Embryology Flashcards
Overall 3 stages of human development before birth 🤰🏽
+ weeks of each stage
1 - Pre-embryonic (1-2)
2- Embryonic (2-8)
3- Fetal (>8)
3 events during the pre-embryonic period
1 - Cleavage
2 - Compaction
3- Beginning of implantation
Where is the oocyte fertilised?
Ampulla
Function of Zona Pellucida
Prevent polyspermy 🙅🏽♀️
Glycoprotein shell
Describe cleavage
The first cell division
Results in 2 blastomere of equal size 🟡🟡
Describe the morula
A ball of 16 TOTIPOTENT cells
Define totipotent
The ability of the cell to become ANY cell type
Clinical use of morula prior to implantation during IVF
PGD (Pre-implantation Genetic Diagnosis) 🩺
Define pluripotent
The capacity of a single cell to become MANY cells
When does cleavage occur?
30 minutes after fertilisation
Time period when ‘compaction’ occurs
Week 1
Describe compaction
1) Formation of the blastocyst cavity
2) Differentiation of cells to:
- inner cell mass (embryoblast)
- Outer cell mass (trophoblast)
When can the blastocyst interact with the uterine surface
After hatching from the zona pellucida
Describe the Week 2 of the pre-embryonic stage
1) Differentiation of
- inner cell mass
- outer cell mass
2) Implantation is completed
3) Utero-placental blood supply is established
What does the outer cell mass differentiate into
- Cytotrophoblast
AND
- Syncytiotrophoblast
What does the inner cell mass differentiate to?
- Epiblast
AND
- Hypoblast
Describe bilaminar disc
- Made of the epiblast and hypoblast
- Separates the amniotic cavity and blastocyst cavity
2 distinct features of the syncytiotrophoblast
- Multinucleated
- No clear cell boundaries
Pre-embryonic structure that develops to the umbilical cord
The Connecting stalk
What pole is the yolk sac formed 🍳
Abrembrionic pole
What pushes the yolk sac from the cytotrophoblast
The Extraembryonic Mesoderm
Describe how uteroplacental circulation is established
- Syncytiotrophoblast embeds embryo maternal uterine
- Syncytiotrophoblast invades the endothelium of maternal sinusoids
- Maternal sinusoids become continuous with syncytial lacunae
Why can ‘spotting’ cause inaccuracy in determining expected delivery date
⬆️ bleeding due to blood flowing from sinusoids to lacunae
Can be confused as normal menstrual bleeding
What merges to form the chorionic cavity 🤔
The spaces in the extraembryonic mesoderm
Describe an ectopic pregnancy
Implantation of embryo at any other site than the uterine body
Describe placenta praevia
+ major complication & treatment
Implantation in the lower uterine segment
Can cause haemorrhage in pregnancy
C-section
What are the 3 titles given to the ‘conceptus’ in order of formation
Zygote ➡️ Morulla ➡️ Blastocyst
Which developmental stage is the most sesnsitive to teratotogenesis
(And why)
Fetal stage because the organ systems are developing at this stage
The CNS is susceptible to teratogenesis in the fetal stage as well because it continues to develop
Outline the 4 processes occurring during the embryonic stage
1 - Gastrulation
2 - Neurulation
3 - Segmentation
4 - Folding
What is gastrulation
The process that establishes the 3 germ layers
Bilaminar to Trilaminar
What are the 3️⃣ germ layers derived from gastrulation
- Ectoderm
- Mesoderm
- Endoderm
What structures are formed at the beginning of gastrulation
- Primitive streak
- Primitive node
What is a potential explanation for the formation of conjoined twins
The formation of 2 primitive streak
What causes the formation of the endoderm and mesoderm
Migration and invagination of epiblast cells underneath the primitive streak
All 3 layers of the trilamiar disc are derived from the …
Epiblast
What is derived from the ectoderm
- Nervous system
- Skin
What is derived from the mesoderm
- Muscle
- Cardiovascular system
- Bone
What is derived from the endoderm
1) Lining of
- GI tract
- Respiratory tract
2) Parenchyma of glands
Define gestational age
Time since last menstruation
Define embryonic age
Gestational age + 2 weeks
How is left-right asymmetry established
Signalling molecules at the primitive node initiated side specific cascades that signal “left” and “right” sidedness
Describe ‘situs inversus’
Complete mirror image of viscera (organs)
Generally causes no problems unless a person presents with both normal and mirror image viscera orientation
How does situs inversus affect cilia
Immotile/abnormal cilia
What is a potential explanation for situ inversus
Immotile cilia during gastrulation
Causing the misdirection of side-specific signalling molecules
Describe neurulation
Formation of the early nervous system
Describe the notochord
Composition, location and function
- Solid tube of cells
- In the Mesdoderm
- Convesion of the overlying ectoderm into neuroectoderm
What structure overlies the notochord following neurulation
The neural tube
Name the 4 parts of the mesoderm
Paraxial
Intermediate
Somatic
Splanchnic
What is occurs the intraembryonic coelom in later development
Organs form within this space
What forms from the intermediate mesoderm
The Genito-Urinary Tract
What forms from the paraxial mesoderm
Somites which then turns into:
- Muscles
- Bones
What arises from the Splanchnic mesoderm
The Viscera of the body (I.e. the organs)
What develops from the somatic mesoderm
Connective tissue
How many somites are there in total following segmentation
31
Why can somites be used to determine developmental stage
Because they appear in an ordered and predictable fashion
What forms from the dermatome
Skin
What forms from myotome
Muscle
What forms from sclerotome
Bones
What are the 2 implications of segmentation
1) Gives rise to repeating structures (e.g. ribs, vertebrae)
2) Guides innervation
Describe the 2 stages of embryonic folding and the resulting external structure
1 - Cephalocaudal folding
The head and tail fold towards the centre
2 - Lateral folding
The edges fold into the centre
Causing ectoderm to completely cover the surface with a small segment of endoderm showing through
When is the conceptus more vulnerable to teratogenesis and why
Embryonic
Organs develop during this time
How does the relationship between the amniotic sac and the embryo change after folding
- Folding pulls the amniotic membrane around the disc
- Suspending the embryo within the amniotic sac
Describe how folding affects the early:
- Gut
- Heart ❤️
- Diaphragm
- Creates the primordium of the gut
- Puts the positioning of the heart in the correct place
- Puts the primordium of the diaphragm in the correct place
At which developmental stage does implantation normally occur
Blastocyst
What is the function of the hypoblast layer
It lines the blastocyst cavity forming the yolk sac
What is the forms from the notochord in later life
The nucleus purposes of intervertebral discs
Why does the buccopharngeal and cloacal membrane have no mesoderm
It gives rise to the future mouth and anus respectively
What does the primitive streak contain and how can these lead to fetal tumours
- They contain stem cells
- Persistence of the primitive streak can lead to differentiation of the cells to many different cell types
Define a zygote
A single diploid cell formed following the fusion of a sperm and an oocyte
