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Embryology > Bilaminar Disc Formation > Flashcards

Bilaminar Disc Formation Flashcards

1
Q

On which day does the blastocyst implant into the uterine lining?

A

around day 5 - 6

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2
Q

What must happen prior to the blastocyst implanting in the uterine lining?

A

It must separate from the zona pellucida

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3
Q

On which day does differentiation of the trophoblast and embryoblast cells begin?

A

day 7

each type of cell differentiates into 2 different types of cell and forms 2 layers

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4
Q

What do trophoblast cells differentiate into?

A

cytotrophoblasts and syncytiotrophoblasts

These will form the support structure

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5
Q

What do embryoblast cells differentiate into?

A

Hypoblasts and epiblasts

These will form the embryo itself

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6
Q

Around day 7, what are the 2 key roles of the syncitiotrophoblast cells?

A
  1. they assist the implantation of the blastocyst into the uterine lining
  2. they prevent the mother’s immune system from attacking the blastocyst
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7
Q

Why is the blastocyst recognised as a foreign body by the mother’s immune system?

A

Half of the genes come from the father

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8
Q

How do the syncitiotrophoblasts hide the blastocyst from the mother’s immune system?

A

They release substances that act on the white blood cells of the immune system to chemically alter it

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9
Q

How do the syncitiotrophoblasts hide the blastocyst from the mother’s immune system in a non-chemical way?

A

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

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10
Q

What cells make up the bilaminar disc?

What is significant about the bilaminar disc?

A

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

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11
Q

Which cells form the dorsal and ventral surfaces of the embryo in the bilaminar disc?

A

The epiblast forms the dorsal surface of the embryo that implants into the uterine lining

The hypoblast forms the ventral surface of the embryo

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12
Q

On what day does the amniotic cavity form?

A

day 8

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13
Q

What is the first stage in formation of the amniotic cavity?

A

Syncitiotrophoblasts lose their cell membranes and form multinucleated cells as they move further into the uterine lining

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14
Q

What is the second stage of amniotic cavity formation?

A

A small fluid-filled cavity forms within the epiblast of the bilaminar disc

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15
Q

On which day does full implantation occur?

A

day 9

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16
Q

What is the coagulation plug?

A

This forms due to where the uterine lining has healed after implantation

It is made from fibrous tissue

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17
Q

What happens to the amniotic cavity after full implantation?

A

It increases in size

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18
Q

What happens to the hypoblast cells after full implantation?

What do they form?

A

They begin to migrate around the inside of the blastocyst cavity

This forms the primary yolk sac

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19
Q

On day 9, where does the embryoblast receive nutrients from?

A

Trophoblastic lacuna

Fluid-filled cavities within the cynsitiotrophoblast layer

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20
Q

How does the embryoblast receive nutrients from the trophoblastic lacuna?

A

Nutrients from the uterine gland will diffuse into the trophoblastic lacuna

They will then diffuse into the embryoblast

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21
Q

On what day does uteroplacental circulation begin to be formed?

A

around 10 to 11 days

22
Q

What is the first stage in development of uteroplacental circulation?

A

The syncitiotrophoblast erodes the walls of maternal capillaries by degradative enzymes

Blood then flows into the trophoblastic lacuna

23
Q

Why can development of the uteroplacental circulation confuse some women?

A

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

24
Q

What do the hypoblast cells form as uteroplacental circulation develops?

A

extraembryonic mesoderm

25
Q

What is the extraembryonic mesoderm?

A

A new layer of connective tissue that forms between the primary yolk sac and the cytotrophoblast

26
Q

what does the extraembryonic mesoderm go on to cover?

A

It develops to cover the whole of the embryo and the amniotic cavity

27
Q

What happens around day 12 - 13 of development?

A

The chorionic cavity forms within the extraembryonic mesoderm

This develops to cover all of the embryo

28
Q

On what day does the definitive yolk sac form?

What is its alternative name?

A

It forms around day 12 - 13

It is also called the secondary yolk sac

29
Q

What is the secondary yolk sac formed from?

A

A second wave of migration by the hypoblast cells

30
Q

What happens to the primary yolk sac as the secondary yolk sac forms?

A

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

31
Q

What happens to the chorionic cavity during formation of the secondary yolk sac?

What are the 3 cavities present by day 13?

A

Chorionic cavity continues to expand

The 3 main cavities are amniotic, definitive and chorionic

32
Q

By the end of week 2, what is the state of the extraembryonic mesoderm?

A

The small amount that is left forms the connecting stalk

33
Q

what is the role of the connecting stalk by the end of week 2?

A

The amniotic cavity and the secondary yolk sac are suspended in the chorionic cavity by the connecting stalk

34
Q

What hormone is secreted by syncitiotrophoblast cells?

A

human chorionic gonadotrophin (hCG)

35
Q

What is the role of hCG in pregnancy?

A

It maintains the uterine lining when levels are high

If there is no fertilised oocyte, the uterine lining is shed

36
Q

What is involved in a pregnancy test?

Why does it work?

A

measuring the levels of hCG in urine

hCG is only secreted in high levels when there are syncitiotrophoblast cells

37
Q

What is the mechanism behind a pregnancy test?

A

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

38
Q

What can often cause a false positive on a pregnancy test?

A

Drugs containing hCG which are used to treat infertility

39
Q

Under what conditions does an ectopic pregnancy occur?

A

It occurs when the fertilised egg implants outside of the uterine cavity

40
Q

What % of pregnancies involve an abnormal implantation site?

What is the most common site of abnormal implantation?

A

2%

The egg is implanted in the ampulla or the uterine tube

41
Q

What happens if the fertilised egg enters the abdominal cavity?

A

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

42
Q

What is placenta previa?

A

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

43
Q

What is the largest consequence of placenta previa and how is it avoided?

A

Severe bleeding in late pregnancy

A C-section occurs before this situation is reached as it can be identified on ultrasound

44
Q

What happens after a fertilised egg has implanted in the uterine tube?

A

The embryo becomes too large and the uterine tube ruptures

This destroys the support structures and is dangerous to the mother

45
Q

Where may the embryo develop in abdominal implantation?

A

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

46
Q

How does lithopaedion occur?

A

When an abdominal pregnancy remains undiagnosed and the foetus dies

47
Q

What usually happens when a foetus dies?

A

The body breaks down and absorbs the dead foetus

48
Q

What happens when a foetus dies that leads to lithopaedion?

Why does this happen?

A

If the foetus is too large to be absorbed, it calcifies

This protects the mother from the necrotic tissue of the foetus

49
Q

What causes a hydatidiform mole?

A

A sperm fertilises an empty oocyte that doesn’t have a nucleus

50
Q

What develops in a hydatidiform mole?

A

The trophoblast develops without any embryonic tissue

51
Q

Why does a hydatidiform mole lead to a false positive on a pregnancy test?

A

Syncitiotrophoblasts cells are present and secreting hCG

Embryology (4 decks)

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