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Axis Formation in Amphibians
How does a radially symmetrical egg
develop the axes of bilateral symmetry?
dorsal
posterior
anterior
ventral
SP
Gray
crescent
Cortical rotation
L
R
2-cell
Nuclei stained
dorsal
Nuclei not stained
ventral
β-catenin (TF) is localized in the
nuclei of dorsal cells due to
cortical rotation.
gastrula
Canonical Wnt Pathway
Dsh binds to and blocks
GSK3. GSK3 degrades βcatenin.
β-catenin activates a transcription factor cascade
that establishes the inducing power of the dorsal lip.
Inject dominant inactive GSK3 in
both blastomeres at 2-cell stage.
GSK3 suppresses dorsal
specification of ventral half.
Inject goosecoid mRNA
http://www.youtube.com/watch?v=BkKWApOAG2g
Unequal radial
holoblastic cleavage
Fate Map of Xenopus laevis
Where is the
mesoderm?
Fate mapping
Walter Vogt,
1929
Gastrulation movements
1. Formation of endodermal bottle cells
2. Convergent extension
3. Crawling of lead cells of sheet
Epiboly
Phayngeal
endoderm moves
to blastocoele
ceiling and
becomes leading
(crawling) edge of
forming
archenteron.
Fibronectin
stained yellow
Early gastrula
Late gastrula
Injection with
integrin binding
fibronectin
peptide
(competitive
inhibition)
Epiboly of ectoderm
Epiboly: cell division and convergent extension
The gray crescent is
necessary for axial
development
baustück
The dorsal lip of the blastopore (the organizer) derives from the
gray crescent and it has the power to ORGANIZE a second
embryo. Thus is primary induction. Spemann and Mangold, 1924
http://www.youtube.com/watch?v=e3HcqGcXls4
Pigmented markers
showed the NT developed
from host ectoderm
http://www.youtube.com/watch?v=e3HcqGcXls4
Depletion of EP-cadherin mRNA
Why a blastocoele?
1. Keep animal cap away from inducing endoderm
2. Provide space into which involuting endoderm and mesoderm can move.
Where does the organizer come from?
The induction of the mesoderm by endoderm
Pieter Nieuwkoop: Nieuwkoop center
Mesodermal induction: Nieuwkoop Center acts in blastula
Maternal mRNA for Veg1 (white) tethered to the vegetal cortex
(Xenopus
nodal related)
TGF-β paracrine
factor
Maternal mRNA
for TF
TGF-β paracrine factor
The Organizer
1. Initiates gastrulation (bottle cells)
2. Autonomously differentiates into the
notochord
3. Dorsalizes the lateral mesoderm to form
somites.
4. Dorsalizes the overlying ectoderm to
form a neural tube. The search for the
inducer.
Is neural induction permissive, rather than instructive?
BMP induces ventral mesoderm and epidermis. Neural
ectoderm and dorsal mesoderm and endoderm require that
BMP be blocked! This is what the inducers do! Does this
explain Holtfreter’s surprising results?
Nodal
Review figure 10.10 A
Find it: Chordin mRNA expression
Chordin is a paracrine factor that blocks BMP-2 and BMP-4 from
binding its receptor
Control
Sox2
expression
(neural
marker)
Morpholino against
BMP-2, 4, and 7)
Morpholino against chordin (and
noggin and follistatin)
Noggin mRNA can
“rescue” UV-irradiated
embryos (belly pieces)
[noggin]
Regional specification
Saxén and Toivonen (film)
Saxén and Toivonin. Neuralizing (head) and
mesodermalizing (trunk and tail) gradients
Please watch Saxen interview