The Placenta Anatomy, Physiology, Function

Question 1

What is Placenta Accreta?

What can incomplete invasion of the Conceptus lead to?

Answer 1

Invasion that is too deep

• Miscarriage
• Infertility
• Placental insufficiency (can lead to Pre-eclampsia)

Question 2

The placenta is the first structure that begins to develop.

In what week does development occur?

What is the Placenta?

Answer 2

Week 2

A specialisation of an area of Fetal Membranes surrounding the fetus

Question 3

Describe what happens as the Amniotic sac enlarges after implantation

(Amniotic sac is simply the Amniotic cavity, as is the Chorionic)

Answer 3

• Yolk sac disappears
• Chorionic sac is displaced
• Amniotic membrane fuses with chorionic membrane to produce a single amniotic cavity

Question 4

What is “water breaking” in labour?

Answer 4

Amniochorionic membrane breaks and amniotic fluid leaks out

Question 5

Initially, the outer surface of the Amniochorionic membrane has projections spread out over the entire surface.

How does this change as growth occurs?

Answer 5

Projections become concentrated into a single disc-like space, which goes on to become the placenta

Question 6

What does Implantation achieve?

Describe this in stages

Answer 6

Establishes the basic unit of exchange from the Conceptus: Villi

• Primary Villi: Early finger-like projections of trophoblast
• Secondary Villi: Invasion of mesenchyme into core
• Tertiary Villi: Invasion of Mesenchyme core by fetal vessels

(They also anchor the placenta)

Question 7

How do Tertiary Villi communicate with maternal blood flow?

Answer 7

Maternal vessels surround the villi

Question 8

Describe the barrier between Fetal and Maternal blood and how it changes as pregnancy progresses

Answer 8

• Initially, thick barrier with full layer of Cyto- and Syncytiotrophoblast.
• Barrier becomes progressively thinner via reduction in no. of Cytotrophoblast
• Eventually only 1 Trophoblast layer thick, but Circulations never mix

Question 9

Why does the Fetal-Maternal blood barrier thin as pregnancy progresses

Answer 9

Increased substance exchange, as fetal needs increase (mainly for brain development)

Question 10

What are Decidua?

How do they carry out their function?

Answer 10

• Endometrial cells that specialise to regulate invasion of the Conceptus once it has implanted
• Via Decidualisation, balanced by factors that promote and inhibit the Decidual reaction

Question 11

Why are Decidua clinically significant?

Answer 11

If implantation occurs where no Decidua are present (Ectopic pregnancy), no inhibition of Decidualisation occurs and Invasion is uncontrolled

Question 12

What can happen if the Decidual Reaction is sub-optimal?

Answer 12

• Pregnancy not maintained (Miscarriage , Infertility)

- Placental insufficiency (Including Pre-eclampsia)

Question 13

Name and describe the action of the maternal and fetal vessels

Answer 13

Maternal;

• Endometrial arteries + veins
• Bathe outside of the Villi

Fetal vessels;

• Bring waste to Villi via 2 Umbilical arteries
• Take O2+nutrients to fetus via 1 Umbilical vein

Question 14

On which side of the Placenta will we see the shiny Amnion and Chorionic Vessel?

What do we see on the maternal aspect?

Answer 14

Fetal aspect (facing towards fetus)

• Cobblestone appearance
• Cotyledons (Functional units, contain chorionic villi)

Question 15

Describe the Endocrine function of the Placenta

Answer 15

Protein hormones;
- Such as HCG, which sustains the Corpus Luteum in Trimester 1

Steroid hormones;

• After 1st Trimester, it takes over the role of the Corpus
• Produces Progesterone and Oestrogen (to keep HPG axis in a pregnancy state)

Question 16

Why is HCG known to be pregnancy-specific?

How is this significant clinically?

Answer 16

Produced by Syncytiotrophoblast

• Can be analysed in urinary and serum testing
• Raised in Trophoblast disease

Question 17

Describe the Maternal Metabolic changes caused by 3 Placental hormones

Answer 17

1. Progesterone- Causes increase in appetite, diverts glucose to fat synthesis
2. Oestrogen- Increase in PRL release
3. Human Chorionic Somatommaotrophin/ Placental Lactogen (hCS/ hPL);
   - Causes Insulin resistance in mother to direct glucose to fetus

Question 18

What are 3 ways transport occurs at the placenta

Suggest substances transported via each way

Answer 18

Simple diffusion;
- Water, gas, electrolytes, Urea and Uric acid

Facilitated diffusion;
- Glucose

Active transport;
- Amino acids, Iron, Vitamins

Question 19

Is Fetal-Maternal gas exchange flow limited or diffusion limited

Fetal O2 stores are small, what can happen if Uteroplacentalcirculation is inadequate?

Answer 19

Flow limited

‘Fetal distress’ (E.g Labour contractions-> Vessel contraction)

Question 20

Describe transfer of Immunity from mother to fetus

Answer 20

• Antibodies can be transported across
• Only IgG immunoglobulin can cross

(Fetal Plasma [IgG] > Maternal Plasma [IgG])

Question 21

When do Teratogen have their greatest effect?

Explain and describe the effects of Teratogens in the Fetal Period (weeks 9-36)

Answer 21

Embryonic period (weeks 3-8)

• Most symptoms have developed and only need to grow
• Effects tend to be less severe, other than in the CNS

Question 22

Why is it significant that pregnancy is considered to be an Immunocompromised state for the mother?

Answer 22

Infections can have more serious consequences in pregnancy

Question 23

Suggest an example of Physiological Teratogenesis and its effects

Answer 23

Blood group incompatibilities such as Rhesus disease;

• Maternal antigens cross barrier and attack Fetal RBCs

Question 24

List 5 Teratogens

Answer 24

• Thalidomide (Limb defects)
• Alcohol (FAS, ARND)
• Smoking
• Medications (Warfarin, ACE Inhibitors, Anti-epileptics)
• Drugs of abuse (Dependency in Fetus and newborn)