Bilaminar Disc Formation Flashcards
On which day does the blastocyst implant into the uterine lining?
around day 5 - 6
What must happen prior to the blastocyst implanting in the uterine lining?
It must separate from the zona pellucida
On which day does differentiation of the trophoblast and embryoblast cells begin?
day 7
each type of cell differentiates into 2 different types of cell and forms 2 layers
What do trophoblast cells differentiate into?
cytotrophoblasts and syncytiotrophoblasts
These will form the support structure
What do embryoblast cells differentiate into?
Hypoblasts and epiblasts
These will form the embryo itself
Around day 7, what are the 2 key roles of the syncitiotrophoblast cells?
- they assist the implantation of the blastocyst into the uterine lining
- they prevent the mother’s immune system from attacking the blastocyst
Why is the blastocyst recognised as a foreign body by the mother’s immune system?
Half of the genes come from the father
How do the syncitiotrophoblasts hide the blastocyst from the mother’s immune system?
They release substances that act on the white blood cells of the immune system to chemically alter it
How do the syncitiotrophoblasts hide the blastocyst from the mother’s immune system in a non-chemical way?
The syncitiotrophoblast cells lose the cell membrane between the cells and form multinucleate cells
There are few gaps between the cells where white blood cells could penetrate to reach the embryo
What cells make up the bilaminar disc?
What is significant about the bilaminar disc?
The epiblasts and the hypoblasts
The bilaminar disc establishes the dorsal and ventral axis of the embryo - the first stage in organisation of the body plan
Which cells form the dorsal and ventral surfaces of the embryo in the bilaminar disc?
The epiblast forms the dorsal surface of the embryo that implants into the uterine lining
The hypoblast forms the ventral surface of the embryo
On what day does the amniotic cavity form?
day 8
What is the first stage in formation of the amniotic cavity?
Syncitiotrophoblasts lose their cell membranes and form multinucleated cells as they move further into the uterine lining
What is the second stage of amniotic cavity formation?
A small fluid-filled cavity forms within the epiblast of the bilaminar disc
On which day does full implantation occur?
day 9
What is the coagulation plug?
This forms due to where the uterine lining has healed after implantation
It is made from fibrous tissue
What happens to the amniotic cavity after full implantation?
It increases in size
What happens to the hypoblast cells after full implantation?
What do they form?
They begin to migrate around the inside of the blastocyst cavity
This forms the primary yolk sac
On day 9, where does the embryoblast receive nutrients from?
Trophoblastic lacuna
Fluid-filled cavities within the cynsitiotrophoblast layer
How does the embryoblast receive nutrients from the trophoblastic lacuna?
Nutrients from the uterine gland will diffuse into the trophoblastic lacuna
They will then diffuse into the embryoblast
On what day does uteroplacental circulation begin to be formed?
around 10 to 11 days
What is the first stage in development of uteroplacental circulation?
The syncitiotrophoblast erodes the walls of maternal capillaries by degradative enzymes
Blood then flows into the trophoblastic lacuna
Why can development of the uteroplacental circulation confuse some women?
Breaking down maternal capillaries leads to some bleeding
Women confuse this with their menstrual period as the bleeding occurs 4 weeks after their last period
What do the hypoblast cells form as uteroplacental circulation develops?
extraembryonic mesoderm
What is the extraembryonic mesoderm?
A new layer of connective tissue that forms between the primary yolk sac and the cytotrophoblast
what does the extraembryonic mesoderm go on to cover?
It develops to cover the whole of the embryo and the amniotic cavity
What happens around day 12 - 13 of development?
The chorionic cavity forms within the extraembryonic mesoderm
This develops to cover all of the embryo
On what day does the definitive yolk sac form?
What is its alternative name?
It forms around day 12 - 13
It is also called the secondary yolk sac
What is the secondary yolk sac formed from?
A second wave of migration by the hypoblast cells
What happens to the primary yolk sac as the secondary yolk sac forms?
As hypoblast cells migrate, they push the primary yolk sac away from itself
The primary yolk sac is only temporary until the second migration of hypoblast cells
What happens to the chorionic cavity during formation of the secondary yolk sac?
What are the 3 cavities present by day 13?
Chorionic cavity continues to expand
The 3 main cavities are amniotic, definitive and chorionic
By the end of week 2, what is the state of the extraembryonic mesoderm?
The small amount that is left forms the connecting stalk
what is the role of the connecting stalk by the end of week 2?
The amniotic cavity and the secondary yolk sac are suspended in the chorionic cavity by the connecting stalk
What hormone is secreted by syncitiotrophoblast cells?
human chorionic gonadotrophin (hCG)
What is the role of hCG in pregnancy?
It maintains the uterine lining when levels are high
If there is no fertilised oocyte, the uterine lining is shed
What is involved in a pregnancy test?
Why does it work?
measuring the levels of hCG in urine
hCG is only secreted in high levels when there are syncitiotrophoblast cells
What is the mechanism behind a pregnancy test?
It uses an immunoassay where antibodies within the stick will bind to hCG if it is present
No other hormones will bind to the antibody to give a positive result
What can often cause a false positive on a pregnancy test?
Drugs containing hCG which are used to treat infertility
Under what conditions does an ectopic pregnancy occur?
It occurs when the fertilised egg implants outside of the uterine cavity
What % of pregnancies involve an abnormal implantation site?
What is the most common site of abnormal implantation?
2%
The egg is implanted in the ampulla or the uterine tube
What happens if the fertilised egg enters the abdominal cavity?
It will fall through a gap between the uterine tube and the ovary
The embryo cannot develop a sufficient blood supply so will not survive
What is placenta previa?
The birthing canal is covered by the placenta
When the baby puts pressure on it, there is a risk of rupture
If it ruptures, the foetus will not have a blood supply
What is the largest consequence of placenta previa and how is it avoided?
Severe bleeding in late pregnancy
A C-section occurs before this situation is reached as it can be identified on ultrasound
What happens after a fertilised egg has implanted in the uterine tube?
The embryo becomes too large and the uterine tube ruptures
This destroys the support structures and is dangerous to the mother
Where may the embryo develop in abdominal implantation?
Rectouterine pouch between the rectum and uterine tube
If it migrates up to the liver is may survive to full term as the liver is very vascular
How does lithopaedion occur?
When an abdominal pregnancy remains undiagnosed and the foetus dies
What usually happens when a foetus dies?
The body breaks down and absorbs the dead foetus
What happens when a foetus dies that leads to lithopaedion?
Why does this happen?
If the foetus is too large to be absorbed, it calcifies
This protects the mother from the necrotic tissue of the foetus
What causes a hydatidiform mole?
A sperm fertilises an empty oocyte that doesn’t have a nucleus
What develops in a hydatidiform mole?
The trophoblast develops without any embryonic tissue
Why does a hydatidiform mole lead to a false positive on a pregnancy test?
Syncitiotrophoblasts cells are present and secreting hCG
