Placenta function and dysfunction Flashcards
What is implantation?
loss of zona pellucida - the trophoblast cells interact with endometrial lining in the uterus - now called the blastocyst - and interacts with increased vasculature and secretory glands for nutrients
Where does implantation occur?
in the superior body of the uterus
What is an ectopic pregnancy?
where implantations occurs outside of the endometrium
What is placenta praevia?
where implantation occurs at a site within the lower uterine segment close to or covering the internal os
What is placenta accreta?
invasion and implantation that is too deep
When does the placenta begin to develop?
2nd week of pregnancy
Why is the placenta so important?
Develops first before any embryological development and the placenta ensures the foetus has all its metabolic needs catered for
What happens in week2?
2 distinct cellular layers emerge from
outer cell mass
- syncytiotrophoblast
-cytotrophoblast
inner cells mass becomes the bilaminar disk
- epiblast
- hypoblast
What does the syncytiotrophoblast do?
secretes HCG to maintain the corpus luteum
What happens to the amniotic and chorionic cavities?
initially following implantation, they are separate. But as the amniotic sac grows, the chorionic cavity disappears and just becomes 1 amniotic cavity
What are chorionic villi and where do they come from?
They come from the trophoblast cells - branch from the syncytiotrophoblast to the functional endometrium
They increase the SA in contact with the maternal blood
-Core of the chorionic villi is connective tissue that foetal blood vessels develop from
How does exchange occur?
The maternal blood vessels surround the chorionic villi to allow exchange to occur
What are they 3 stages of its development?
primary - finger-like projections of the cytrotrophoblast extend into the syncytiotrophoblast and branch
secondary - mesenchyme from the embryo invades cytotrophoblast of the primary villi and extends upwards
tertiary - foetal blood vessels form from the mesenchyme that link with foetal circulation formed by the 3rd week
the placenta now has a blood supply from the mother and the fetus and effect exchange of nutrients gases and waste products can occur
What does implantation achieve?
- establishes exchange
- anchors the placenta
- establishes maternal blood flow within the placenta
What is the barrier between maternal and fetal blood flow?
single layer of trophoblast (syncytiotrophoblast)
- starts of relatively thick as full layer of cytotrophoblast and synctiotrophoblast
- as pregnancy progresses, becomes thinner as loss of cytotrophoblast cells leaving just 1 layer of trophoblast
What is the decidua and the decidual reaction?
Decidua is the term given to the endometrial cells that have becomed specialised following the decidual reaction. They are a thick layer of modified mucous membrane which lines the uterus during pregnancy. The modification allows modulation of the degree of invasion of the conceptus once implanted
What happens if implantation occurs in an area with no decidua?
There is no decidual reaction so no control on degree of invasion - nothing to stop it invading too deep
This occurs in tubal ectopic pregnancy
What are the maternal blood vessels called and what do they do?
the endometrial arteries and veins - bathe the outside of the villi in maternal blood for exchange to occur
What are the foetal vessels called and what do they do?
through the umbilical arteries (pleural), waste is brough to the villi and oxygen and nutrients is taken to the fetus via the umbilical vein (singular)
What are the broad functions of the placenta?
- endocrine
- metbaolic changes
- transport
What hormones are produced by the placenta?
Protein; -hCG Steroid; -progeseteron -oestrogen
What is the role of hCG and where is produced?
produced by the syncytiotrophoblast (pregnancy specific)
supports the secretory function of the CL
only produced in the first 2 months of pregnancy
Why is hCG a marker of pregnancy?
it is secreted in maternal urine
Why does the placenta also produce progesterone and oestrogen?
When the placenta becomes large enough it can take over secretion from the CL (after the first trimester) - keeps the HPG axis in the pregnant state
What metabolic changes occur in the mother and how do they occur?
increase appetite due to increased progesterone - allows increased fat deposition to help support the fetus and breastfeeding
hPL - creates diabetogenic state to cause insulin resistance in the mother increasing the glucose availability to the fetus
What are the transport functions of the placenta?
- gas exchange
- immunity transfer
What molecules cross the membrane by simple diffusion?
water, electrolytes, urea, uric acid, gases
What molecules cross the membrane by facilitated diffusion?
glucose
What is the rate of gaseous exchange limited by?
flow limited NOT diffusion limited so adequate uteroplacental circulation is required
What happens if the uteroplacental circulation is compromised in labour?
contractions can lead to compression of the blood vessels causing fatal distress
What substances are transported across the placenta by AT?
AA, ions, vitamins - via transporters expressed by the syncytiotrophoblast
What immunoglobulins can cross the placenta?
only IgG via receptor-mediated endocytosis
Why do they need to cross the placenta?
so when the baby is born they have SOME defence against infection
What are tetrogens?
a drug/factor that causes malformation of an embryo
When do teratogens have their greatest effect/
in the first few weeks of pregnancy (especially 3-8 as this is the key time for development) - in the foetal period, there is less risk of damaging the fetus as they are just growing (except for CNS)
What are the harmful substances that can cross the placenta?
- thalomide (limb defects)
- alcohol (CNS)
- theraputic drugs
- drugs of abuse (dependancy)
- maternal smoking
Why is maternal smoking bad for the fetus?
Baby can’t get enough oxygen as the CO binds to Hb and moves into the fetus which means the fetus get lets O2 causing distress
