Embryology Flashcards
Outline the stages leading up to fertilisation:
- Oocyte released from ovary
- Travels down Fallopian tube
- Fertilised in ampulla
How long is the gestation period?
- 40 weeks give or take 2 depending on circumstance.
What is a fertilised oocyte known as?
- Zygote
Where is the ideal implantation site?
- Posterior uterine wall.
How long is an oocyte and a sperm viable for?
- Oocyte: 24hrs
- Sperm: 3 days
What occurs at 30 hours after fertilisation and what does it result in?
- Cleavage (1st cell division)
- Two blastomeres of equal size
What surrounds the two blastomeres?
- Glycoprotein shell: zona pellucida
What happens after the two blastomeres are formed?
- A MORULA forms (more cell division, identical cells)
- Totipotent
Name an assisted reproductive technique and a test for genetic diseases.
- Oocytes fertilised in vitro - Morula is transferred to uterus
- Pre-implantation genetic diagnosis (PGD) - Cell removed from morula and tested for serious genetic diseases before transferral into uterus.
What is compaction?
- Formation of first cavity
- Outer cell mass: Trophoblast
- Inner cell mass: Embryoblast
- Embryoblast masses at top of cell leaving a cavity: Blastocyst
What is hatching?
- Blastocyst hatches from zona pellucida
- No longer constrained, free to grow
- Can interact with uterine wall to implant.
How does implantation occur?
- Embryoblast side of cell joins to uterine epithelium.
During week 2 the inner and outer cell masses change, how so?
- Trophoblast: 1) Syncytiotrophoblast (transporting membrane)
2) Cytotrophoblast (stem cell layer: repair) - Embryoblast: Bilaminar disk: - epiblast (upper) & hypoblast (lower)
What are the two cavities surrounding the Bilaminar disk?
- Amniotic cavity (above)
- Blastocyst cavity/yolk sac (below)
The Bilaminar disk is suspended and supported by what two things?
- Suspended by connective stalk
- Supported by chorionic cavity
What is the point of implantation?
- Establishes maternal blood flow within placenta
- Establishes basic structural unit of materno-foetal exchange
What is implantation exactly?
- Uterine epithelium breached and conceptus implants within uterine stroma.
Name two implantation defects.
- Ectopic pregnancy
- Placenta praevia
What is ectopic pregnancy?
- Implantation of conceptus not in uterus (Fallopian tube)
- Peritoneal/ovarian
- Life threatening
What is placenta praevia?
- Implantation in the lower uterine segment
- Haemorrhage
- Requires C-section delivery
On day 9 of development, two poles form, what are these and what else occurs?
- Embryonic pole: rapid development of syncytiotrophoblast
- Abembryonic pole: primitive yolk sac formation
- Yolk sac is in contact with cytotrophoblast layer
On Day 11 if development the yolk sac is pushed away from the cytotrophoblast by what and what happens to this?
- Acellular extraembryonic reticulum
- Converted into an extraembryonic mesoderm layer by cell migration.
What happens on day 12 of development?
- Sinusoids are invaded by syncytiotrophoblast.
- Lacunae become continuous with sinusoids
- Uteroplacental circulation begins.
- Uterine stroma prepares for support of embryo.
On day 13 of development a secondary yolk sac forms, how does this occur?
- Pinches off from primitive yolk sac.
What percentage of births are:
a) lost in weeks 2-3
b) diagnosed but miscarried
c) recurrent miscarriages
a) 50%
b) 15%
c) 1%
Why may intrauterine growth restriction occur?
- Poor maternal nutrition
- O2 supply to foetus
What is pre-eclampsia?
- High blood pressure
- High protein content of pregnant women
- Leads to life threatening seizures.
What is the role of gastrulation?
- Production of 3 germ layers
- Trilaminar disk
- Axes
What are the 4 different axes of the body?
- Posterior/anterior
- Superior/Inferior
- Dorsal/ventral
- Left/right
What is the primitive streak, node and pit?
- Primitive node is located at the cranial end of the streak and the pit is the centre of the zygote.
When does the primitive streak regress and what direction does development occur?
- When the 3 germ layers have been established
- Cranium -> Caudal
How is the Trilaminar disk formed from the Bilaminar disk?
- Migration and invagination of cells
- Cells spread cephalad (towards the head) and laterally.
- Displacement of hypoblast layer to form mesoderm
How does the mouth and anus form during gastrulation?
- Cells don’t invaginate at the head and end of the streak.
What is the notochord’s role in development?
- Drives NS development.
- Basis for axial skeleton
- Defines phylum cordata
How is the notochord formed and what’s its role?
- Epiblast migrating through the cranial part of the primitive pit
- Forms a solid rod with signalling purposes.
What is situs inverta, and why does it occur?
- Mirror image of normal for internal organs
- Results from immotile cilia
- Only problems if mix of reverse and normal.
How can twinning occur?
- Embryo splits after cleavage.
- Two embryos with their own placenta
Or - Embryo cell mass duplicates
- Share a placenta
What is teratogenesis?
- When normal embryonic development is disrupted
- Each organ has its own ‘sensitive window’
Name 3 teratogenic agents.
- Thalidomide
- Rubella
- Alcohol
What is neuralation?
- Notochord driven induction of ectoderm to form NS
- Neural plate -> neural tube which forms brain and spinal chord
How does the neural plate change to form the neural tube?
- Notochord signals to cause overlying ectoderm to thicken
- ‘Slipper’ shaped neural plate
- Edges elevate out of the plane of the disk and curls towards each other creating the neural tube.
After neurulation what does the mesoderm split into and what is the space between these two layers called?
- Splits into somatic and splanchnic mesoderm
- Intraembryonic coelom
What are somites?
- Organisation of paraxial mesoderm into segments
How are somites formed?
- Craniocaudal sequence, 3 pairs a day from day 20 -> 42/44 pairs
- Some disappear to leave 31 pairs
What is the next step after 31 pairs form?
- Organised degeneration: ventral walls break down to form a sclerotome ( hard tissue section)
- Further organisation of dorsal portion forms combined dermomyotome
What happens to the dermomyotome?
- Myotome (muscle section) proliferates and migrates
- Dermatome (skin section) disperses
What does segmentation give rise to?
- Vertebrae
- Ribs
- Intercostals
- Spinal cord segments
What is embryonic folding?
- Head fold then a tail fold: cephalocaudal folding
- Lateral folding (other edges to leave a small gap for the umbilical cord)
- This is driven by expansion of neural tube
What does embryonic folding achieve?
- Creates a ventral wall around disks
- Pulls amniotic membrane around disk (embryo becomes suspended within amniotic sac)
- Pulls connecting stalk ventrally.
- Creates primordium of gut from yolk sac
- Puts heart and primordium of diaphragm into right place
- This creates a new cavity within the embryo
