11- Implantation, the Placenta and Hormonal Changes in Pregnancy Flashcards
What do we need for implantation to occur?
A fully developed blastocyst
- fully expanded
- hatched out from the zona pellucida
A receptive endometrium
- thickened endometrial lining
- expression of embryo receptivity markers
What are the 2 definitive cell lineages that are formed at the blastocyst stage and what do they cells give rise to?
Trophoblast -
- outside cells
- gives rise to placenta
Embryoblast or inner cell mass (ICM) -
- inside cells
- gives rise to the foetus
How is the blastocyst labelled?
The position where the inner cell mass is concentrated is known as the embryonic pole and the opposite end is the abembryonic pole
What are the 3 stages of implantation?
- Apposition - the positioning of the blastocyst close to the endometrium
- Attachment - the attachment of the blastocyst to the endometrium
- Invasion - the blastocyst burrows into the endometrium and implants its self
What happens on days 7-8 on the implantation timeline?
Blastocyst attaches itself to the surface of the endometrial wall (decidua basalis).
Trophoblast cells start to assemble to form a Syncytiotrophoblast in order facilitate invasion of the decidua basalis.
What happens on days 9-11 on the implantation timeline?
Syncytiotrophoblast further invades the endometrial wall and by day 11 its almost completely buried in the decidua (endometrial wall)
What happens on day 12 on the implantation timeline?
Decidual reaction occurs - remodelling of the cells of the endometrium driven by high progesterone levels
High levels of progesterone result in the enlargement and coating of the decidual cells in glycogen and lipid-rich fluid.
This fluid is taken up by the Syncytiotrophoblast and helps to sustain the blastocyst early on before the placenta is formed.
What happens around the 14th day in the implantation timeline?
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 known as the Junctional Zone.
What is the Junctional Zone known as?
The circulatory foundation for the formation of the placenta
What happens on day 17 after fertilisation to form the placenta?
The foetal mesoderm cells start to form blood vessels within the villi. Capillaries connect with blood vessels in the umbilical cord. The villi grows larger in size and develops into the Chorionic Frondosum. At this point the endothelial cell wall and Syncytiotrophoblast (villi) lining separate maternal and foetal red blood cells
When does the decidual septa form?
They form in the 4th and 5th month of pregnancy they form as they divide the placenta into 15-20 regions known as Cotyledons
How is each cotyledon supplied?
They are supplied by numerus maternal arteries facilitating the maternal foetal exchange
How does the placenta facilitate maternal and foetal exchange?
Takes up:
- oxygen and glucose
- immunoglobulins
- hormones
- toxins (in some cases)
Drops off:
- carbon dioxide
-waste products
What are the functions of the placenta?
- 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.
Why is the placenta good at its job?
- Huge maternal uterine blood supply – low pressure (allows for an efficient filtration system)
- Huge surface area in contact with maternal blood.
- Huge reserve in function.
What are some disorders of the placenta?
- Pre- eclampsia
- Placental Abruption
- Placental Previa
Describe Pre-eclampsia
It occurs in 3-4% of al pregnancies and occurs after 20 weeks in gestation and up to 6 weeks after delivery.
It results in placental insufficiency - inadequate maternal blood flow to the placenta during pregnancy
It causes new onset maternal hypertension and proteinuria
It is characterised by the narrowing of the maternal spiral arteries supplying blood to the placenta - reduced blood flow to the placenta
What are the risk factors of Pre-eclampsia?
- First pregnancy
- Multiple gestation
- Maternal age >35yo
- Hypertension
- Diabetes
- Obesity
- Family history of pre-eclampsia
Pre- eclampsia + seizures = eclampsia (life-threatening)
How does pre-eclampsia cause hypertension?
Due to the constriction of blood vessels there is reduced blood supply to the placenta this causes the release of pro-inflammatory proteins that enter the maternal circulation and cause endothelial cell dysfunction. This causes vasoconstriction in blood vessels which reduces the blood supply to other organ systems, this affects the kidneys which disrupts glomerular filtration to retain more salt and retains more fluid, the combination of this causes hypertension
How does pre-eclampsia cause proteinuria?
Due to the reduced blood flow in the kidney this causes glomerular damage so the filtration system is damaged and proteins are not filtered and they enter the urine causing proteinuria
How does pre-eclampsia cause seizures?
Due to vasoconstriction there is high blood pressure in the brain which causes a distorted blood supply in the brain causing disruption in brain activity leading to seizures
What is placental abruption?
The premature separation of all or part of the placenta. Its symptoms include vaginal bleeding and pain in the back and the abdomen
What is the cause of placental abruption?
Its caused by the degeneration of maternal arteries supplying blood to the placenta.
Degenerated vessels rupture causing haemorrhage and separation of the placenta.
What are the risk factors of placental abruption?
- 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
What are the maternal and foetal complications of placental abruption?
Maternal complications:
- Hypovolemic shock
- Sheehan Syndrome (Perinatal Pituitary Necrosis)
- Renal failure
- Disseminated Intravascular Coagulation (from release of thromboplastin)
Foetal complications:
- Intrauterine hypoxia and asphyxia
- Premature birth
What is Placental Previa?
- Placenta implants in lower uterus, fully or partially covering the
internal cervical openings - Associated with increased chances of pre-term birth and foetal
hypoxia. - May be caused due to the endometrium in the upper uterus not being fully vascularised
What are the risk factors of placenta previa?
- Previous caesarean delivery
- Previous uterine/endometrial surgery
- Uterine fibroids
- Previous placenta previa
- Smoking & recreational drug use
- Multiple gestation
- Maternal age >35yo
What is the function of hCG?
It maintains the corpus luteus, it has LH like properties that allow it to bind to LH receptors on the corpus luteus to produce oestrogen and progesterone
What is the ratio in which oestrogen and progesterone is secreted from the corpus luteus?
low oestrogen to progestogen ratio
the low ratio of oestrogen is necessary for maintaining pregnancy
Describe the changes that occur in hormone secretion during pregnancy
During the first 7 weeks of pregnancy the corpus luteum hCG, oestrogen and progesterone.
At around 7 weeks the placenta takes over hormone secretion and hCG production is reduced and hPL is produced by the Syncytiotrophoblast cells in the placenta and is secreted by the placenta
How does the placenta take over progesterone production and secretion?
Cholesterol and LDL from the mother is taken in by the placenta and is converted into pregnenolone and then into progesterone and is secreted by the placenta back to the mother
How does the placenta take over oestrogen production and secretion?
Oestrogens are synthesised via the conversion of foetal adrenal androgens.
Cholesterol and LDL reserves in the foetus are converted into pregnenolone and is converted into DHEAS in the adrenal gland this is then transported back into the placenta and is converted into oestrogens by enzymes and is secreted back into the mother by the placenta
What is hPL?
hPL - Human placental lactogen
counter acts the affect of maternal insulin
synthesised by the Syncytiotrophoblast cells and is secreted by the placenta after week 7
What changes occur during pregnancy due to the rise in oestrogen and progesterone?
- increase in blood volume
- shallow breathing
- increased urinary output
- mood changes
- nausea and taste changes
- loosened ligaments
- breast changes
- darkened skin around the nipple
