Early Embryonic Development Flashcards
What are the steps in development of a human being?
\+ Fertilisation \+ Single cell \+ Ball of cells \+ Cavities and two layered disc \+ Three layered disc with middle layer \+ Segments in middle layer \+ Tube forms from two outer layers (nervous system and gut) \+ Fully formed body form/organs
Where does fertilisation occur?
In the ampulla of the uterine tube within 12 hours of ovulation.
What kind of cells are sperm and ovum?
Haploid (23)
What is restored by fertilisation?
The diploid number of chromosomes (46)
What is cleavage?
+ First stage of embryogenesis
+ Repeated mitotic division of a zygote into blastomeres that cluster to a compact mass called morula.
+ Continues until blastula is formed.
When does initiation of cleavage occur?
Within 36 hours of fertilisation
What is a feature of cells up until the blastocyst stage?
They are totipotent
What is a blastula?
A blastula in which some differentiation of cells has occured
When does ovulation to implantation occur?
The first week of development
When does compaction begin?
At 8 cells ( central cells linked by gap junctions)
What is a morula?
A solid ball of cells with inner and outer layers (16+ cells)
What happens to the morula approx. 3-4 days after fertilisation?
+ Transported along the uterine tube and arrives at the uterus (~30-40 cells)
+ Zona pellucida disappears
+ Outer cells divide to become trophectoderm, inner cells become inner cell mass
What happens after 4.5 days?
+ Cavities form in the inner cell mass
+ Blastocyte is formed from trophoblast and inner cell mass
+ Still free unattached in uterus
When are cells totipotent up until?
The blastocyte stage
During what stage is it preferred that cells be sampled at for genetic testing?
+ Trophectoderm (5-6 days) now preferred over blastomere sampling
+ Previously inner cell mass sampling was used over this method
+ Searching for gene disorders or chromosome abnormalities
What is involved during binding of the embryo to the uterus wall?
+ Embryonic pole attaches
+ Down regulation of anti-adhesion molecule MUC-1 (epithelium)
+ Embryo allows binding via selectins to glyco-components on epithelial cells of the uterus
+ Similar mechanism to white blood cell adhesion to blood vessel walls
+ Integrins, laminin and fibronectin involved in initial penetration
What occurs after ~5.5-6 days?
Binding of the embryo to the uterus wall
What main events happen after ~6-7 days?
+ Implantation of blastocyst starts
+ Trophoblast becomes the “invasive” syncytiotrophoblast and cytotrophoblast
+ Syncytiotrophoblast multinucleate syncytium invasion via metalloproteases
+ Immunosuppressionof host/graft and graft/host reactions
What main events happen on day ~8?
+ Trophoblast divides and part becomes “invasive” syncytiotrophoblast and cytotrophoblast
+ Two layers form in the embryo: epiblast and hypoblast
+ The amniotic cavity BEGINS to form as a space formed within the epiblast
What happens on day ~9?
+ Hypoblast cells (yellow) coat the blastocyte cavity to form the primary yolk sac
+ Spaces develop within the syncytiotrophoblast
+ Amniotic cavity has formed as a space formed within the epiblast
+ Extraembryonic mesoderm will soon form deep to trophoblast
What happens during days ~11 - 12?
+ The embryo has split into two laters (bilayered)
+ Blastocyte has burrowed into the endometrium
+ Syncytiotrophoblast (SCT) cells erode through walls of large maternal capillaries which bleed into spaces - primitive placental circulation
+ New layer between yolk sac and cytotrophoblast
+ Chronic cavity will form by fusion of spaces in extra embryonic mesoderm
What happens by the end of week 2 (when a woman expects her period)?
+ 2 layers develop in the trophoblast
- syncytiotrophoblast
- cytotrophoblast
+ 2 layers in the inner cell mass
- epiblast (ectoderm)
- hypoblast (endoderm)
+ 2 cavities form
- amniotic cavity
- chorionic cavity
What is preeclampsia?
+ Maternal hypertension and proteinuria after 20 weeks
+ Cytotrophoblast invasion into spiral arteries of uterus is defective (occurs later in pregnancy up to 20-22 weeks)
+ Spiral arteries keep their muscular walls and act as high resistance vessels —> high blood pressure
What is the most common site of ectopic implantation?
Ampulla (tube rupture causes haemorrhage)
