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describe the yolk in the egg:
- telolecithal (mainly yolk)
- little disk of cytoplasm (blastodisc)
what are the layers in the egg?
- area opaca - outside dark rim, touches the yolk, peripheral ring of blastoderm cells
- area pellucida - inside, lighter, forms the whole embryo
- marginal zone - in between these 2 layers
describe the cleavage events:
only happens in the blastodisc, discoidal meroblastic cleavage - creates single cell layer called blastoderm - which then divides into 5 cell layers thick and is linked together by tight junctions - blastoderm cells absorb water from albumen and secrete it into space between themselves and the yolk, creating the subgerminal cavity
beginning steps in gastrulation:
- 1) area pellucida cells delaminate forming a new layer called the hypoblast, and old layer is called the epiblast. cells that migrate cluster together into hypoblast islands and are sometimes called primary hypoblast
- 2) deep posterior marginal zone cells migrate in and push the primary hypoblast cells anteriorly (secondary hypoblast cells)
- 3) 2 layers of blastoderm are connected at marginal zone cells near area opaca
- 4) blastocoel forms in between the 2 layrs
what is the chic equivalent to the dorsal lip?
describe how it forms:
- the primitive streak
- 1) thickening of epiblast at posterior edge of area pellucida is called Koller's sickle
- 2) these cells extend anteriorly into a streak due to intercalation and convergent extension
- 3) primitive groove forms and it is where cells ingress into the blastocoel to pass into deep layers of the embryo
what is Hensen's node?
it is in thickening of cells in the anterior portion of the primitive streak
what is homologous to what?
- primitive streak - blastopore lips
- primitive groove - blastopore
- Hensen's node - dorsal lip/organizer
how does the primitive streak set up the axes?
- set up anterior to posterior
- cells migrate through its dorsal side and move to the ventral side
- separates left portion of embryo from the right
cell fate of cells that migrate through Hensen's node:
- first cells - pharyngheal endoderm (anterior and ventral)
- next - head mesenchyme and prechordal plate (anterior)
- next - chordamesoderm
*these would be endodermal and mesodermal cells that migrate through the streak
look at slide 15 in powerpoint
when are they told to be meso/endoderm and which axis they will become?
- told endo/meso before they enter the streak
- and then when they go through the streak is when theyre told anterior/posterior and D/V
what is the factor that tells the cells to enter the streak and then keep on moving?
FGF8, expressed by cells in the streak
big pic of cells migrating through the streak:
endoderm and mesoderm migrate through first and set up in an A- P fashion:
1) pharyngeal endo and head meso - induce anterior parts of the brain
2) & notocord - induces hindbrain and spinal cord
finally - epiblast is completely ectodermal
how is endo/meso/ecto set up? maybe ask her to clarify this
- cells that ingress through the anterior part of the streak - endoderm, head mesoderm, and notochord
- cells pass through posterior portions of the streak - mesodermal tissues
- cells that displace hypoblast - meso, end up wrapping around and forming gut tube
- cells that never ingress through streak - ectoderm
what is the linear progression from A to P
pharyngeal endoderm - head process - hensen's node - primitive streak - area pellucida - area opaca
what is going on at the same time as extension of the primitive streak?
anterior migration of secondary hypoblast cells
after the streak extends what does it do and what happens?
the streak regresses and leaves behind the dorsal axis
what happens to the ovum as it is traveling through hen's reproductive tract?
gravity causes it to rotate - lighter components of yolk lie beneath one side of blastoderm - the end of the blastoderm that tips up becomes the posterior marginal zone cells of the embryo (where the streak starts)
what is the PMZ analogous to?
what happens when PMZ cells are implanted into anterior region ?
induces primitive streak and Hensen's node
what factors cause the MZC to be like the niewkoop center?
- entire MZ cells secrete Wnt - activates Beta catenin (rim around yolk)
- PMZ - secretes Vg1
- Vg1 and Wnt together - induce Nodal in the future epiblast next to Koller's sickle
- nodal is needed to induce the streak
what prevents primitive streak formation in the primary hypoblast cells?
- Cerberus - because it is an antagonist of Nodal
- as the primary hypoblast cells move anteriorly they take cerberus with it, so the posterior region is free to express Nodal and the streak can form
What does the streak itself secrete to prevent more streaks from forming?
it secretes Lefty (a nodal antagonist)
implanting what will cause a second embryonic axis to form?
what is the equivalent of the organizer in birds? where does it come from? and how does it fx?
- Hensens node - comes from location anterior to the PMZ cells
- - secretes BMP inhibitors (chordin, noggin, Shh, and nodal) - dorsalize the ectoderm and mesoderm, but this alone is not sufficient for dorsalization... need FGFs!!!
what do FGFs do?
what secretes them?
say "dont be ventral epidermis" and also tell ectoderm to "be neural!"
- synthesized in the hypoblast and in Hensen's node precursor cells just prior to gastrulation
what are the 4 specific things that FGFs do?
- 1) specifies mesoderm tissue as cells migrate through streak (in collaboration with nodal, it turns on genes brachyury and TBx6 that say be mesoderm)
- 2) separates mesoderm from neuralation: induces epiblast cells to express gene called Churchill, which activates SIP1 - which inhibits the epiblast cells from ingressing through the streak - so they stay put and become neural plate cells
- 3) specify neural genes: sip1 interacts with smad1
- 4) induce ERN1 & Sox3 - induce neural tissue
- - look at page 298!!
what is molecular mechanism of FGF?
it is a ligand that binds to a receptor tyrosine kinase (dimer formation on a membrane) that when bound - phosphorylates tyrosine which turns gene transcription on/off
How if Left/Right symmetry regulated?
what is located in the midline?
- On the left: Shh (originally everywhere, but after streak reaches max length it is only expressed on the left) - activates Cerberus - activates Nodal, which represses Snail but activates Pitx2
- On the right: Activin shuts down Shh. Activin + BMP activate Fgf8 - blocks cerberus
big pic: on the left you have Nodal
, but on the right you have neither
is in the midline and keeps cerberus over on the left
what is Reiger's syndrome in humans?
you have a homozygous loss of Pitx2 so you have asymmetry abnormalities