Embryonic membranes and cavities + placenta Flashcards
Role of the extra embryonic mesoderm
(Occurs during week 2)
Forms a layer that lines the chorionic cavity and contributes to the formation of 2 membranes :
- the amnion (amniotic epithelium + extra-embryonic mesoderm)
- the chorion (extra-embryonic mesoderm + trophoblast)
Describe the development of the yolk sac
The primary yolk sac is derived from the hypoblast
The secondary yolk sac is from from the epiblast during gastrulation
Development of embryonic membranes and cavities during week 4
Amniotic cavity enlarges faster than the chorionic cavity and eventually obliterates it
The amnion and chorion join and the foetus is free to float in the amniotic fluid
Development of embryonic membranes and cavities during weeK 6
Chorion consists of trophoblast and extra-embryonic mesoderm up until this point
It now specialises and become :
Smooth (chorion laeve) at the abembryonic pole
Irregular (chorion frondosum) at the embryonic pile where the placenta is forming
Label this diagram of the decidua (differentiated endometrium)
1 decidua parietalis (uninvolved bit)
2 decidua capsularis (where embryo implants - between the blastocyst and uterine cavity)
3 decidua basalis (between the blastocyst and the musculature)
Name the areas of the uterus where the embryo should implant in
Decidua capsularis (in front and covering the embryo) Decidu basalis (behind embryo)
Main developments in week 8
Expanding embryo and its covering membranes begin to obliterate the uterine cavity
Decidua capsularis fuses with decidua parietalis
what are the lacunae ?
Cavities that form in the syncytiotrophoblast ; they eventually become the placenta
Development of embryonic membranes and cavities during month 3
Embryo floats in expanded amniotic cavity
Uterine cavity completely obliterated
Amnion and chorion have formed the amniochorionic membrane
Explain how uteroplacental circulation is established
Days 9-11 the penetration site is plugged by a fibrin clot
the lacunae become filled by maternal vessels ; plug is fully removed by 8-12 weeks
foetal villi grow into the lacunae and develop into fetal blood vessels which connect with the umbilical arteries/veins
this allows exchange of gases (but not of blood|)
Name the 4 components of the feto-maternal barrier
- Syncytiotrophoblast
- cytotrophoblast
- mesoderm
- endothelium of foetal vessels
Where does exchange between the foetus and mother take place
Through the branches of the stem villi which float in the intervillous spaces
villi that extent from the edge of the embryo to the edge of the trophoblast anchor the vessels in place
Placental formation week 4 post fertilisation
Branching of stem villi into intermediate and terminal villi
septa bind cotyledon
How does exchange between the embryo and the mother occur during the first trimester
diffusion across the amnion rather than through villi as there are few if them and the vessels are central (so large diffusion distance)
nutrition provided mainly via the glands in the decidua
Exchange between embryo and mother 2nd and 3rd trimester
Mainly via the villi as diffusion across amnion not sufficient to meet metabolic demands
Describe the inter vascular barrier during the first trimester
Few large villi with central vessels
barrier comprised of:
layer of syncytiotrophoblast
complete layer of cytotrophoblast
substantial amount of mesoderm
endothelium of foetal vessels
describe the intervascular barrier in the 3rd trimester
many small villi with peripheral fetal vessels
barrier now comprises of :
syncytiotrophoblast layer
isolated cytotrophoblast cells
little mesoderm
endothelium of foetal vessels
