Embryology 2

Question 1

Day 5: What crucial step occurs on day 5 which will mean the embryo will start to grow.

Answer 1

Hatching. As the embryo is dividing, it moves from the ampulla into the uterus. At this point the Zona Pellucida is removed.

Question 2

Why is week 2 described as the week of 2s?

Answer 2

Both the embryoblast and trophoblast differentiate into 2 cell types.
The embryoblast differentiates into the epiblast and the hypoblast.
The trophoblast differentiates into cytotrophoblast and the syncytiotrophoblast.

Question 3

What is the role of syncytiotrophoblasts?

Answer 3

1. Hides the embryo from the mother’s immune system. It does this by secreting factors that hide the embryo from the mother’s immune system. The cells are multinucleated, the cells have very little space between. The immune cells can there not get between the cells to get to the embryo.
2. Anchor the embryo to the endometrium.
3. Establish the uteroplacental circulation.

Question 4

How does the embryo implant into the uterus?

Answer 4

The syncytiotrophoblasts bury there way into the endometrium and anchor to it. As a result, the embryo backs into the endometrium.

Question 5

What is the bilaminer disc?

Answer 5

The embryonic disc made up of the hypoblast and epiblast.

Question 6

How is a dorsal/ventral axis established in the embryo?

Answer 6

The epiblasts make up the dorsal surface and the hypoblasts make up the ventral surface.

Question 7

• Describe the process of full implantation of the blastocyst and the formation of the amniotic cavity and yolk sac

Answer 7

Now fully implanted, a coagulation plug forms - it becomes fibrous and heals over. The syncytiotrophoblasts have now covered the entire embryo. The amniotic cavity forms between the the epiblast cells.
The hypoblast migrate over the blastocyst cavity. Once the hypoblast have migrated around the whole cavity, the cavity is known as the primary yolk sac.

Question 8

How does the embryoblast receive nutrients without a placental circulation.

Answer 8

The syncytiotrophoblats encroach on the swollen uterine gland to gain nutrients from the mother to the embryo.

Question 9

How is the uteroplacental circulation set up?

Answer 9

The syncytiotrophoblasts break down the walls of the maternal capillary. The nutrients empty into trophoblastic lacuna before diffusing into the embryo.

Question 10

Where does the chorionic cavity form in week 2?

Answer 10

A new layer of connective tissue derived from the yolk sac cells forms between the primary yolk sac and cytotrophoblast – extraembryonic mesoderm. This also forms the connecting stalks which will go onto form the umbilical cord. In this the chorionic cavity forms. It is the largest cavity so far.

Question 11

What is the definitive yolk sac? How does it form?

Answer 11

The hypoblasts cell continue to migrate and push the old cells out of the way. Once a new layer of hypoblasts surrounds the cavity, it is known as the definitive (secondary) yolk sac.

Question 12

Describe the production of human chorionic gonadotrophin and its importance in pregnancy testing

Answer 12

Syncytiotrophoblasts secrete hCG. Since there are many of these cells, the concentration of hCG reaches so high, it is excreted in urine. As a result this can be used to detect a pregnancy. The concentration of hCG doubles every 48 hours initially.

Question 13

Where is the normal implantation sites for an embryo? Give examples of abnormal sites? What is the effect of abnormal implantation?

Answer 13

An embryo will usually implant in the endometrium.
Abnormal sites include:
- Mesentery (The embryo can implant here as a fertilised egg can drop between the space between the fimbriae and the ovary. It can then drop into the peritoneal cavity)
- Cervix
- The Fallopian tube
- The Ampulla

There will reach a point in development and cannot survive. This can get so big, it can rupture the uterine tube. The mother’s life is therefore here at risk as she is haemorrhaging due to the connection of blood supplies. An implantation of the embryo outside the uterus is known as an ectopic pregnancy. This can be picked up in prenatal screening.

Question 14

What is placenta pervia?

Answer 14

Placenta praevia is when the placenta attaches inside the uterus but near or over the cervical opening. The position of the baby can rupture the placenta and endanger the mother and baby.

Question 15

What is a lithopedion?

Answer 15

It is a rare phenomenon which occurs most commonly when a foetus dies during an abdominal pregnancy, is too large to be reabsorbed by the body, and calcifies on the outside as part of a foreign body reaction, shielding the mother’s body from the necrotic tissue, thereby preventing infection.

Question 16

Describe the formation of a hydatidiform mole.

Answer 16

When an empty egg (egg without a nucleus) is fertilised by a sperm. This leads to a blastocysts made up of only trophoblasts. This suggests that paternal genes may be largely involved with the production of supportive structures. Since there are syncytiotrophoblasts, there is still a large production of hCG and so a hydatidiform mole gives a false positive on a pregnancy test.