Gastrulation and Folding Flashcards
where does the primitive streak appear?
appears in midline at caudal end of epiblast
what is gastrulation?
movement of epiblast through the primitive streak to form the three 1’ germ layers
the intraembryonic mesoderm layer forms where?
between definitive endoderm (hypoblast) + epiblast -> epiblast now called ectoderm
What is associated with the epiblast at the end of week 2?
amniotic cavity
what is associated with the hypoblast at the end of week 2?
Definitive yolk sac
movement of the epiblast cells through primitive pit displaces what and from what?
displaces hypoblast cells and forms definitive endoderm
the tip of the primitive streak is called the what?
The primitive node
why is the primitive node important?
It is important in gastrulation as it is a small indentation where the site of future mouth where embryo starts to form.
where does the primitive streak appear?
in midline at caudal end of epiblast.
how many layers are formed after gastrulation?
3 layers
What is key in the formation of the germ layers?
migration
how to epiblast cells migrate?
Some cells move and loose connection with neighbours and migrate towards the primitive streak. They then dive down to form a new layer and populate the hypoblast layer. This then pushes the original two layers apart and forms a new layer in the centre.
Cells migrate through what structure?
primitive streak
Do cells migrate to cranial end of cell?
yes
why do cells also migrate to cranial end of cell?
so complete generation of new layers can occur through whole embryo.
The hypoblast is replaced with what when cells migrate?
Definitive endoderm
What layer is formed between the endoderm and epiblast when migration of cells occur?
Intraembryonic mesoderm
the epiblast becomes what after migration?
ectoderm
cells that migrate through node to cranial part migrate where?
midline, straight line
as the cells grow to form the notochord do they retain a connection with ectoderm?
yes
what are the three stages of notochord formation?
Hollow tube -> flattened plate-> solid rod
describe briefly how the notochord is formed.
Starts out as a hollow tube and moves downwards and connects with the endoderm and fuses. It then retracts away (brief period), as it separates it looses its lumen and stops being a tube. The Tod runs bang in the middle and runs the whole length of embryo.
why is the notochord important?
- solid rod
- acts as an axis
- positioned to direct development
- highly inductive
- development of vertebral column and neural tube
- gives an anchor point
what is the fate of the ectoderm?
- epidermis of skin
hair, nails - nervous system
brain, spinal cord, peripheral nerves
what are the 3 regions of the mesoderm?
- paraxial
- intermediate
- lateral plate
the lateral plate splits into what 2 regions?
somatic mesoderm and visceral mesoderm
describe the paraxial region of the mesoderm
formed on either side of notochord. Segments to form somites. Contributes to axial skeleton, voluntary muscle, parts of dermis
Describe the intermediate mesoderm
next to paraxial mesoderm only in caudal regions because only contributes to urogenital system
what does the splanchnic part cover?
covering of visceral organs and cardiovascular system.
what does the somatic part cover?
lining of body wall, dermis, parts of limbs
what is the fate of the endoderm?
- lining of the gut tube
- lining of respiratory tract
- lining of the bladder and urethra
what happens on day 18 of the development of the mesoderm?
spaces called coelom form in lateral plate mesoderm. form two layers, somatic and visceral.
what happens on say 21 of the development of the mesoderm?
paraxial mesoderm forms somites.
when is the first sign of the CNS?
day 18
signals around the notochord cause what?
the ectoderm cells to differentiate into neuroectoderm.
as neural plate grows what happens to the primitive streak?
it degresses
what happens on day 20 of the neural tube formation?
fold is more pronounced. Neural plate becomes broader at cranial end for brain.
Moves into embryo from centre causing it to fold. As it folds up it pulls ectoderm with it.
what happens on day 22 of neural tube formation?
edges meet and they fuse. Paraxial mesoderm somites begin to form around neural tube.
where is the first closure point of the neural tube?
cervical region
what happens on day 23 of neural tube formation?
Almost closed tube completely. Yolk sac opening narrows as folding of neural tube occurs.
what is the opening called at the cranial end of the neural tube?
cranial neuropore
what is the opening called at the caudal end of the neural tube?
caudal neuropore
when does the cranial neuropore first close?
around day 24/25
when does the caudal neuropore first close?
around day 26-28
what neuropore closes first?
Cranial
what are neural crest cells?
specialised group of cells multi potent stem cells.
when the neural tube forms what happens to the neural crest cells?
when tube forms they loose connections + migrate throughout embryo.
where do neural crest cells migrate?
- schwann cells
- adrenal gland
- chromatin cells
- teeth
- bones of skull
- melanocytes in skin
- dorsal root ganglia
- enteric system
describe the stages of lateral folding
- Endoderm folds downwards + pulls visceral m. to form the gut tube
- Ectoderm folds downwards + pulls somatic m. to form embryonic body cavity
- note: not complete fusion of edges at site of yolk sac to leave a connection
- creates intraembryonic body cavity
describe the stages of longitudinal folding
- Head region folds ventrally
- Oropharyngeal membrane + mesoderm that forms heart tube move to correct plot
- caudal region fold ventrally
- caudal folding allows connecting stalk to move to umbilical region.
