Development- Body and Coelom Formation Flashcards
What did the evolution of extra-embryonic or fetal membranes allow?
Allowed reptiles, mammals, and birds to move away from aquatic environments and to evolve to land
How does the embryonic disc become covered by extra-embryonic membranes?
Body folding
Function of extra-embryonic membranes
- Supply/storage of nutrients
- Gas exchange
- Excretion of wastes
- Mechanical protection
- Immune protection
- Hormone production (mammals)
What are the extra-embryonic membranes?
- Chorion
- Amnion
- Yolk Sac
- Allantois
Yolk sac
- Connected to midgut by yolk duct (yolk duct wall continuous with gut)
- Mediates nutrition in developing birds and reptiles
- No yolk within the membrane in higher mammals. But still important as it is the first hematopoietic organ (blood cell formation) in mammals and a source of primordial germ cells (or stem cells which will eventually form oocytes/sperm cells)
What forms the yolk sac wall?
- Formed from endoderm and visceral mesoderm (splanchnic)
- Therefore termed Splanchnopleure
Chorion
- Outermost extra-embryonic membrane
- Functions in gas exchange, respiration etc
What is the chorion derived from?
- Derived from trophectoderm (near embryonic disc) and somatic mesoderm
- Therefore termed somatopleure
Where is the chorion in reptiles and birds?
Up against the egg shell
Steps of body folding cranial to caudal
- Somatopleure pushes up forming chorioamniotic folds that wrap around the embryo forming the amnion and eventually the amniotic cavity
- Splanchnopleure pinches inwards forming primitive gut, and the yolk sac and yolk duct
Amnion and its importance
The most evolutionary significant membrane to form
Surrounds the embryo proper and provides an aquatic micro-environment which permits embryogenesis in water analogous to evolutionary ancestors.
- Prevents dessication
- Acts as a shock absorber
Mesamnion (chorioamniotic raphe)
- Seam. Site of fusion of somatopleure/ chorioamniontic folds
- In pigs and ruminants, fusion.
- In horses and carnivores, no fusion occurs. The chorion and amnion connection is lost in horses and carnivores. This allows allantois to move in between the chorion and amnion and wrap around the fetus.
Allantois
Acts to store urinary wastes form embryo’s developing urinary system and mediates gas exchange
Size difference of allantois in reptiles vs. mammals
Reptiles and birds: can be quite large and apposed to chorion keeping toxic by-products of metabolism away from the embryo
Mammals: size of allantois depends on how well nitrogenous wastes are removed by chorionic placenta, but it can fill the entire extra-embryonic coelom
Why does the head bend so much during cranial-caudal folding?
Head bends to create enough space for full development of large brain