S6) Placental Function and Dysfunction

Question 1

In week 2 of embryological development, two distinct cellular layers emerge from the outer cell mass.

Identify them

Answer 1

• Synctiotrophoblast
• Cytotrophoblast

Question 2

What happens to the inner cell mass during week 2 of embryological development?

Answer 2

Inner cell mass becomes the bilaminar disk:

• Epiblast
• Hypoblast

Question 3

When does implantation begin and end?

Answer 3

• Commencement: day 6
• Completion: day 9

Question 4

What are the 2 cavities in the embryo?

Answer 4

• Amniotic cavity
• Yolk sac

Question 5

What has happened by the end of week 2 of embryological development?

Answer 5

• The conceptus has implanted
• The embryo and its two cavities are suspended by the connecting stalk within a supporting sac called the chorionic cavity

Question 6

What is the fate of the embryonic spaces after the end of week 2?

Answer 6

• Yolk sac disappears
• Amniotic sac enlarges
• Chorionic sac is occupied by the expanding amniotic sac

Question 7

What does implantation achieve?

Answer 7

• Establishes the basic unit of exchange
• Anchors the placenta
• Establishes maternal blood flow within the placenta

Question 8

Identify and describe the 3 forms of villi created by implantation

Answer 8

- 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 9

What is meant by the statement ‘implantation is interstitial’

Answer 9

Implantation is interstitial as the the uterine epithelium is breached and the conceptus implants within the stroma

Question 10

Briefly, how does the placenta change as the foetus develops?

Answer 10

The placental membrane becomes progressively thinner as the needs of the foetus increase

Question 11

What are chorionic villi?

Answer 11

- Chorionic villi are villi that sprout from the chorion to provide maximum contact area with maternal blood

• They contain foetal blood vessels

Question 12

What is the placenta?

Answer 12

The placenta is an organ, specialised from the chorionic membrane, that connects the developing foetus to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange

Question 13

Identify 2 things that can go wrong with implantation

Answer 13

• Implantation in the wrong place
• Incomplete invasion

Question 14

Identify 2 clinical conditions which can result from incomplete invasion

Answer 14

• Placental insufficiency
• Pre-eclampsia

Question 15

Identify 2 clinical conditions which result from implantation at an inappropriate site

Answer 15

• Ectopic pregnancy
• Placentae praevia

Question 16

Describe implantation in ectopic pregnancy and the consequences of such

Answer 16

• Implantation at site other than uterine body (peritoneal/ ovarian)
• Very quickly becomes life-threatening emergency

Question 17

Describe implantation during placenta praevia and the consequences of such

Answer 17

• Implantation in the lower uterine segment
• Can cause haemorrhage in pregnancy
• Requires C-section delivery

Question 18

Describe how the invasion during implantation is controlled

Answer 18

• Transformation of the endometrium in the presence of a conceptus
• Conceptus becomes the decidua
• The decidual reaction provides the balancing force for the invasive force of the trophoblast

Question 19

What is the decidual reaction?

Answer 19

• The decidual reaction is the set of changes in the endometrium of the uterus that prepare it for implantation of an embryo
• These changes include swelling of stromal cells due to accumulation of glycogen and other nutrients

Question 20

What happens if the decidual reaction is sub-optimal?

Answer 20

Sub-optimal decidual reactions can lead to a range of adverse pregnancy outcomes

Question 21

Describe the structure of the chorionic villus

Answer 21

Question 22

Over the course of pregnancy, the ‘placental’ barrier becomes thinner.

Describe this in terms of the chorionic villi

Answer 22

• First trimester villus: thicker barrier (blue)
• Third trimester villus: barrier at optimal “thinness” (pink)

Question 23

Identify the 2 classes of hormone produced by the placenta

Answer 23

• Protein
• Steroid

Question 24

Identify 2 steroid hormones produced by the placenta

Answer 24

• Progesterone
• Oestrogen

Question 25

Identify 4 protein hormones produced by the placenta

Answer 25

- Human chorionic **gonadotrophin** (hCG) - Human chorionic **somatomammotrophin** (hPL/hCS) - Human chorionic **thyrotrophin** - Human chorionic **corticotrophin**

Question 26

Describe the production, excretion and function of hCG

Answer 26

- **Production**: produced during first 2 months of pregnancy by synctiotrophoblast - **Excretion**: excreted in maternal urine - **Function**: mimics the action of LH and supports the secretory function of corpus luteum

Question 27

What do the placental steroid hormones do?

Answer 27

- Responsible for maintaining the pregnant state - Placental production takes over from corpus luteum by the 11th week

Question 28

Explain how placental hormones influence maternal metabolism

Answer 28

- **Progesterone** increases appetite - **hCS** increases glucose availability to foetus

Question 29

Identify and describe the transport functions of the placenta

Answer 29

- **Simple diffusion:** H₂O, electrolytes, gases, urea & uric acid - **Facilitated diffusion:** glucose

Question 30

Describe gas exchange in the placenta

Answer 30

- Simple diffusion - Flow-limited, not diffusion-limited

Question 31

Why is it important to maintain adequate uteroplacental circulation?

Answer 31

Foetal O₂ stores are small therefore maintenance of adequate uteroplacental circulation is essential

Question 32

There are specific “transporters” expressed by the syncytiotrophoblast. Identify 3 substances which are transported

Answer 32

- Amino acids - Iron - Vitamins

Question 33

The foetal (and newborn) immune system is immature and thus needs the transfer of passive immunity. Describe this

Answer 33

- **Passive immunity** is a receptor-mediated and immunoglobulin class-specific process involving the transfer of the IgG antibody only - IgG concentrations in foetal plasma exceed those in maternal circulation

Question 34

Identify 5 substances which can cross the placenta and their effects

Answer 34

- **Thalidomide**: limb defects - **Alcohol**: FAS and ARND - **Therapeutic drugs** *e.g. warfarin, ace inhibitors, anti-epileptic drugs* - **Drugs of abuse:** dependency in the fetus and newborn - **Maternal smoking:** smaller placenta, smaller babies

Question 35

Teratogens can access the foetus by crossing the placenta. Briefly highlight the effects of teratogenesis in the following period of development: - Pre-embryonic - Embryonic - Foetal

Answer 35

- **Pre**-**embryonic** – lethal effects - **Embryonic** – extremely sensitive (narrow windows for some systems) - **Fetal** – ± sensitive