Transcript Developmental Biology, 9e

V. Organizing Power and Axis Formation
A. Background Information
B. Invertebrates
1.
2.
3.
4.
5.
Sea Urchins
Snails
Tunicates
C. Elegans
Drosophila melanogaster
C. Vertebrates
1.
2.
3.
4.
The Frog
Zebrafish
The Chick Embryo
Mammals
• Part of these processes is the
determination of axes in the organism
– The first few cleavages may produce little or
no directionality to the embryo
– It starts at varying stages in various animals
and can result from different mechanisms
Remember our primary axes....
Figure 5.8 Fate map and cell lineage of the sea urchin Strongylocentrotus purpuratus
Step 1: Specification of Micromeres
Two Big Changes: Specified to become skeletogenic mesenchyme
Specified to become “Organizer” for other cells
egg
disheveled expression blocks B-catenin degradation
-catenin’s job
NML
ALL
All
endo
and
meso
NONE
All
ecto
Step 2: “Organizing Power”
• Secrete Wnt-8 into autocrine loop
• Wnt-8Blimp-1B-cateninWnt-8
• Paracrine “early signal” induces macromeres and
vegetal cells to differentiate to vegetal endoderm
– Unknown signal as of yet
• Delta-Notch juxtacrine signal induces non-skeletogenic
mesenchyme
• Wnt-8 makes a come-back to induce invagination
Axis Determination
• Anterior-Posterior: Cytoplasmic determinants in the egg
cytosol, such as disheveled and B-catenin
• Left-Right: Nodal expression (TGF-B family member)
• Dorsal-Ventral: unclear
Spiral cleavage in molluscs
The spirally cleaving mollusks have
a strong autonomous specification
from cytoplasmic determinants in egg.
Step 1: Polar lobe formation
The polar lobe
is a cytoplasm
outpouching
from the egg
prior to cleavage
It isolates critical
determinants into
only one of the
first cell pair.
TF’s associated
with the lobe
turn CD into
“The Organizer”
Figure 5.27 Association of decapentaplegic (dpp) mRNA with specific centrosomes of Ilyanassa
Decapentaplegic is TGF-B family member used to
induce specific cell fates secreted by the Organizer
The Organizer induces mesodermal and endodermal
fates in cells that would otherwise remain ectodermal
MAP kinase activity activated by D-quadrant snail blastomeres
Figure 5.30 MAP kinase activity activated by D-quadrant snail blastomeres (Part 2)
Normal
MAPK Blocked
Axis Determination
• Anterior-Posterior: Cytoplasmic determinants in the lobe
• Left-Right: Nodal expression (TGF-B family member)
• Dorsal-Ventral: Cytoplasmic determinants in the lobe
Bilateral, Holoblastic Cleavage of the Tunicate
The 8-cell embryo is
already autonomously
specified for cell fates
Figure 5.35 Cytoplasmic rearrangement in the fertilized egg of Styela partita
Fertilization rearranges cytoplasmic determinants
1. Animal pole cytosol determines ectoderm
2. B-catenin presence determines endoderm (like urchins)
3. Macho-1 in yellow crescent determines muscle cells
Figure 5.38 Antibody staining of -catenin protein shows its involvement with endoderm formation
Wherever B-catenin shows up, endoderm is formed
Figure 5.37 Autonomous specification by a morphogenetic factor
Where Macho-1 shows up tail muscle will form
Zinc-finger TF for muscle actin, myosin, TBX-6
Also TF for Snail TF which blocks notochord induction
Conditional Specification also plays a role
Integrates with the autonomous specification patterns
Axis Formation accomplished prior to cleavage!
Fertilization rearranges cytoplasmic determinants
determines
dorsal-ventral
Left-right: unclear
but nodal shows it later
determines
anterior-posterior
Rotational, Holoblastic Cleavage in the nematode Caenorhabditis elegans
hermaphrodite
Figure 5.42 The nematode Caenorhabditis elegans (Part 2)
Both autonomous and conditional
specification at work early on.
