5.3 Role of the Placenta

Question 1

What is the foetal origin of placenta called (8-10 days post conception)?

Answer 1

Chorion frondosum (consists of trophoblast cells and a layer of extra-embryonic mesoderm)

Question 2

What is the maternal origin of placenta called (8-10 days post concepetion) ?

Answer 2

Decidua (altered uterine lining in pregnancy → known as decidua basalis at site of implantation)

Question 3

[Days 8-10 post conception]
The trophoblast consists of the cytotrophoblast (inner) and syncytiotrophoblast (outer):

Cytotrophoblast: Consists of stem cells → give rise to new trophoblast cells:
• May differentiate into more specialised cytotrophoblast cells (e.g. interstitial, endovascular)
• May fuse together (losing their cell boundaries) to form the _____________

Syncytiotrophoblast: Multinucleated mass from fusion of cytotrophoblast cells:
• Invades the endometrium → forms ________________
• Main source of human chorionic gonadotrophin (hCG) → maintains the _________________ (allowing ongoing hormone production to support early pregnancy)

Answer 3

syncytiotrophoblast;

chorionic villi ;

corpus luteum

Question 4

From days 12 to 15 post-conception, the primary chorionic villi (finger-like projections with inner cytotrophoblast and outer syncytiotrophoblast) begin to form:
• Serve as exchange between foetal and maternal blood
• Endometrial stromal cells (decidual cells with ____________________ shape) fill up with glycogen and lipids → supply energy to the trophoblast cells
• Lacunae within the syncytiotrophoblast fill with maternal blood (due to invasion)

Answer 4

polyhedral

Question 5

[Days 20-21 post conception]
The lacunae fuse to form the lacunar network (becomes intervillous space of the placenta), which bathes the chorionic villi in maternal blood.

Secondary (early week 3): Layer of __________________ grows within the centre of the primary chorionic villi (finger-like projections of the trophoblast)

Tertiary (late week 3): Foetal blood vessels develop from the mesodermal core within the secondary villi (i.e. foetal blood in villus; maternal blood around villus):
• Villi surrounds the conceptus (foetus)

Anchoring (from tertiary): Cytotrophoblast cells from some of the tertiary villi grow towards the decidua basalis to form the cytotrophoblast shell (layer connecting the ____________________) → anchoring/stem villi:
• Branch into intermediate then terminal villi
• Branch villi float in the
________________→ platform for exchange between the foetal vessels and maternal blood
• Further penetration of the decidua is controlled by immunological and chemical mechanisms
• Nitabuch’s layer: zone of _____________________

Answer 5

extra-embryonic mesoderm;

villi to the decidua basalis;

intervillous space ;

fibrinoid degeneration between the compact and spongy layers of the decidua basalis

Question 6

[Day 21- 4 months post conception]

From day 21 up to 4 months of embryonic development, villi surrounding different parts of the foetus develop differently

• On surface of decidua basalis: Continue growing → ___________________
• Covering rest of conceptus: Degenerate → ___________________

Cytotrophoblast cells invade and alter the initially tightly coiled spiral arteries of the uterus to create the blood supply of the placenta:

Changes
- 3 – 12 weeks: Invasion of the arteries at the level of the decidua
- 14 – 16 weeks: Invasion further into the ________________
- By 20 weeks: Process completed
• Invasion causes destruction of smooth muscle and elastic fibres in the arterial wall and the endothelium is replaced by ____________________
• Transforms arteries from tightly coiled spiral-shaped vessels into ________________ → increased blood flow to the intervillous space of the placenta

Answer 6

placenta (chorion frondosum);

chorion laeve (fuses with the decidua parietalis lining on the opposite side of the uterus in week 20 to obliterate the uterine cavity);

upper third of the myometrium;

endovascular trophoblast cells;

dilated low resistance high capacitance funnel-shaped flaccid vessels

Question 7

how does the foetal side of the placenta look like?

Answer 7

• Appears smooth (covered in amnion; foetal vessels lie underneath amnion)
• Split into 15 – 28 lobules (foetal cotyledons)
• Umbilical cord: about 50cm long and 1.5cm in diameter at term (contains 2 arteries and 1 vein) → originally 2 veins (1 remains and becomes dilated; the other disappears around 6 weeks post-conception)

Question 8

How does the maternal side of placenta look like?

