Embryology Lecture 1 - Early Embryonic Development 1 Flashcards Preview

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

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


Where is the ideal site for implantation?

The posterior wall of the uterus


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

1-2 weeks = Pre-embryonic

3-8 weeks = Embryonic

9-38 weeks = Foetal


How is pregnancy weeks calculated?

From date of last menstrual period

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


Define 'Zygote'

A fertilised ovum/oocyte


Define 'Cleavage' in relation to a fertilised ovum

When does the first instance of cleavage occur?

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


What is the zona pellucida?

What is its function?

The glycoprotein shell surrounding an ovum/morula/blastocyst

Prevents polyspermy


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

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


Define 'Ovary'

The female reproductive organ producing ova for fertilisation


Define 'Fallopian tube'

A tube the ovum travels down to reach the uterus


Define 'Uterus'

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


How is a blastocyst formed?

Compaction of the cells that make up the morula


What is the trophoblast?

Where is it found?

What is formed from the trophoblast?

A layer of cells found in the early conceptus/blastocyst

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


What is an embryoblast?

Where is it found?

What structures will be formed from the embryoblast?

An aggregation of cells found in the early conceptus/blastocyst

Will go on to form the bilaminar disk


What is Implantation?

When does it occur?

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


What is the cytotrophoblast?

Where is it derived from?

It's the placental membrane around the yolk sac

Derived from the trophoblast


What is the syncytiotrophoblast? 

Where is it derived from?

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

Derived from the trophoblast


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

Compaction - Day 4

Hatching - Day 5

Implantation - Day 6-7


Describe the events of day 4 of pre-embryonic development

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

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)



This pictures shows what structure?

What structures are labelled?

This is a blastocyst

Labels (from top left clockwise)


Blastocyst cavity

Zona pellucida



Describe the events of day 5 of pre-embryonic development

Hint: Hatching

Blastocyst hatches from the zona pellucida

No longer constrained and free to enlarge

Can now interact with the unterine surface to implant


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

Implantation begins

The blastocyst now has 107 cells

- 8 will make the embryo

- 99 will begin development of the foetal membrane


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

Mention the differentiation of the blastocyst cells

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:



Epiblast and Hypoblast (Bilaminar disk)


Syncytiotrophoblast and Cytotrophoblast


Describe the process of implantation

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

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


What are the two major implantation defects?

Outline the results of these defects

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


Label this diagram

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

Labels from top left clockwise:





Blastocyst cavity

Bilaminar disk

Amniotic cavity

At the beginning of week 2 of development


Describe the events of day 9 of pre-embryonic development

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


Label this diagram

When would you expect the conceptus to look like this?

Labels from top clockwise:


Primitive yolk sac

Amniotic cavity


Day 9 of development


Describe the events of day 11 of pre-embryonic development

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


Describe the events of day 12 of pre-embryonic development

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


Describe the events of day 13 of pre-embryonic development

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


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

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


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?

Labels from top right clockwise (black boxes only):


Maternal sinusoid

Connecting stalk

Amniotic cavity

Secondary yolk sac

Chorionic cavity