Embryology Lecture 1 - Early Embryonic Development 1 Flashcards
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)
Embryoblast
Blastocyst cavity
Zona pellucida
Trophoblast
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:
Embryoblast:
Epiblast and Hypoblast (Bilaminar disk)
Trophoblast:
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:
Syncytiotrophoblast
Cytotrophoblast
Epiblast
Hypoblast
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:
Lacunae
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):
Lacunae
Maternal sinusoid
Connecting stalk
Amniotic cavity
Secondary yolk sac
Chorionic cavity
