ToB Embryology 1 - Pre-Embryonic Period Flashcards Preview

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Flashcards in ToB Embryology 1 - Pre-Embryonic Period Deck (82)
1

How early in development does the first differentiation take place?

Within the first 2 weeks

2

What is the first differentiation that occurs?

- Intra-embryonic tissues
- Extra-embryonic tissues

3

What is meant by 'intra-embryonic tissues'?

Tissues that will become part of the body

4

What is meant by 'extra-embryonic tissues'?

Tissues which will not become part of the body, but will support the development of the body

5

Why is there a difference between the length of complete gestation, and the length of pregnancy term?

Complete gestation is 38 weeks, but pregnancy weeks are calculated from the last menstrual period, ie conception weeks + ~2 weeks = 40 weeks

6

How long is the pre-embryonic period?

2 weeks

7

What are the 3 main stages from fertilisation to birth?

1) Pre-embryonic
2) Embryonic
3) Fetal

8

What is the name of the first division after fertilisation, and what does it produce?

'Cleavage' to produce 2 blastomeres

9

What is the name of the group of cells which makes contact with the endometrial lining of the uterus?

Blastocyst

10

At what point of the fallopian tube is the oocyte fertilised?

At the ampulla (end nearest the ovary)

11

What is the name of a fertilised oocyte?

Zygote

12

Where is the ideal site for implantation?

Posterior uterine wall, within the endometrial lining of the uterus

13

How long is an oocyte viable for?

1 day

14

How long is a sperm viable for?

3 days

15

How soon does cleavage occur after fertilisation?

About 30hrs

16

Why are the 2 blastomeres (created via cleavage) the same size as the oocyte?

The cells cannot expand as they are encased by the zona pellucida (glycoprotein shell)

17

Define zona pellucida:

Thick glycoprotein membrane surrounding the ovum before implantation, which prevents polyspermy

18

How are other sperm prevented from reaching the oocyte after it has been fertilised?

They are excluded by the zona pellucida

19

Define morula:

Group of totipotent blastomere cells, formed after cleavage and compaction, distinct from a blastocyst due to lack of cavity.

20

Why are the cells within the morula described as totipotent?

As the cells are identical at this point, they all have the capacity to become any cell point.

21

During IVF, what is the name of the group of cells which will be transferred into the uterus?

Morula

22

When can Pre-implantation Genetic Diagnosis occur?

During the morula stage of IVF, before cells are transferred into the mother

23

What process allows embryos to be screened for serious heritable conditions?

Pre-implantation Genetic Diagnosis (PGD), during IVF

24

Define the embryonic period of development:

Period 3-8 weeks after fertilisation, during which all the structures and systems are built

25

Define the fetal period of growth:

Period 9-38 weeks after fertilisation, during which growth is the focus (as all structures and systems have been built). Physiological maturation of the systems occurs.

26

Define the pre-embryonic period:

Period 0-2 weeks after fertilisation, during which cleavage and the 1st differentiation takes place, and implantation begins.

27

Define zygote:

A fertilised oocyte

28

Describe cleavage:

The 1st division of the zygote, into two masses known as blastomeres. First cleavage happens ~30 hours after fertilisation

29

Define ovary:

The female reproductive organ producing oocytes/eggs for fertilisation

30

Define fallopian tube:

Either of a pair of tubes along which oocytes/eggs travel along from the ovaries to the uterus

31

Define uterus:

Female reproductive organ where implantation of the zygote and growth of the embryo occurs

32

Define gestation:

The carrying of an embryo/fetus inside a female reproductive organ.

33

At what point after fertilisation does a blastocyst form?

During day 4 after fertilisation

34

How is a morula different to a blastocyst?

-Morula contains identical totipotent cells, whereas the blastocyst contains 2 differentiated types of cells
- Blastocyst contains a cavity, the morula does not

35

What is the outer layer of the blastocyst?

Zona pellucida

36

What is the name of the cell formed after the 1st differentiation event?

Blastocyst

37

What types of cells does the 1st differentiation event produce?

1) Trophoblast cells = outer cell mass
2) Embryoblast cells = inner cell mass

38

How are trophoblast cells different to embryoblast cells?

Trophoblast cells make up the outer cell mass of the blastocyst,and will become supporting tissue (never part of the body).
Embryoblast cells make up the inner cell mass, and will produce the embryo and fetus.

39

What event causes the cells of an embryo to change from totipotent to pluripotent?

Compaction

40

Define totipotent:

The capacity of a cell to become any cell type.

41

Define pluripotent:

The capacity of a cell to become one of many cell types

42

What is the difference between totipotency, and pluripotency?

Totipotent cells have the capacity to become ANY cell type, whereas pluripotent cells have the capacity to become one of MANY cell types.

43

With every cell division, why do the blastomeres become smaller?

They are limited by the zona pellucida

44

What word describes the degradation of the zona pellucida, allowing the blastocyst to enlarge?

Hatching

45

At what point after fertilisation does 'hatching' take place?

Day 5 after fertilisation

46

What is required to occur, before the blastocyst can begin implantation?

