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Chaper 11, Opener
Discoidal meroblastic cleavage in a chick egg
Blastodisc resulting from cleavage
Formation of the three-layered blastoderm of the chick embryo
Formation of the three-layered blastoderm of the chick embryo
Formation of the three-layered blastoderm of the chick embryo
Cell movements of the primitive streak and fate map of the chick embryo
Cell movements of the primitive streak and fate map of the chick embryo
Zona pellucida has changed from a disc to
a pear shape and Hensen’s node appears.
Cell movements of the primitive streak and fate map of the chick embryo
Cell movements of the primitive streak and fate map of the chick embryo
Migration of endodermal and mesodermal cells through the primitive streak
Migration of endodermal and mesodermal cells through the primitive streak
Chick gastrulation 24–28 hours after fertilization
24 hrs primitive
streak full extension
25 hrs two somite stage
Chick gastrulation 24–28 hours after fertilization
27 hrs four somite stage
28 hrs Hensen’s node is caudal
Specification of the chick anterior-posterior axis by gravity
Induction of a new embryo by transplantation of Hensen’s node
Formation of Hensen’s node from Koller’s sickle
Green – anterior cells = Hensen’s node and chordamesoderm
Red - posterior cells= posterior region of primitive streak
Pre-streak embryo
Gene expression in the primitive streak
If Hensen’s node is the Organizer, what signals lead to the node’s induction?
Expresses Vg1
and Nodal =
Nieuwkoop center
Both Nieuwkoop
center and
Organizer genes
expressed
Expresses chordin and
Cgoosecoid = Organizer
Gene expression in the primitive streak
Signals required to establish A-P axis - Summary
In posterior marginal zone before primitive streak
Wnt-8c all of posterior marginal zone.
Vg1 localized to posterior marginal zone.
Both signals required to induce the expression of Nodal and
set up the Organizer on the anterior portion of Koller’s
sickle.
Nodal and FGF, expressed by Koller’s sickle, are required for
the formation of the streak and a functional Hensen’s node.
Once the primitive streak is formed and Hensen’s node
becomes functional, Hensen’s node expresses genes
associated with The Organizer in amphibians, like chordin
and noggin.
Possible contribution to chick neural induction by the inhibition of BMP signaling
Noggin is expressed in Hensen’s node. Its expression inhibits
BMP signaling
** In chick embryos repression of BMP signal does not seem to
be sufficient for neural induction – FGF signaling required.
Roles of FGF signaling in a chick’s embryo:
1. Specify (induce) mesoderm
2. After mesoderm ingresses stop ingression of cells
to allow neural place cells to stay outside.
3. Help bring about neurulation by making
antagonists of BMP signal stronger.
•FGFs are synthesized in Hensen’s node precursor cells
prior to gastrulation. As far as we know FGF signaling is
not involved in gastrulation in the amphibian embryo.
Model by which FGFs regulate mesoderm formation and neurulation
Epiblast
Stage X = one layer
blastoderm
Fgf8 induces pre-neural genes
Cerberus inhibits Nodal
ERNI and Sox3 – pre neural
Genes induced by Fgf8
Stage 1 = primitive
streak
Cerberus signal goes forward=
Nodal expression = mesoderm
induction
Model by which FGFs regulate mesoderm formation and neurulation
Stage 4 = complete primitive streak,
induction of Churchill.
Curchill induces Sip1 which
blocks mesoderm induction.
Stage 4 (end) = no more mesoderm
ingression.
Neuroectoderm
induced.
Anterior-posterior patterning in the chick embryo
Retinoic acid, secreted by
posterior mesoderm,
induces caudalization.
In amphibians the
caudalizing signal seems
to be Wnt.
Expression of retinaldehyde
dehydroxylase-2 – synthesis
of retinoic acid
Expression of retinoic acid
degradation enzymes
Hoxb1 expression marks
the anterior boundary of
the posterior cells
Pathway for left-right asymmetry in the chick embryo
Nodal = signal
Pitx2 = transcription factor
Unanswered question:
What limits ActivinB
expression to right side
of the embryo?
Development of a human embryo from fertilization to implantation
Comparison of early cleavage in (A) echinoderms and amphibians and (B) mammals
Cleavage of a single mouse embryo in vitro
Compaction
Morula – 16 cells
Blastocyst
Schematic diagram showing the derivation of tissues in human and rhesus monkey embryos
Tissue formation in the human embryo between days 7 and 11
Tissue formation in the human embryo between days 7 and 11
Tissue formation in the human embryo between days 7 and 11
11.33 Tissue formation in the human embryo between days 7 and 11 (Part 4)
Amnion structure and cell movements during human gastrulation
Amnion structure and cell movements during human gastrulation
A-P axis and notochord formation in the mouse
AVE = anterior visceral
endoderm expresses
anterior markers like
Cerberus
Axis and notochord formation in the mouse
Anterior-posterior patterning in the mouse embryo
Evolutionary conservation of homeotic gene organization and transcriptional expression in fruit
flies and mice
Axial skeletons of mice in gene knockout experiments
Left-right asymmetry in the developing human
Left-right axis
1. frogs – starts with the localization of Vg1 on the left
2. chicks – starts with the suppression of sonic hedgehog
on the right
3. mammals – starts with the ciliary movement of NVPs
(nodal vesicular parcels) from the node to
the left side. NVPs contain Sonic hedgehog
protein.
The end of the pathway, the activation of Nodal and
expression of txn factor Pitx2 on the left side is common
to all.
Situs formation in mammals
Ciliated cells in the mammalian node
Situs formation in mammals