Answer 8

• Appears rough (not covered in amnion
• Split into 20 lobes (maternal cotyledons) separated by decidual septa
• Decidual septa: folds of the decidual plate which form due to regional variations in the growth (due to traction on the decidual plate caused by anchoring villi which have a relatively slow growth rate)

Question 9

Umbilicard cord: The two umbilical arteries wrap around the large umbilical vein in a spiral fashion → arterial pulsations help to massage blood back along the umbilical vein:
• Coiling helps to protect from tension and compression of the cord
• Vessels are packed and protected by _____________________

Abnormal umbilical artery regression: one of the umbilical arteries may regress, leaving only 2 vessels in the umbilical cord

• Number of vessels in the cord is counted at a screening ultrasound and at delivery
• 2 vessels in the cord may be a sign of ________________

Answer 9

Wharton’s jelly (thick mucinous substance;

other foetal abnormalities (especially of the heart)

Question 10

The __________________ are the functional units of the placenta (each containing one anchoring villus and its branches):
• Presence of anchoring villi stabilises villous trees → branches into 3 – 5 intermediate villi then 10 – 12 terminal villi
o Contain foetal vessels which receive foetal blood from chorionic vessels
o ______________ float freely in the intervillous space (main site of exchange)
• ___________________within the villi regress as gestation progresses
• _______________________ s occupy most of the cross-sectional area of the villus with presence of microvilli on the surface of chorionic villi → maximise surface area
• At term: only a thin layer of syncytiotrophoblast and a few scattered cytotrophoblasts in terminal villi separate the foetal capillaries from maternal blood

Answer 10

foetal cotyledons;

Terminal villi;

Inner cytotrophoblast layer;

Foetal capillaries and sinusoid

Question 11

The placental bed is composed of the ________________ lying below the placenta:
• Invaded by ______________ cells → changes to the endometrial cells and spiral arteries
• Contains cells of the maternal immune system (leukocytes, macrophages, granular lymphocytes) → important role in implantation and placentation

Answer 11

decidua and myometrium;

trophoblast

Question 12

[Maternal placental circulation]

Maternal systole: blood spurts at high pressure (~80mmHg) from more than _________________ into the intervillous space
• Forces the blood up towards the chorionic plate → hits the chorionic plate → dissipates blood laterally and slows the flow
• Blood then pours down again with reduced force → bathes _________ (allows time for exchange)
• Drains through _________________
• Factors affecting maternal blood flow: changes in spiral arteries, maternal arterial BP, intrauterine pressure, presence and pattern of ___________________ (obstructs venous outflow)

Answer 12

100 spiral arteries;

villi;

veins in the decidual/basal plate;

uterine contractions

Question 13

[Foetal Placenta Circulation]
Foetal blood flow (~110 – 115mL/kg; __________________ in the placental circulation) occurs via the umbilical cord:
• Umbilical arteries: carry __________________ → branch several times in the placenta → tiny network of capillaries in the chorionic villi
• Umbilical vein: ______________ enters the venous network in the villi and carried to the foetus via the umbilical vein

Answer 13

40% of foetal cardiac output;

deoxygenated blood from the foetus to the placenta;

oxygenated blood

Question 14

Effective gas transfer is essential within the placenta as it acts as the point of respiration for the foetus (lungs are non-functional in utero):
• Facilitated by partial pressure gradient, Hb oxygen affinity and double Bohr effects

Partial pressure gradient: Oxygen is a small molecule which moves by simple diffusion → direction of movement is influenced by ________________
• Maternal blood entering the intervillous space has ____________________ → uptake of O2 causes drop to ______________ → travels via umbilical vein (~30mmHg)

High O2 affinity of HbF: Foetus achieves _______________ despite low pO2 + lower affinity for CO2 than maternal Hb → encourages exchange

Double Bohr effect
Bohr effect: Hb has reduced ability to bind O2 under ________________________

Others

• High cardiac output of foetus (relative to its size)
• High HbF concentration (50% more than adult)
• Acid-base balance: H+, HCO3-, lactic acid can diffuse across the placenta → foetus can buffer excess acid with HCO3- reserves (except in __________________ → foetal acidosis and distress)

Answer 14

pressure gradient;

pO2 of ~100mmHg (97% saturation);

~35mmHg (65%);

~85% saturation;

acidic conditions (due to displacement by high CO2)

significant maternal acidosis e.g. ketoacidosis, dehydration in labour

Question 15

What are the nutrients tat transfer via simple diffusion?

