Implantation, placentation and hormone changes in pregnancy

Question 1

What is the blastocyte?

Answer 1

Blastocyst (an embryo that is at the stage of implantation, usually formed at day 5-6, post-fertilisation)

Question 2

What cell lineages is the blastocyte made up of?

Answer 2

Trophoblast (Purple - forms the placenta) and Embryoblast or inner cell mass (Green - forms the foetus).

Question 3

What is the Blastocoel?

Answer 3

Also present inside the blastocyst is a fluid-filled cavity known as the Blastocoel.

Question 4

Describe the 2 poles of the blastocyte

Answer 4

The embryoblast is concentrated at the ‘embryonic pole’ while the opposite pole (where trophoblast cells are concentrated) is known as the ‘abembryonic pole’

Where the inner cell mass is known as the embryonic pool and the opposite end is called the abembryonic pool.

Question 5

What do we need for implantation to occur?

Answer 5

2. A receptive endometrium at the secretory phase of the MS
   • Thickened endometrial lining.
   • Expression of embryo receptivity markers.

Question 6

Label the blastocyte

Answer 6

On image

Question 7

What do the inner cell mass and trophoblast form?

Answer 7

Inner cell mass forms the foetus and the trophoblast forms the placenta.

Question 8

What causes the blastocyst to hatch?

Answer 8

The developed blastocyte will then hatch out of the zona pellucida. At the abembryonic end there are digestive enzymes which digest the zona pellucida. This is sensed by the blastocyte which then undergoes contractions and rhythms to move out of the area of digestion. It herniates out of it.

Question 9

What are the 3 stages of implantation?

Answer 9

1. Apposition – positions of the blastocyte so it is ready to attach to the uterus wall (endometrium.)
2. Attachment – blastocyte attaches to the endometrium.
3. Invasion – the blastocyte burrows into the endometrium

Question 10

Describe what happens at day 7-8

Answer 10

• Blastocyst attaches itself to the surface of the endometrial wall (decidua basalis).
• Trophoblast cells start to assemble to form a Syncytiotrophoblast (faciliatates the invasion of the endometrium lining) in order facilitate invasion of the decidua basalis.

Question 11

Describe what happens at day 9-11

Answer 11

• Syncytiotrophoblast further invades the decidua basalis and by Day 11 its almost completely buried in the decidua.

Question 12

Describe what happens on day 12

Answer 12

• Decidual reaction occurs (remodelling of the cells of the endometrium). High levels of progesterone result in the enlargement and coating of the decidual cells in glycogen and lipid-rich fluid. This is driven my progesterone.
• This fluid is taken up by the Syncytiotrophoblast and helps to sustain the blastocyst early on before the placenta is formed.

Question 13

Describe what happens on day 14

Answer 13

• Cells of the Syncytiotrophoblast start to protrude out to form tree-like structures known as Primary Villi, which are then formed all around the blastocyst.
• Decidual cells between the primary villi begin to clear out, leaving behind spaces known as Lacunae.
• Maternal arteries and veins start to grow into the decidua basalis. These blood vessels merge with the lacunae – arteries filling the lacunae with oxygenated blood and the veins returning deoxygenated blood into the maternal circulation.
• Blood-filled lacunae merge into a single large pool of blood connected to multiple arteries and veins. This is known as the Junctional Zone.

Question 14

What contributes towards the formation of the placenta?

Answer 14

The placenta is co-created by the mother and foetus, with contributions from endometrial as well as embryonic cells/tissue.

Question 15

Describe the development of the placenta

Answer 15

• Around day 17, foetal mesoderm cells start to form blood vessels within the villi – a basic network of arteries, veins and capillaries. Capillaries connect with blood vessels in the umbilical cord (formed around week 5).
• Villi grows larger in size, develops into the Chorionic Frondosum.
• At this point, endothelial cell wall and Syncytiotrophoblast (villi) lining separate maternal and foetal red blood cells.
• On ultrasound, chorionic cavity shows up as a large dark space. Used to identify a pregnancy even before a foetus can be seen.

