak expands to create the primitive node (circular depression)
- The depression continues along the midline of epiblast towards the caudal end of the primitive streak forming a primitive groove. (invagination of primitive streak)
How is the trilaminar embryonic disc formed? (3)
- First, cells of the epiblast migrate medially and through primitive groove into hypoblast (displacing them- they become part of the yolk sac instead) to form embryonic endoderm (MET process).
- Later cells move into the space btw epiblast and endoderm to become embryonic mesoderm.
- Cells left in the epiblast become embryonic ectoderm.
What is the primitive node and what is its importance?
- Fluid filled groove (cells at the bottom have cilia which rotate in same direction)
- Important for setting up left-right symmetry.
What are th 3 hypotheses for breaking symmetry?
- Morphogen hypothesis
- Nodal vesicular parcel hypothesis
- Two cilia hypothesis
Explain the morphogen hypothesis. (2)
- Growth factors/molecules are secreted into the fluid and are pushed to the left.
- They bind to receptors on the cells on the left and activate signal transduction pathways telling them they are on the left side.
Explain the nodal vesicular parcel hypothesis.
- Similar to morphogen hypothesis but molecules are in vesicles.
Explain the two cilia hypothesis. (3)
- There are 2 types of cilia (ones at the ring don’t rotate, ones at the bottom rotate)
- Rotating node push fluid from right to left causing left cilia to bend to the left which activates signal transduction to let it know its on the left.
- No fluid bending right nodes so it knows its on the right.
How does the notochord form? (4)
- Formed through migration of cells through the primitive node (mesodermal).
- This forms a hollow tube which extends towards the cranial end of the embryo and form cartilage structure in the middle of embryonic disc.
- While primitive streak regresses and lengthens towards the caudal end.
- The hollow tube fuses with the endoderm to form the notochordal plate which eventually come together formaing the notochord.
What is the importance of neural tube?
- Important for induction of neural tube (neuralation).
How does the neural plate form? (3)
- Induced by notochord.
- Ectodermal cells differentiate into thick plate of pseudostratified, columnar neuroepithelial cells (neuroectoderm).
- It resides btw the cranial end of embryo and the primitive node.
What is neurulation?
Process of neural plate becoming neural tube.
How does the neural tube form? (2)
- Notochord give signals to cells in ectoderm to start thickening (neural plate) and invaginating to form a neural groove.
- The neural fold on either side of the neural grove progress towards each other and fuse.
What is the neural crest and what do they become? (3)
- Particular group of cells which reside at the peaks of the neural folds.
- They form a layer over the top of the neural tube.
- These cells develop into most of the peripheral nervous system.