Blastocyst must hatch out of the zona pellucida, to allow interaction of the trophoblast cells with the uterine wall.

47

After hatching, how many cells are present in the conceptus?

107 cells

48

How many of the 107 cells present after hatching will make the embryo, and how many will become fetal membranes?`

8 cells will make the embryo
99 will become fetal membanes

49

At what point after fertilisation does implantation begin?

Day 6-7 after fertilisation

50

During the early stages of development, which has priority: the making of the embryo, or the making of the placenta?

Formation of the placenta, to be able to support the developing embryo

51

Why is week 2 known as the 'week of two's'?

The outer and inner cell masses both differentiate into 2 different cell types, (4 different cell types present by end of week 2)

52

During week 2, what happens to the trophoblast cells?

They differentiate into:
1) Syncytiotrophoblast cells, or
2) Cytotrophoblast cells

53

What is the role of the syncytiotrophoblast cells?

-Form a syncytium of multi-nucleated cells
-Produce and release lytic enzymes to break down part of the endometrial epithelial cells
-Penetrate basal lamina and invade maternal sinusoids, producing a uteroplacental circulation
-Surround embryo when implantation is complete
-Allow efficient diffusion

54

Define cytotrophoblast cells:

Non-invasive cells which encase the embryo during implantation, and form the placental membrane

55

What group of cells within the blastocyst become the bilaminar disk?

Embryoblast cells (inner cell mass)

56

What 2 groups of cells make up the bilaminar disc?

1) Epiblast cells
2) Hypoblast cells

57

During week 2, what happens to the embryoblast cells?

They differentiate into:
1) Epiblast cells
2) Hypoblast cells

58

Describe the embryo at the end of the 2nd week:

- Consists of epiblast and hypoblast cells,
- Two cavities formed: amniotic cavity, and the yolk sac
- In the center of the two cavities, the epiblast and hypoblast cells are joined to form the bilaminar disc
- Is suspended within the chorionic cavity via a connecting stalk

59

What type of cells surround the amniotic cavity?

Epiblast cells

60

In which cavity will the embryo develop, and gestation will occur?

Amniotic cavity

61

After implantation, what happens to the degraded endometrial lining of the uterus?

The invasion point is filled with a fibrin plug

62

At what point after fertilisation is implantation complete, and a fibrin plug inserted into the endometrial lining of the uterus?

Day 9-10 after fertilisation

63

What does the conceptus implant into?

The uterine stroma, through the endometrial lining of the uterus

64

How does the generation of a uteroplacental circulation change the support of the embryo?

The support of the embryo will change from histiotrophic (tissue fluid simple diffusion) to haemotrophic (relies on nutrients from maternal blood)

65

What are the 4 main conditions linked to implantation defects?

1) IUGR (Inter Uterine Growth Restriction)
2) Pre-eclampsia
3) Ectopic pregnancy
4) Placenta praevia

66

Define ectopic pregnancy:

A pregnancy that is not occurring in the uterus

67

Why is an ectopic pregnancy life-threatening?

If invasive implantation takes place somewhere other than the uterus, can cause haemorrhage elsewhere in the pelvis.

68

What is placenta praevia?

When the placenta grows across the cervix

69

What causes placenta praevia?

The placenta will grow wherever implantation takes place, so if implantation occurs in the lower uterine segment there is a high risk of the placenta growing across the cervix.

70

How will delivery differ between a normal pregnancy, and a pregnancy with placenta praevia?

Pregnancies with placenta praevia usually require Caesarian section delivery, due to very high risk of haemorrhage.

71

What are the two poles of the conceptus immediately after implantation?

Embryonic pole = first side of embryo to implant, so side where lacunae form
Abembryonic pole = last part of embryo to implant, so side next to fibrin plug

72

At which pole of the conceptus does the primitive yolk sac form?

Abembryonic pole

73

Which type of cells form the primitive yolk sac membrane?

Hypoblast cells

74

At which pole of the conceptus do lacunae first form?

Embryonic pole

75

What causes the primitive yolk sac membrane to be separated from the cytotrophoblast layer?

The primitive yolk sac secretes a primitive proteinaceous acellular extraembryonic reticulum

76

How is the extraembryonic mesoderm formed?

The primitive yolk sac secretes an acellular extraembryonic reticulum, which is later converted to an extraembryonic mesoderm via cell migration

77

At what point after fertilisation is the uteroplacental circulation developed?

Day 12 (week 2)

78

What are the function of the lacunae in the syncytiotrophoblast?

The syncytiotrophoblast invades the maternal sinusoids, resulting in the lacunae filling with maternal blood = uteroplacental circulation.

79

At what point after fertilisation does the definitive/secondary yolk sac form?

Day 13 (week 2)

80

How is the definitive/secondary yolk sac formed?

Some of the primitive yolk sac (at the abembryonic pole) pinches off

81

How is the chorionic cavity formed?

Spaces within the extraembryonic mesoderm merge, to form the cavity which encases the embryo

82

Why is there often vaginal bleeding around 2 weeks after fertilisation?

1) The invasive process of implantation may cause bleeding
2) The fusing of the maternal sinusoids with the embryonic lacuna may cause bleeding