Life Before Birth Flashcards
What are the 3 developmental stages of life before birth? what are their timings and what happens briefly?
Pre-implantation stage: week 1 (fertilised eggs implanted in uterus)
Embryonic stage: week 2-8 (organogen)
Fetal stage: week 9-38 (growth and development)
Describe cleavage?
When zygote undergoes a process of mitotic division and the cells in the embryo are cleaved and multiply in number but size of zygote remains the same.
Why is it necessary that the zygote does not increase in size?
So that zygote can pass through the isthmus (narrowest part of the Fallopian tube)
What helps protect the zygote from premature implantation?
Zygote is surrounded by the ZONA PELLUCIDA - which is a tough, transparent coat made up of glycoproteins. This helps ti along the Fallopian tube.
What is a morula?
Cluster of around 16 cells/blastomeres
Describe morula formation and when doe sit occur?
Occurs in day 4 after fertilisation
As cells continue to multiply, they run out of space and cluster together. They maximise contact with one another and form tight junctions to communicate with one another.
Describe blastocyst formation and when does it form
Day 5
Cells in the embryo either differentiate to form:
Inner cell mass (which forms the embryo and extra embryonic cells)
OR
Trophoblast cells (which forms the placenta)
Describe the structure of a blastocyst
Trophoblast cells line the outside
Clump of inner cell mass in one side
Blastocyst Cavity is filled with fluid (enters via the ZONA PELLUCIDA)
What triggers the hatching process and how does it occur?
Cells start to run out of nutrients and need to implant on the endometrium.
Inner cell mass begins to proliferate and fluids builds up in the blastocyst cavity. the pressure eventually causes the blastocyst to hatch out of the zone pellucid to allow implantation
When does the hatching process occur?
Days 5-7
What week and days does implantation occur in?
Week 2
Days 7-12 after fertilisation
What is implantation?
Interaction between mother’s endometrium and implanting embryo
Which cells implant into the endometrium first?
trophoblast cells
What do trophoblast cells differentiate into? which ones are inner and outer?
Syncytiotrophoblast cells (outer) Cytotrophoblast cells (inner)
What does the bilaminar embryonic disc comprise of?
Epiblast
Hypoblast
Give examples of abnormal implantation
Ectopic pregnancy: embryo implants itself in the Fallopian tube
Ectopic sites: embryo escapes the Fallopian tube and sticks onto external surface of uterus
Describe the 2 cavities around the bilaminar disc
Amniotic sac: extends from the epiblast, cavity filled with amniotic fluid which surround and protects developing embryo
Yolk sac: extends from the hypoblast, providing nutrients to the embryo in week 2-3. Disappears at around week 20
List extra embryonic membraneS?
Amnion
Chorion
Yolk sac
Describe chorion, what is it made from?
Made from trophoblast and mesoderm cells. It contributes to the formation of the fetal component of the placenta. Only seen in early pregnancy as it disappears when the amnion gets bigger.
Define gastrulation and what week does it occur in?
Gastrulation is the process of cell division and migration, resulting in formation of three germ layers (trilaminar embryo)
What are 3 important structures in gastrulation?
Primitive streak
Notochord
Neural tube
What is the primitive streak?
Thickening of cells in the caudal end of the embryo
Describe the process of invagination
Cells in the epiblast move towards the primitive streak and push down where the PS is. The cells then push outwards and form a layer underneath called the Mesoderm.
Describe what happens after imagination and what the 3 germ layers are
After invagination, cells replace the hypoblast to form the endoderm (bottom layer).
Cell that lie between the endoderm and the epiblast = mesoderm
Remaining cells in the epiblast form the ectoderm (top layer)
