7. Early Embryonic Development - The Pre-embryonic Period

Question 1

What is the blastocyst?

Answer 1

The formation of a cavity.

Question 2

What is the pre-embryonic period?

Answer 2

The first two weeks of development, formation of morula and then blastocyst and when implantation begins.

Question 3

Where should implantation happen?

Answer 3

On the posterior uterine wall.

Question 4

Where is the oocyte released from?

Answer 4

The ovary.

Question 5

Where does the oocyte get fertilised by the sperm?

Answer 5

In the ampulla of the Fallopian tube.

Question 6

When does cleavage happen?

Answer 6

30 hours after fertilisation.

Question 7

What happens in cleavage?

Answer 7

The zygote divides into 2 blastomer ew of equal size, half the size of the original zygote.

Question 8

What is the zone pellucida?

Answer 8

The glycoprotein shell of the zygote.

Question 9

What does totipotent mean?

Answer 9

The capacity to become any cell type.

Question 10

In in vitro fertilisation, when is the morula transferred into the uterus?

Answer 10

After dividing to the 4 or 8 cell stage.

Question 11

What happens in pre-implantation genetic diagnosis?

Answer 11

A cell is removed from the morula and tested for serious heritable condition prior to transfer of the embryo into the mother.

Question 12

What is the outer cell mass called and what does it give?

Answer 12

Trophoblast, it gives supporting cells, membrane and the placenta.

Question 13

What is the inner cell mass called and what does it give?

Answer 13

Embryo blast and it gives the embryo.

Question 14

Define pleuripotent.

Answer 14

The capacity to become one of many cell types.

Question 15

What happens in hatching in week 1 of early embryonic development?

Answer 15

The blastocyst hatches from the zona pellucida and now is free to enlarge. It can interact with the uterine surface to implant.

Question 16

How many cells does the conceptus have when it implants?

Answer 16

107- 8 to make the embryo and 99 to begin development of the fatal membranes and placenta.

Question 17

Why is week 2 of early embryonic development known as the week of twos?

Answer 17

Two distinct cellular layers energy from the outer cell mass: synctiotrophoblatst and cytotrophoblast, and the inner cell mass becomes the bilaminar disk: epiblast and hypo blast.

Question 18

What has happened by the end of the 2nd week?

Answer 18

The conceptus has implanted and the embryo, with its two cavities, will be suspended within a supporting sac.

Question 19

What is the purpose of implantation?

Answer 19

To establish maternal blood flow within the placenta so changes of the embryo from histiotrophic (support from tissues) to haemotrophic (support from maternal blood flow) is supported.

Question 20

What is meant by the implantation is interstitial?

Answer 20

The uterine epithelium is breached and the conceptus implants within the uterine stroma.

Question 21

What are four conditions linked to implantation defects?

Answer 21

IUGR (interuterine growth restriction), pre-eclampsia, ectopic pregnancy and placenta praevia.

Question 22

What is an ectopic pregnancy?

Answer 22

Implantation at another site than the uterine body, often the Fallopian tubes, that can be peritoneal or ovarian and can become a life-threatening emergency.

Question 23

What is placenta praevia?

Answer 23

Implantation in the lower uterine segment that can cause haemorrhaging in pregnancy and requires a C-section delivery.

Question 24

What happens on day 9 of early embryonic development?

Answer 24

There is rapid development of synctiotrophoblast, embryonic pole. Also primitive yolk sac is formed, abembryonic pole. The yolk sac membrane is in contact with cytotrophoblast.

Question 25

What happens on day 11 of early embryonic development?

Answer 25

The primitive yolk sac membrane is pushed away from cytotrophoblast layer by an acellular extraembryonic reticulum. The reticulum later converts to extraembryonic mesoderm by cell migration.

Question 26

What happens on day 12 of early embryonic development?

Answer 26

The maternal sinusoids are invaded by synctiotrophoblast and the lacunae become continuous with sinusoids. Uteroplacental circulation begins. The uterine stroma prepares for support of the embryo.m

Question 27

What happens on day 13 of early embryonic development?

Answer 27

Formation of the secondary yolk sac, from pinching off from the primitive yolk sac.

Question 28

What happens on day 14 of early embryonic development?

Answer 28

Spaces within the extraembryonic mesoderm merge to form the chorionic cavity. The embryo and its cavities are suspended by the connecting stalk.

Question 29

Why can bleeding around day 14 of early embryonic development be confused with menstrual bleeding?

Answer 29

Because it is a very vascular and invasive process where there is a breach of endometrium.

Question 30

What is the blastocoele?

Answer 30

The first cavity, formed as a result of compaction.

Question 31

What is the amniotic sac?

Answer 31

Formed from spaces within the epiblast.

Question 32

What is the primitive yolk sac?

Answer 32

Formed by hypoblast lining blastocoele.

Question 33

What is the secondary yolk sac?

Answer 33

Formed within the primitive yolk sac.

Question 34

What is extraembryonic coleum?

Answer 34

Formed from spaces within extraembryonic reticulum and mesoderm.

Question 35

What percentage of zygotes are lost in the first 2/3 weeks?

Answer 35

50%

Question 36

What percentage of diagnosed pregnancies will miscarry?

Answer 36

15%.

Question 37

What percentage of women will have recurrent miscarriages?

Answer 37

1%.

Question 38

What is the morula?

Answer 38

When the fertilised oocyte develops into a multicellular ball.