Embryology 1

Question 1

What are the three periods from fertilisation until birth?

Answer 1

Pre-embryonic
Embryonic
Fetal

Question 2

What is the pre-embryonic period? Name the three steps.

Answer 2

The first two weeks of development

1. Cleavage (formation of morula)
2. Compaction (formation of blastocyst)
3. Implantation begins

Question 3

Briefly outline fertilisation.

Answer 3

1. An oocyte is released from the ovary.
2. Travels along the Fallopian tube
3. Here in the Fallopian tube in the ampulla it is fertilised by the sperm
4. The fertilised oocyte is called the zygote.
5. Implantation occurs

Question 4

What is the ideal site for implantation?

Answer 4

The posterior uterine wall.

Question 5

What happens after the zygote is formed, and what is the product called?

Answer 5

Cleavage and this results in 2 blastomeres of equal size. Also the zona pellucida is formed.

Question 6

What is the zona pellucida?

Answer 6

A glycoprotein shell that prevents further sperm to try to fertilise the oocyte.

Question 7

What happens after the 2 blastomeres are formed?

Answer 7

The blastomeres continue to divide into 4 upto 8 cells to form a morula. This is called the cleavage.

Question 8

Why is the morula important?

Answer 8

Because it is totipotent. This means that it has the capacity to become any cell.

Question 9

How does the morula relate to in vitro fertilisation.

Answer 9

The oocyte is fertilised and allowed to develop into a morula, then it is transferred into the uterus.
Pre-implantation genetic diagnosis can be used to safely remove a cell from the morel to test for heritable conditions prior to transfer the embryo into the mother.

Question 10

Where does the sperm fertilise the oocyte?

Answer 10

In the ampulla of the Fallopian tube.

Question 11

What is the fertilised oocyte called?

Answer 11

A zygote

Question 12

What is the divided zygote called?

Answer 12

Blastomeres

Question 13

What happens after the formation of the morula?

Answer 13

This step is called compaction which is the formation of first cavity and thereof a blastocyst.

Question 14

What is the inner cell mass in the blastocyst called?

Answer 14

Embryoblast

Question 15

What is the outer cell mass that lines the blastocyst cavity called?

Answer 15

Trophoblast

Question 16

What happens after compaction regarding the potency of the cells?

Answer 16

From being totipotent the cells are now only pluripotent. This means that they can’t differentiate into all cells but still many cells.

Question 17

What happens after compaction?

Answer 17

Hatching where the zona pellucida is broken down.

Question 18

What are the two most important consequences of the breakdown of the zona pellucida?

Answer 18

The blastocyst is free to enlarge now.

It can also interact with the uterine surface to implant itself.

Question 19

What happens following the hatching?

Answer 19

Implantation of the blastocyst.

Question 20

Briefly outline the process of fertilisation to implantation.

Answer 20

Zygote is formed. Divide into two to form 2 blastomeres, the zona pellucida also forms here.
Cleavage occurs to form the morula (4-8 cells)
After this compaction occur where a blastocyst is formed and the first cavity is formed as well.
Hatching occurs which breaks down the zona pellucida rendering the blastocyst able to enlarge and implant into the (favourably the) posterior uterine wall.

Question 21

What happens during week 2?

Answer 21

Differentiation into two distinct cellular layers.

Question 22

What two distinct cellular layers emerge?

Answer 22

An outer cell mass and an inner cell mass.

Question 23

What are the two parts of the outer cell mass?

Answer 23

Syncytiotrophoblasts

Cytotrophoblasts

Question 24

What are the two parts of the inner cell mass?

Answer 24

Epiblasts

Hypoblasts

Question 25

What is the bilaminar disc?

Answer 25

The two-layered disc that consists of epiblasts and hypoblasts.

Question 26

What is the amniotic cavity?

Answer 26

A cavity lined by epiblasts.

Question 27

Why is the implantation so important?

Answer 27

It establishes maternal blood flow within the placenta to allow the embryo to be supported from maternal circulation. It establishes the basic structural unit of matron-fetal exchange of nutrients.

Question 28

Give two example of conditions linked to implantation defect.

Answer 28

Placenta praevia | Ectopic pregnancy

Question 29

What is ectopic pregnancy?

Answer 29

When implantation has occurred anywhere else but the uterine wall. This can be life threatening very quickly.

Question 30

What is placenta praevia?

Answer 30

When implantation occurs in the lower uterine segment. | This can cause haemorrhage in pregnancy and requires C-section delivery.

Question 31

Where can embryonic pole be found? What happens here after implantation?

Answer 31

Above the amniotic cavity and the epiblasts as well as the cytotrophoblasts. This is faced away from the uterine wall. Here occurs rapid development of syncytiotrophoblasts which proliferate quickly.

Question 32

Where can the abembryonic pole be found? What happens here after implantation?

Answer 32

The part closest to the uterine wall. Here the primitive yolk sac is formed.

Question 33

What happens after the proliferation of syncytiotrophoblasts and the formation of the yolk sac?

Answer 33

The yolk sac is pushed away from the cytotrophoblasts and a reticulum is formed. This reticulum is later converted into extra embryonic mesoderm.

Question 34

After this there is an important step occurring. Briefly outline it.

Answer 34

Syncytiotrophoblasts invade the maternal sinusoids. This establishes a uteroplacental circulation in order for the embryo to be able to take use of the maternal blood circulation.

Question 35

What is the next step?

Answer 35

Formation of a secondary yolk sac which is pinched off from the primitive yolk sac.

Question 36

Now by the end of the second week, what does the embryo look like?

Answer 36

From in to out. The bilaminar disc consisting of epiblasts (upper) and hypoblasts (lower). The epiblasts line the amniotic cavity. The hypoblasts line the yolk sac. This formation is found surrounded by the chorionic cavity. The chorionic cavity is lined by the cytotrophoblasts which in their turn are lined by syncytiotrophoblasts that have invaded the maternal sinusoids.

Question 37

Why are the cytotrophoblasts and syncytiotrophoblasts important?

Answer 37

Because they form placental membranes.

Question 38

Why are the epiblasts and hypoblasts important?

Answer 38

Because they form the bilaminar disc that will become the foetus eventually.