Implantation, placentation and hormone changes in pregnancy

Question 1

what 2 things do we need for implantation to occur?

Answer 1

1. a fully fledged blastocyst
   - Fully expanded
   - Hatched out from the zona pellucida
2. a receptive endometrium
   - Thickened endometrial lining
   - Expression of embryo receptivity markers

Question 2

blastocyst structure?

Answer 2

1. made up of 2 cell lineages
   - trophoblasts, outer layer, forms the placenta
   - embryoblast, inner cell mass, forms the foetus
2. blastocoel is the fluid filled cavity

Question 3

what does the blastocyst bathe in and when does it hatch?

Answer 3

bathes in uterine fluid, begins to hatch around the end of day 5

Question 4

when does the zona pellucida become thinner?

Answer 4

after full blastocyst expansion

Question 5

how is hatching achieved?

Answer 5

• enzymes dissolve the ZP at the abembryonic pole

- series of rhythmic expansions and contractions cause the blastocyst to bulge out of the ZP

Question 6

summary of the stages of implantation:

Answer 6

apposition
-blastocyst positions itself close to the uterine wall or endometrium

attachment
-trophoblast cells attach to endometrium

invasion
-trophoblast cells multiply, invade into the endometrium and bury the blastocyst

Question 7

is implantation tightly regulated?

Answer 7

Really tight regulated window of implantation, about 24-36 hours where implantation can take place

Question 8

timeline of implantation

Answer 8

days 7-8

• blastocyst attaches to the surface of the endometrial wall (decidua basalis)
• trophoblast cells assemble and form a syncytiotrophoblast to facilitate invasion of decidua basalis

day 9-11

• syncytiotrophoblast further invades the decidua basalis
• by Day 11 almost completely buried in the decidua

day 12

• decidual reaction occurs
• high progesterone levels cause enlargement and coating of the decidual cells in glycogen and lipid-rich fluid
• fluid taken up by the syncytiotrophoblast, helps sustain the blastocyst early on before placenta formation

Question 9

what happens to the blastocyst at around day 14?

Answer 9

junctional zone formation

• cells of the blastocyst protrude outwards and form tree-like structures called primary villi which form all around the blastocyst
• decidual cells behind the primary villi clear out forming lacunae
• maternal arteries and veins grow into the decidua basalis, and merge with the lacunae, arteries fill the lacunae with oxygenated blood
• veins return deoxygenated blood
• blood filled lacunae merge into a large pool of blood connected to many arteries and veins - junctional zone

Question 10

is the placenta created by the mother?

Answer 10

no, the placenta is co-created by the mother and the foetus

-contributions from the endometrium and embryonic cells

Question 11

formation of the placenta

Answer 11

• foetal mesoderm cells form blood vessels within the villi
• villi grow in size and develop into the Chorionic Frondosum
• endothelial cell wall and Syncytiotrophoblast (villi) lining separate maternal and foetal red blood cells

Question 12

what forms in the fourth and fifth months of pregnancy?

Answer 12

decidual septa

Question 13

what is the role of the decidual septa?

Answer 13

divides the placenta into 15-20 regions called Cotyledons.

• all supplied by different spiral arteries
• gives a larger SA in contact with maternal blood, more efficient with nutrient exchange

Question 14

is the placenta formed in the upper or lower uterus?

Answer 14

upper

Question 15

maternal foetal exchange?

Answer 15

foetus takes up oxygen and glucose, immunoglobins and hormones
-drops off co2 and waste products

Question 16

functions of the placenta?

Answer 16

• 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 17

Why is the placenta good at it’s job?

Answer 17

Huge maternal uterine blood supply – low pressure.

Huge surface area in contact with maternal blood.

Huge reserve in function.

Question 18

name 3 placental disorders?

Answer 18

• pre-eclampsia
• placental abruption
• placenta previa

Question 19

explain pre-eclampsia

Answer 19

• 3-4% of pregnancies
• constriction of maternal spiral arterioles which supply blood to the foetus
• results in placental insufficiency
• placenta responds to narrowing by releasing pro-inflammatory proteins which enter maternal circulation
• vasoconstriiiton, hypertension
• glomerular damage and proteinuria

Question 20

what is placental insufficiency?

Answer 20

inadequate maternal blood flow to the placenta during pregnancy

Question 21

risk factors for pre-eclampsia

Answer 21

First pregnancy
Multiple gestation
Maternal age >35yo
Hypertension
Diabetes
Obesity
Family history of pre-eclampsia

Question 22

placental abruption

Answer 22

• premature separation of all or part of the placenta
• caused by degeneration of maternal arteries supplying blood to the placenta
• arteries rupture
• haemorrhage and placental separation

Question 23

risk factors of placental abruption

Answer 23

• 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
• Hypertension from severe pre-eclampsia

Question 24

complications of placental abruption?

Answer 24

COMPLICATIONS – MATERNAL
Hypovolemic shock
Sheehan Syndrome (Perinatal Pituitary Necrosis)
Renal failure
Disseminated Intravascular Coagulation (from release of thromboplastin)

COMPLICATIONS – FOETAL
Intrauterine hypoxia and asphyxia
Premature birth

Question 25

explain how placental abruption causes coagulation?

Answer 25

Degeneration of blood vessels means clotting factors released into blood circulation, causing coagulation

Question 26

explain what is meant by hypovolemic shock and why it causes Sheehan Syndrome

Answer 26

heart doesn’t have enough blood to circulate to all the regions of the body - not enough blood supplied to the pituitary so it dies. Loss of secondary sexual features

Question 27

what is placenta previa?

Answer 27

placenta implants in the lower uterus, partially covering the lower os
-associated with increased chances of pre-term birth and foetal hypoxia

Question 28

in placenta previa, why might the placenta implant in the lower uterus?

Answer 28

endometrium in the upper uterus might not be well vascularised

caesarean delivery or uterus surgery can cause upper uterus to be less vascularised, and when the blastocyst is trying to find an implantation point it will move to a more vascularised area.

Question 29

once the blastocyst has implanted what is secreted?

Answer 29

trophoblasts start to secrete hcg which binds to LH receptors on CL, causing synthesis and secretion of oestrogen and progesterone
-progesterone produced in much higher amounts than oestrogen

Question 30

when does serum HCG reach peak level?

Answer 30

by 9-11 weeks

Question 31

when does the placenta take over from the CL?

Answer 31

week 7

Question 32

what does the placenta synthesise?

Answer 32

• synthesises oestrogens from foetal androgens from the foetal adrenal cortex
• placenta synthesises progesterone from maternal cholesterol

Question 33

beta-HCG levels

Answer 33

Very rapid increase, gradual fall in HCG