Placenta

Question 1

Why does establishment of the placenta take precedence in early embryonic development?

Answer 1

To ensure support for the pregnancy

Question 2

What is meant by haemomonochorial in relation to the placenta?

Answer 2

The placenta is haemomonochorial

One layer of trophoblast ultimately separates maternal blood from foetal capillary wall

Question 3

Give some information on implantation in regards to the placenta.

Answer 3

Implantation is interstitial
- The uterine epithelium is breached and the conceptus
implants within the stroma
The placental membrane becomes progressively thinner as the needs of the fetus increase

Question 4

What are the aims of implantation in regards to the placenta?

Answer 4

Establish the basic unit of exchange:
- Primary villi: early finger-like projections of trophoblast
- Secondary villi: invasion of mesenchyme into core
- Tertiary villi: invasion of mesenchyme core by fetal
vessels

Anchor the placenta:
- Establishment of the outermost cytotrophoblast shell

Establish maternal blood flow within the placenta

Question 5

Give a reference of timing to the different stages of placenta development.

Answer 5

Day 13 - Primary
Day 15-16 - Secondary
Day 23 - Tertiary

Question 6

Tell me a bit about implantation defects. (location)

Answer 6

Ectopic pregnancy
- Implantation at site other than uterine body (most
commonly fallopian tube.
- Can be peritoneal or ovarian
- Can very quickly become life-threatening emergency

Placenta praevia

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

Question 7

How does the endometrium prepare for implantation in regards to the placenta?

Answer 7

“pre-decidual” cells

Elaboration of spiral arterial blood supply

Question 8

Describe decidualisation.

Answer 8

The decidual reaction provides the balancing force for the invasive force of the trophoblast

• Ectopic pregnancy
• Conditions characterised by excessive invasion

Question 9

Describe the remodelling of the spiral arteries.

Answer 9

Creation of low resistance vascular beds
Maintains the high flow required to meet fetal demand
- Particularly late in gestation

Question 10

Tell me a bit about an implantation defect. (completion)

Answer 10

Incomplete invasion

• Placental insufficiency
• Pre-eclampsia (hypertension and high protein in urine)

Question 11

Tell me about twinning and it’s relationship to the placenta.

Answer 11

The degree to which membranes are shared in monozygotic twins can vary

You can have:

• Two amnions & two chorions
• Two amnions but a shared connection
• Amnion and chorion are both shared

Question 12

Describe the 1st trimester placenta.

Answer 12

Placenta established
Placental “barrier” is still relatively thick
Complete cytotrophoblast layer beneath syncytiotrophoblast

Question 13

Describe the term placenta.

Answer 13

Surface area for exchange dramatically increased
Placental “barrier” is now thin
Cytotrophoblast layer beneath syncytiotrophoblast

Question 14

Tell me about the umbilical cord vessels & the fetal circulation.

Answer 14

Two umbilical arteries
- Deoxygenated blood from fetus to placenta
One umbilical vein
- Oxygenated blood from placenta to fetus

Question 15

What are the functions of the placenta?

Answer 15

Metabolism:

• Placental synthesis of:
  
  • Glycogen
  • Cholesterol
  • Fatty acids

Endocrine:

• Hormones produced by the placenta are:
  
  • Protein
    
    • Human chorionic gonadotrophin (hCG)
    • Human chorionic somatomammotrophin
    • Human chorionic thyrotophin
    • Human chorionic corticotrophin
  • Steroid
    
    • Progesterone
    • Oestrogen

Transport:

• Simple diffusion
  
  • Molecules moving down a concentration gradient:
    
    • Water
    • Electrolytes
    • Urea & uric acid
    • Gases
  • Facilitated diffusion
    
    • Applies to glucose transport

Question 16

What is hCG?

Answer 16

Human chorionic gonadotrophin
Produced during the first 2 months of pregnancy
Supports the secretory function of the corpus luteum
Produced by syncytiotrophoblast therefore is pregnancy specific
Excreted in maternal urine therefore used as the basis for pregnancy testing
Trophoblast disease
- Molar pregnancy (hydatidiform mole)
- Choriocarcinoma

Question 17

What are the placental steroid hormones and what is their purpose?

Answer 17

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

Question 18

How do placental hormones influence maternal metabolism?

Answer 18

Progesterone
- Increased appetite
hCS/hPL
- Increases glucose availability to fetus

Question 19

Describe gas exchange across the placental membrane?

Answer 19

Simple diffusion

Flow-limited but not diffusion-limited

Fetal O2 stores are small therefore maintenance of adequate flow essential

Question 20

Describe active transport across the placenta.

Answer 20

Specific “transporters” expressed by the syncytiotrophoblast

• Amino acids
• Iron
• Vitamins

Question 21

Explain passive immunity.

Answer 21

Fetal immune system is immature
Receptor-mediated proves, maturing as pregnancy progresses immunoglobulin class-specific
- IgG only
- IgG concentration in fetal plasma exceed those in maternal circulation

Question 22

What are the consequences of the transport function of the placenta?

Answer 22

The placenta is not a true “barrier”
Teratogens access the fetus via the placenta
Unintentional outcomes from physiological process

Question 23

What drugs are harmful in pregnancy?

Answer 23

Thalidomide
Alcohol
Therapeutic drugs
Drugs of abuse
Maternal smoking

Question 24

What can go wrong with antibody transport in regards to the placenta and fetus?

Answer 24

Haemolytic disease of the newborn

• Rhesus blood group incompatibility of mother and fetus
• Now uncommon because of prophylactic treatment

Question 25

What infectious agents are a risk in fetus’?

Answer 25

Varicella zoster
Cytomegalovirus
Treponema pallidum
Toxoplasma gondii
Rubella