molecular basis of morphogenesis
TRANSCRIPT
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1.Presumptive areas of blastula are predestined for a
specific part in future development.
Transplantation experiments:
1) autoplastic- piece of an embryo transplanted to
another site in the same embryo
2) homoplastic- host (recipient) and donor individuals
are of the same species
3) heteroplastic- host and donor species belong to the
same genus
4) xenoplastic- host and donor species belong to
different genera.
Only autoplastic and homoplastic transplants are
successful in adult vertebrates.
Success in xenoplastic transplants have been achieved in
invertebrates, and between embryos of salamanders and
frogs, mammals and birds.
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2. Presumptive areas have definite normal fates or
prospective significance.
At the time of the stages used in theseexperiments, their fates have not been fixed
3. Ability of the parts of an early embryo to develop in
more than one way is called prospective potency.
Prospective potency of epidermis and neural
system areas of gastrula embryo are practicallyidentical (includes mesodermal, endodermal
tissues), in spite of their different prospective
significance. Grafted material develops in
conformity with its new surroundings. An epidermis area of an early gastrula was
heteroplastically transplanted into the neural
system area of another embryo in the same stage
of development, and vice versa.
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Transplanted epidermis became neural plate of
host; reverse transplantation showed an oppositeresult.
Presumptive ectoderm transplanted into marginal
zone of early gastrula: graft was drawn into
blastopore and found in host interior
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4. At the end gastrulation, prospective potencies ofpresumptive epidermis and neural system areas will
be narrowed down to their prospective significance.
The fixing of the fate of a part of the embryo is
called determination.
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5. The determination of parts of the ectoderm
depends on the surroundings in which theendodermal cells find themselves, and does not
come from causes inherent in the ectoderm itself.
Hilde Mangold- heteroplastic transplant of the
dorsal lip of the blastopore of an early gastrula
to the lateral lip of the blastopore of the host early gastrula resulted to a graft which gave rise
to a secondary embryo in the host.
Comparison of traits reveal that both donor and
host embryos participated in formation of parts
of secondary embryo.
Graft influenced development of certain parts of
embryo (induction); source of influence is called
inductor.
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6. Contact with archenteron roof (chordomesoderm)
determines development of mid-dorsal ectoderm into
the neural tube. Grafts taken from dorsal lip of blastopore and adjoining
marginal zone induce neurulation in early gastrula.
7. Reacting cells must be competent, a particular state
that enable them to differentiate under the influence of
the inductor. Competence of host cells to inductor is highest in the
early gastrula, declines in late gastrula, and fades away
at neurulation. Absence of inductor causes neural
ectoderm to differentiate into epidermis.
8. Due to its ability to induce a complete 2ndary embryowhen transplanted, the amphibian dorsal lip of the
blastopore (also the posterior edge of blastodisc in
fishes, or the anterior half of the primitive in birds) is
called the primary organizer.
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9. The capacity of a certain part of the egg to serve as
the organization center is not fixed at the start of
development, but introduced gradually. Appearance of a definite amphibian organization
center is dependent on interaction of cortical and
subcortical cytoplasm of gray crescent, and the
presence of yolky cytoplasm of vegetal
hemisphere.
The primitive streak in birds is dependent on
underlying hypoblast for its formation;
discrepancies in the orientation of the epiblast
and hypoblast result to primitive developing inaccord with hypoblast orientation.
10.Tissues of mammalian (rabbit) gastrula have
competence for neural induction with chick primitive
streak as an inductor, and vice-versa.
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1. Regional differentiations in the normal embryos are
controlled by a balance between neuralizing andmesodermalizing substances distributed in the form
of gradients.
a.Regional inductions:
Archencephalic (forebrain, eyes and nose
rudiments)- neuralizing (dorsalizing or activating)factor alone
Deuterencephalic (hindbrain and ear vesicles)- low
mesodermalizing + neuralizing factor
Mesodermal (notochord, muscle, kidney and limb-bud) mesodermalizing (caudalizing or
transforming) factor alone
Spinocaudal (trunk organs and tailbud)- high
mesodermalizing factor + low neuralizing factor
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The dorsal lip of blastopore of early gastrula contains head
inductor; same structure at late gastrula contains spinocaudalinductor. The gradient of the neuralizing substance is highest
mid-dorsally and declines towards the lateral and ventral parts
of the embryo. Mesodermalizing substance is concentrated at
the posterior end of embryo and forms a declining gradient both
in the anterior and lateral direction.
