Embryology Lecture 1 - Early Embryonic Development 1

Question 1

What occurs between release of an ovum and the zygote entering the uterus.

Answer 1

Ovum is released from the ovary

Travels toward the uterus through the oviduct

Is normally fertilised in the ampulla of the oviduct

Fertilised oocyte is called a zygote

Question 2

Where is the ideal site for implantation?

Answer 2

The posterior wall of the uterus

Question 3

Define the pre-embryonic, embryonic and foetal stages of development

Answer 3

1-2 weeks = Pre-embryonic

3-8 weeks = Embryonic

9-38 weeks = Foetal

Question 4

How is pregnancy weeks calculated?

Answer 4

From date of last menstrual period

Therefore pregnancy is said to last 40 weeks, but only 38 weeks post-fertilisation

Question 5

Define ‘Zygote’

Answer 5

A fertilised ovum/oocyte

Question 6

Define ‘Cleavage’ in relation to a fertilised ovum

When does the first instance of cleavage occur?

Answer 6

The dividing of cells into two masses known as blastomeres, first cleavage happens ~30 hours after fertilisation

Question 7

What is the zona pellucida?

What is its function?

Answer 7

The glycoprotein shell surrounding an ovum/morula/blastocyst

Prevents polyspermy

Question 8

Define ‘Morula’ How are they formed? What is special about the cells of the morula?

Answer 8

A dividing ball of cells surrounded by a zona pellucida

The result of cleavage of the fertilised ovum

Each cell is totipotent, can differentiate into any other kind of cell

Question 9

Define ‘Ovary’

Answer 9

The female reproductive organ producing ova for fertilisation

Question 10

Define ‘Fallopian tube’

Answer 10

A tube the ovum travels down to reach the uterus

Question 11

Define ‘Uterus’

Answer 11

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

Question 12

How is a blastocyst formed?

Answer 12

Compaction of the cells that make up the morula

Question 13

What is the trophoblast?

Where is it found?

What is formed from the trophoblast?

Answer 13

A layer of cells found in the early conceptus/blastocyst

Will later form the supporting structures for the embryo (Placenta)

Question 14

What is an embryoblast?

Where is it found?

What structures will be formed from the embryoblast?

Answer 14

An aggregation of cells found in the early conceptus/blastocyst

Will go on to form the bilaminar disk

Question 15

What is Implantation?

When does it occur?

Answer 15

The attatchment of the blastocyst to the wll of the uterus at day 6-7

Question 16

What is the cytotrophoblast?

Where is it derived from?

Answer 16

It’s the placental membrane around the yolk sac

Derived from the trophoblast

Question 17

What is the syncytiotrophoblast?

Where is it derived from?

Answer 17

Made up of cells that invade the maternal sinusoids (irregular blood vessels) resulting in early uteroplacental circulation

Derived from the trophoblast

Question 18

What are the 3 major events of week one of pre-embryonic development and on what days do they occur?

Answer 18

Compaction - Day 4

Hatching - Day 5

Implantation - Day 6-7

Question 19

Describe the events of day 4 of pre-embryonic development

What happens to the differentiation capacity of the cells of the conceptus after compaction?

Answer 19

Blastomeres making up the morula compact and form the blastocyst (compaction)

After compaction cells are no longer totipotent, but become pluripotent (can differentiate into many, but not all cell types)

Question 20

This pictures shows what structure?

What structures are labelled?

Answer 20

This is a blastocyst

Labels (from top left clockwise)

Embryoblast

Blastocyst cavity

Zona pellucida

Trophoblast

Question 21

Describe the events of day 5 of pre-embryonic development

Hint: Hatching

Answer 21

Blastocyst hatches from the zona pellucida

No longer constrained and free to enlarge

Can now interact with the unterine surface to implant

Question 22

Describe the events of days 6-7 of pre-embryonic development

Describe the cellular composition of the blastocyst at this point in development

Hint: Implantation

Answer 22

Implantation begins

The blastocyst now has 107 cells

• 8 will make the embryo
• 99 will begin development of the foetal membrane

Question 23

Give an overview of the events of week two of pre-embryonic develoment

Mention the differentiation of the blastocyst cells

Answer 23

Both the embryo and placenta begin to form, at the earliest stages the placenta has priority

The bilaminar disk and amniotic cavity are formed

Differentiation of cells occurs:

Embryoblast:

Epiblast and Hypoblast (Bilaminar disk)

Trophoblast:

Syncytiotrophoblast and Cytotrophoblast

Question 24

Describe the process of implantation

Answer 24

The uterine epithelium is breached and the conceptus implants in the uterine stroma

Implantation is therefore interstitial (invasive and embedded in surrounding tissue)

Question 25

What are the two major implantation defects?

Outline the results of these defects

Answer 25

Ectopic pregnancy:

Implantation at site other than the uterus, most commonly in the fallopian tubes

Can very quickly become a life threatening emergency

Placenta Praevia:

Implantation in the lower uterine segment, placenta grows across the cervical opening

Can cause haemorrhage in pregnancy

Requires C-section delivery

Question 26

Label this diagram

When would you expect to the conceptus to look like this?

Answer 26

Labels from top left clockwise:

Syncytiotrophoblast

Cytotrophoblast

Epiblast

Hypoblast

Blastocyst cavity

Bilaminar disk

Amniotic cavity

At the beginning of week 2 of development

Question 27

Describe the events of day 9 of pre-embryonic development

Answer 27

At the embryonic pole:

There is rapid development of the syncytiotrophoblast

At the abembryonic pole:

The primitive yolk sac is formed by Heuser’s membrane spreading across the blastocyst cavity

Yolk sac membrane is in contact with the cytotrophoblast layer

Question 28

Label this diagram

When would you expect the conceptus to look like this?

Answer 28

Labels from top clockwise:

Lacunae

Primitive yolk sac

Amniotic cavity

Day 9 of development

Question 29

Describe the events of day 11 of pre-embryonic development

Answer 29

Primitive yolk sac embrane is pushed away from the cytotrophoblast by an acellular extraembryonic reticulum

Reticulum will later be converted to the extra embryonic mesoderm via cell migration

Question 30

Describe the events of day 12 of pre-embryonic development

Answer 30

Maternal sinusoids invaded by the syncytiotrophoblasts

Lacunae in the syncytiotrophoblast become continuous with the maternal sinusoids

Uteroplacental circulation begins

Uterine stroma prepares to support the embryo

Question 31

Describe the events of day 13 of pre-embryonic development

Answer 31

A small amount of the primitive yolk sac is pinched off, whatever remains is now referred to as the secondary yolk sac

Question 32

Describe the events of day 14 of pre-embryonic development (end of the second week)

Answer 32

Spaces within the extraembryonic mesoderm merge to form the chorionic cavity

The connecting stalk suspends the embryo and its cavities in the chorionic cavity

The connectiong stalk is a column of mesoderm, the furutre umbilical cord

Bleeding around now can be confused with menstrual bleeding

Question 33

Label this diagram, you only need to worry about the lines with black boxes at the ends

When would you expect the conceptus to look like this?

Answer 33

Labels from top right clockwise (black boxes only):

Lacunae

Maternal sinusoid

Connecting stalk

Amniotic cavity

Secondary yolk sac

Chorionic cavity