Placenta & Foetal Circulation Flashcards

1
Q

Why does the placenta need to be inspected following birth?

A
  • 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
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2
Q

When does the foetal period begin and what happens during this time?

A
  • 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
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3
Q

What are the 2 main roles of the placenta?

A
  1. nutrient and gas exchange
  2. hormone production
  • initially hormone production is achieved via the corpus luteum from the oocyte
    • the placenta takes over when this begins to regress
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4
Q

What happens to the placenta as the foetus enters week 9 of development?

A
  • 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
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5
Q

What makes up the foetal component of the placenta?

A

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
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6
Q

What makes up the maternal component of the placenta?

A

the uterine endometrium

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7
Q

What is placenta praevia?

Can it be screened for?

A
  • 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
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8
Q

What is placenta accreta and why does it occur?

What is the associated risk with this condition?

A
  • 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
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9
Q

What are some potentially harmful substances that are able to cross the placenta?

A
  • 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:
  1. drugs (e.g. thalidomide)
  2. alcohol
  3. some viruses (e.g. HIV, rubella, toxoplasmosis, Zika, CMV)
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10
Q

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?

A
  • 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
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11
Q

How does production of hCG vary during pregnancy?

A
  • 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
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12
Q

What are the 3 main hormones produced by the syncitiotrophoblast?

A
  1. progesterone
  2. oestrogen
  3. somatomammotropin
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13
Q

When does the placenta produce progesterone?

A
  • 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
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14
Q

How does production of oestrogenic hormones by the placenta change over time?

What are the roles of these hormones?

A
  • 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
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15
Q

What is the role of somatomammotropin (placental lactogen) secreted by the placenta?

A
  • 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
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16
Q

As well as hormone production, what are the other 3 key roles of the syncytiotrophoblast?

A
  1. modulating the immune response
  2. implantation
  3. breaking down maternal capillaries
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17
Q

What is the main difference between the trophoblast and embryoblast?

A

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
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18
Q

What processes occur on day 7 of development?

A
  • 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
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19
Q

What is the significance of the syncytiotrophoblast cells being multinucleated?

A
  • 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
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20
Q

How far has the blastocyst implanted by day 9 of development?

A
  • the blastocyst is more deeply embedded in the endometrium
  • the penetration defect in the surface epithelium has been closed by a fibrin coagulum
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21
Q

What changes take place in the trophoblast on day 9 of development?

A
  • vacuoles appear within the syncytium
  • the vacuoles fuse together to form large trophoblastic lacunae** within the **syncytium
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22
Q

What changes take place in the hypoblast during day 9 of development?

A
  • 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)
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23
Q

What is meant by the decidual reaction?

When does this occur and what is its purpose?

A
  • 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
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24
Q

How far has the blastocyst implanted by day 11-12 of development?

A
  • 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
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25
Q

How is the arrangement of the trophoblast different on day 11-12 of development?

A
  • 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
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26
Q

During day 11-12 of development, what happens to the syncytiotrophoblast layer?

A
  • 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
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27
Q

What new population of cells appears during day 11-12 of development?

A
  • 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
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28
Q

What cavities develop within the extraembryonic mesoderm between days 12-13?

A
  • 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)
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29
Q

How can the extraembryonic mesoderm be further divided?

A
  • the EEM lining the cytotrophoblast and amnion is the the extraembryonic somatic mesoderm
  • the EEM lining covering the yolk sac is the extraembryonic splanchnic mesoderm
30
Q

What can happen around day 13 that can lead to inaccuracy in determining the expected delivery date?

A
  • the surface defect in the endometrium has usually healed by day 13
  • occassionally, some bleeding occurs at the implantation site as a result of increased blood flow into the lacunar spaces
  • this bleeding occurs near the 28th day of the menstrual cycle so can be confused with normal menstrual bleeding
31
Q

What happens in the hypoblast around day 13 of development?

A
  • there is a second wave of migration of hypoblast cells that proliferate to form a new cavity within the primitive yolk sac
  • this cavity is the secondary / definitive yolk sac
  • during its formation, large portions of the primitive yolk sac are pinched off and these portions remain as exocoelomic cysts
    • these are found within the extraembryonic coelom or chorionic cavity
32
Q

What happens to the chorionic cavity around day 13 of development?

A
  • the chorionic cavity increases in size
  • the EEM lining the inside of the cytotrophoblast layer is now the chorionic plate
  • the only place the EEM traverses the chorionic cavity is in the connecting stalk
    • with the development of blood vessels, this stalk becomes the umbilical cord
33
Q

What changes take place in the trophoblast at the same time the chorionic cavity begins to form (around day 13)?

A

formation of primary villi

  • formation of the extraembryonic mesoderm signals back to the cytotrophoblast layer to induce further proliferation
  • the cytotrophoblast proliferates locally to form extensions that grow into the overlying syncytiotrophoblast
  • the extensions of cytotrophoblast grow out into the blood-filled lacunae and carry a covering of syncytiotrophoblast with them
34
Q

What does a primary villus consist of?

