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

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.

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:

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

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)

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

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

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

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

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

Unincubated Chicken egg

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

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

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

24 hr chick (neurulation)

48-hr Chick

56-hr Chick

96-hr Chick

Fetal Membranes of the Chick