Placenta & Foetal Circulation Flashcards
(71 cards)
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
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
3
Q
What are the 2 main roles of the placenta?
A
- 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
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
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
6
Q
What makes up the maternal component of the placenta?
A
the uterine endometrium
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
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
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:
- drugs (e.g. thalidomide)
- alcohol
- some viruses (e.g. HIV, rubella, toxoplasmosis, Zika, CMV)
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
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
12
Q
What are the 3 main hormones produced by the syncitiotrophoblast?
A
- progesterone
- oestrogen
- somatomammotropin
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
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
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
16
Q
As well as hormone production, what are the other 3 key roles of the syncytiotrophoblast?
A
- modulating the immune response
- implantation
- breaking down maternal capillaries
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
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
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
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
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
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)
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
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
25
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**
26
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_**
27
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
28
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)
29
How can the extraembryonic mesoderm be further divided?
* 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
What can happen around day 13 that can lead to inaccuracy in determining the expected delivery date?
* 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
What happens in the hypoblast around day 13 of development?
* 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
What happens to the chorionic cavity around day 13 of development?
* 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
What changes take place in the trophoblast at the same time the chorionic cavity begins to form (around day 13)?
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
What does a primary villus consist of?
* 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
How do secondary villi form?
* 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
How does a tertiary villus form?
* 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
What vessels do the capillaries within the tertiary villi make contact with?
* 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
How do the tertiary villi develop even further in the 9th week?
* 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
How does maternal blood pass from the trophoblastic lacunae into the chorionic vessels?
* 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
When does the transition from secondary to tertiary villi occur?
* 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
What is the decidual plate?
Why is it important?
* 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
What are decidual septa?
* 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
What is the difference between a free and an anchoring villus?
**_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
What changes occur during endovascular invasion of maternal spiral arteries by the cytotrophoblast?
* 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 l**_arge with low resistance_** allowing increased quantities of maternal blood to reach the intervillous spaces
45
What changes occur in the syncytium around day 21?
* 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
What is meant by the junctional zone?
* 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
48
How do tertiary villi develop initially?
How do their characteristics change over time?
* 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
When does the embryo bulge into the uterine lumen?
What is the uterine lining at this point called?
* the embryo begins to bulge into the uterine lining during the **2nd month**
* the uterine lining covering this bulge is the **_decidua capsularis_**
50
When USS screening is performed during pregnancy, what aspects of the placenta should be assessed?
Why is this important and is it compulsory?
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
What are all placental abnormalities associated with?
* vaginal bleeding in the second half of pregnancy
52
What is the definition of placenta praevia?
What is it directly correlated with?
defined as the **_placenta overlying the endocervical os_**
it is directly correlated with the number of previous C-sections
53
What are the risk factors for placenta praevia?
* previous spontaneous / elective pregnancy terminations
* previous uterine surgery
* increased maternal parity
* increased maternal age
* smoking
* cocaine use
* multiple gestations
* previous placenta praevia
54
What is the main complication of placenta praevia?
What more serious consequences can this lead to?
* 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
How can placenta praevia be detected and managed?
* it can be **_detected on USS_** and birth should be via **_C-section_** to minimise the risk of bleeding
56
What is meant by vasa praevia?
* 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
What are the potential risks of vasa praevia?
* 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
How is vasa praevia detected and managed?
* diagnosed via USS and use of colour Doppler
* delivery via C-section before the onset of labour due to rupture of membranes
59
What is meant by placenta accreta?
* 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
What are the 3 conditions that lie under the spectrum defined as placenta accreta?
**_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
What are the risk factors for the placenta accreta spectrum?
* 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
What are the main complications associated with the placenta accreta spectrum?
* 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
What arteries deliver blood to the intervillous spaces?
How often does this occur and how is the blood removed?
* 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
In late pregnancy what is the placental barrier comprised of?
Can foetal cells cross this barrier?
* 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
How is thinning of the placental barrier to reduce the diffusion distance acheived?
1. loss of the cytotrophoblast cells
2. thinning of the endothelial lining of the embryonic vessels
66
When is the placenta expelled?
What diameter should it be?
* the placenta is expelled around 30 minutes after the neonate
* it should be around **20cm** in diameter
67
What makes up the foetal side of the placenta?
* 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
What makes up the maternal side of the placenta?
* 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
Why do changes in the foetal circulation need to occur after birth?
* 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
What is the role of the ductus venosus?
* **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
What is the role of the foramen ovale and ductus arteriosus?
* 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***