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3. Any factor that affects the gradient affect the whole
morphogenetic system.
Defects spread from the front end backward: noserudiments and forebrain, eyes, posterior parts of
brain, gill clefts, ear vesicles.
Raise the level of the dorsoventral gradient by
treatment with sodium thiocyanate: increase in size of
neural system.
Depressing the level of the dorsoventral gradient
gradient at its highest level by exposing embryo to
magnesium or lithium chloride or removing part of the
archenteron roof beneath anterior end of nervoussystem results to injured embryos that develop
cyclopia. Cyclopia and similar defects may result from
toxicosis of mother during early gestation, e.g.
contact with German measles illness or Rubella.
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Babies who have too little of the Sonic hedgehog protein
will be born without noses, and in the most severe cases
cyclopia. An abundance of Shh produces eyes that are far
apart, an abnormally wide nose, or even duplicated facialfeatures, called diprosopus. This can range from only two
noses, to a complete full second face with two mouths, two
noses, and up to four eyes. Each nose has only a single
nostril leading into the sinuses.
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4. Gray crescent area is
the center of gradient
for archencephalicdevelopment:
UV irradiation of gray
crescent caused
microcephaly,
anencephaly. Injection of oocyte
nucleus material or
cytoplasm of
untreated fertilizedegg into the
blastocoele restore
embryo to normal
development.
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Induction is region specific. Otto Mangold
transplanted 4 successive regions of archenteron
roof of late gastrula newt embryos into the
blastocoeles of early gastrula embryos.
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Spemann and Mangold showed that the dorsal lip of the
blastopore constitutes an organizer that has the ability to:
initiate gastrulation movements; dorsalize ectoderm intoneural ectoderm; cause neural plate to become neural
tube; become dorsal mesoderm and notochord; dorsalize
surrounding mesoderm into lateral mesoderm.
The field of molecular biology was refined in the late
1980s, enough to provide a starting point to understand
what genes encoded the products of the organizer.
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In the unfertilized egg, VegT and Wnt 11 mRNA are concentrated in
the vegetal pole. Powered by microtubules and motors atfertilization, the cortical rotation displaces Wnt 11 mRNA opposite
to the sperm entry point. This is crucial for giving the dorsal side
regional identity, and leads to the formation of a signaling center
in the vegetal region called the Nieuwkoop center.
AMPHIBIAN INDUCTION
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Vegetally localized mRNA ofVegT and Vg1, are endoderm-
inducers. VegT directs synthesis of Xnr proteins which aremesoderm-inducing signals. Dorsalizing factors Wnt 11
and Xnr activate the Wnt pathway that leads to the
accumulation of-catenin in the dorsal side. -catenin
acts with VegT to produce higher levels of Xnr.
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Translocation of the Dishevelled protein to the dorsal side of the
egg during rotation stabilizes -catenin in the dorsal cells of the
embryo, which establishes the Nieuwkoop center.
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F-catenin complexes
with Tcf3 which
activates theexpression of the
siamois gene.
The siamois product
and a TGF- signal
((Vg1/ nodalsignaling pathway)
activate the
goosecoid gene
that specifyorganizer genes.
Siamois also
interacts with Xnr to
activate goosecoid.
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The organizer works by secreting inhibitors of
BMP4, e.g., Noggin, chordin, andfollistatin. They
dorsalize the adjacent mesoderm by inhibiting the
ventralizing signal. What is the ventralizing signal?
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BMP signals are powerful ventralizing factors that induce the
expression of epidermal-specific genes. Varying inductionsare created by interaction of BMP4 with BMP antagonists
from the organizer, e.g. low BMP doses activate muscle
formation; intermediate levels instruct cells to become
kidney; high levels activate blood cell and CT formation.
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In the absence ofBMP
signaling, neuralizing
transcription factors
(X1pou2, SoxD) are
produced.
These factors in turn
activate the neurogenin
gene for the transcription
factor NeuroD that causethe differentiation of the
cell into a neuron.