If cells are separated:
P1 will develop autonomously
Stem cell divisions are meridional
Founder cell divisions are equatorial
AB requires input from P lineage
• Autonomous specification in P1
– SKN-1, PAL-1 and PIE-1 TFs from egg
– as P1 divides these determine daughter fates
– P lineage becomes “Organizer”
• Conditional specification in AB
– P2 secretes Wnt family member MOM-1 to
induce endodermal specification in AB lineage
– P2 use Delta-Notch signals to induce
ectodermal fates in AB lineage
Axis Determination in C. elegans
Anterior-Posterior axis
is determined by egg shape
Which end is posterior
is determined by sperm
(the closest end is back)
Sperm CYK-4
activates egg rho,
actin rearrangement
causes assymetric
first cleavage division
AB division leads to both dorsal ventral and left-right axes
Assymetrical division of AB-MS forces AB dorsal and MS ventral
Delta-notch recognition between daughters of AB and MS gives left-right
Cytoskeletal rearrangement also pushes P-granules into the germ line
The cells of the blastula have specified fates in Xenopus.....
Gastrulation changes all of that,
.....afterwards all cell fates are determined!
Development of “Organizing Power” at the dorsal blastopore lip
The bottle cells
get the ball rolling
but the real power
is conferred on the
first cells through
the blastopore.
The dorsal mesoderm keeps the power to determine other cell’s
fates throughout gastrulation: “Spemann’s Organizer”
This ability to determine
cell fates is called...
“Primary Embryonic Induction”
The dorsal lip cells first have to become competent to be “Organizer”
Cortical rotation shifts
disheveled, GBP, Wnt-11
to dorsal side of embryo
The area of Dsh
accumulation is
seen as a gray
crescent in some
amphibian embryos
β-catenin starts out everywhere in the embryo but only survives
GSK3 in the dorsal portion due to Dsh, GBP and Wnt-11
The dorsal vegetal cells of the Nieuwkoop Center turn on “Organizer”
Wnt and Vg-1 (TGF-B family)
induce pre-dorsal lip mesoderm
FGF needed for all mesoderm
Figure 7.22 Summary of events hypothesized to bring about induction of the organizer in the dorsal
mesoderm
Nodal
Vg-1
Figure 7.23 Vegetal induction of mesoderm (Part 2)
So, what can the “Organizer” do?
• Initiate gastrulation
• Become the notochord and other dorsal
mesoderm
• Dorsalize ventral mesoderm into paraxial
mesoderm, somites, etc.
• Dorsalize the ectoderm into the neural
plate and neural tube
Figure 7.26 Localization of chordin mRNA
The “Organizer” is induced prior to gastrulation
Dorsal blastopore lip
Blastopore
Dorsal mesoderm
Continues to organize events throughout its own differentiation
Interestingly, the primary mechanism is by means of inhibition....
Presumably, the Wnt, FGF and RA signals arise from endoderm and ectoderm
Without the “Organizer”
you get mainly skin and gut
Figure 7.31 Cerberus mRNA injected into a single D4 blastomere of a 32-cell Xenopus embryo
induces head structures as well as a duplicated heart and liver
Don’t underestimate the power of the “Organizer”!
Axis Formation
• Dorsal-Ventral: sperm penetration and
cortical rotation
• Anterior-Posterior: migration direction of
the dorsal mesoderm
• Left-Right: nodal expression exclusively on
left side of the lateral plate mesoderm
Nodal expression causes Pitx2 expression
Nodal and Pitx2 on left
Injected on both sides
Relationships between the frog and chick “Organizers”
• The hypoblast = dorsal vegetal cells
• Koller’s sickle = pre-dorsal lip mesoderm
• Hensen’s node = dorsal blastopore lip and
dorsal mesoderm
• Primitive streak = blastopore
Formation of Hensen’s node from Koller’s sickle
Wnt and FGF from the hypoblast
induce Koller’s sickle epiblast
Figure 8.10 Induction of a new embryo by transplantation of Hensen’s node (Part 1)
Possible contribution of inhibition of BMP signaling
Appears to be similar to the frog....
In the chick, the hypoblast plays a large role much like the frog endoderm
Figure 8.8 Specification of the chick anterior-posterior axis by gravity
Anterior-Posterior axis parallels the rotation inside the shell
Rostral-Caudal (Anterior-Posterior) axis extension in chick embryos
The combination of
positional specification,
complex signaling and
TF (Hox, etc.) expression
is thought to cause axis.
Left-right asymmetry in the chick embryo
This is farther along
Nodal and Pitx2 again are implicated