The Placenta

Question 1

Which cells of the blastocyst develop into the fetal membranes? In which week of embryology does this occur?

Answer 1

Synctiotrophoblast and Cytotrophoblast

Occurs in the week of 2’s

Question 2

Which cells produce hcG?

Answer 2

Syncytiotrophoblast

Question 3

How does communication between the endometrium and the conceptus start?

Answer 3

Syncytiotrophoblast cells being the communication with the endometrium at day 6 of pregnancy

Question 4

What happens to the yolk sac, amniotic sac and chonrionic sac as the embryo grows?

Answer 4

• Yolk sac dissapears
• Amniotic sac enlarges
• Chorionic sav occupies by expanding amniotic sac

Question 5

What are chorionic villi?

Answer 5

Projections from the trophoblast that establish an inital unit of exchange with the endometrium and anchor the placenta

Material blood vessels surround the villi to allow the exchange to occur

Question 6

Distinguish between primary, secondary and tertiary villi?

Answer 6

• Primary villi - early finger like projections of trophoblast
• Secondary villi - invasion of mesenchyme into core
• Tertiary villi - invasion of mesenchyme core by fetal vessels

Question 7

How does the barrier between fetal and maternal circulations change throughout pregnancy?

Answer 7

1st Trimester- thick barrier with a full layer and cytotrophoblast and syncytiotrophoblast

As pregnancy progesses → placental membrane becomes progessively thinner as the metabolic demands of the fetus increase

Eventually one layer of trophoblast separates maternal and fetal blood

Question 8

What defects can occur in implantation?

Answer 8

• Implantation in the wrong place
  
  • ​ectopic
  • placenta praevia
• Incomplete invasion
  
  • placental insufficiency
  • pre-eclampsia

Question 9

Explain the main principles of ectopic pregnancies and placenta praevia

Answer 9

Question 10

How is the implantation invasion controlled?

Answer 10

The Decidual reaction provides a balancing force for how far the trophoblast can invade

Question 11

With regards to decidual reaction, why is ectopic pregnancy so dangerous?

Answer 11

There is no decidual reaction outside of the uterus, therefore in ectopic pregnancies invasion goes unchecked leading to haemorrhage

Question 12

What happens if the decidual reaction is sub- optimal?

Answer 12

If invasion is not deep enough the pregnancy may not be maintained or there is a spectrum of placental insufficiency including pre-eclampsia

Question 13

What blood vessles exist in the umbilical cord?

Answer 13

• 2 umbilical arteries - carry deoxygenated blood from fetus → placenta
• 1 umbilical vein - carries oxygenated blood from placenta → fetus

Question 14

What hormones are produced by the placenta?

Answer 14

Protein hormones:

• hCG
• human chorionic somatomammotrophin
• human chorionic thyrotrophin
• human chorionic corticotrophin

Steroid hormones:

• progesterone
• oestrogen

Question 15

What is the function of Human Chorionic Gonadotrophin (hCG) and where is it produced?

Answer 15

• Produced by the syncytiotrophoblast in the 1st 2 months of pregnancy
• Supports the secretory function of the corpus luteum in the 1st trimester

Question 16

Why does the placenta produce steroid hormone?

Answer 16

Placenta secretes oestrogen and progesterone

Maintains the pregnancy state → takes over production from the corpus luteum at week 11

Question 17

How can placental hormones influence maternal metabolism?

Answer 17

• Progesterone → increases appetite to lay down fat stores
• hCS/ hPL → makes mother insulin resistant to increase glucose available for the fetus

Question 18

How do molecules move across the placenta?

Answer 18

• Mostly by simple diffusion: water, electrolytes, urea & uric acid and gases
• Glucose moved by facilitated diffusion
• Active transport: amino acids, iron, vitamins

Question 19

How is gas exchange limited across the placenta?

Answer 19

Flow limited, not diffusion limited

Therefore adequate flow is essential to maintain fetal O₂ stores

Question 20

Which class of maternal antibodies can diffuse across the placenta?

Answer 20

IgG only

Transported by receptor mediated endocytosis

Question 21

Which harmful substances can cross the placenta?

Answer 21

• Thalidomide
• Alcohol
• Therapeutic drugs: anti-epileptic, Warfarin, ACE inhibitors
• Drugs of abuse
• Maternal smoking

Question 22

At which stage in embryological development is teratogenesis the most harmful?

Answer 22

Weeks 3-8, in the embryonic stage

Events less severe from weeks 9 onwards