Establishment of axis in animals

Name: Malaika Muskan
Roll no. 875
University roll no. 023793
Semester 6th
Bs zoology
Session (2017-2021)
Submitted to:
prof. Dr. sagheer ahmed
Topic:
Establishment of body axes
Govt. college of science, wahdat road
Lahore
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Axes formation
 Axes formation follows gastrulation and this process
controlled by the specific set of genes that decide
which cell develop into specific structure of body.
 Egg + sperm zygote (sequential cleavage and
differentiation ) blastula gastrula
(gastrulation) axes formation
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Blastula formation
From zygote to blastula which further differentiate into three layers.
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Axes formation:
 Three type of axes form during the development of
animal’s embryo:
1) dorsal-ventral axis
2) Anterior-posterior axis
3) Left-right or lateral-medial axis
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Axes Function
Dorsal-ventral axis Separate the front of body from the back
Anterior-posterior axis Determines the position of mouth in front and the anus at the rear
Left-right axis Creates a mirror-like symmetry of our extremities and left-right
asymmetry of our organs.
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Why axis formation is important ?
 Axis formation is very fundamental for the normal
embryonic development.
 For instance: CNS develops along the dorsal axis with
large concentration of neuronal tissues __ brain __ at
anterior axis of embryo.
 Left-right axis deals with our extremities etc.
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Dorsal-ventral axis:
Neurulation;
 DEFINITION:
“Neurulation is the folding process of embryo which
includes the transformation of neural plate to neural
tube. The embryo at this stage is neurula.”
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Neurulation
 The process begin when the notochord induce the
formation of central nervous system by signaling the
ectoderm germ layer above it to form the thick and flat
neural plate. The neural plate folds itself and form the
neural tube which ultimately develop into brain and
spinal cord, eventually forming central nervous
system.
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Neural tube formation
 Different portions of neural tube form by the two
different processes:
1) primary neurulation
2) secondary neurulation
Primary Neurulation: The neural plate creases
inward until the edge come in contact and fuse.
Secondary Neurulation: The tube form by hollowing
out of the anterior of the solid precursor
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Primary neurulation
 Primary Neurulation divides the ectoderm into three
cell types:
1) Internally located neural tube
2) Externally located epidermis
3) Neural crest cells
 Primary neurulation begins when the neural plate
formed.
 The edge of neural plate start to thicken and lift
upward, forming the neural fold.
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Primary neurulation
 The neural plate remains grounded, allowing the U-
shaped neural groove to form.
 The neural fold pinch inside towards the middle line
of embryo and fuse together to form the neural tube.
15

