Implantation and placentation Flashcards
How is a placenta formed?
Fetal membranes become closely attached to uterine wall- facilitates gas, nutrient and waste exchange
Explain the development of the extraembryonic membranes in mammals
- Primitive endoderm forms beneath ICM and grows downwards forming the yolk sac.
- Mesoderm develops between endoderm and ICM forming a sac around the yolk sac & forming amniotic folds.
- Mesoderm fuses with trophoectoderm to form chorion
- Amniotic folds continue to grow and fuse enclosing embryo in amniotic fluid.
- Yolk sac regresses as allantois expands.
What forms the amniotic folds?
The mesoderm
What forms the chorion?
The trophoectoderm and mesoderm fusing
What does the amnion allow?
- Symmetrical growth/ protection of foetus
- Temperature control and foetal movement
In marsupials what forms the placenta?
Yolk sac
What are the 3 stages of implantation?
- Apposition: blastocyst or foetal membrane become closely apposed to uterine lining
- Adhesion: biochemical interaction between molecules on the trophoblast and epithelium
- Firm attachment OR invasion of trophoblast into uterus: Dependant on species
Which species undergo invasive/ non invasive implantation?
- Cows/ sheep = non invasive: elongation/ apposition
- In horses/ primates which have a spherical embryo you see invasive attachment where it invades the trophoblast
What is the difference between hemotroph and histotroph?
- Hemotroph= nutrition from circulating maternal blood
- Histotroph= nutrition from endometrium
What do non deciduate/ deciduate mean?
- Deciduate: at birth some of the maternal tissue is shed as well as the fetal components
- Non deciduate: fetal/ maternal components separate at birth
What are the maximum possible fetal/ maternal tissue layers?
- 3 possible foetal layer: foetal endothelium, foetal CT, foetal trophoblast
- 3 possible maternal layers: maternal epithelium, maternal CT, maternal endothelium
Describe the 3 types of placentation
1= Epitheliochorial (sows): 6 layers
2= Endotheliochorial (dogs): trophoblast cells have invaded the uterus epithelium and so trophoblast cells are in direct contact with some of the maternal blood vessels (4 layers)
3= Haemochorial: most invasive types, no layer on maternal side at all= you get pooling of maternal blood washing up against foetal membranes
(N.B. fetal layer number remains the same)
What is the functional unit of the foetal placenta?
Chorionic villous
What are the names of the different placentas called regarding the chorionic villi?
- Diffuse= villi evenly distributed throughout membrane and INTERDIGITATE with maternal endometrium (sow/ mare)
- Cotyledonary= the chorionic villi are clustered
- Zonary= wide band of chorionic villi down the centre (dog)
Which species has microcotyledons?
The mare
Describe placentation in the sow
- Epitheliochorial placentation
- Diffuse
- Non -deciduate
Describe placentation in the ewe and cow
- Synepitheliochorial (epithelial chorial placenta which is more specialised)
- Cotyledonary
- Non - deciduate
What are BNCs?
Found in ruminant synepitheliochorial placentas.
-Allow a syncytium to form
What is a placentome?
Comprised of a fetal cotyledon and a maternal caruncle.
Which species have a convex placentome and which have a concave placentome?
Convex= cow/ giraffe Concave= sheep / goat
Describe placentation in the horse
- Epithelichorial placentation
- Diffuse
- Non- deciduate
What is the chorionic girdle?
-Junction between yolk sac and the chorioallantois
What are endometrial cups?
- Found in the horse
- They develop from the chorionic girdle cells following localised invasion of the uterus.
- They secrete eCG
Why do endometrial cups stop producing eCG?
- The cup cells are foreign to the mother so her immune system attacks them, casing them to slough off the uterus thus they produce no more eCG
Describe placentation in the dog
- Enddtheliochorial placentation (4 layers)
- Zonary
- Deciduate
How many zones does the zonary placenta consist of?
- Transfer zone: nutrient transfer
- Pigmented zone- local regions of maternal haemorrhage/ necrosis where iron transport occurs
- Allantochorion= poor vascularity
Describe decidualisation
- Endometrial stromal cells proliferate and enlarge to become decidual cells
- The decidua is the maternal placenta
Describe fetal circulation
- OXYGENATED BLOOD= umbilical vein
- DEOXYGENTATED BLOOD= umbilical artery
- Foramen ovale: blood rich in oxygen bypasses liver, goes directly to CaVC, the blood enters right atrium of the heart via the foramen ovale.
- Ductus arteriosis: venous blood flow back to the heart= RA to RV, most doesn’t enter pulmonary artery so passes into aorta via the ductus arteriosus.
