Placenta

Question 1

Describe the implantation of the placenta. (6)

Answer 1

Implantation begins on day 6 of the pregnancy, when the trophoblast cells interact with the endometrial lining following hatching from the zona pellucida. The blastocyst becomes embedded in the endometrium to interact with the vasculature and secretory glands.
This should occur on the superior posterior body of the uterus.

Question 2

Explain 3 clinical conditions relating to implantation. (6)

Answer 2

Outside the endometrium - ectopic - trophoblast invades through the wall of wherever it is. 
Lower uterine segment / over internal os - placenta previa
Placental insufficiency (in this case from incomplete invasion) - pre-eclampsia is the hypertensive response to this because the placenta panics due to its low blood supply and raises systemic blood pressure.

Question 3

Describe the development of the placenta. (4)

Answer 3

Initially, the embryo and it’s cavities (amniotic and yolk sac) are suspended from the connecting stalk within the chorionic cavity.
The amniotic sac enlarges and fuses with the chorionic cavity to become the amniotic cavity.
Projections around the outer surface of the membrane of the amniotic cavity (where the connecting stalk attaches) thicken and form a disc, which will become the placenta.

Question 4

Describe the development of the foetal circulation, and the connections between maternal and foetal blood. (3)

Answer 4

Chorionic villi emerge from the trophoblast with a syncytiotrophoblast outer layer and a connective tissue core. This is where foetal blood develops.
Maternal blood vessels surround these villi, allowing exchange to occur with no blood contact.

Question 5

Describe how the endometrium regulates invasion during implantation. (1)

Answer 5

The cells of the endometrium are specialised to modulate the degree of invasion though decidualisation.

Question 6

Explain why ectopic pregnancies can lead to life threading haemorrhages. (3)

Answer 6

If the invasion is occurring somewhere other than the endometrium, there is no decidual reaction present, so invasion is unchecked. This can invade into blood vessels and cause haemorrhage

Question 7

Explain the importance of the endocrine functions of the placenta. (6)

Answer 7

Produces hCG which maintains the corpus luteum in the first trimester - produces oestrogen and progesterone.
Produces oestrogen and progesterone in the later trimesters to keep the HPG axis in a ‘pregnancy state’ to prevent ovulation.
Produces human placental lactogen which can create gestational diabetes to increase the amount of glucose available to the foetus.

Question 8

Explain the importance of maintaining good placental blood flow. (3)

Answer 8

The gas exchange in the placenta is the foetus’ only way to get oxygen, and is flow limited, so any reduction in blood flow will limit the amount of oxygen. Can lead to starving of oxygen if placental vessels are compressed eg in labour.

Question 9

Explain the advantages and disadvantages of the placenta being really effective at exchanging substances between maternal and foetal blood. (4)

Answer 9

Advantages - easier for gas exchange, IgG transfer of immunity for the foetus.
Disadvantages - teratogens like drugs or alcohol can cross the placental barrier and cause damage to the foetus, especially in the first trimester.

Question 10

Explain why pregnancy is classed as an immunocompromised state for the mother. (1)

Answer 10

Immune system damped down so we don’t reject the baby.