Answer 15

Water, O2, fatty acids, fat-soluble vitamins (A, D, E, K), urea, CO2

Question 16

What are the nutrients tat transfer via facilitated diffusion?

Answer 16

Glucose, lactate

Question 17

What are the nutrients tat transfer via active transport?

Answer 17

Amino acids, fatty acids, iron, water-soluble vitamins, calcium, phosphate

Question 18

What are the nutrients tat transfer via endocytosis?

Answer 18

Very large substances (e.g. antibodies, LDL, hormones)

Question 19

Glucose is the major source of energy for the foetus (serves 90% of its needs) → ________________ becomes progressively thinner while placental surface area increases with gestation:
• Increased quantity of glucose is transferred as gestation/foetal demand increases
• At term: ~10g of glucose/kg body weight is required everyday → excess is converted into glycogen (stored in ______________) and fat (deposited around ___________________)
o Provides energy for foetal metabolism and regulates _________________ after birth → preterm babies lack these stores (more prone to complications)

Complex nutrients are broken down into simple components before transfer (e.g. proteins and fats broken into amino acids and fatty acids):
• Reconstituted from their components in the chorionic villi

Answer 19

trophoblast layer;

foetal liver;

heart and scapula;

body temperature

Question 20

The rate of transfer of drugs is governed by the drug characteristics and placental factors:
• Greatest risk of teratogenicity is during ___________________ → risk of structural defects is much lower after this time (but some drugs are still harmful to the foetus in other ways)

Answer 20

organogenesis (between 17 – 20 days)

Question 21

What hormones are synthesised by the placenta to maintain decidua?

Answer 21

oestrogen, progesterone

Question 22

What hormones are synthesised by the placenta to stimulate mammary glands?

Answer 22

oestrogen, HCS, HCG

Question 23

What hormones are synthesised by the placenta to maintain corpus luteum?

Answer 23

HCG

Question 24

What hormones are synthesised by the placenta to inhibit uterine contractions?

Answer 24

progesterone

Question 25

What hormones are synthesised by the placenta to promote energy release for foetus?

Answer 25

HCS

Question 26

What hormones are synthesised by the placenta to soften cervix and pelvic ligaments?

Answer 26

relaxin

Question 27

how does the placenta protect from pathogens?

Answer 27

Maternal IgG is able to cross the placenta to protect foetus:
• Most bacteria are too large to cross the placenta → those that try are often engulfed by macrophages
• Some viruses are able to cross the placenta → dealt with by the maternal IgG in the foetal circulation

Question 28

how does the placenta prevent immune rejection?

Answer 28

Poorly understood mechanism; involves two parts:
• Trophoblast cells are involved in disguising the foetus from the maternal immune system
• hCG has immunosuppressive effect → prevents recognition of the conceptus as a foreign body

Question 29

[Placental abnormalities]
Abnormal Placentation: The umbilical cord inserts into the placenta centrally → clinically significant if very off-centre

• Marginal insertion: Insertion of the cord ___________________
• Velamentous insertion: Cord vessels run in the _________________ → leaves vessels exposed in the membrane without protection of ______________
  • Vessels become more vulnerable to rupture (especially if there is associated vasa previa)
• Vasa previa: Vessels run in the membranes over the cervical os → may lead to stillbirth due ____________________
• _________________: Rare; placenta has accessory lobe separated by membranes
• Placenta circumvallate: Placenta is smaller than the ____________ → trophoblast invades the decidua further and deeper laterally → _______________ of the placental surface:
  • Usually of no clinical consequence (but may occasionally lead to antepartum/intrapartum haemorrhage)

Answer 29

at the edge of the placenta;

foetal membranes before inserting into the placenta;

Wharton’s jelly;

to haemorrhage with spontaneous/artificial rupture of the membranes

Placenta succenturia;

chorionic plate;

ridged appearance

Question 30

[Placental abnormalities]

Location
- Placenta previa: insertion in the _________________ → antepartum/postpartum haemorrhage
• Contraindication for vaginal delivery if ____________

Penetration Placenta accreta/increta/percreta (depending on depth of invasion) → massive haemorrhage as placenta does not separate easily at delivery
•______________: chorionic villi enter the myometrium
• _________________: invasion deeper into the myometrium
• ____________________: invades through myometrium (even into bladder/rectum)

Answer 30

lower segment of the uterus covering or very close to the cervical os;

placenta <2cm from os;

Placenta accreta;

Placenta increta;

Placenta percreta