placenta/pregnancy 2

Question 1

why are spiral arteries crucial for the success of pregnancy

Answer 1

spiral arteries carry the maternal blood which perfuses the placenta and placenta extracts oxygen and nutrients for the fetus

-haemochorial nutrition

Question 2

what does the embryo eat while the spiral arteries are plugged

Answer 2

uterine gland contains glandular ‘milk’

• this is secreted into the intervillous space
• consumed as a major energy source in the first few weeks of pregnancy after initial implantation

= histiotrophic nutrition

Question 3

define villous

Answer 3

branch of the placenta

Question 4

define villous cytotrophoblast

Answer 4

trophoblast progenitor cell type found mainly in the 1st trimester underlying the syncytiotrophoblast

Question 5

define syncytiotrophoblast

Answer 5

surface layer of the placenta formed by fusion of villicytotrophoblast
syncytotrophoblast doesnt replicate but is replaced by fusion of additional villicytotrophoblast

Question 6

extravillous cytotrophoblast

Answer 6

differentiated cells that have migrated out of the villous placenta towards the maternal tissues

Question 7

structural changes in the placenta with gestational age

Answer 7

1. during early pregnancy stroma of the villi become more cellular and more vascularised
2. during 2nd trimester villous cytotrophoblast thins down
3. during 3rd trimester viloous cytotrophoblast is spare
4. branching of villi increases
5. size of placenta increases

Question 8

diameter of pacenta throughout

Answer 8

12 weeks = 56mm

38 weeks = 220mm and 0.5kg

Question 9

types of decidua

Answer 9

decidua basalis - decidua underlying the implantation site

decidua capsularis - decidua overlying the implantation site

decidua peritalis - decidua around the remainder of the uterus

Question 10

maternal contributions to the placenta

Answer 10

as gestation progresses the amniotic cavity enlarges obliterating the uterine cavity

the decidua capsularis then fuses with the decidua peritalis

Question 11

placental membranes

Answer 11

amnion - avascular covers cord and placenta (closes to baby)

chorion - fetal vessels (sits next to amnion)

Question 12

umbilical cord

Answer 12

formed from the yolk sac and allantois

vessels of the cord are derived from the allontois

• 2 arteries
• 1 vein (carries oxygenated blood)
• between the 3 vessels is whartons jelly
  - consists of a network of myofibroblasts and spaces are filled with mucopolysaccharides (*keeps cord turgid)

Question 13

importance of whartons jelly

Answer 13

keeps cord turgid and prevents compression of vessels in umbilical cord

Question 14

placenta adaptations to increase transport

Answer 14

1. villous structure is tortuous with a large surface area
2. syncytiotrophoblast has a microvillous surface (slows down movement of maternal blood across placenta)
3. in 3rd trimester most villi are small tertiary villi
4. in the 3rd trimester the fetal capillaries are closely apposed to the syncytiotrophoblast

Question 15

fetal Hb

Answer 15

adapted to ‘suck’ oxygen from maternal blood

fetal Hb has a greater affinity for oxygen

at a standard pO2

• fetal = 80% saturated
• adult = 50% saturated

fetal blood has more Hb and can carry more oxygen than maternal

Question 16

the Bohr effect

Answer 16

as maternal blood picks up fetal metabolites the pH lowers
-thus, the affinity for O2 decreases and dissociation of O2 increases

the converse occurs of the fetal side
-double Bohr effect

Question 17

the haldane effect

Answer 17

capacity of Hb to bind CO2 is related to the amount of bound O2
-thus, if O2 is lost from maternal blood capacity of maternal blood for CO2 increases

the converse occurs of the fetal side
-double Haldane effect

Question 18

function of amniotic fluid

Answer 18

1. bouyant medium allows symmetric growth
2. cushions the embryo/fetus
3. prevents adhesions of the fetus with the membranes
4. allows the fetus to move - muscle development
5. development of GI/respiratory tracts - breathing and swallowing (practise movements)

Question 19

origins of amniotic fluid

Answer 19

1. initially ultrafiltrate of maternal plasma
2. major fetal contribution
3. 20+ weeks fetal urine and surface of placenta and cord

Question 20

volumes of amniotic fluid over time

Answer 20

10 weeks = 30mls

35 weeks = 1L

human fetal urine output estimated at approx 500-1200mls/day

Question 21

amniotic fluid recycling

Answer 21

fluid leaves the amniotic cavity mainly by fetal swallowing

fluid can also move across the fetal skin (prior to keratinisation at 24 weeks)

fluid can move across the fetal membranes into the maternal circulation or into the fetal vessels of the placenta and umbilical cord

Question 22

polyhydramnios

Answer 22

excessive amniotic fluid possibly due to loss of swallowing

found in many cases of diabetic pregnancy

Question 23

oligohydramnios

Answer 23

lack of amniotic fluid potentially due to kidney problems

Question 24

clinical importance of amniotic fluid

Answer 24

amniocentesis

chorionic villi sampling

used to screen for anomalies

Question 25

what does the placenta usually prevent transmission of

Answer 25

HepB
Rabies
Measles
Malaria - can cause clogging of placenta

Question 26

what is the placenta not a barrier against

Answer 26

HIV
small pox and other related viruses
Rubella
Toxoplasmosis - from cats and raw meats

Question 27

drugs in pregnancy

Answer 27

ethanol - crosses the placenta may cause FAS

recreational drugs - may cause IUGR and development delays

paracetamol & aspirin - crosses placenta but it safe

heparin - doesnt cross placenta

warfarin - crosses placenta causes fetal malformations