Implantation, the placenta and hormonal changes during pregnancy Flashcards
What is required for implantation to occur?
- A fully developed blasocyst
- A receptive endometrium
What are the two cell lineages which make up the blasocyst?
- Embyroblast/inner cell mass (gives rise to foetus)
- Trophoblast (gives rise to the placenta)
What is the fluid filled cavity in the blastocyst called?
Blastocoel
What does the trophoblast form?
Trophoblast forms the placenta
What does the embyroblast form?
Foetus
What do we require for implantation (in detail)?
-
Fully developed blastocyst
- Fully expanded + hatched out from the zona pellucida
-
A receptive endometrium
- Thickened endometrial lining, expression of embryo receptivity markers
Where does the blastocyst hatch out from?
Blastocyst hatches out of the zona pellucida
Give a brief description of what occurs after fertilisation?
- Oocyte is released from the ovary into the fallopian tube via ovulation from the dominant follicle during the LH surge
- Fertilisation occurs in the ampulla of the uterine tube
- This results in two pro-nuclei forming which then fuse
- The cells begin to divide into 2 cells, then 4 then 8
- Series of cell divisions occurs = MORULA FORMATION
- From this you get blastocyst formation where the cells differentiate into trophoblast and embryoblast cells
- During blastocyst formation and expansion, the zona pellucida starts to shrink
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- During blastocyst formation and expansion, the zona pellucida starts to shrink
What day does the blastocyst hatch from the zona pellucida?
Day 6-7 (check notes??)
What are the three stages of implantation?
- Apposition - close positioning of the blastocyst to the endometrium
- Attachment - cells of trophoblast attaching blastocyst to endometrium
- Invasion - trophoblast cells invade endometrium and completely implant the embryo
What happens on day 7-8 of 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 to facilitate invasion of the decidua basalis
What happens on day 9-11 of the implantation timeline?
Syncytiotrophoblast further invades the decidua basalis and by Day 11 its almost completely buried in the decidua.
What happens at around day 12 of implantation?
- Decidual reaction occurs.
- High levels of progesterone (released from corpus luteum) 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 day 14 of the implantation time line?
- Cells of the syncytiotrophoblast starts to protude out to form tree like structures known as primary villi - around the blastocyst
- Decidual cells between the primary villi clear out leaving empty spaces called lacunae
- The junctional zone formes which is when the maternal arteries and veins grow into the decidua basalis merging with the lacunae filling it with oxygenated blood and the veins returning deoxygenated blood to maternal circulation
What is the junctional zone?
Circulatory foundation for formation of the placenta
What creates the placenta?
Co-created by both maternal and foetal contributions from endometrial as well as embryonic cells/tissues
What is the foetal and maternal contribution to the placenta?
Foetal contribution = chorionic frondosum
Maternal contribution = maternal spinal arteries
What occurs at day 17?
Growth of the placenta
- The foetal mesoderm cells form blood vessels within the villi (basic network of arteries, veins and capillaries)
- Capillaries connect with the blood vessels in the umbilical cord
- Primary villi grow larger in size - develops into the chorionic frondosum
- At this point - endothelial cell wall and syncytiotrophoblast (vili) lining seperate maternal and foetal red blood cells
What seperates maternal and foetal red blood cells?
The endothelial cell wall and syncytiotrophoblast (villi)
What happens once the umbillical cord is formed?
Once the umbilical cord is formed it connects to the capillaries formed by the mesoderm which feed into the junctional zone - supplied by the maternal circulation
What are cotelydons and how are they formed?
What is their function?
Divisions of the placenta by the decidual septa forming 15-20 regions = COTYLEDONS
- Numerous maternal arteries supply blood to each cotyledon, facilitating efficient maternal foetal exchange
Describe exchange of the placenta maternal and foetal
-
The placenta takes up
- O2 and glucose
- Immunoglobulins
- Hormones
- Toxins (some cases)
-
The placenta will drop off
- CO2
- Waste products (urea and NH3)
Where does the placenta form?
In the upper uterus
What does the umbillical cord contain?
Contains two arteries and one vein
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.
- Site of endocrine secretion e.g. hCG, oestrogens, progesterone, HPL, cortisol.
Why is the placenta good at its function?
