Embryology- pre-embryonic phase (weeks 1 to 2)

Question 1

How does one cell become a multicellular body?

Answer 1

1. Growth
2. Morphogenesis - development of form and structure
3. Differentiation - specialisation for function

Question 2

Define the stages of human development before birth

Answer 2

1. Pre-embryonic
2. Embryonic
3. Fetal

Question 3

Which weeks post-fertilisation make up the pre-embryonic stage?

Answer 3

Weeks 1-2

Question 4

Which weeks post-fertilisation make up the embryonic stage?

Answer 4

Weeks 3-8

Question 5

Which weeks post-fertilisation make up the fetal stage?

Answer 5

Weeks 9 - birth (38 weeks plus or minus 2 (is normal))

Question 6

What is the start point which is used to calculate the weeks of the stages of development?

Answer 6

These are calculated from the LMP (last menstrual period). Therefore on average the term of pregnancy is these conception weeks plus 2 (term of pregnancy is 40 weeks) because typically fertilisation occurs 2 weeks into the menstrual cycle.

Question 7

Which three developmental processes occur in the pre-embryonic stage of development?

Answer 7

1. Cleavage - formation of morula by first mitosis
2. Compaction - formation of blastocyst
3. Implantation begins - blastocyst makes contact with the endometrium of the uterus

Question 8

Where is the oocyte generally fertilised?

Answer 8

In the ampulla of the fallopian tube

Question 9

What is the name of a fertilised oocyte?

Answer 9

zygote

Question 10

What is the ideal site for implantation of the blastocyst?

Answer 10

Posterior uterine wall

Question 11

Why is there a small window for fertilisation?

Answer 11

An oocyte is only viable for 1 day and sperm for 3 days

Question 12

What happens during cleavage in the pre-embryonic period?

Answer 12

The first mitotic division.
Results in two blastomeres of equal size surrounded by a glycoprotein shell called the zona pellucida. Blastomeres contained within the zona pellucida are collectively called the morula when they have divided to form 8 blastomeres.

Question 13

What is the name of the glycoprotein shell that surrounds the blastomeres?

Answer 13

Zona pellucida

Question 14

What is the morula?

Answer 14

Eight blastomeres and their surrounding zona pellucida.

Question 15

What is important about the ability of the cells in the morula?

Answer 15

They are totipotent - have the capacity to become any cell type.

Question 16

At what stage do assisted reproductive techniques transfer the fertilised oocyte into the uterus?

Answer 16

When it has divided to the 4-8 cell stage (morula)

Question 17

How does pre-implantation genetic diagnosis (PGD) test for heritable conditions?

Answer 17

A cell is safely removed from the morula and tested, prior to transfer of the embryo into the mother

Question 18

What is the defining occurrence during the pre-embryonic process of compaction?

Answer 18

Formation of the first cavity - blastocyst cavity

Question 19

What is the name of the total cell mass once compaction has occurred?

Answer 19

blastocyst

Question 20

What are the two different tissue types that occur during the first differentiation event, compaction?

Answer 20

Inner cell mass “embryoblast”

Outer cell mass “trophoblast”

Question 21

What does the inner cell mass go on to form?

Answer 21

the embryo

Question 22

What does the outer cell mass go on to form?

Answer 22

the supporting tissues of the embryo - placenta and membranes

Question 23

What is the name of the first cavity that is formed in the pre-embryonic period?

Answer 23

Blastocyst cavity

Question 24

What is the difference in the ability of the cells before and after the compaction event in the pre-embryonic phase of pregnancy?

Answer 24

Before - totipotent: capacity to become any cell type

After - pluripotent: capcity to become 1 of many cell types

Question 25

After cleavage, hatching occurs. What is meant by the term hatching?

Answer 25

The blastocyst breaks free from the zona pellucida and is no longer constrained, therefore it can enlarge. The cells of the outer cell mass are now able to make contact with the endometrium.

Question 26

Define conceptus

Answer 26

The product of conception: embryo plus associated membranes

Question 27

When does implantation begin?

Answer 27

Week 1

Question 28

Week 2 is called the week of twos. Why?

Answer 28

Both the outer and inner cell mass form two distinct cellular layers: Outer cell mass: syncytiotrophoblast and cytotrophoblast Inner cell mass (bi-laminar disk): epiblast and hypoblast

Question 29

In week two what two cell types make up the bilaminar disk?

Answer 29

Epiblast and hypoblast

Question 30

In week two what two cell types does the outer cell mass differentiate into?

Answer 30

Synctiotrophoblast | Cytotrophoblast

Question 31

What two cavities have formed by the end of week 2?