A
  • a core of cytotrophoblast cells** with a **covering of syncytiotrophoblast cells
  • these extend into the trophoblastic lacunae to form intervillous spaces between the primary villi
35
Q

How do secondary villi form?

A
  • cells of the extraembryonic mesoderm penetrate the core of the primary villus and grow toward the decidua
  • this produces a secondary villus - a core of EEM, covered by a layer of cytotrophoblast cells then a covering of syncytiotrophoblast cells
36
Q

How does a tertiary villus form?

A
  • extraembryonic mesodermal cells in the core of the villus begin to differentiate into blood cells** and **small blood vessels
  • this forms the villous capillary system (chorionic vessels) that play a role in exchanging nutrients with maternal blood within the trophoblastic lacunae
37
Q

What vessels do the capillaries within the tertiary villi make contact with?

A
  • capillaries within the tertiary villi come into contact with capillaries developing within the chorionic plate** and **connecting stalk
  • these vessels establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo
38
Q

How do the tertiary villi develop even further in the 9th week?

A
  • the tertiary villi continue to mature to form free villi
  • these are even further projections of the tertiary villi that further increase the surface area for efficient exchange of nutrients
39
Q

How does maternal blood pass from the trophoblastic lacunae into the chorionic vessels?

A
  • the chorionic vessels are formed from the extraembryonic mesodermal core of the tertiary villi
  • around this core there is a layer of cytotrophoblasts, followed by a layer of syncytiotrophoblasts
  • the syncytiotrophoblast cells line the trophoblastic lacunae, which contain blood from the spiral arteries of the uterus
  • syncytiotrophoblast cells degrade maternal spiral arteries, which causes blood to leak into the trophoblastic lacuna
40
Q

When does the transition from secondary to tertiary villi occur?

A
  • when the mesodermal core of the villus differentiates into blood vessels
  • chorionic arteries and veins from the embryo will pierce the EEM to develop within it
41
Q

What is the decidual plate?

Why is it important?

A
  • the decidual plate is part of the uterine lining where the placenta will form
  • the cytotrophoblast and syncytiotrophoblast will implant into the uterine lining up to a certain extent
  • this implantation usually stops at the decidual plate
42
Q

What are decidual septa?

A
  • they are wedge-like projections between groups of villi on the maternal side of the placenta
  • they partially separate the maternal side of the placenta into regions called cotyledons
43
Q

What is the difference between a free and an anchoring villus?

A

Anchoring villi:

  • extend from the chorionic plate (EEM + overlying trophoblast layers) to the decidua basalis (decidual plate)

Free villi:

  • branch from the sides of the stem villi and through which exchange of nutrients and other factors will occur
44
Q

What changes occur during endovascular invasion of maternal spiral arteries by the cytotrophoblast?

A
  • endovascular invasion by the cytotrophoblast replaces the epithelial cells in the vessel walls to produce “hybrid vessels”
  • this transforms the arteries from being small with high resistance to large with low resistance allowing increased quantities of maternal blood to reach the intervillous spaces
45
Q

What changes occur in the syncytium around day 21?

A
  • the syncytium becomes thinner** and the **cytotrophoblasts are lost from the villi
  • this means that the placental barrier now consists only of the vessel endothelium and the syncytium
46
Q

What is meant by the junctional zone?

A
  • the junctional zone is a specific layer of the decidua basalis at which the trophoblast stops invading
  • at this point, maternal and foetal tissues intermingle
47
Q
A
48
Q

How do tertiary villi develop initially?

How do their characteristics change over time?

A
  • in the early weeks of development, villi cover the entire surface of the chorion
  • as the pregnancy advances, villi on the embryonic pole continue to grow and expand and give rise to the chorion frondosum
    • this is the “bushy chorion” assoicated with the decidua basalis
  • the villi on the abembryonic pole degenerate so this side of the chorion (chorion laeve) is now smooth
49
Q

When does the embryo bulge into the uterine lumen?

What is the uterine lining at this point called?

A
  • the embryo begins to bulge into the uterine lining during the 2nd month
  • the uterine lining covering this bulge is the decidua capsularis
50
Q

When USS screening is performed during pregnancy, what aspects of the placenta should be assessed?

Why is this important and is it compulsory?

A
  1. location
  2. size
  3. vascularity / blood flow
  4. vessels
  • undetected placental abnormalities can lead to death of the mother and/or foetus if allowed to deliver spontaneously
  • whilst women may not want prenatal screening tests, they should be strongly advised to have an USS to assess the placenta
51
Q

What are all placental abnormalities associated with?

A
  • vaginal bleeding in the second half of pregnancy
52
Q

What is the definition of placenta praevia?

What is it directly correlated with?

A

defined as the placenta overlying the endocervical os

it is directly correlated with the number of previous C-sections

53
Q

What are the risk factors for placenta praevia?

A
  • previous spontaneous / elective pregnancy terminations
  • previous uterine surgery
  • increased maternal parity
  • increased maternal age
  • smoking
  • cocaine use
  • multiple gestations
  • previous placenta praevia
54
Q

What is the main complication of placenta praevia?

What more serious consequences can this lead to?

