Implantation and placentation Flashcards
What foetal membranes surround/are attached to the developing embryo?
- Amnion
- Yolk sac
- Allantois
- Chorion
Amnion
- Innermost, fluid-filled membrane in reptiles, birds and mammals
- Allows symmetrical growth and protection of the foetus
- Allows temperature control and foetal movement
Yolk sac
- Extension of the midgut
- In reptile and bird eggs, this membrane surrounds the yolk
- In marsupials, this becomes the definitive placenta
- In higher mammals, this is transient with a definitive role in early mammals
Allantois
- Sac from the hindgut
- This connection becomes the umbilical cord
Chorion
- Outer membrane
- This become the foetal contribution to the placenta
- All placentas are chorionic at some stage of development
What type of placenta do marsupials have?
Choriovitelline placenta
- It is called this become the yolk sac is involved
Describe how a placenta is formed
- Foetal membranes become closely attached to the uterine wall to facilitate the physiological exchange of gases, nutrients and waste products
Describe the 3 stages of implantation
- Apposition: blastocyst/foetal membranes become closely apposed to the uterine epithelium
- Adhesion: a complex biochemical interaction between molecules on the trophoblast and epithelium
- Firm attachment/invasion of the trophoblast into the uterus - this depends on the species
How can we classify placentas?
- Through the foetal extraembryonic membranes involved
- Through source of nutrition (haemotrophic vs histotroph)
- Whether maternal tissue is shed at birth (deciduate vs. non-deciduate)
- Histological structure
- Macroscopic nutrient exchange area: where are the chorionic villi
- Number of layers between foetal and maternal blood circulations
Haemotrophic
nutrition comes from circulating maternal blood
Histotroph
nutrition comes from the endometrium
What are the possible layers separating maternal and foetal blood - foetal?
- Foetal endothelium
- Foetal connective tissue
- Foetal trophoblast
What are the possible layers separating maternal and foetal blood circulations - maternal?
- Maternal epithelium
- Maternal connective tissue
- Maternal endothelium
What is the functional unit of the placenta?
Chorionic villus
This is where the nutrient exchange occurs
What type of placentation is found in the sow?
- True epitheliochorial - 6 layers
- Diffuse
- Non-deciduate
Villi are spread all over the membrane. Villi meet maternal tissue in crypts which is where exchange occurs.
What are BNGC and what is their function?
Binuclear granulated cells
Move across from the foetal side and fuse with a maternal cell, emptying the contents of their granules into maternal circulation. These contents are primarily placental lactogen.
Why is the cow/ewe placenta known as synepitheliochorial?
It is epitheliochorial but the movement and fusion of BNCGs creates a kind of syncytium at the maternal-foetal interface.
What is the difference between cow/giraffe and sheep/goat placentas?
Difference is shape of placentome
- Cow, giraffe = convex placentome
- Sheep, goat = concave placentome
What structure is present in horse placentas?
Microcotyledons
(Not the same as cotyledons in the sheep and cow)
These are distributed all over the entire allantochorionic membrane and allow clusters of villi to penetrate the mare’s endometrium
What is the chorionic girdle and what is its significance?
Chorionic girdle: significant site where the allantochorion first touches the uterus. It meets the yolk sac here.
This means the mare effectively has 2 placentas at the same time:
- Allantochorion
- Yolk sac - effectively a choriovitelline placenta
Eventually the yolk sac all but disappears and the allantois dominates.
Bunches of cells invade the mare’s uterus at the chorionic girdle, forming endometrial cups.
What type of placentation is found in the bitch and queen?
- Endotheliochorial placentation (4 layers total)
- Zonary and deciduate
