Embryonic membranes and cavities and the placenta Flashcards
What does the trophoblast differentiate into
- Cytotrophoblast
- syncytiotrophoblast
What does the trophoblast contribute to the formation of
Extraembryonic mesoderm
What is the amnion
The extra-embryonic mesoderm that covers the amniotic epithelium
What is the chorion
The extra-embryonic mesoderm that lines the trophoblast
What does the hypoblast produce
Produces cells which form the primitive (primary) yolk sac
What forms the definitive (secondary) yolk sac and when
Epiblast at the time of gastrulation
What week does the fetus float around in the amniotic fluid
Week 4
What happens in week 4
- amniotic cavity enlarges faster than the chorionic cavity and gradually obliterates it
- eventually the amnion and chorion join and the fetus floats in the amniotic fluid
What is the chorion made of
-trophoblast and extra-embryonic mesoderm
What does the chorion become
- Smooth (chorion leave)
- irregular (chorion frondosum)
Where is the smooth chorion
At the abembryonic pole
Where is the irregular chorion
At the embryonic pole where the placenta is developing
Where does the conceptus develop
In the region of the decider basalis
What happens in week 8
- The expanding embryo and its coverings begin to obliterate the uterine cavity
- THe decidua capsularis eventually fuses with the decidua parietalis
In the third month, what happens to the amnion and chorion
They have formed the amniochorionic cavity
In the second week, what is the blastocyst partially embedded in
Endometrial stroma
IN days 9-11, what is the blastocyst embedded in the penetration site by
A fibrin clot
IN days 9-11, what happens at the embryonic pole
Lacunae form in the syncytiotrophoblast
What produces more cyncytiotrophoblast
Cytotrophoblast mitosis
What does the production of syncytiotrophoblast do
Erodes and plus and remodels maternal vessels and when the plug is removed, a functioning uteroplacental circulation is established
What happens in day 13
Lacunae are filled with maternal blood
when do fetal blood vessels form?
Day 13
What develops into fetal blood vessels and how
Villi (of fetal origin) grow into blood-filled lacunae and these come to develop fetal blood vessels which connect to the umbilical arteries and veins
What does the feto-maternal barrier initially consist of
- syncytiotrophoblast
- cytotrophoblast
- MEsoderm
- endothelium of fetal vessels
In week 2, what is the placenta like
When placental villi first form, they are called primary stem villi. They have a core of cytotrophoblast covered by a layer of syncytiotrophoblast
Whats the placenta like in week 3
Secondary villi have developed a core of extra-embryonic mesoderm inside the 2 trophoblast layers
-Tertiary villi will then also develop fetal blood vessels within the mesodermal one
How are anchoring villi formed
-cytotrophoblast breaks through syncytium and overgrows it
Whats happened at the end of week 4
- PLacental septum forms
- Cotyledon bounded by septa
- There is a branching of stem villi into intermediate and terminal villi
What’s the intervascular barrier
The tissue layers between maternal and fetal bloods
What is the placenta like in the first trimester
- VIlli few but large
- Central fetal vessels (so exchange surfaces are small and distances are large)
-The barrier comprises; a layer of syncytiotrophoblast a complete layer of cytotrophoblast substantial mesoderm the endothelium of fetal vessels
What is the placenta like in the third trimester
- Villi are numerous and smal
- wit peripheral fetal vessels
The barrier now comprises; the syncytiotrophoblast isolated cytotrophoblast layer little mesoderm the endothelium of fetal vessels
