The placenta

Question 1

Where does the placenta come from?

Answer 1

• Begins to develop in second week of development
• Early development there is focus on ensuring development of foetal membranes
• Cannot have a healthy pregnancy without a healthy placenta

Question 2

What are the foetal membranes?

Answer 2

• The sacs supporting the embryo/foetus
• The placenta

Question 3

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

Answer 3

• 2 distinct layers emerge from outer cell mass (syncytiotrophoblast and cytotrophoblast)
• And inner cell mass becomes bilaminar disc (epiblast and hypoblast)

Question 4

What happens on day 6 of embryo development?

Answer 4

• Implantation begins
• Move into endometrial layer (maternal tissue)
• Blastocoele is genetically different structure
• Must be communication between cells of foetus and cells of mother to allow implantation

Question 5

What happens on day 9 of embryo development?

Answer 5

• Early embryo is fully embedded in endometrium
• Placenta development is now vital to maintain integrity of embryo

Question 6

What has happened by the end of the 2nd week of embryo development?

Answer 6

• Conceptus has implanted
• Embryo has 2 cavities: amniotic cavity and yolk sac
• Sacs are suspended by a connecting stalk within chorionic cavity

Question 7

What is the fate of the embryonic spaces?

Answer 7

• Yolk sac disappears (part is pinched off to form primitive gut)
• Amniotic sac enlarges as embryo grows bigger
• Chorionic sac is occupied by expanding amniotic sac

Question 8

What does implantation achieve?

Answer 8

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

Question 9

What is the function of the different types of villi?

Answer 9

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

Question 10

Outline implantation

Answer 10

• Interstitial: uterine epithelium is breached and conceptus implants within stroma
• Placental membrane becomes progressively thinner as needs of foetus increase (large size of foetal brain requires more energy)
• Maternal blood separated from foetal capillary wall by layer of trophoblast
• 2 circulations never mix

Question 11

What is a chorionic villus?

Answer 11

• Placenta is a specialisation of chorionic membrane
• Finger-like projections of trophoblast
• Inner connective tissue core containing foetal vessels
• Very good for exchange
• Foetal vessels can actively receive nutrients across cell membranes

Question 12

What are the different types of implantation defects?

Answer 12

• Implantation in the wrong place (ectopic pregnancy, placenta praevia)
• Incomplete invasion (placental insufficiency, pre-eclampsia)

Question 13

Outline ectopic pregnancy

Answer 13

• Implantation at site other than uterine body
• Fallopian tube most common site
• Can be peritoneal or ovarian
• Quickly becomes life-threatening emergency

Question 14

Outline placenta praevia

Answer 14

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

Question 15

What controls how the embryo implants?

Answer 15

• Endometrium transforms to decidua in presence of conceptus
• Decidual reaction provides balancing force for invasive force of trophoblast

Question 16

What happens to the decidua in ectopic pregnancy?

Answer 16

• There is no decidua in fallopian tubes
• Therefore no control
• Trophoblast can invade into intraperitoneal region
• Causes haemorrhage

Question 17

What happens if decidual reaction is suboptimal?

Answer 17

• Can lead to a range of adverse pregnancy outcomes
• E.g. pre-eclampsia, preterm birth

Question 18

What is a cotyledon?

Answer 18

• Functional units containing chorionic villi
• Allows transport

Question 19

How does the chorionic villus change with time?

Answer 19

• First trimester villus creates thicker barrier
• Third trimester villus - barrier at optimal thinness to meet energy requirements of larger foetus
• Complete loss of syncytiotrophoblast
• Marginalisation of foetal capillaries so they sit right next to villus surface

Question 20

What are the umbilical cord vessels?

Answer 20

• 2 umbilical arteries bring deoxygenated blood from foetus to placenta
• 1 umbilical vein brings oxygenated blood from placenta to foetus

Question 21

What is the endocrine function of the placenta?

Answer 21

Produces:
- Human chorionic gonadotrophin
- Human chorionic somatomammotropin
- Human chorionic thyrotrophin
- Human chorionic corticotrophin
- Progesterone
- Oestrogen

Question 22

Outline the production of hCG

Answer 22

• Produced during first 2 months of pregnancy
• Supports secretory function of corpus luteum
• Produced by syncytiotrophoblast
• Therefore is pregnancy specific
• Excreted in maternal urine

Question 23

What is trophoblast disease?

Answer 23

• Cancer derived from trophoblast
• E.g. molar pregnancy and choriocarcinoma
• Leads to high hCG

Question 24

What is the function of the placental steroid hormones?

Answer 24

• Progesterone and oestrogen
• Responsible for maintaining the pregnant state
• Placental production takes over from corpus luteum by the 11th week

Question 25

Which placental hormones influence maternal metabolism?

Answer 25

- Progesterone leads to increased appetite - hCS increases glucose availability to foetus - This means that you can begin to see insulin resistance in mother - Foetus is prioritised and gets preferential glucose for energy

Question 26

Outline the transport functions of the placenta?

Answer 26

- Simple diffusion due to blood flow - e.g. water, electrolytes, urea, uric acid, gases - Facilitated diffusion (applies to glucose transport)

Question 27

Outline how gas exchange occurs over the placenta

Answer 27

- Simple diffusion - Flow-limited, not diffusion-limited - Foetal O2 stores are small therefore maintenance of adequate flow is essential

Question 28

How does active transport occur over the placenta?

Answer 28

- Specific transporters are expressed by the syncytiotrophoblast - Amino acids - Iron - Vitamins

Question 29

Outline how passive immunity is transferred from mother to foetus

Answer 29

- Foetal and newborn immune system is immature - Receptor-mediated process, maturing as pregnancy process - Immunoglobin class-specific - IgG only - IgG concentrations in foetal plasma exceed those in maternal circulation

Question 30

Outline the pathophysiology of placental transport

Answer 30

- Placenta is not a true barrier - Substances can pass across placenta out of our control - Teratogens can access foetus via placenta - Unintentional outcomes from physiological process - E.g. haemolytic disease of newborn secondary to Rhesus incompatibility of mother and foetus

Question 31

What substances can cross the placenta and harm the foetus?

Answer 31

- Thalidomide - limb defects - Alcohol - FAS and ARND - Anti-epileptic drugs - Warfarin - ACE inhibitors - Drugs of abuse - Maternal smoking

Question 32

Outline teratogenesis

Answer 32

- Timing is key - Pre-embryonic exposure has lethal effects - Embryonic period - embryo is highly sensitive - After embryonic period risk of structural defects is very low - Except CNS