Pre-neurulation

Question 1

Explain how exactly the moment of fertilization happens.

Answer 1

1. Sperms reach the egg
2. Sperm attempts to bind to Zona Pellucida (outer layer of the egg, external to egg membrane) via the binding protein ZP3 on oocyte
3. Acrosome reaction -> the sperm releases enzyme which break down Zona Pelludica
   - acrosome = head of the sperm
4. Once the “path” is set the sperm can get through -> reaching plasma membrane of secondary oocyte
5. Inside, it binds to the egg membrane (cortical reaction) => pronuclei fuse
   -> depolarization of the oocyte, release of contents from granules towards the membrane
   -> causes changes to ZP3 binding sites
   -> no more sperms can bind anymore

Question 2

How does the process continue further (up until the fusion with the uterus wall)?

Answer 2

From the day 1 - pro-nuclei fuse (one male, one female) = zygote
-> division begins (cleavage) -> two-cells (day 2) -> doubling until it reaches Morula (tight group of 16 cells) -> cells start to migrate towards the edges of the membrane creating Blastocyst cavity -> after approxiametaly 6-7 days Blastocysts hatches (release of the cells out of the zona pellucida) -> “freed” cells attach to the uterus wall = Nidation

Question 3

How does in-vitro fertillization work? What’s the difference between the older and newer approach(+why)?

Answer 3

Egg of a mother gets fertilized by sperms of the father in a Petri dish -> the cell culture is left to extend as it would in the human body -> at one point based on the Gardners grading system Blastocyst is put back into the mother -> and let to attach itself

• The main difference now is that we’re using blastocysts that already got out of the Zona Pellucida while it used to be given in the hatching phase
  -> this increases the chances of successful pregnancy

Question 4

Explain Nidation.

Answer 4

Hatched blastocyst attaches to the uterine mucosa (thickens to protect the embryo) and passes through the endometrium = nesting -> embryoblast starts to differentiate into 2 germ layers
- Epiblast and Hypoblast (walls will form yolk sack)
- Apart from that Amniotic cavity also forms (to protect the fetus from e.g. trauma to the mother’s abdomen)

Question 5

How does the blastocyst look like before it enters endometrium?

Answer 5

Question 6

What does gastrulation signify?

Answer 6

= process by which embryo develops from 2-germ layered blastula into 3-germ layered gastrula
- Epiblast -> Ectoderm
- Cells in the primitive streak (initially from the primitive pit) differentiate and migrate away to form mesoderm (and displace hypoblast further)
- Hypoblast -> Endoderm

Question 7

What is the result of Gastrulation? And what can we expect from these layers?

Answer 7

• dorsal: the amnion cavity
• ventral: the yoke sac

Question 8

What has been recently discovered about the yoke sac (functions, later replacement)?

Answer 8

= transient extraembryonic organ
- delivers vital functions of the embryo before other organs develop
- Hematopoiesis = formation of blood cellular components and Hematopoitic stem cells
- Non-hematopoitic: creation of immune cells, metabolism, coagulation (i.e. blood clotting), erythroid cell mass regulation

-> once the time comes it gets replaced with the final organs
- liver = metabolism, coagulation
- bone marrow = hematopoisis
- kidneys = regulation

Question 9

What is EPO? Why is important? Who produces it?

Answer 9

EPO = erythropoietin
- first producer = yoke sack -> later astrocytes
- oxygen dependent
- upregulated in hypoxia (i.e. low levels of oxygen -> thus blood vessels must try harder to transport the small amount that’s there)