Gastrulation

Question 1

Formation of primitive streak

Answer 1

at 3rd week the primitive streak forms on the surface of the epiblasts –> 15th - 16th day –> primitive groove and cranially the primitive node which then invaginates to form the primitive pit (a depression)

Question 2

Formation of the trigerminal disc

Answer 2

Endoderm
cells of the epiblast migrate to the primitive streak and invaginate to displace the hypoblasts to the yolk sac wall and are replaced by the endoderm– GIT system

Mesoderm
cells that remain between the epiblasts and the endoderm –> lose mesenchymal cells (mesoderm)

Ectoderm
form the remaining cell of the outer shell
–> dermis, nails, hair etc..

Question 3

Formation of the notochord and notochordal plate

Answer 3

1. primitive pit cells that will invaginate between the ectoderm and the endoderm - notochord process
2. some cells will undergo apoptosis (canalization) - notochordal canal; extend to the prechodal plate
   - between the tip of the notochord and the oropharyngeal memebrane
   invaginate into the hypoplasts and form the notochordal plate (17th day) –> replaced by the endoderm causes the plate to detach and forming the definitive notochord
3. notochord releases signals to the dorsal ectoderm to thicken = neural plate ( at the cranial part)

Question 4

derivatives of the ectodermal germ layer

Answer 4

Formation of the neural tube
- neural plate will extend tot he caudal direction and lateral edges will elevate= neural folds and fusing together and forming the neural tube –> fuse from the middle than move caudally and cranially

Question 5

Neural crest cells

Answer 5

derived cells form the neural folds that detach
- epith cells that undergo the meschymal transition and travel to the mesoderm which contibutes to the periheral nervous system

1. dorsally; thorugh the ectoderm to the skin ofr the production of the melanocytes
2. ventrally; to the viserocranium areas for the formation of the face and skull
   can also for the sensory ggl and schwann cells

Question 6

Mesoderm derivatives

Answer 6

day 17th
paraxial mesoderm and the lateral plate mesoderm

paraxial mesoderm –> somitomeres
-> mesodermal cells in a concentric whorls
-> cranially associate with the neural plate and form the neuromeres for the mesenchye of the head
-> caudally organizes into somites = axail skelton
1. sclerotome= tendons, cartilage and bone components
2. myototme = muscle components
3. dermatome = epiderms component

42-44 pairs, begin to form in the 20 day until the 5th week

intermidiate
- connecton of the paraxial with the laterla palte mesoderm –> diff into sturctures of the urigenital system

lat plate
parietal + ectoderm form the lat n ventral wall of the body
- mesothelium serous memebrane - lines the caities pericarlal, pleural and secretes serous
visceral layer + endoderm form the gut wall of the body
- serous membrane around each organ

Question 7

Endoderm

Answer 7

forms the gut tube and its organs
ant - forgut
- bound to the orophanengeal memebrane (4th week it ruptures which forms an open communication of the amniotic cavity to the primitive gut.
tail - hinge gut
- bound at the cloacal memebrane that breaks down at the 7th week ) to form the oppening of the anus
between both is the midgut that communicates with the yolk sac thorugh the yolk stalk
also the endoderm lines the secondary yolk sac