Placenta & Foetal Circulation Flashcards
Why does the placenta need to be inspected following birth?
- a thorough inspection of both the embryonic and maternal sides of the placenta needs to be conducted
- this is to determine whether the placenta is fully intact and ensure no abnormalities are present
When does the foetal period begin and what happens during this time?
- the foetal period begins at the start of week 9 and continues until birth
- it is characterised by maturation of tissues and organs and rapid growth of the body
What are the 2 main roles of the placenta?
- nutrient and gas exchange
- hormone production
- initially hormone production is achieved via the corpus luteum from the oocyte
- the placenta takes over when this begins to regress
What happens to the placenta as the foetus enters week 9 of development?
- the demands of the foetus for nutritional and other factors increase as it enters week 9
- major changes in the placenta occur as it matures to allow for efficient exchange across a thin membrane
- there is an increase in surface area between foetal and maternal compartments to facilitate exchange
- the disposition of foetal membranes is altered as amniotic fluid production increases
What makes up the foetal component of the placenta?
foetal component consists of trophoblast cells and the extraembryonic mesoderm
- trophoblast cells are responsible for forming foetal support tissues
- the extraembryonic mesoderm (chorionic plate) is also part of the support structures
- it is “extraembryonic” so is not formed by the 3 germ layers that develop during gastrulation
What makes up the maternal component of the placenta?
the uterine endometrium
What is placenta praevia?
Can it be screened for?
- an abnormal implantation site leads to the placenta developing over the cervix
- it can be screened for via USS and Doppler can be used to determine whether there is sufficient blood supply
What is placenta accreta and why does it occur?
What is the associated risk with this condition?
- when the embryo implants into the uterine lining, it is important that invasion of support structures only occurs to a certain extent
- it needs to be sufficient to anchor the embryo and provide sufficient amounts of blood to the foetus
- if too much invasion occurs, some placenta can be left within the uterus** after birth, causing **vaginal bleeding
What are some potentially harmful substances that are able to cross the placenta?
- the placenta is efficient at filtering the blood to protect the foetus from harmful substances, but it is not 100% effective
- some harmful factors that can cross the placenta to enter foetal circulation are:
- drugs (e.g. thalidomide)
- alcohol
- some viruses (e.g. HIV, rubella, toxoplasmosis, Zika, CMV)
What part of the placenta is involved in hormone production?
Why can the ovaries be removed from a woman after 4 months of pregnancy without causing an abortion?
- the syncytiotrophoblast takes over from the corpus luteum in hormone production
- ovaries can be removed after the 4th month without causing an abortion as the corpus luteum has become redundant
How does production of hCG vary during pregnancy?
- hCG is produced by the syncytiotrophoblast during the first 2 months of pregnancy
- it supports the corpus luteum and has a role in regulating the mother’s immune response
- so much hCG is produced that it enters the urine and can be used in pregnancy testing
What are the 3 main hormones produced by the syncitiotrophoblast?
- progesterone
- oestrogen
- somatomammotropin
When does the placenta produce progesterone?
- production gradually increases until the end of the 4th month when progesterone production is taken over by the placenta
- at this stage, the placenta produces progesterone in sufficient amounts to maintain the pregnancy if the corpus luteum is damaged / removed
How does production of oestrogenic hormones by the placenta change over time?
What are the roles of these hormones?
- the placenta mainly produces estriol
- production increases throughout pregnancy until just before it ends, when a maximum level is reached
- high levels of oestrogens stimulate uterine growth** and **breast development
What is the role of somatomammotropin (placental lactogen) secreted by the placenta?
- it is a growth-hormone like substance that gives the foetus priority over maternal blood glucose
- this makes the mother somewhat diabetogenic
- it also promotes breast development for milk production
As well as hormone production, what are the other 3 key roles of the syncytiotrophoblast?
- modulating the immune response
- implantation
- breaking down maternal capillaries
What is the main difference between the trophoblast and embryoblast?
Trophoblast:
- the support structures (including placenta) are formed from this layer
- this forms the OCM that will implant into the uterine lining
Embryoblast:
- this forms the ICM that goes on to form the embryo proper
What processes occur on day 7 of development?
- the bilaminar disc is formed from the epiblast and hypoblast cells
- the trophoblast has further differentiated into the cytotrophoblast and syncytiotrophoblast
- the syncytiotrophoblast anchors the embryo onto the uterine lining to allow for implantation
What is the significance of the syncytiotrophoblast cells being multinucleated?
- they are multinucleated as they have lost some of their membranes
- this means that it is more difficult for immune cells to penetrate the syncytiotrophoblast layer to reach the bilaminar disc
How far has the blastocyst implanted by day 9 of development?
- the blastocyst is more deeply embedded in the endometrium
- the penetration defect in the surface epithelium has been closed by a fibrin coagulum
What changes take place in the trophoblast on day 9 of development?
- vacuoles appear within the syncytium
- the vacuoles fuse together to form large trophoblastic lacunae** within the **syncytium
What changes take place in the hypoblast during day 9 of development?
- a wave of flattened cells originating from the hypoblast form the exocoelomic (Heuser) membrane
- this membrane migrates around the entire blastocyst cavity and lines the inner surface of the cytotrophoblast
- Heuser’s membrane together with the hypoblast forms the primitive yolk sac (extracoelomic cavity)
What is meant by the decidual reaction?
When does this occur and what is its purpose?
- the decidual reaction occurs after implantation and involves a change in morphology of the uterine lining
- the uterine lining becomes more vascular and packed with nutrients
- the uterine gland becomes enlarged, allowing for diffusion of nutrients
- the maternal capillaries (sinusoids) become engorged
How far has the blastocyst implanted by day 11-12 of development?
- the blastocyst is completely embedded within the endometrial stroma
- the surface epithelium almost entirely covers the original defect in the uterine wall
- the blastocyst produces a slight protrusion in the lumen of the uterus
How is the arrangement of the trophoblast different on day 11-12 of development?
- the trophoblastic lacunae within the syncytium have enlarged
- lacunar spaces within the syncytium form an interconnected network that is more evident at the embryonic pole
- at the abembryonic pole, the trophoblast still consists mainly of cytotrophoblast cells
During day 11-12 of development, what happens to the syncytiotrophoblast layer?
- the syncytiotrophoblasts penetrate deeper into the stroma and erode the endothelial lining of maternal capillaries
- these capillaries are congested and dilated and are known as sinusoids
- syncytial lacunae become continuous with the sinusoids and maternal blood enters the lacunar system
- the syncytiotrophoblast continues to erode more and more sinusoids causing maternal blood to flow through the trophoblastic system and establishing the uteroplacental circulation
What new population of cells appears during day 11-12 of development?
- cells of the extraembryonic mesoderm appear between the inner surface of the cytotrophoblast and the outer surface of the primitive yolk sac
- these cells are derived from yolk sac cells
- they form a fine, loose connective tissue that eventually develop to form a layer surrounding the entire embryo
What cavities develop within the extraembryonic mesoderm between days 12-13?
- several cavities form within the extraembryonic mesoderm which become confluent to form the chorionic cavity (extraembryonic cavity)
- the chorionic cavity surrounds the primitive yolk sac (except where the germ disc is connected to the trophoblast by the connecting stalk)