Fertilization and Placental development Flashcards
Fertilization and beyond
- Sperm and Ovum meet in Fallopian Tube (Uterine Tube) (usually ampulla) 12-24 hours after ovulation.
- Fusion occurs and 2nd meiotic division occurs
•Acrosome reaction makes ovum impermeable to other sperm
•End- Zygote- has diploid (46 chromosomes)
Zygote to blastomere
Initially, you have the zygote which divides into 2 cells, and then 4 cells, 8 cells (Morula, about 72hrs), the blastocyst will then form the baby in about 4 days after fertilisation.,
Days 4-5
- The morula develops a cavity and becomes known as a blastocyst.
- Blastocyst thins out and becomes the trophoblast – you can see the trophoblast. around the outside of the cell and it is what turns into the placenta.
- The rest of the cells move (are pushed up) to form the inner cell mass. This creates an embryonic pole.
The blastocyst has now reached the uterine lumen and is ready for implantation
Day 6-7 Bilaminar disc of the embryo
- Inner cell mass differentiates into two layers: epiblast and hypoblast.
- These two layers are in contact.
- Hypoblast forms extraembryonic membranes and the primary yolk sac
- Epiblast forms embryo
- Amniotic cavity develops within the epiblast mass
Days 16 +
Bilaminar disc develops further by forming 3 distinct layers (this process is known as gastrulation
- Initiated by primitive streak.
- The epiblast becomes known as ectoderm
- The hypoblast is replaced by cells from the epiblast and becomes endoderm
- The epiblast gives rise to the third layer the mesoderm.
Embryo – the 3 germ layers
- The hypoblast degenerates. The epiblast gives rise to all three germ layers.
- The embryo folds to create the adult pattern
The development of the placenta
The Syncytiotrophoblast cells are very important for the placenta. They invade into the uterus, into the myometrium, they invade the spiral arteries which are the mum’s arteries inside the myometrium and starting the formation of the primary/secondary and tertiary villi.
Formation of the Placenta
- Syncytiotrophoblast invades decidua (endometrium)
- Cytotrophoblast cells erodes maternal spiral arteries and veins
- Spaces (lacunae) between the fill up with maternal blood
- Followed by mesoderm that develops into fetal vessels
- Aiding the transfer of nutrients, O2, across a simple cellular barrier
- Syncytiotrophoblast invades decidua (endometrium)
- Cytotrophoblast cells erodes maternal spiral arteries and veins
•Big Spaces called (lacunae) develop and the fill up with maternal blood
- Followed by mesoderm that develops into fetal vessels
- Aiding the transfer of nutrients, O2, across a simple cellular barrier
IN THE IMAGE.
The top image shows what should be happening and the image below shows what happens in pre-eclampsia and growth restriction.
- In the picture above, you can see the Syncytiotrophoblast cells eroding the maternal blood vessels and we end up with a nice and wide space for blood flow.
- In the picture below, the Syncytiotrophoblast cells are not going in as deep and the invasion of the arteries isn’t happening, The spiral artery is much narrower, it didn’t turn into a nice blood-filled space like it should have been. It also has higher resistance which makes it harder for the blood to flow between mum and baby. This leads to a lot of complications in pre-eclampsia and growth restrictions.
Cytotrophoblast cells (CTB) VS Syncytiotrophoblast cells (STB)
The Placenta as an Endocrine Organ
- Produces Human chorionic gonadotrophin (HCG) which maintained the corpus luteum of pregnancy
2• produces progesterone and oestrogen. the corpus luteum starts this role and the placenta takes over when the corpus luteum degrades.
- Produces Human placental lactogen (HPL) which alter maternal lipid and carbs metabolism.
basically helps. with growth and lactation.
The placenta has many more functions. Many more!
Placental Barrier
Maternal blood in the lacunae (the space) in direct contact with syncytiotrophoblasts
Mono layer of syncytiotrophoblast/cytotrophoblast/fetal capillary epithelium is all that separates the fetal and maternal blood
Cytotrophoblasts decrease as the pregnancy advances (not needed)
The barrier thins as pregnancy advances leading to a greater surface area for exchange (over 10m2 )
Transfer Across the Placenta. what get s transferred?
1. Gases – oxygen and carbon dioxide by simple diffusion
2. Water and electrolytes
3. Steroid hormones
4. Proteins poor – only by pinocytosis. This is how antibodies is transferred from mother to baby. that is why we ask mothers to get immunised because the IgG can cross the placenta.
Transfer of maternal antibodies IgG -starts at 12 weeks – but occurs mainly after 34 weeks therefore lack of protection for premature infants.
pinocytosis, is a mode of endocytosis in which small particles suspended in extracellular fluid are brought into the cell through an invagination of the cell membrane.
Named Parts of the Decidua
don’t have to memorise
Placenta
The top image shows the maternal surface; the bit that is attached to the uterus, the bottom is the fetal surface.
Different types of Umbilical cord insertions
normally the cord comes out of the placenta and sometimes it can come out of the edge
Vasa praevia
Vasa praevia is a condition in which fetal blood vessels cross or run near the internal opening of the uterus. These vessels are at risk of rupture when the supporting membranes rupture, as they are unsupported by the umbilical cord or placental tissue.
The cord insertion is running across the cervix and this is very important to know because when the membranes rupture, you get bleeding from the fetal blood vessels that are going over the cervix and this can be fatal for the baby.
Velamentous insertion of the umbilical cord refers to insertion of the cord into the membranes rather than directly into the placenta. Fetal vessels then traverse the membranes freely, unprotected by Wharton’s Jelly or placental tissue, to insert into the placenta.
Placenta Accreta
This is a spectrum of abnormality where the placenta invades abnormally and is unable to separate after the baby is delivered.
- you can see a normal placenta on the left where you have a normal connection between maternal and fetal circulation
- In placenta ACCRETA; the placenta is going too deep so at the time of delivery, its very hard to separate the placenta and this can lead to a massive hemorrhage.
- INCRETA (invading through the myometrium, almost to the outside of the uterus and this placenta will never come out) and PERCRETA (worst form, goes all the way through the myometrium through the uterus and can invade into surrounding organs like bowel , bladder, blood vessels and this will alsmot certainly end in a hysterectomy) are more extreme versions of ACRETA.
- if you have a scar on your uterus, you are more at risk of placenta accrete, i.e if the placenta attaches on the scar, i.e people who have had a previous C section. the more cessarians you have the more the risk of a placenta acreta. or even some surgically mamanged miscarriages can also be a risk factor for accreta.
- by the time you have had 3 cesarian, you have about 25% risk of a placenta previa.
Placental abruption
This is where the placenta starts to detach from the uterus. It can completely detach. This can cause a massive haemorrhage in pregnancy. the difference between this and placenta praevia bleeding is that placenta abruption is usually very painful while placenta praevia is usually painless.
It is very dangerous mum and baby.
Placenta in multiple pregnancies
Not to be memorised
