Chic Development

• telolecithal (mainly yolk)
• little disk of cytoplasm (blastodisc)

outside in:

• 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

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

• 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

• 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

it is in thickening of cells in the anterior portion of the primitive streak

• primitive streak - blastopore lips
• primitive groove - blastopore
• Hensen's node - dorsal lip/organizer

• 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

• 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

• 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

FGF8, expressed by cells in 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

• 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

pharyngeal endoderm - head process - hensen's node - primitive streak - area pellucida - area opaca

anterior migration of secondary hypoblast cells

the streak regresses and leaves behind the dorsal axis

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)

niewkoop center

induces primitive streak and Hensen's node

• 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

• 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

it secretes Lefty (a nodal antagonist)

Hensen's node

• 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!!!

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

• 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!!

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

• 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 and PitX2, but on the right you have neither

Lefty is in the midline and keeps cerberus over on the left

you have a homozygous loss of Pitx2 so you have asymmetry abnormalities