Placentation

Question 1

What is the function of the yolk sac?

Answer 1

1. The yolk sac acts as the first site of maternal-foetal exchange (sometimes known as the yolk sac placenta). It is highly vascularised and is the first site of maternal-foetal exchange.
2. First site of haematopoiesis.
3. Secretes numerous proteins essential to survival of the embryo, including α-fetoprotein.

Question 2

What are the characteristics of the yolk sac that allow it to carry out its function?

Answer 2

1. Expression of various nutrient transporters (e.g. GLUT1)
2. Numerous microvilli increasing surface area of absorption
3. Good blood supply

Question 3

What is the process of development of the amnion?

Answer 3

1. The epiblast expands to surround the amniotic cavity, forming the amnion.
2. As the conceptus continues to develop, the amnion expands to completely surround the embryo.
3. Eventually, the amnion fuses with the overlying chorion, obliterating the extraembryonic coelum and the yolk sac.

Question 4

What are the functions of amniotic fluid?

Answer 4

1. Cushions against mechanical trauma
2. Prevent fetus drying out

Question 5

What are the 2 surfaces of the placenta?

Answer 5

1. Chorionic plate: Faces fetus and attaches to umbilical cord
2. Basal plate: Apposed to decidua basalis of endometrium and partakes in exchange.

Question 6

What is meant by a “haemochorial” placenta?

Answer 6

Maternal blood comes into direct contact with fetal blood as fetal blood passes though villous trees extending from the chorionic plate. However, there is no direct mixing between maternal and fetal blood.

Question 7

What are the layers of the trophoblast?

Answer 7

1. Syncytiotrophoblast
2. Cytotrophoblast

Question 8

What is significant about the development of the syncytiotrophoblast?

Answer 8

The syncitiotrophoblast is non-proliferative itself and expands as a result of cellular contributions from the cytotrophoblast.

Question 9

What is the sequence of events that occur during placental development?

Answer 9

1. As the STB expands, it erodes into the decidua.
2. As it expands, it incorporates blood vessels and endometrial glands, resulting in spaces appearing.
3. These spaces eventually coalesce to form lacunae, filled with maternal blood and glandular secretions.
4. Columns of STB between lacunae are the STB trabeculae (primary villi).
5. As the STB invades the decidua, mesoderm also invades CTB, forming finger-like projections into STB trabeculae (secondary villi).
6. As time progresses, mesoderm at centre of secondary villi differentate to form blood vessels (tertiary villi).
7. Further branches project into the lacunae, which are now referred to as the intervillous spaces.

Question 10

What is the structure of the placental villous tree?

Answer 10

1. Stem villous: Extends from the chorionic plate.
2. Intermediate villi: Extends into intervillous spaces where terminal villi arise and where most exchange occurs.
3. Anchoring villi: Extends into the basal plate and holds placenta in place.

Question 11

As development continues, which parts of the villus tree undergo the most expansion?

Answer 11

Terminal villi

Question 12

What is the nature of chorionic villus growth throughout development?

Answer 12

1. Initially, the villi develop around the whole of the chorion.
2. However, they eventually regress over the abembryonic pole (pole opposite embryonic pole) to form smooth chorion levae.
3. On the embryonic pole, they continue to grow, giving the chorion frondosum.
4. This allows the abembryonic pole to rupture during birth and provide route of exit for fetus.

Question 13

When does the fetal-maternal circulation become established?

Answer 13

10-12 weeks of pregnancy

Question 14

How is the fetus supplied with nutrients during course of development?

Answer 14

0-10/12 weeks: Histotrophic nutrition (placenta-independent)

>10/12 weeks: Haemotrophic nutrition

Question 15

What is histotrophic nutrition?

Answer 15

Uptake of oviductal, uterine secretions and cell debris by the trophoblast and yolk sac.

Question 16

What is the function of histotrophic nutrition during development?

Answer 16

Allows anaerobic conditions to be maintained during early development.

Question 17

Why is it to maintain anaerobic conditions during early pregnancy?

Answer 17

1. Embryogenesis can take place with minimum risk of ROS production, which may lead to teratogenesis.
2. Embryonic stem cells are adapted to proliferating in low O₂ environments.

Question 18

How is histotrophic nutrition maintained up to 10 weeks despite the fetal-maternal circulation forming ~week 4?

Answer 18

Endovascular extravillous trophoblasts form dilations blocking maternal spiral arteries.

Question 19

What is haemotrophic nutrition?

Answer 19

Exchange of gases, nutrients and waste occurs directly between maternal and fetal circulations.

Question 20

What are the different stages of development for the conceptus?

Answer 20

1. Embryonic period (0-8 weeks, 1^st trimester): This stage involves differentiation of embryonic stem cells to produce the different types of tissues found in the neonate.
2. Fetal period (8-42 weeks, 2^nd & 3^rd trimesters): This stage involves growth and development of the different tissue types developed in embryonic period.

Question 21

What is the gross structure of the uterine blood supply?

Answer 21

1. the uterine and ovarian arteries run in the lateral walls of the uterus.
2. These arteries each give ~10 branches (arcuate arteries) that anastomose with each other and encircle the uterus.

3, From the arcuate arteries, basal arteries arise and penetrate the endometrium, giving off the spiral arteries that supply the placenta.

Question 22

What is the purpose of the highly coiled nature of spiral arteries?

Answer 22

Allows for distension of the uterus as the fetus grows in size.

Question 23

What is the process of maternal spiral artery conversion?

Answer 23

1. Migration of endovascular extravillous trophoblasts into the walls of the distal ends of spiral arteries, as well as to some extent the interstitial extravillous trophoblasts cells that migrate through surrounding stroma, mediates conversion.
2. This migration causes loss of smooth muscles from distal ends, causing formation of “terminal dilations” in process of conversion.

Question 24

What is the structure of a placenal lobule?

Answer 24

‘Inverted wine glass’ lobular chorionic villi each with a maternal spiral artery opening into the centre.

Question 25

What are the consequences of spiral artery conversion in terms of maternal blood flow?

Answer 25

Reduce the pressure of maternal blood entering placenta but maximise volume of blood delivered. This may be to ensure that: 1. Rate of blood is slow for efficient exchange to occur. 2. Conceptus is protected from being dislodged by excess pressure.

Question 26

What is the most common clinical conditions related to failure of spiral artery conversion?

Answer 26

1. Pre-eclampsia 2. IUGR