Early Embryology Flashcards
Where does fertilisation occur?
Ampulla of uterine tube
What type of cells are the ovum and sperm?
Haploid cells
How does the sperm get into the ovum?
The sperm has a specialised lysosome in its head which dissolves the acrosomal membrane, the sperm’s protective shell around its head
It undergoes capacitation (maturation) and the acrosome reaction (described above) to be able to enter the maternal cytoplasm
What are the two layers surrounding the ovum?
Zona pellucida and Corona radiata
What happens in the ovum once the sperm enters?
- The sperm decondenses and forms a pronuclei
- The male and female pronuclei fuse together
- Maternal and paternal chromosomes line up on spindle equator
- Chromatids separate, forming two separate cells
What changes inside the embryo during the first 3-4 days?
Cell numbers increase but cell size decreases
When does cleavage occur?
36 hours after fertilisation
What is compaction and when does it occur?
Individual cells become less visible, cells start to communicate between each other through gap junctions - anterior and posterior axis of embryo is estabilished
Begins at 8 cells
What is the morula stage?
- Reached after 3/4 days, becomes a solid ball of 16+ cells
- The morula is transported along the uterine tube and arrives at the uterus approximately 3/4 days after fertilisation (30-40 cells)
- Zona pellucida disappears before implanting
- Outer cells divide to become trophectoderm and inner cells, combining into the inner cell mass
What happens 4.5+ days after fertilisation?
Blastocyst stage
- Cavities (blastocoel) form in inner cell mass
- Blastocyst is made up of the trophoblast and inner cell mass
- Embryo is still free and unattached in the uterus
- Cells are totipotent until the blastocyst stage
What are blastomeres?
Cells of the embryo at the cleavage stage
What is the process of cleavage after fertilisation?
Cell division without growth
Where do they remove cells from for genetic testing?
What are they looking for?
- 5-6 days old trophectoderm
- Safer than blastomere or inner cell mass sampling
- They are looking for single gene disorders or chromosome abnormalities
What happens 5.5-6 days after fertilisation?
Binding at the uterus epithelium by the embryonic pole
- Epithelium of the uterus becomes more sticky during this period since expression of MUC-1 (that makes it non-sticky) is turned off
- Binding is via selectins of the embryo to the glyco-component on epithelial cells of the uterus
- Similar mechanism to while blood cell adhesion to blood vessel walls
- Integrins, lamina and fibronectins are involved in initial penetration of implantation
What happens 6-7 days after fertilisation?
Implantation of blastocyst begins
- Trophoblast splits into syncytiotrophoblast and cytotrophoblast
- Trophoblast becomes the “invasive”, pushes through the uterus epithelium wall to create a network of cytoplasm with nuclei in it a.k.a. syncytiotrophoblast multinucleates syncitium invasion via metalloproteases
- Immunosuppression of host/graft reactions, so embryo is tolerated
What is syncyntiotrophoblast?
Bits of trophoblast fused together; contains lots of nuclei without cell walls
What happens 8 days after fertilisation?
- Two layers form in the embryo: epiblast and hypoblast, which will eventually develop into the ectoderm and endoterm
- Amniotic cavity begins to form as a space within the epiblast
What happens 9 days after fertilisation?
- Hypoblast cells coat the blastocyst cavity to form the primary yolk sac
- Spaces develop within the syncytiotrophoblast
- Amniotic cavity is formed
- Extraembryonic mesoderm slowly starts to form below trophoblast (syncytritrophoblast and cytotrophoblast)
What happens 11-12 days after fertilisation?
- Extraembryonic mesoderm forms with spaces, that will eventually form the chorionic cavity
- Embryonic disc has two layers bilayered
- Syncytiotrophoblast (SCT) cells erode through the walls of maternal capillaries which bleed into the spaces of SCT
What happens at the end of week 2 after fertilisation?
2 layers develop in the trophoblast
- Syncytiotrophoblast
- Cytotrophoblast
2 layers form in the inner cell mass
- Epiblast - ectoderm
- Hypoblast - endoderm
2 cavities form
- Amniotic cavity
- Chorionic cavity
The remnant of the primary yolk sac can sometimes hang around in an adult
What is preeclampsia?
Defect in the invasion process
- Cytotrophoblast tried to invade spiral arteries of the mother but can’t - defective
- Spiral arteries keep their muscular walls and act as high resistance vessels, increasing blood pressure
What is ectopic implantation?
Embryo leaves the ampulla and implants into the peritonial cavity
- Most common site is ampulla due to tube rupture
Ectopics more likely in inflammatory disease of pelvis
What are the causes of congenital malformations?
Genetic - chromosome/single mutation/deletions (20-25%)
Multifactorial or unknown (65-75%)
Environmental
- Drugs/chemicals (nicotine, alcohol, chemotherapy)
- Infectious agents (rubella, toxoplasma)
- Ionising radiation
How does large alcohol consumption affect the developing embryo?
The neurotubes/neuro crest cells don’t work as well, resulting in:
- Growth retardation
- Central nervous system
- Effects on facial development
How are the axes determined in the embryo?
Dorsal/ventral
- determined in the oocyte
Left/right
- determined by cilia movement in the node
Rostral/caudal