In the presence ofBMP
signaling, epidermalizing
transcription factors(Msx1, GATA1, Vent) are
generated, leading to the
activation of the pathway
enabling the cell to
become a keratinocyte.
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1. In chick embryos, activation of various secreted
factors (Vg1, Nodal, Wnt8C, FGF8 and Chordin);
and transcription factors: (brachyury and goosecoid)
adjacent to the site of streak formation, is required for
the coordinated function of the organizer.
Removal of the hypoblast in the chick results in
correctly patterned ectopic streaks, suggesting that
the hypoblast may inhibit formation of the primitive
streak by secreting an antagonist to Nodal.
2. BMPs do not inhibit neural induction, nor does ectopic
expression ofchordin in the non-neural epiblast causeneural induction.
3. Recent evidence suggests that the anterior visceral
endoderm (AVE) is providing these signals for anterior
neuron production.
AVIAN INDUCTION
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4. FGFs produced in Hensen's node and primitive
streak, appear to induce trunk and hindbrain
neuronal expression in the epiblast cells. Other posteriorizing signals (Wnt3a, retinoic acid,
eFGF) can influence the anterior-posterior
specification of the neural tube.
In the head region, an additional set of proteins
(Cerberus, Frzb, Dickkopf) block these Wnt signal
from the ventral and lateral mesoderm.
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5.Ciliary beating around the
primitive node displaces
FGF8 to the left, causing the
expression of nodal andlefty-1 on the left side of the
streak, thereby activating
the Pitx2 gene responsible
for rotation of the gut and
stomach, spleen and lunglobation.
6.The D-V axis is defined by
the embryonic-abembryonic
(trophoblast opposite ICM)
poles. In birds, gravity iscritical in determining the A-
P axis, while pH differences
appear crucial for
distinguishing dorsal from
ventral.
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In mammals the
primitive node is anembryonic organizer.
(remove HN: no neural
tissue development;
transplant HN to host
embryo: 2nd neuralaxis is induced). The
mammalian node is
not a classic organizer
because it can't act
alone to induce theembryonic axis but
requires other anterior
germ tissues to be
fully effective.
Progressive cellular interactions
characterizes the organizer
activity of the Hensen's node
region in lower vertebrates.
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In mammals, there are 2 signaling centers: one in
the node, and one in the AVE (hypoblast), which
becomes part of the head organizer.
Prechordal plate (head organizer) pass through the
node and become associated with the endoderm
caudal to the oropharyngeal membrane.
They are followed by the chordamesoderm(precursor of the notochord), which is the major
axial signaling center of the trunk.
In birds, the prechordal plate alone can induce head
parts, but in mammals,
the prechordal platemesoderm sustains the
induction of the anterior
visceral endoderm.
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The expression of Nodalfrom the primitive streak drives
the expression of Shh and Goosecoid.
Goosecoid- activates genes of organizer. With SHH,
goosecoid activates expression of HNF-3B, which also
results in the expression of Noggin and Chordin.
Noggin, chordin, follistatin- BMP antagonists from the
organizer, they dorsalize mesoderm. They induce only
forebrain and midbrain types of tissues.
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BMP4- secreted throughout the bilaminar disc, acts
with FGF to ventralize mesoderm into intermediate and
lateral plate structures. The fate of the entire ectodermis dependent upon BMP concentrations: high levels
result in epidermis formation, lower levels at the border
of the neural plate and nonneural ectoderm induce the
neural crest. BMP inhibition leads to neural plate
induction. HNF-3F - from the organizer, this is required for node
formation, initiation of notochord function, and
establishment of midline structures cranial to the node.
T /brachyury - induced by products of HNF-3F and
goosecoid genes, they are necessary for normalmovements of mesodermal cells through the primitive
streak (T gene mutants have gross caudal body defects
in humans). Brachyury also antagonizes BMP4 to
dorsalize mesoderm in caudal regions of the embryo.
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To produce a head, the
BMP4 and Wntsignals are
blocked by transcriptionfactors Otx-2 and Lim-1 (a
homeobox gene); and the
signaling molecule
Cerberus-related 1 HeSXL
(mutants produceheadless phenotype).
