Session 8: The Placenta Flashcards
When does implantation occur?
Day 6
What cells interact with what to allow implantation to occur?
Trophoblast cells interact with the endometrial lining
Trophoblasts are two types of cells, which?
Cytotrophoblasts (inner) and syncytiotrophoblasts (outer)
Where does implantation typically occur?
In the superior body of the uterus.
What is placenta praevia?
If implantation occurs at a site withing the lower uterine segment e.g. close to or covering the internal os.
Explain the relevance of C-section scars and implantation.
Implantation can occur in old C-section scars, they are not usually viable.
What is placenta accreta?
When implantation has occured too deep.
What can placenta accreta as well as incomplete invasion of the conceptus lead to?
Miscarriage
Placental insufficiency
Pre-eclampsia
When does the placenta begin to develop?
In week 2
What does the placenta develop from?
The trophoblasts of the conceptus.
Explain the development of the fetal placenta.
After following implantation the amniotic membrane and the chorionic membrane are two separate membranes.
However as the amniotic enlarges the chorionic sac becomes displaced and the amniotic membrane will then fuse with the chorionic membrane. This forms the amniotic cavity.
Projections on the outer surface of the membrane are initially balanced around the entire surface of the membrane. But when the amniotic sac enlarges the projections become concentrated to one area into a single disc like space.
These concentrated projections will become the placenta.
What does the maternal placenta develop from?
Maternal uterine tissue.
What is the fetal placenta also called?
Chorion frondosum
What is the maternal placenta also called?
Decidua basalis
What is the initial unit of exchange in implantation between the mother and foetus?
The chorionic villi.
What are primary villi?
Early finger-like projections of trophoblasts
What are secondary villi?
Invasion of mesenchyme into the core of the villi.
What are tertiary villi?
The invasion of mesenchyme core by foetal vessels.
Functions of the chorionic villi.
Allow for exchange of nutrients as well as anchoring of the placenta.
Explain the exchange of nutrients and other substances from mother to foetus.
Foetal blood vessels in the chorionic frondosum exchange substances with the maternal vessels found just outside.
There is a barrier between the maternal and foetal blood flow which is a single layer of trophoblasts. This allows for optimal transport.
It is important to know however that the two circulations never mix.
Explain how the anatomy of the barrier changes as the pregnancy progresses.
In first trimester the barrier is relatively thick with a full layer of cytotrophoblasts and syncytiotrophoblasts.
As the pregnancy progresses there is a reduction in number of cytotrophoblasts to allow for more efficient transport.
What is the decidua?
Cells of the endometrium that become specialised to modulate the degree of invasion of the conceptus.
(Decidualisation)
Why is decidualisation important?
Because if the conceptus implants in a place without decidua there is no inhibition and no control over the degree of invasion.
Also if implantation occurs and the decidual reaction is not enough, it can lead to miscarriage, infertility or placental insufficiency like pre-eclampsia.
Where in the endometrial wall does implantation occur?
It is interstitial and implants in within the stroma.
What are the maternal blood vessels called that are found outside of the villi and important for exchange between mother and foetus?
Endometrial arteries
What are the important fetal blood vessels for exchange between foetus and mother?
Two umbilical arteries
One umbilical vein
What do the umbilical arteries carry?
Deoxygenated blood from foetus to placenta
What does the umbilical vein carry?
Oxygenated blood from placenta to supply foetus.
What type of hormones does the placenta produce?
Protein hormones and steroid hormones.
Give an example of a protein hormone that the placenta produces.
Human chorionic gonadotrophin (hCG).
What is hCG produced by specifically?
Syncytiotrophoblasts
Function of hCG.
Sustain the corpus luteum in the first trimester.
Give examples of steroid hormones produced by the placenta.
Oestrogen and progesterone.
Functions of the steroid hormones produced by the placenta.
This happens after the first trimester as the placenta has grown enough to not be dependent on the corpus luteum anymore. The placenta takes over the function of the corpus luteum and produces oestrogen and progesterone to keep the HPG axis in a pregnant state.
Give a function of progesterone on maternal metabolism.
Promotes an increase in appetite to allow increased fat deposition to support foetus and breastfeeding later on.
Give examples of trophoblast disease.
Molar pregnancy also called hydatidiform mole.
Choriocarcinoma
Give an example of another important hormone produced by the placenta.
Human placental lactogen (hPL)
Function of human placental lactogen.
Creates a diabetogenic state leading to increased insulin resistance of the mother. This means that more glucose will be available to the foetus.
Modes of transport at the placenta.
Simple diffusion (passive)
Facilitated diffusion
Active transport
Give examples of simple diffusion at the placenta.
Water
Electrolytes
Urea and uric acid
Gases
Give examples of facilitated diffusion at the placenta.
Glucose transport
Give examples of active transport at the placenta.
Amino acids
Iron
Vitamins
How is gas exchange limited at the placenta?
By flow and not by diffusion.
Why is it important to not that gas exchange is flow limited at the placenta?
This means that it requires adequate uteroplacental circulation.
This can be compromised during labour e.g. and contraction can lead to compression of the blood vessels and lead to foetal distress.
The foetal immune system is immature and not adequate for survival. How is this issue overcome?
Antibodies can be transported across the placenta from maternal circ to foetal circ.
This is a receptor-mediated process.
Can all immunoglobulins be transported across the placenta?
No only certain types
Give examples of immunoglobulins that can be transported cross the placenta.
Only IgG
This means that the concentrations of IgG in the foetal circ is higher than in maternal circ.
Why might the exchange of substances across the placenta pose a problem?
Because it is not a true barrier. This means that harmful substances called teratogens can cross the barrier and harm the foetus.
When do teratogens have the greatest effect?
At early stages of the pregnancy from 3-8 weeks when the conceptus is in its embryonic stage.
Effects of teratogens in the foetal period (9-36 weeks)
Most systems have developed, however there can still be effects.
Specifically in the CNS as this has not fully developed.
What is Rhesus disease?
When blood group incompatibilities occurs.
This leads to maternal antigens crossing into the foetal circulation and attacking foetal blood cells.
It’s an haemolytic disease
Give examples of teratogens.
Thalidomide
Alcohol
Therpeutic drugs like; anti-epileptic drugs, warfarin and ACE inhibitors
Drug abuse
Smoking
Effects of thalidomide
Limb defects
Effects of alcohol
FAS and ARND
Effects of teratogens in the pre-embryonic stage.
Lethal effects
What are the two parts of the placenta?
Maternal placenta
Fetal placenta
What is the fetal placenta also called?
Chorion frondosum
What is the maternal placenta also called?
Decidua basalis
Change in the barrier of exchange in first trimester compared to third trimester.
Thick in the first trimester with undifferentiated mesoderm as well as cytotrophoblasts.
Becomes thinner as pregnancy progresses and ends up with capillary endothelium and syncytiotrophoblasts.
Why would you measure hCG?
Test for pregnancy
Trophoblast disease like molar pregnancy or choriocarcinoma.
Also in testicular cancer
Where can the active transporter need in exchange between foetus and mother be found?
On the syncytiotrophoblasts.
What are the endometrial arteries also called?
Spiral arteries
