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gastrulation (chickens)
blastodisk: initial divisions, always tilted-gravity determines orientation,
mesoblastic discoidel
cytokinesis furrow
part of meroblastic discoidel cleavage in chickens
subgerminal cavity
space between the yolk and the epiblast layer where the hypoblast develops

koller’s sickle
more dense area of cells underneath the area pellucida, occurs after laying, where the primitive streak grows from
area opaca
outer layer of the embryo
area pellucida
the inner area of the embryo

epiblast
ectoderm + mesoderm + endoderm
specification of the primitive streak stage 1
wnt8c is expressed around marginal zone
vg1 (TGFb ligand) expressed on posterior side marginal zone
cerberus expressed from hypoblast —> seqeusters and inhibits wnt8c and Vg1

specification of primitive streak stage 2
proliferation of cells from area opaca form the endoblast next to the hypoblast —> pushes cerberus expression farther away allowing wnt8c and Vg1 activation
expressed wnt8c and Vg1 + FGF expressed from the growing Koller’s sickle —> activate NODAL (TGFb)

specification of primitive streak stage 3
NODAL expression defines the primitive streak
NODAL forms the organizer: organizes cells into mesodermal and endodermal cells (needed and sufficient for orientation of embryo)

Hensen’s node
node between notochord and primitive streak (AP)