7. embryonic membranes and cavities and the placenta Flashcards
what contributes the formation of extra-embryonic mesoderm?
trophoblast and yolk sac
Cavities form in the extra-embryonic mesoderm so that it forms a mesodermal lining of a new cavity known as
the chorionic cavity or extraembryonic mesoderm
The extra-embryonic mesoderm forms a layer which lines the chorionic cavity and contributes to 2 membranes:
- Extra-embryonic mesoderm covers the amniotic epithelium and forms the amnion
- Extra-embryonic mesoderm also lines the trophoblast to form the chorion.
What happens to the amniotic and chorionic cavity at week 4?
The amniotic cavity enlarges faster than the chorionic cavity and eventually oblierates it.
They eventually join, and the fetus floats in the fluid of the amniotic cavity
What happens to the chorion at week 6?
It becomes smooth (chorion laeve) at the abembryonic pole
becomes irregular (chorion frondosum) at the embryonic pole where the placenta is developing
which part of the uterine wall does the conceptus develop?
decidua basalis
what happens at week 8 to the uterine wall?
the expanding embryo begins to obliterate the uterine cavity.
the decidua capsularis eventually fuses with the decidua parietalis
What happens to the embryo and cavities at month 3
the embryo floats in the expanded amniotic cavity (hold 1 litre of fluid per term)
the uterine cavity has obliterated
decidua capsularis has fused with decidua parietalis
amnion and chorion have formed the amniochorionic membrane
What happens with regards to placenta formation in days 9-11
The blastocyst is further embedded in the endometrial stroma and the penetration site is plugged by a fibrin clot
Lacunae form in the syncytiotrophoblast at the embryonic pole
day 12-13 placenta formation
cytotrophoblast mitosis produces a multinucleated syncytiotrophoblast which further invades the endometrium
this erodes and plugs maternal vessels and when the plugs remove (12 weeks) a functioning uteroplacental circulation is established
placenta formation day 16
villi of fetal origin grow into blood-filled lacunae ad these come to develop fetal blood vessels which connect with umbilical arteries and veins
allows gas exchange (but not exchange of blood)
week 3-4 of placenta formation:
where does exchange of gases take place
through branches of the stem villi which float in the intervillous spaces
other villi anchor vessels in place
week 3-4 placenta formation:
what does the feto-maternal barrier initially consist of:
- syncytiotrophoblast
- cytotrophoblast
- mesoderm
- endothelium of fetal vessels
it is of FETAL origin
where does the placenta develop
region of decidua basalis
fourth month: what happens to the germ layers
cytotrophoblast and mesoderm regress
syncytiotrophoblast and endothelium is left between fetal and maternal blood
Primary stem villi
week 2 when placental villi first form
core of cytotrophoblast layer covered by a layer of syncytiotrophoblast
secondary stem villi
developed core of extra-embryonic mesoderm inside 2 trophoblast layers
tertiary villi
developed fetal blood vessels within mesodermal core
The intervascular barrier
tissue layers between maternal and fetal bloods
First trimester:
what does the barrier consist of
- villi is few but large
- central fetal vessels (small exchange surface and large distance)
a layer of syncytiotrophoblast,
a complete layer of cytotrophoblast,
substantial mesoderm
the endothelium of fetal vessels
Third trimester:
what does the barrier consist of
- villi are numerous and small
- with peripheral fetal vessels (so exchange surfaces are large and distances small)
the syncytiotrophoblast layer
isolated cytotrophoblast cells
little mesoderm
the endothelium of fetal vessels
describe the placenta at term (9months)
- discoidal (15-25cm) weighs 450-600g
- maternal aspect divided into 15-20 cotyledons
- maternal blood enters via cotyledons with spiral arteries and leaves by uterine veins
- the intervascular barrier is mainly syncytiotrophoblast and vascular endothelium
- large SA but is thin
