function of the placenta

Question 1

structure of spiral arteries

Answer 1

• highly coiled, allowing for expansion
• highly muscular

Question 2

describe spiral artery remodelling

Answer 2

• EVTs (extravillous trophoblast cells) migrate from anchoring villi and invade into spiral arteries
• invasion of EVTs into spiral arteries > loss of vascular smooth muscle and endothelial cells lining them, leaving a fibrinoid material
• results in vessel dilation and loss of vasoreactivity > slower blood flow and less blood pressure

Question 3

how do EVTs reach spiral arteries

Answer 3

• migrate through decidua (interstitial)
• migrate through spiral arteries (endovascular)

Question 4

what happens when trophoblast is too invasive

Answer 4

accreta: attach to myometrium
increta: further into myometrium
percreta: into & beyond uterus, maybe to other organs e.g. bladder

Question 5

structure of villi in early placenta

Answer 5

• low SA:V because minimal branching
• surrounded by outer syncytiotrophoblast and inner cytotrophoblast
• within stroma core, a few small calibre fetal capillaries
• surrounded by intervillus space filled with uterine gland secretions and a low level of maternal blood

Question 6

describe the structure and properties of the placenta villi in the later half of pregnancy

Answer 6

1. nuclei of syncytiotrophoblast clustered so remaining portion of the cell = thin diffusional barrier
2. cytotrophoblast no longer consistent
3. terminal villi increasingly formed in 2nd half of gestation and highly vascularised
4. fetal capillaries not in centre core of villi but directly abut trophoblast to reduce diffusional distance

Question 7

how the placenta increases diffusion rate

Answer 7

• villus surface area 12-14m2, amplified 7x by microvilli on outer syncytiotrophoblast cell membrane
• diffusional barrier in villi is 2-3um thick, composed of syncytiotrophoblast + fetal endothelium

Question 8

do all nutrients and gases go to the fetus

Answer 8

no, placenta consumes at least 30% of total O2

Question 9

oxygen exchange in the placenta

Answer 9

• oxygen readily diffuses across because maternal circulation has a higher oxygen tension
• and CO2 diffuses from fetus > mother because higher tension
• but saturation & content similar

Question 10

how is oxygen saturation and content similar

Answer 10

• fetal Hb has higher affinity
• double Bohr shift in placenta, so ~10% is transferred in a fetal direction (fall in maternal pH due to uptake of fetal CO2 causes release of maternal O2, rise in fetal pH due to removal of CO2 leads to uptake of O2)

Question 11

facilitated diffusion in placenta

Answer 11

GLUT1 on syncytiotrophoblast facing maternal circulation

Question 12

how is AA uptake regulated

Answer 12

• fetus can sense when maternal and therefore fetal AA levels are low
• sends a signal to the placenta to upregulation the correct transporter

Question 13

carrier systems in the placenta for AA transport

Answer 13

A/ASC: Na+ dep, neutral amino acids (alanine, glycine AG)
XAG: Na+ dep, acidic amino acids (DE)
L: Na+ indep, neutral (leu, phe)

Question 14

receptor mediated endocytosis in the fetus

Answer 14

• used for IgG
• bind to receptors in microvilli and become concentrated in coated pits at their base, then endocytosed and transported
• more is found in later gestation

Question 15

what can be detected from the fetus in the mother

Answer 15

• trophoblast cells
• cell-free DNA and RNA from the placenta
  (don’t know why for both)