1. From fertilisation to placenta formation

Question 1

What day does fertilisation occur on?

Answer 1

We count fertilisation as day 1

Question 2

What is a Morula and on what day does it form?

Answer 2

1. The fertilised cell divides again and again until it forms a group of 16 totipotent (able to become all tissues and placenta) cells
2. Happens on approx day 3

Question 3

What is a Blastocyst and when does it form?

Answer 3

1. Outer cells of Morula undergo compaction via creation of tight junctions = trophoblast
2. Separation of outer cell layer from inner cell layer = inner cell mass
3. Within the trophoblast layer = inner cell mass and a blastocyst cavity
4. Happens on approx day 5

Question 4

What has to happen before the blastocyst implants into the uterine wall?

Answer 4

1. Hatching from the zona pellucida

Question 5

What does the trophoblast and inner cell mass give rise to respectively?

When does this happen?

Answer 5

1. The fetal components of the placenta
2. The embryo
3. Day 8

Question 6

The outer cell mass (i.e. the trophoblast) and inner cell masses divide into what respectively?

Answer 6

1. The trophoblast divides into the Syncytiotrophoblast (outside) and cytotrophoblast (on the inside)
2. The inner cell mass divides into the bilaminar disk consisting of the epiblast (nearer the cytotrophoblast layer) and the hypoblast (nearer the blastocyst cavity)

Question 7

What cavity is formed by the hypoblast cells and extracoloemic membrane?

What cavity is formed by the epiblast cells and the migrated epiblast cells around top of cytoblast cells?

Answer 7

1. It lines what was previously called the blastocoele (blastocyst cavity), which is now called the ‘Primary Yolk Sac’
2. Amniotic cavity

Question 8

When and where does implantation occur?

Answer 8

1. Day 9
2. Endometrium of uterine wall

Question 9

What is the next thing to form after implantation? Where and how?

When?

Answer 9

1. The chorionic cavity (i.e. extraembryonic cavity)
2. The hypoblast produces extra cells that form a new cavity within the extracolaemic cavity - the new cavity is called the secondary yolk sac (note, much smaller than primary)
3. End of second week
4. Connecting stalk = made from extraembryonic mesoderm = connects embryo to the placenta

Question 10

What is the placenta?

Answer 10

1. Specialisation of the amniochorionic membrane
2. 2 parts: foetal placenta (develops from the syncytiotrophoblast) and maternal placenta (develops from maternal uterine tissue)

Question 11

How and when does the placenta form?

Answer 11

1. Lacunae begin to appear within the syncytiotrophoblast
2. Cells of syncytiotrophoblast erode endothelial lining of the maternal capillaries creating sinusoids so that maternal blood can enter the lacunae
3. Cytotrophoblast extend forming chorionic villi, invade the endometrium and allow transfer of nutrients from maternal blood to fetal blood
4. Note: within the villi comprised of trophoblast, we have mesoderm which gives rise to the CVS system (therefore fetal vessels developing within the villi) so now Oxygen can travel through the trophoblast, cross fetal capillary endothelium to get into fetal circulation.

Question 12

What is the chorionic frondosum?

Answer 12

1. The larger part of the placenta.
2. The other side is called the chorionic laeve