placenta/pregnancy 2 Flashcards
why are spiral arteries crucial for the success of pregnancy
spiral arteries carry the maternal blood which perfuses the placenta and placenta extracts oxygen and nutrients for the fetus
-haemochorial nutrition
what does the embryo eat while the spiral arteries are plugged
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
define villous
branch of the placenta
define villous cytotrophoblast
trophoblast progenitor cell type found mainly in the 1st trimester underlying the syncytiotrophoblast
define syncytiotrophoblast
surface layer of the placenta formed by fusion of villicytotrophoblast
syncytotrophoblast doesnt replicate but is replaced by fusion of additional villicytotrophoblast
extravillous cytotrophoblast
differentiated cells that have migrated out of the villous placenta towards the maternal tissues
structural changes in the placenta with gestational age
- during early pregnancy stroma of the villi become more cellular and more vascularised
- during 2nd trimester villous cytotrophoblast thins down
- during 3rd trimester viloous cytotrophoblast is spare
- branching of villi increases
- size of placenta increases
diameter of pacenta throughout
12 weeks = 56mm
38 weeks = 220mm and 0.5kg
types of decidua
decidua basalis - decidua underlying the implantation site
decidua capsularis - decidua overlying the implantation site
decidua peritalis - decidua around the remainder of the uterus
maternal contributions to the placenta
as gestation progresses the amniotic cavity enlarges obliterating the uterine cavity
the decidua capsularis then fuses with the decidua peritalis
placental membranes
amnion - avascular covers cord and placenta (closes to baby)
chorion - fetal vessels (sits next to amnion)
umbilical cord
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)
importance of whartons jelly
keeps cord turgid and prevents compression of vessels in umbilical cord
placenta adaptations to increase transport
- villous structure is tortuous with a large surface area
- syncytiotrophoblast has a microvillous surface (slows down movement of maternal blood across placenta)
- in 3rd trimester most villi are small tertiary villi
- in the 3rd trimester the fetal capillaries are closely apposed to the syncytiotrophoblast
fetal Hb
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
the Bohr effect
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
the haldane effect
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
function of amniotic fluid
- bouyant medium allows symmetric growth
- cushions the embryo/fetus
- prevents adhesions of the fetus with the membranes
- allows the fetus to move - muscle development
- development of GI/respiratory tracts - breathing and swallowing (practise movements)
origins of amniotic fluid
- initially ultrafiltrate of maternal plasma
- major fetal contribution
- 20+ weeks fetal urine and surface of placenta and cord
volumes of amniotic fluid over time
10 weeks = 30mls
35 weeks = 1L
human fetal urine output estimated at approx 500-1200mls/day
amniotic fluid recycling
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
polyhydramnios
excessive amniotic fluid possibly due to loss of swallowing
found in many cases of diabetic pregnancy
oligohydramnios
lack of amniotic fluid potentially due to kidney problems
clinical importance of amniotic fluid
amniocentesis
chorionic villi sampling
used to screen for anomalies
what does the placenta usually prevent transmission of
HepB
Rabies
Measles
Malaria - can cause clogging of placenta
what is the placenta not a barrier against
HIV
small pox and other related viruses
Rubella
Toxoplasmosis - from cats and raw meats
drugs in pregnancy
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