A
  • complications are direct consequences of maternal haemorrhage
  • blood loss is associsted with:
    • increased need for transfusion
    • hysterectomy
    • maternal death
    • sepsis
    • thrombophlebitis
    • ICU admission
  • most of the foetal complications result from pregnancies being premature
55
Q

How can placenta praevia be detected and managed?

A
  • it can be detected on USS and birth should be via C-section to minimise the risk of bleeding
56
Q

What is meant by vasa praevia?

A
  • this describes foetal vessels that run through foetal membranes over or near the endocervical os that are unprotected by the placenta or umbilical cord
  • these vessels are at risk of rupture when supporting membranes rupture
57
Q

What are the potential risks of vasa praevia?

A
  • it has a high mortality rate as foetal vessels can tear during spontaenous labour, leading to foetal-neonatal haemorrhage
  • it is associated with increased risk of preterm birth
58
Q

How is vasa praevia detected and managed?

A
  • diagnosed via USS and use of colour Doppler
  • delivery via C-section before the onset of labour due to rupture of membranes
59
Q

What is meant by placenta accreta?

A
  • this occurs when the placenta becomes abnormally adherent to the uterine wall
  • there is direct attachment of trophoblast cells to the myometrium without intervening decidua
  • usually the trophoblast cells should stop invading the uterus when they reach the decidua basalis
60
Q

What are the 3 conditions that lie under the spectrum defined as placenta accreta?

A

Placenta accreta:

  • the trophoblast attaches directly to the myometrium with no intervening decidua
  • the placenta is attached too deeply into the uterine wall

Placenta increta:

  • the trophoblast invades into the myometrium
  • the placenta is attached into the uterine muscle

Placenta percreta:

  • the trophoblast invades through the myometrium, beyond the serosa and into surrounding structures (e.g. bladder)
61
Q

What are the risk factors for the placenta accreta spectrum?

A
  • the major risk factor is previous C-section or surgery that may damage the endometrium
  • smoking
  • IVF
  • increased maternal age
  • multiparity
  • placenta praevia
  • a short interval between prior C-section and subsequent pregnancy
62
Q

What are the main complications associated with the placenta accreta spectrum?

A
  • most maternal complications are the result of maternal haemorrhage:
  1. disseminated intravascular coagulation
  2. multiorgan failure
  3. need for additional surgery (e.g. hysterectomy)
  4. thromboembolism
  5. death
  • neonatal complications are mainly the result of preterm birth
63
Q

What arteries deliver blood to the intervillous spaces?

How often does this occur and how is the blood removed?

A
  • around 100 spiral arteries deliver around 150ml of blood to the intervillous spaces
  • this blood is replaced 3-4 times each minute
  • as the pressure drops within the intervillous spaces, blood flows back into the endometrial veins
64
Q

In late pregnancy what is the placental barrier comprised of?

Can foetal cells cross this barrier?

A
  • the placental barrier is composed of a syncytial membrane and the endothelial lining of the capillaries within the villi
  • there is no mixing of maternal and foetal blood, but sometimes a few foetal cells can cross the thin placental barrier
  • these cells are syncytiotrophoblasts that undergo apoptosis and enter the trophoblastic lacuna
  • the presence of these cells can be used in prenatal screening
65
Q

How is thinning of the placental barrier to reduce the diffusion distance acheived?

A
  1. loss of the cytotrophoblast cells
  2. thinning of the endothelial lining of the embryonic vessels
66
Q

When is the placenta expelled?

What diameter should it be?

A
  • the placenta is expelled around 30 minutes after the neonate
  • it should be around 20cm in diameter
67
Q

What makes up the foetal side of the placenta?

A
  • this consists of the chorionic plate and chorionic vessels that are covered by amnion
  • the chorionic vessels converge towards the insertion of the umbilical cord, which is usually central
68
Q

What makes up the maternal side of the placenta?

A
  • this is characterised by the presence of 15-20 cotyledons (wedge-shaped pieces of septa)
  • it must be fully examined after birth to ensure it is intact
  • if any part of the placenta is left behind, it can form a connection with the mother’s vascular system and result in haemorrhage
69
Q

Why do changes in the foetal circulation need to occur after birth?

A
  • there is some mixing of blood within the foetal circulation, which needs to be avoided after birth
  • structural / anatomical adaptations that exist in foetal circulation need to close up shortly after birth
70
Q

What is the role of the ductus venosus?

A
  • umbilical veins carrying oxygenated blood bypass the liver via the ductus venosus
  • they enter the inferior vena cava which is carrying poorly oxygenated blood
  • the IVC empties into the right atrium, which also receives venous blood from the superior vena cava
71
Q

What is the role of the foramen ovale and ductus arteriosus?

A
  • blood entering the right atrium from the IVC and SVC passes into the left atrium via the foramen ovale
  • the blood then passes into the left ventricle and aorta
  • any blood that reaches the right ventricle is ejected into the pulmonary trunk
  • as the developing lungs do not require much blood, the majority of this blood bypasses the lungs and enters the aorta via the patent ductus arteriosus