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Flashcards in Embryology Deck (42)
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What occurs at day 0?

Fertilisation of the oocyte by sperm in the ampulla of the Fallopian tube to give a zygote.


What happens between fertilisation and compaction?

Cleavage first occurs to give 2 blastomeres surrounded by a zona pellucida. Further mitotic divisions then occur to give a morula.


What happens during compaction and at around what day?

Day 4.
The cells differentiate and become pluripotent to give:
- inner cell mass
- trophoblast
- blastocyst cavity


What occurs shortly after compaction? (~ day 5)

Hatching of the blastocyst from the zona pellucida = free to grow and interact with uterine surface.


How many cells does the embryo have when implantation begins on day 6-7?



What occurs on day 8?

- trophoblast develops into cytotrophoblast and syncytiotrophoblast
- inner cell mass develops into epiblast and hypoblast
- formation of amniotic cavity.


When and how is implantation completed?

~ day 9-10

Syncytiotrophoblast secretes enzymes which allow penetration of uterine wall.
This is invasive so a fibrin plug develops to block entry site.


What does implantation achieve?

Support of the embryo is now haemotrophic and the embryonic tissue is interstitial with the maternal tissue.


What are the 2 common implantation defects?

1). Ectopic pregnancy - where the embryo implants anywhere other than the posterior uterine wall.

2). Placenta praevia - where they embryo implants in the lower uterine segment. Due to chance of haemorrhage (due to placentas rich blood supply) C sections must be performed.


What development occurs on day 9?

Rapid mitotic division of cells in the cytotrophoblast which then migrate to the syncytiotrophoblast and fuse to those cells.

At the abembryonic pole, the hypoblast cells form a membrane that lines the cytotrophoblast = primary yolk sac


What forms the extraembryonic reticulum and when?

Around day 11, the yolk sac is pushed away from the cytotrophoblast by extra embryonic reticulum, which will later form the extra embryonic mesoderm.
It then grows to surround the hypoblast and yolk sac.


What occurs between the formation of the extraembryonic reticulum at around day 11 and the differentiation of extraembryonic mesoderm around day 14?

The maternal sinusoids invade and become continuous with lacunae = uteroplacental circulation begins.
Secondary yolk sac also pinches off from the primitive.


How does extraembryonic mesoderm differentiate and when?

Day 14
Extraembryonic cavities formed by extraembryonic reticulum merge to form chorionic cavity.
Some of the mesoderm differentiates into a column to form the connecting stalk


Where and when does the primitive streak appear?

At the start of the 3rd week.
On the dorsal surface of the epiblast.


In what direction does the primitive streak appear?

Craniocaudal direction.


What is occurring in the primitive streak?

1). Epiblast cells migrate upwards towards the streak, flatten then invaginate between the epiblast and hypoblast. This displaces the hypoblast and creates a third layer = trilaminar disc. The cells continue to spread laterally and cephalad.


What does the fate of the epiblast depend on?

Where in the primitive steak they invaginate.


How does the primitive streak set the axes of the embryo?

Cillia on the primitive streak waft molecular signals from the primitive node, which ensures correct axial development. This initiates side-specific signalling cascades = right or lefthandedness decided.


Where do prenotochordal cells originate?

From the epiblast cells that migrate through the cranial part of the primitive pit.


What is the notochord and its purpose?

It is a solid rod of cells that form down the midline of the trilaminar disc.
- defines the axis of the embryo
- drives neurulation


What is left behind after the notochord has regressed?

The nucleus pulposus if the intervertebral discs.


What occurs during neurulation?

The ectoderm above the notochord begins tho thicken in response to signals the notochord releases. This creates a neural plate, in which the edges curl up until they meet, to form the neural tube.


What does mesoderm differentiate and become organised into?

- paraxial mesoderm
- lateral plate
--> splanchnic mesoderm
--> somatic mesoderm
- intermediate mesoderm
- intraembryonic coelom


What does the intraembryonic coelom provide?

Space for the body systems to grow.


What does splanchnic mesoderm give?

Smooth musculature, vasculature and CT of gut.


What does somatic mesoderm give?

The CT of limbs and contributes to the anterior-lateral body wall.


How is paraxial mesoderm organised and at what rate?

Organised into 31 pairs of somites at a rate of around 3 pairs per day in the craniocaudal direction.
Appear as a block of regular mesoderm cells surrounding a cavity.


What does paraxial mesoderm somites go on to form?

Axial skeleton, dermis and muscles of the limb and ventral body wall.


What does organisation of mesoderm into somites give us?

Gives rise to repeating structures (e.g. Ribs, vertebrae, intercostal muscles etc.)
Guides innervation.


How is sclerotome produced and what is it?

Sclerotome comes from organised degeneration, where the ventral wall of the somite breaks down. It gives rise to bone/ hard tissue such as the vertebrae and ribs.