1. Development of foetal membranes in birds and mammals
VAN 504, Lecture 08
Development of foetal membranes in birds and mammals

2. Function of Fetal Membranes and Fluids
Yolk Sac – Originates from fetal midgut. Nutrient supply for early embryo. Absorbs uterine sectretions from endometrium to stimulate early embryonic development.
Amnion – Innermost membrane directly surrounding the fetus. Protects from injury and provides lubrication for parturition. Prevents lung collapse and opens digestive tract.
Allantois – Originates from the gut and forms the umbilicus. Supports blood vessels. Reservoir of nutrients and wastes.
Chorioallantois: product of fusion between the allantoic and chorionic membranes
Chorion –Outermost membrane in direct contact with uterine tissue. Becomes vascularized by allantoic vessels. Site of hormone production, nutrient and gas exchange.

3. Fetal membranes:
Four fetal membranes develop in a conceptus. Two arise from the trophoblast layer of the blastocyst (and are continuous with the somatopleure of the embryo). Two arise from the inner cell mass of the blastocyst (and are continuous with splanchnopleure of the embryo); these two planchnopleure membranes are vascular. The four foetal membranes are:

5. 1.Chorion — forms the outer boundary of the entire conceptus (from trophoblast)
Outermost membrane
Attachment to mother

6. 2. Amnion — encloses the embryo within a fluid-filled amnionic cavity; formed by folds of chorion in domestic mammals (in humans, amnion forms by cavitation deep to a persistent trophoblast)
Non-vascular, fluid filled
Fluid produced by fetus
Protective cushion
Ruptures at birth (not breaking of water)

7. Blood vessels Fuses with chorion
3. Allantois — develops as an outgrowth of hindgut splanchnopleure (originates from innercell mass). Allantois grows to fill the entire extra-embryonic coelom, with fluid-filled allantoic cavity in domestic mammals. The outer surface of allantois binds to the inner surface of chorion (and the outer surface of amnion). The allantois is highly vascular and provides the functional vessels of the placenta, via umbilical vessels.
Blood vessels
Fuses with chorion
Allantochorion or chorioallantois
Brings blood vessels to chorion

8. In birds to nourish embryo
4. Yolk sac — continuous with midgut splanchnopleure (develops early with hypoblast formation from inner cell mass). Supplied by vitelline vessels, it forms an early temporary placenta in the horse and dog. Yolk sac is most important in egg laying vertebrates.
Note: The term conceptus refers to the embryo or fetus plus its fetal membranes.
In birds to nourish embryo
In mammal atrophies but source of blood cells and primordial germ cells

10. Developmental Adaptations of Amniotes
Embryos of birds, other reptiles, and mammals develop in a fluid-filled sac in a shell or the uterus
Organisms with these adaptations are called amniotes

11. During amniote development, four extraembryonic membranes form around the embryo:
The chorion functions in gas exchange
The amnion encloses the amniotic fluid
The yolk sac encloses the yolk
The allantois disposes of waste products and contributes to gas exchange

12. Extraembryonic membranes in birds
Amnion
Allantois
Embryo
Amniotic cavity with amniotic
fluid
Albumen
Shell
Chorion
Yolk (nutrients
Yolk sac

13. Unincubated Chicken egg

14. Structure of Egg chorion- outer-most membrane, aids in gas exchange
allantois- exchange of O2 and CO2. Also a storage site for wastes
amnion- sac containing amniotic fluid which surround the embryo, provides cushion and acts as a shock absorber
yolk sac- source of food for the embryo. Blood vessels in the yolk sac transport food to the developing embryo

15. chorion- outer-most membrane, aids in gas exchange
allantois- exchange of O2 and CO2. Also a storage site for wastes
amnion- sac containing amniotic fluid which surround the embryo, provides cushion and acts as a shock absorber
yolk sac- source of food for the embryo. Blood vessels in the yolk sac transport food to the developing embryo
Embryo

16. Chicken Egg outer layer of thin albumen germinal disc yolk chalaza
shell
inner shell membrane
outer shell membrane
air cell yolk membranes
dense albumen
inner layer of thin albumen
chalaziferous layer

18. 24 hr chick (neurulation)

19. 48-hr Chick

20. 56-hr Chick

21. 96-hr Chick

22. Fetal Membranes of the Chick