Secondary neurulation
 In secondary Neurulation, neural fold completely
pinch and cells of neural plate form a cord-like
structure that migrate inside the embryo and hollows
to form the tube.
 This neural tube give rise to CNS; brain and spinal
cord (ganglion).
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Early brain development:
 The anterior part of neural tube develop int0 three part of
brain by the neuroepithelial cells division:
 forebrain (prosencephalon)
 midbrain (mesencephalon) and
 hindbrain (rhombencephalon)
 And spinal cord under the control of some signaling
molecule.
Forebrain___ cerebrum, hypothalamus and optic vesicle.
Hindbrain ____ metencephalon (pons nd cerebellum) and
myelencephalon (medulla oblangata).
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Regulation of neurulation:
The following genes and factors help in the regulation
of Neurulation:
o Shh (sonic hedgehog gradients)
o Patched transmembrane protein
o Smoothened , 7-transmembrane domain receptor
o Gli proteins
The neurulation initiate by the Shh. Neural plate form
under the influence of Shh, protein expressed by
notochord.
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Key terms:
 The SHH provides instructions for making a protein called
sonic hedgehog(shh). The protein functions as a chemical
signal that is essential for embryonic development.
 Patched is a transmembrane protein receptor that plays an
obligate negative regulatory role in shh siganling, essential
gene in embryogenesis.
 Smoothened is a g-protein coupled receptor, component
of shh signaling pathway.
 Gli proteins are the transcriptional effectors of the
hedgehog signaling pathway,play a key role in
embryogenesis.
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Regulation:
 Shh signals through a unique process.
 In the absence of Shh, the patched transmembrane
protein prevent the activation of smoothened, a 7-
transmembrane domain receptor.
 In the absence of smoothened activity, Gli proteins are
processed into repressor forms that inhibit gene
transcription.
 Shh binds to patched-1 to allow the activation of
smoothened, which both activate the Gli protein and
attenuate the repressor Gli formation.
 Dorsal-ventral patterning is imparted to the developing
neural tube by Shh secreted from the notochord.
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Anterior-posterior axis:
 The anterior-posterior axis was the first embryonic
axis to arise in evolution since it allowed animals to
move unidirectionally. In modern bilaterians, the AP
axis corresponds to the head-tail axis.
 Different animals use a wide variety of mechanism to
create this axis in the early embryo.
 Here we will discuss about vertebrates (zebrafish and
mouse) to examine different strategies used to form
the AP axis.
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zebrafish:
 AP patterning in the vertebrate embryo can be roughly
divided into two major phases:
a) Initiation phase: embryo is generally divided into
head and body
b) Elaboration phase: body progressively forms towards
the posterior end, forming the trunk and tail ( since
this process involves the formation of muscles tissue
blocks called somites we will refer to this as the
somitogenesis stages.)
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Initiation verses elaboration:
 In zebrafish, the initiation phase occurs prior to the
start of gastrulation, such that by the start of
gastrulation, the different territories of the final body
plan can be roughly mapped onto the embryo.
 The mesoderm of the head, which comprises the part
of a very important signaling center called, the
organizer, is first specified near the equator on what is
defined as the dorsal side of embryo. These cells
migrate towards the animal pole during gastrulation,
where the brain forms.
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Initiation versus elongation:
 The most posterior cells will migrate during
gastrulation towards the vegetal pole where they form
a structure called tailbud, such that by the end of
gastrulation, the AP axis will align with the animal-
vegetal pole.
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REGULATION:
 The patterning of the neural ectoderm along the anterior-
posterior axis in the zebrafish appears to be result of the
interplay of:
FGF
Wnts
Retinoic acid
 This regulation of anterior posterior axis appears to be
coordinated by retinoic acid-4-hydroxylase, an enzyme that
degrades RA.
 The gene encoding this enzyme, cyp26, is expressed at the
decided anterior end of embryo.
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Key terms:
 FGF is fibroblast growth factors that are the family of
cell signaling proteins that are involved in a wide
variety of processes, most noticeable in the normal
development.
 Wnts are secreted factors that regulate growth,
motility, and differentiation during embryonic
development. They activate diverse signaling cascades
inside the target cells.
 Retinoic acid is the metabolite of vitamin A1, that
mediates the functions of vitamin A1, required for
growth and development.
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Regulation:
 Retinoic acid act as a morphogen, regulating cell
properties depend on its concentration. Cell receiving
very little RA express anterior gene; cell receiving high
level of RA express posterior gene; and those cell
receiving the intermediate level of RA express genes
characteristics of cells between the anterior and
posterior regions. This morphogen is extremely
important in hindbrain, where different level of RA
specify different type of cells along the anterior-
posterior axis.
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Regulation:
 The retinoic acid-4-hydroxylase prevents the
accumulation of retinoic acid at anterior side, blocking
the expression of posterior genes there.
 This inhibition is reciprocated, since the posteriorly
expressed FGFs and Wnts inhibit the expression of
cyp26 gene, as well as inhibiting the expression of the
head specifying gene Otx2. This mutual inhibition
creates a border between the zone posterior gene
expression and the zone of the anterior gene
expression.
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Left-right axis:
 The series of embryonic development events that
distinguish the left and right side of embryo along its
anterior-posterior and dorsal-ventral axes include the
direction of axial rotation, morphogenesis of
individual organs, and organ placement.
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Regulation:
 In all vertebrates studied, the right-left sides differ
both anatomically and developmentally.
 In fish, the heart is on left side and there are different
structures in the left and right sides of the brain.
 Moreover, as in other vertebrates, the cells on the left
side of body are given that information by the NODAL
signaling and by the Pitx2 transcription factor.
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Regulation:
 In zebrafish, the NODAL structure cilia that control
left-right asymmetry is a transient fluid –filled
organ called kupffer’s vesicle.
 Blocking ciliary function by preventing the synthesis
of dyenin resulted in abnormal left-right axis
formation. Cilia are responsible for the left-side
specific activation of NODAL signaling cascade.
NODAL target genes are critically important in
instructing asymmetric organ migration and
morphogenesis in the body.
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Establishment of axis in animals

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