Implantation and placentation

Question 1

How is a placenta formed?

Answer 1

Fetal membranes become closely attached to uterine wall- facilitates gas, nutrient and waste exchange

Question 2

Explain the development of the extraembryonic membranes in mammals

Answer 2

• 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.

Question 3

What forms the amniotic folds?

Answer 3

The mesoderm

Question 4

What forms the chorion?

Answer 4

The trophoectoderm and mesoderm fusing

Question 5

What does the amnion allow?

Answer 5

• Symmetrical growth/ protection of foetus

- Temperature control and foetal movement

Question 6

In marsupials what forms the placenta?

Answer 6

Yolk sac

Question 7

What are the 3 stages of implantation?

Answer 7

• 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

Question 8

Which species undergo invasive/ non invasive implantation?

Answer 8

• Cows/ sheep = non invasive: elongation/ apposition

- In horses/ primates which have a spherical embryo you see invasive attachment where it invades the trophoblast

Question 9

What is the difference between hemotroph and histotroph?

Answer 9

• Hemotroph= nutrition from circulating maternal blood

- Histotroph= nutrition from endometrium

Question 10

What do non deciduate/ deciduate mean?

Answer 10

• Deciduate: at birth some of the maternal tissue is shed as well as the fetal components
• Non deciduate: fetal/ maternal components separate at birth

Question 11

What are the maximum possible fetal/ maternal tissue layers?

Answer 11

• 3 possible foetal layer: foetal endothelium, foetal CT, foetal trophoblast
• 3 possible maternal layers: maternal epithelium, maternal CT, maternal endothelium

Question 12

Describe the 3 types of placentation

Answer 12

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)

Question 13

What is the functional unit of the foetal placenta?

Answer 13

Chorionic villous

Question 14

What are the names of the different placentas called regarding the chorionic villi?

Answer 14

• 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)

Question 15

Which species has microcotyledons?

Answer 15

The mare

Question 16

Describe placentation in the sow

Answer 16

• Epitheliochorial placentation
• Diffuse
• Non -deciduate

Question 17

Describe placentation in the ewe and cow

Answer 17

• Synepitheliochorial (epithelial chorial placenta which is more specialised)
• Cotyledonary
• Non - deciduate

Question 18

What are BNCs?

Answer 18

Found in ruminant synepitheliochorial placentas.

-Allow a syncytium to form

Question 19

What is a placentome?

Answer 19

Comprised of a fetal cotyledon and a maternal caruncle.

Question 20

Which species have a convex placentome and which have a concave placentome?

Answer 20

Convex= cow/ giraffe
Concave= sheep / goat

Question 21

Describe placentation in the horse

Answer 21

• Epithelichorial placentation
• Diffuse
• Non- deciduate

Question 22

What is the chorionic girdle?

Answer 22

-Junction between yolk sac and the chorioallantois

Question 23

What are endometrial cups?

Answer 23

• Found in the horse
• They develop from the chorionic girdle cells following localised invasion of the uterus.
• They secrete eCG

Question 24

Why do endometrial cups stop producing eCG?

Answer 24

• 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

Question 25

Describe placentation in the dog

Answer 25

• Enddtheliochorial placentation (4 layers)
• Zonary
• Deciduate

Question 26

How many zones does the zonary placenta consist of?

Answer 26

• Transfer zone: nutrient transfer
• Pigmented zone- local regions of maternal haemorrhage/ necrosis where iron transport occurs
• Allantochorion= poor vascularity

Question 27

Describe decidualisation

Answer 27

• Endometrial stromal cells proliferate and enlarge to become decidual cells
• The decidua is the maternal placenta

Question 28

Describe fetal circulation

Answer 28

• 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.