8 Embryology 1 Flashcards
What are the 3 stages in embryology?
Pre-embryonic (week 1-2)
Embryonic (week 3-8)
Foetal (week 9-38)
How are pregnancy weeks calculated?
From date of last menstrual period so conception weeks +2, term is 40 pregnancy weeks
What happens in fertilisation?
Oocyte released from ovary
Travels along Fallopian tube
Fertilised by sperm in ampulla of Fallopian tube
Fertilised oocyte called zygote
What is the ideal site for implantation of the zygote?
Posterior uterine wall
What are the 4 stages in week 1?
Cleavage
Compaction
Hatching
Implantation begins
Describe what happens in cleavage
Formation of morula
- first mitotic division
- begins 30 hours after fertilisation
- results in 2 blastomeres of equal size
- zona pellucid forms - a glycoprotein shell
What kind of stem cells are in cleavage stage?
Totipotent (can become any cell)
Describe what happens in the compaction stage
Formation of blastocyst
- formation of first cavity
- now called a blastocyst
What type of stem cells are in compaction stage?
Pluripotent (can become one of many cell types)
What is the structure of a blastocyst?
Inner cell mass (embryoblast)
Trophoblast (surrounding layer of cells)
Blastocyst cavity (blastocoele)
Zona pellucida surrounds entire structure
Describe what happens in the hatching stage
Blastocysts hatches from zona pellucida
Can enlarge and interact with uterine surface to implant
What happens to the approximately 100 cells in conceptus?
8 will make embryo
Remainder develop into foetal membranes
What are the 2 main stages in week 2?
Differentiation
Implantation
Describe what happens in differentiation stage
2 distinct cellular layers emerge from outer cell mass and inner cell mass
Outer cell mass - syncytiotrophblast, cytotrophoblast
Inner cell mass - epiblast, hypoblast
What becomes the bilaminar disk?
Inner cell mass, made up of epiblast and hypoblast
Describe what happens in implantation
Uterine epithelium is breached, conceptus implants within uterine stroma
Established maternal blood flow within placenta
Establishes basic structural unit of maternal-foetal exchange
What are the 2 main implantation defects?
Ectopic pregnancy
Placenta praevia
What is an ectopic pregnancy?
Implantation at site other than uterine body (usually Fallopian tube)
Can be peritoneal or ovarian
Can become life-threatening emergency
What is placenta praevia?
Implantation in lower uterine segment
Can cause haemorrhage
Require C-section delivery
Placenta partially or completely covers cervix
What is the embryonic pole and when does it occur?
Rapid development of syncytiotrophoblast in week 2, day 9
What is the abembryonic pole and when does it occur?
Primitive yolk sac formed in week 2, day 9
Yolk sac membrane in contact with cytotrophoblast layer
What happens in week 2, day 11?
Primitive yolk sac membrane pushed away from cytotrophoblast layer by acellular extra embryonic reticulum
Reticulum later converted to extra embryonic mesoderm by cell migration
What happens in week 2, day 12?
Maternal sinusoids invaded by syncytiotrophoblast
Lacunae continuous with sinusoids
Uteroplacental circulation begins
What happens in week 2, day 13?
Formation of secondary yolk sac (definitive yolk sac) - pinches off from primitive yolk sac
Yolk sac bigger than amniotic cavity
What happens in week 2, day 14?
Spaces within extraembryonic mesoderm merge to form chorionic cavity
Embryo and cavities suspended by connecting stalk - future umbilical cord
Bleeding could be confused with menstrual bleeding
What is a blastocoele?
First cavity, formed as result of compaction
What is the amniotic sac?
Formed from spaces within epiblast
What is the primitive yolk sac?
Formed by hypoblast lining blastocoele
What is the secondary yolk sac?
Formed within primitive yolk sac
What is the extraembryonic coelem (chorionic cavity)?
Formed from spaces within extraembryonic reticulum and mesoderm