Question 16

What vessels does the umbilical cord contain?

What does it drop off and take?

Answer 16

• Umbilical cord normally contains two arteries and one vein.

On image

Question 17

What are the functions of the placenta? (5)

Answer 17

• Provision of maternal O2, CHO, fats, amino acids, vitamins, minerals, antibodies.
• Metabolism e.g. synthesis of glycogen.
• Barrier e.g. bacteria, viruses, drugs etc.
• Removal of foetal waste products e.g. CO2, urea, NH4, minerals.
• Endocrine secretion e.g. hCG, oestrogens, progesterone, HPL, cortisol

Question 18

Why is the placenta good at it’s job?

Answer 18

• Huge maternal uterine blood supply – low pressure.
• Huge surface area in contact with maternal blood.
• Huge reserve in function.

Question 19

What is PRE-ECLAMPSIA?

When does it occur?

What is it?

What does it cause?

If you get seizures what is it called?

How does it cause proteinuria?

Answer 19

PRE-ECLAMPSIA:
• 3-4% of pregnancies.
• ≥20 weeks gestation (up to 6 weeks after delivery).
• Results in placental insufficiency – inadequate maternal blood flow to the placenta during pregnancy.
• Placental insufficiency – inadequate maternal blood flow to the placenta during pregnancy. This in turn compromises the transfer of necessary nutrients to support the development of the foetus.
• Pre-eclampsia usually occurs after around 20 weeks gestation. Could still develop after delivery – up to 6 weeks after delivery.
• Causes new onset maternal hypertension and proteinuria.
• Symptoms range from mild to life-threatening.
RISK FACTORS
• First pregnancy
• Multiple gestation
• Maternal age >35yo
• Hypertension
• Diabetes
• Obesity
• Family history of pre-eclampsia
Pre-eclampsia + seizures = Eclampsia.
• Primary cause is still unclear.
• Characterised by the narrowing of the maternal spiral arteries supplying blood to the placenta.
This is what happens in response to a reduced blood supply:

Increased retention of fluid results in high blood pressure.
Vasoconstriction results in distorted blood supply to the brain causing the siezures shown in the previous slide

Question 20

What is a PLACENTAL ABRUPTION and what are the symptoms?

What are the risk factors?

Answer 20

Premature separation of all or part of the placenta.

Symptoms include vaginal bleeding and pain in the back and abdomen

• Blunt force trauma e.g. car crash, fall
• Smoking & recreational drug use – risk of vasoconstriction and increased blood pressure.
• Multiple gestation
• Maternal age >35yo
• Previous placental abruption

Question 21

What are the causes of placental abruption?

Answer 21

• Caused by the degeneration of maternal arteries supplying blood to the placenta.
• Degenerated vessels rupture causing haemorrhage and separation of the placenta.

Question 22

What are the maternal and fetal complications of a placental abruption?

Answer 22

• Hypovolemic shock
• Sheehan Syndrome (Perinatal Pituitary Necrosis)
• Renal failure
• Disseminated Intravascular Coagulation (from release of thromboplastin)
• Intrauterine hypoxia and asphyxia
• Premature birth

Question 23

What is PLACENTA PREVIA?

What is it associated with?

Answer 23

• Placenta implants in lower uterus, fully or partially covering the internal cervical os.
• Associated with increased chances of pre-term birth and foetal hypoxia.

Question 24

What are the risk factors of PLACENTA PREVIA?

Answer 24

•	Previous caesarean delivery
•	Previous uterine/endometrial surgery
•	Uterine fibroids
•	Previous placenta previa 
•	Smoking & recreational drug use
•	Multiple gestation
•	Maternal age >35yo
Cause still unclear.
? Endometrium in the upper uterus not well vascularised

Question 25

Describe the hormonal changes throughout pregnancy

Answer 25

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Question 26

What are the other effects of pregnancy changes

Answer 26

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