- Huge maternal uterine blood supply – low pressure. (allows to filter substances efficiently)
- Huge surface area in contact with maternal blood. (cotyledons)
- Huge reserve in function
What does pre-eclampsia cause?
- Results in placental insufficiency
- Maternal hypertension
- Characterised by the narrowing of the maternal spinal arteries - supplying blood to the placenta
- Proteinuria (glomerular damage due to hypertension)
What is eclampsia?
Medical emergency - where you have pre-eclampsia and seizures
What are the risk factors of pre-eclampsia?
- First pregnancy
- Multiple gestation
- Maternal age < 35 years old
- Hypertension
- Diabetes
- Obesity
- Family history of pre-eclampsia
How does pre-eclampsia cause proteinuria and maternal hypertension?
- Pro-inflammatory proteins enter the mother’s circulation causing endothelial cells dysfunction.
- Causes vasoconstriction, hypertension and glomerular damage (if there is glomerular damage = proteinuria)
What is placental abruption?
Premature seperation of all or part of the placenta
What are the symptoms of placenta abruption?
Vaginal bleeding and pain in the back + abdomen
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
- Hypertension from severe pre-eclampsia
What causes placental abruption?
- Degeneration of maternal arteries supplying blood to the placenta
- Degenerated vessels rupture causing a haemorrhage and seperation of the placenta
What are the maternal complications of placental abruption?
- Hypovolemic shock (heart doesn’t have enough blood to circulate other organs in the body)
- Sheehan Syndrome (Perinatal Pituitary Necrosis) (degeneration of pituitary gland due to not enough blood, loss of secondary sexual features e.g decreased breast size and loss of pubic hair)
- Renal failure
- Disseminated Intravascular Coagulation (from release of thromboplastin)
What are the foetal complications of placental abruption?
- Intrauterine hypoxia and asphyxia
- Premature birth
What is placenta previa?
When the placenta implants in the lower uterus fully or partially covering the internal cervical os
Normally placenta would implant in the upper uterus
What is placenta previa associated with?
Increased chances of per-term birth and foetal hypoxia
What is the cause of placenta previa?
Cause still unclear.
However, correlations with individuals who have previous caesarean delivery or uterine/endometrial surgery. This is because the areas which have been operated on become less vascularised resulting in the embryo/blastocyst to implant in areas with are more vascularised e.g lower uterus
What are 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
Briefly summarise the hormonal changes in pregnancy
- Once trophoblast cells have implanted into the endometrium they will start secreting beta hCG
- This binds to LH receptors on the corpus luteum
- = synthesis and secretion of oestrogen and progesterone
- Week 7 the placenta starts to take over and there is a decline in hCG due to degeneration of the corpus luteum
- However, there is continous production of oestrogen + progesterone
- Also production of hPL (human placental lactogen)
- hPL makes the mother more resistant to maternal insulin = more glucose available for mother and foetus
What is important about the release of progesterone and oestrogen?
Low ratio of oestrogen: progesterone ratio
This is to maintain pregnancy and prevent the maturation of any other follicles in the ovary
What is the role of hPL?
hPL makes the mother more resistant to maternal insulin (insulin decreases blood glucose levels)
- This makes glucose more readily available to the mother and foetus
When is beta hCG detected and by what?
Beta hCG is detected in the blood stream as early as day 9 and is the basis of urinary qualitative pregnancy tests
What is serum hCG useful in detecting?
Monitors early pregnancy complications e.g ectopic pregnancy, miscarriage
When does the placenta take over producing hormones?
At around week 7 when there is degeneration of the corpus luteum
What does the placenta synthesise oestrogens from?
What does the placenta synthesise progesterone from?
What is pregnenolone converted into and where?
From foetal androgens from the foetal adrenal cortex
Maternal cholesterol
Converted into progesterone in placental mitochondria
What roles do oestrogen and progesterone have in pregnancy?
- Shallow breathing
- Increased blood volume
- Increased urinary output
- Breast changes
- Loosened ligaments
- Nausea and taste changes
- Mood changes
Describe how the levels of these hormones change during pregnancy
- hCG
- hPL
- Oestrogen and progesterone
- hCG = Week 10 - hCG is at the highest levels by the end of the weeks of gestation the hCG are at the lowest
- hPL = increases gradually from week 5 to week 40 (end of gestation)
- Oestrogen + Progesterone = increases gradually over 40 weeks of gestation
