11.8 The embryonic shield as organizer in the fish embryo

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11.8 The embryonic shield as organizer in the
fish embryo

• Shield tissue
• can convert lateral and ventral mesoderm (blood
  and connective tissue precursors) into dorsal
  mesoderm.
• can cause the ectoderm to become neural rather
  than epidermis.

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11.9 Axis formation in the zebrafish embryo
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11.14 Discoidal meroblastic cleavage in a chick
egg
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11.15 Formation of the three-layered blastoderm
of the chick embryo (Part 1)
- Most of the cells of the area pelucida remain
at the surface, forming a epiblast. - Other area
pelucida cells have delaminated and migrated
individually into the subgerminal cavity to form
the polyinvagination islands (primary hypoblast),
an archipelago of disconnected clusters
containing 5-20 cells each.
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11.15 Formation of the three-layered blastoderm
of the chick embryo (Part 2)

• A sheet of cells from the posterior margin of the
  blastoderm migrates anteriorly and pushes the
  primary hypoblast cells anteriorly, therefore
  forming the secondary hypoblast (endoblast).
• - Epiblast and hypoblast are joined together at
  the marginal zone of the area opaca, and the
  space between the layers forms a blastocoel.

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11.15 Formation of the three-layered blastoderm
of the chick embryo (Part 3)

• The primitive streak cells form a layer between
  the hypoblast and epiblast cells.
• The primitive streak become a definitive region
  of the epiblast, with cells migrating through it
  to become the mesoderm and the endoderm.

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11.16 Cell movements of the primitive streak and
fate map of the chick embryo (Part 1)
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11.16 Cell movements of the primitive streak and
fate map of the chick embryo (Part 2)
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11.16 Cell movements of the primitive streak and
fate map of the chick embryo (Part 3)
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11.16 Cell movements of the primitive streak and
fate map of the chick embryo (Part 4)
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11.17 Migration of endodermal and mesodermal
cells through the primitive streak
As soon as the primitive streak has formed,
epiblast cells begin to migrate through it and
into the blastocoel.
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11.19 Specification of the chick
anterior-posterior axis by gravity
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11.20 Formation of Hensens node from Kollers
sickle
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11.21 Induction of a new embryo by
transplantation of Hensens node (Part 1)
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11.21 Induction of a new embryo by
transplantation of Hensens node (Part 2)
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11.22 Gene expression in the primitive streak
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11.24 Model by which FGFs regulate mesoderm
formation and neurulation (Part 1)
A) Fgf8 induces pre-neural genes and Nodal is
being blocked by Cerberus
B) The hypoblast is displaced by the endoblast
allowing Nodal to function. Nodal and Fgf8
induced Bra and Tbx6 to specify the mesoderm and
initiate the ingression of mesoderm cells through
the primitive streak.
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11.24 Model by which FGFs regulate mesoderm
formation and neurulation (Part 2)
C) Continued Fgf8 expression activates
Churchill. Churchill can activate the SIP1 which
probably blocks the Smad1 signal and prevents
further ingression of cells through the primitive
streak.
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11.26 Pathway for left-right asymmetry in the
chick embryo (Part 1)
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11.26 Pathway for left-right asymmetry in the
chick embryo (Part 2)
Cerberus
Nodal
Pitx2
R L
L R