WNT3a and FGF - induce
caudal neural plate
structures: hindbrain and
spinal cord. Retinoic acid - organize
the cranial-to-caudal axis
by regulating expression
of homeobox genes.
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Hox genes pattern the
anterior-posterior axis
and help tospecifypositions along that axis.
IfHox genes are knocked
out, segment-specific
malformations can
arise.o Causing the ectopic
expressions of the
same genes can alter
the body axis.o Homology of gene
structure and expressions between Drosophila and
mammalian Hox genes suggest that this patterning
mechanism is evolutionarily ancient.
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Expression patterns of
genes that regulate somitedifferentiation. SHH and
noggin, secreted by thenotochord and floor plate of
the neural tube, cause the
ventral part of the somite to
form sclerotome and toexpress PAX1, which in turn
controls chondrogenesisand vertebrae formation.
WNT proteins from the
dorsal neural tube activate
PAX3, which demarcates thedermomyotome. WNT
proteins also direct thedorsomedial portion of the
somite to form epaxial
(back) muscles and to
express the muscle-specificgene MYF5.
The middorsal portion of the somite is
directed to become dermis by
neurotrophin 3 (NT-3) expressed by thedorsal neural tube. Hypaxial (limb and
body wall) musculature is derived from the
dorsolateral portion of the somite underthe combined influence of activating WNT
proteins and inhibitory BMP4 protein,
which together activate MYOD expression.
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VEGF induced by Shh, this induces
hemangioblasts to form hematopoietic stem
cells, the precursors of all blood cells. Peripheralhemangioblasts differentiate into angioblasts,
the precursors to blood vessels.
PDGF and TGF directs maturation and
modeling of the vasculature until the adultpattern is established.
Notch pathway - induced by VEGF expression,
this specifies arterial development and
suppression of venous cell fate through
expression of ephrinB2.
PROX1 - induces lymphatic vessel differentiation.
Blood vessel and nerve outgrowth appear to
involve guidance factors (e.g., NGFs).
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Blood vessels form in two
ways: vasculogenesis(top), in which vessels
arise from blood islands,
and angiogenesis
(bottom), in which new
vessels sprout from
existing ones. Duringvasculogenesis, FGF2
binds to its receptor on
subpopulations ofmesoderm cells and
induces them to form
hemangioblasts. Then,under the influence of
vascular endothelial
growth factor (VEGF)acting through two
different receptors, these
cells become endothelial
and coalesce to form
vessels.
Angiogenesis is also regulated by VEGF,
which stimulates proliferation of endothelialcells at points where new vessels will
sprout from existing ones. Final modeling
and stabilization of the vasculature areaccomplished by PDGF and TGF-.
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Chordin, activin, nodal, sonic
hedgehog are also involved
in the L-R asymmetry of
certain body parts. The
protein activin, a member of
the TGF- signaling family,
directs development of parts
in the right axis. There are over 24 genes
currently known for L-R
assymetry, and gene
mutations downstream in the
cascade are responsible forright-left switching for
various organs (e.g., heart as
seen in situs inversus,
Kartageners syndrome).
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The Vg1 protein
activates a Nodal
protein solely on the
left side of the body. Asin other vertebrates,
the Nodal protein
activates expression of
Pitx2, which is critical
in distinguishing left-
sidedness from right-sidedness in the heart
and gut tubes.
Activation of transcription factor Pitx2 regulates genes involved in
lateral plate mesoderm differentiation. Other factors such as Shh,
FGF8, Lrd and Kifare also involved. The nodal flow model suggeststhat morphogens and signaling molecules involved in defining the L-
R axes is driven by the action of cilia that cause molecules to flow
from right to left across the primitive node. Cells in the node with
mutations that affect ciliary action (e.g. null mutations for Lrd which
encodes ciliary dynein) alter L-R symmetry.
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ERRORS OF GASTRULATION
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Spinal cordAbnormalities
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Little Samuel Armas was diagnosed with spina bifida and would not survive if
removed from his mother's womb. His mother knew of Dr. Bruner's remarkable
surgical procedure: he performs these special operations while the baby is still in
the womb. 'The tiny hand of the 21-week-old fetus emerges from the mother's uterus
to grasp the finger of Dr. Joseph Bruner as if thanking the doctor for the gift of life.
Our God is an awesome God!
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