Reproduction: Implantation, placentation and hormone changes during pregnancy Flashcards
What 2 things do you need for implantation to occur?
- A fully developed blastocyst - fully expanded and hatched out of zona pellucida
- A receptive endometrium - thickened endometrial lining and has blastocyst receptivity markers
How is the fully developed blastocyst able to hatch out of the zona pellucida?
- Enzymes dissolve the zona pellucida at abembryonic pole (opposite end from developing embryo)
- Series of expansions and contractions allow blastocyst to bulge out of zona pellucida
What are the stages of implantation?
- Apposition
- Attachment
- Invasion
Describe the first events of implantation once the blastocyst gets close to endometrial wall
- Blastocyst attaches itslef to the surface of the endometrial wall (decidua basalis)
- Once attached to the decidua basalis the trophoblast cells begin to form a syncytiotrophoblast to help blastocyst invade decidua basalis
What is a syncytiotrophoblast?
A multi-nucleated single cell made up of massive amounts of trophoblast cells
What occurs a result of the formation of the syncytiotrophoblast?
- Syncytiotrophoblast continues to invade decidua basalis until entire trophoblast is almost completely buried within the decidua basalis
What occurs once the blastocyst is completely buried within the decidua basalis?
- Decidual reaction occurs - Cells within the decidua begin to enlarge and become coated with fluid rich in lipids and glycogen
- Fluid is taken up by trophoblast cells of blastocyst which helps sustain these cells before blastocyst develops connection with placenta
What triggers the decidual reaction?
High progesterone levels due to secretions of progesterone from corpus luteum
What processes occur to the blastocyst after the decidual reaction?
- Cells of the syncytiotrophoblast form tree-like structures called primary villi which surrond blastocyst
- Primary villi begin to digest cells of the decidua basalis which form empty paces within the decidua called lacunae
What happens after the formation of the lacunae within the decidua basalis?
- Maternal arteries and veins grow into the decidua basalis
- These blood vessels then merge with the lacunae and so the lacunae become filled with blood
- Blood-filled lacunae then merge into a single pool of blood known as the junctional zone
What organ does the functional zone form the basis of?
Functional zone froms basis of placenta
Explain what processes occur during the first stages of placental formation
- Foetal mesoderm cells start to form blood vessels within the primary villi surronding the blastocyst
- The primary villi also enlarge to form the chorionic frondosum
At the beginning stages of placental formation what layers seperate the maternal and foetal circulations?
- Lining of the primary villi
- Endothelial cell lining of the decidua basalis
What does the umbilical cord connect to once it’s formed within the developing embryo?
Once umbilical cord forms it the connects to the capillaries within the primary villi that are formed by the mesoderm
What happens to the placenta during 4th/5th months of pregnancy?
- The decidual septa form and as a result they divide the placenta into regions called cotyledons
What advantages are there of each cotelydon having its own maternal blood supply?
- It makes the maternal-foetal gas exchange much more efficient
- This is because it increases surface area in which maternal-foetal gas exchnage can take place
What are some of the functions of the placenta?
- Metabolism e.g. synthesis of glycogen.
- Barrier to infection
- Removes foetal waste products
- Site of endocrine secretion e.g. hCG, oestrogens, progesterone, HPL, cortisol.
Why is the placenta able to carry out its functions effectively?
- Has a huge maternal uterine blood supply that’s at low pressure - this allows it to filter substances more efficiently
- Has a huge surface area in contact with maternal blood
What are the main symptoms of pre-eclampsia?
- Maternal hypertension
- Proteinurea - excess proteins within the urine
How does pre-eclampsia affect the placenta?
- Results in placental insufficiency which is inadequate maternal blood flow to the placenta during pregnancy
How can pre-eclampsia develop during pregnancy?
- In normal pregnancies spiral arteries that supply placenta dilate to allow more blood flow
- However, in pre-eclampsia these spiral arteries narrow due to them being fibrous
- This limits the blood supply to the placenta
How does the placenta respond to the lack of blood flow caused by pre-eclampsia and what does this result in?
- Placenta secretes pro-inflammatory proteins which then enter the maternal circulation
- Once in the maternal circulation the pro-inflammatory proteins cause the dysfunction of endothelial cells of the blood vessels of the mother
How does the dysfunction of the endothelial cells of the maternal blood vessels cause hypertension and proteinurea?
- Endothelial cell dysfunction causes Vasoconstriction of arteries
- This vasoconstriction results in hypertension
- Vasoconstriction affects arteries that supply kidneys and by extension the capillaries of the glomerulas
- This results in impaired glomerular filtration which leads to proteinurea
What is placental abruption?
Separation of all or part of the placenta from the endometrium
What are the symptoms of placental abruption?
- Vaginal bleeding
- Pain in the back and abdomen
What are some of the risk factors of placental abruption?
- Blunt force trauma
- Smoking/recreational drugs
- Hypertension due to pre-eclampsia
Explain how placental abruption can occur?
- Degeneration of maternal arteries supplying blood to the placenta.
- Degenerated vessels rupture causing haemorrhage and separation of the placenta.
What maternal complications can arise due to placental abruption?
- Hypovolemic shock - Isn’t enough blood for heart to pump around entire body
- Sheehan syndrome - degeneration of the pituitary gland
- Renal failure
How does placental abruption lead to sheehan syndrome?
- Due to hypovolemic shock there’s isn’t enough blood and oxygen being supplied to pituitary so it degenerates
What foetal complications can arise from placental abruption?
- Intrauterine hypoxia and asphyxia
- Premature birth
What is placenta previa?
Occurs wehn the placenta implants in the lower uterus and either partially or fully covers interal os of cervix
Once the blastocyst implants within the endometrium what hormone does it secrete?
Trophoblast cells of blastocyst secrete hCG
What is the effect of the secretion of hCG from the trophoblast cells?
- hCG binds to LH receptors on corpus luteum causing corpus luteum to secrete oestrogen and mainly progesterone
At what week of pregnancy does the placenta begin to take over secretion of oestrogen and progesterone from the corpus luteum?
Week 7
Apart from oestrogen and progesterone what other hormone is produced by the placenta?
hPL (Human placental lactogen)
During pregnancy why do you see a massive rise in hCG levels early on and then a rapid fall later on in pregnancy?
- Rapid rise due to the fact that hCG needed to keep corpus luteum alive and in early stages of pregnancy corpus luteum is what secretes oestrogen and progesterone
- Later on in pregnancy placenta takes over oestrogen and progesterone secretion from corpus luteum so corpus luteum no longer needed
- This results in massive decline in hCG levels