Answer 31

Amniotic cavity and yolk sac

Question 32

Which of the three processes in pre-embryonic development finishes by the end of week 2?

Answer 32

Implantation

Question 33

What is the name of supporting sac in which the embryo is suspended at the end of the pre-embryonic phase?

Answer 33

Chorionic cavity

Question 34

How is the amniotic cavity formed?

Answer 34

By the secretions of cells...

Question 35

Which of the two cavities formed within the embryo, does the embryo ultimately grow into?

Answer 35

amniotic cavity

Question 36

Why can there be a bit of bleeding during week 2 of development which can sometimes be mistaken for a light menstruation?

Answer 36

Implantation is an invasive process - the blastocyst breaches the mother's endometrium using enzymes.

Question 37

How is the breached endometrium during implantation repaired?

Answer 37

fibrin plug

Question 38

What is meant by 'implantation allows support of the embryo to change from histiotrophic to haemotrophic?

Answer 38

It means that support is able to change from simple diffusion to access to the maternal circulation - allowing significant growth to occur.

Question 39

Name 2 conditions that are linked to implantation defects

Answer 39

Ectopic pregnancy | Placenta praevia

Question 40

What implantation defect has occurs in ectopic pregnancies?

Answer 40

Implantation at site other than uterine body (most commonly fallopian tube but can be peritoneal or ovarian). Can be caused by cilia problems or blockages in the fallopian tubes.

Question 41

Why can ectopic pregnancy very quickly become a life-threatening emergency?

Answer 41

Haemorrhage There is no endometrium and so when the invasive part of development occurs, the conceptus can invade large blood vessels and cause high blood loss.

Question 42

What implantation defect occurs in placenta praevia?

Answer 42

Implantation in the lower uterine segment.

Question 43

Why is placenta praevia a problem?

Answer 43

The degree of severity depends on how low in the uterus it has implanted. It can cause haemorrhage and requires a C-section delivery.

Question 44

Which of the layers syncytiotrophoblast or cytotrophoblast is found on the outer layer of the conceptus?

Answer 44

Synctiotrophoblast layer: synctio- means transport, whereas cyto- means cellular. Synctiotrophoblast is involved in the formation of lacunae which fill with the maternal blood supply.

Question 45

What are the name of the pockets that develop in the syncytiotrophoblast?

Answer 45

lacunae

Question 46

What is the name of the cavity formed when the hypoblast layer spread out from the bilaminar disk and lines the cytotrophoblast layer?

Answer 46

Primitive yolk sac

Question 47

In week 2 the primitive yolk sac membrane (hypoblast layer) is pushed away from the cytotrophoblast layer by an acellular extraembryonic reticulum (glycoprotein, protein ground substance). What is this reticulum later converted to by cell migration?

Answer 47

Extraembryonic mesoderm

Question 48

In week 2, the maternal sinusoids are invaded by syncytiotrophoblast and the lacunae become continuous with the sinusoids. The uterine stroma prepares for support of the embryo. What type of circulation can now occur?

Answer 48

Uteroplacental circulation

Question 49

What is a sinusoid?

Answer 49

A small irregular blood vessel found in certain organs e.g. the liver.

Question 50

At the end of week 2 the secondary yolk sac is formed. How does this happen?

Answer 50

A small part of the yolk sac is pinched off into the chorionic cavity. The remaining yolk sac is called the secondary or definitive yolk sac.

Question 51

The connecting stalk is formed from a column of mesoderm. What will it become in the future?

Answer 51

The umbilical cord

Question 52

What is the name of the first cavity formed as a result of compaction?

Answer 52

Blastocoele

Question 53

What is the name of the sac formed from spaces within the epiblast?

Answer 53

Amniotic sac

Question 54

What sac is formed by hypoblast lining the blastocoele?

Answer 54

Primitive yolk sac

Question 55

What is the name of the sac formed within the primitive yolk sac?

Answer 55

Secondary yolk sac or definitive yolk sac

Question 56

What cavity is formed from spaces within extraembryonic reticulum and mesoderm?

Answer 56

Chorionic cavity/ extraembryonic coelom

Question 57

During which weeks are 50% of all zygotes lost?

Answer 57

Weeks 2-3

Question 58

What percentage of diagnosed pregnancies will miscarry?

Answer 58

15%

Question 59

What percentage of woman suffer from recurrent miscarriage?

Answer 59

1%

Question 60

What are the names of the three conceptus stages that it goes through in the pre-embryonic phase?

Answer 60

zygote -> morula -> blastocyst