In this PPT I completed that interesting topic , molecular embryology discussing this time molecular regulation of some other systems in the developing embryo, wishing that I could make this as simple as possible.

1. By:
Dr. Khaled El Masry
Assistant Lecturer of Human Anatomy &
Embryology
Mansoura Faculty of Medicine, Mansoura University,
Mansoura , Egypt.
Molecular Embryology
2013
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All Data and Diagrams included
in this presentation are basically
derived from our highly
valuable reference of
Embryology
(Langman's Medical
Embryology 12th ed. - T.
Sadler (Lippincott, 2012)
, recommending you all to refer
to it for more details….

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Introduction

4. Molecular Biology has opened the doors to new ways
to study Embryology and to enhance our
understanding of Normal and Abnormal development.
Sequencing the Human Genome, together with
creating techniques to investigate gene regulation at
many levels of complexity, has taken Embryology to
the next level.
Thus, from Anatomical to Biochemical to
MOLECULAR level, the story of Embryology has
progressed, and each chapter has enhanced our
knowledge.
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5.  Genes are contained in a complex of DNA and proteins called chromatin, and
its basic unit of structure is the nucleosome. Chromatin appears tightly coiled
as beads of nucleosomes on a string and is called heterochromatin.
 For transcription to occur, DNA must be uncoiled from the beads as
euchromatin. Genes reside within strands of DNA and contain regions that can
be translated into proteins, called exons, and untranslatable regions, called
introns.
 A typical gene also contains :
1. a promoter region that binds RNA polymerase for the initiation of transcription;
2. a transcription initiation site, to designate the first amino acid in the protein;
3. a translation termination codon; and
4. a 3′ untranslated region that includes a sequence (the poly A addition site)
that assists with stabilization of the mRNA.
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7.  The RNA polymerase binds to the promoter region that usually
contains the sequence TATA, the TATA box. Binding requires
additional proteins called transcription factors. Methylation of
cytosine bases in the promoter region silences genes and
prevents transcription.
 Different proteins can be produced from a single gene by the
process of alternative splicing that removes different introns
using spliceosomes. Proteins derived in this manner are called
splicing isoforms or splice variants. Also, proteins may be
altered by post- translational modifi cations, such as
phosphorylation or cleavage.
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9.  Induction is the process whereby one group of cells or tissues (the
inducer) causes another group (the responder) to change their fate.
The capacity to respond is called competence and must be conferred
by a competence factor.
 Many inductive phenomena involve epithelial– mesenchymal
interactions.
 Signal transduction pathways include a signaling molecule (the
ligand) and a receptor.The receptor usually spans the cell membrane
and is activated by binding with its specific ligand.
 Activation usually involves the capacity to phosphorylate other
proteins, most often as a kinase. This activation establishes a cascade
of enzyme activity among proteins that ultimately activates a
transcription factor for initiation of gene expression.
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11.  Cell-to-cell signaling may be paracrine, involving diffusable
factors, or juxtacrine, involving a variety of nondiffusable factors.
 Proteins responsible for paracrine signaling are called paracrine
factors or growth and differentiation factors (GDFs).
 There are four major families of GDFs:
FGFs, WNTs, hedgehogs, and TGF-bs.
 In addition to proteins, neurotransmitters, such as serotonin (5HT)
and norepinephrine, also act through paracrine signaling, serving as
ligands and binding to receptors to produce specific cellular
responses.
 Juxtacrine factors may include products of the extracellular
matrix, ligands bound to a cell’s surface, and direct cell-to-cell
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12. Molecular Regulation of
Gastrulation
(Establishment of the Body Axes)
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Establishment of the body axes;
anteroposterior, dorsoventral, and left-right, takes
place before and during the period of gastrulation.
Cells at the prospective cranial end of
the embryo in the anterior visceral
endoderm (AVE) express the
transcription factors OTX2, LIM1, and
HESX1 and the secreted factor
cerberus that contribute to head
development and establish the
cephalic region.
Goosecoid, expressed in the node, regulates chordin
expression, and this gene product, together with noggin and
follistatin, antagonizes the activity of BMP4, dorsalizing mesoderm
into notochord and paraxial mesoderm for the head region.
Once the streak is formed and gastrulation is progressing, BMP4 is
secreted throughout the bilaminar disc and acts with FGF to
ventralize mesoderm into intermediate and lateral plate mesoderm.
Later, expression of the Brachyury (T) gene antagonizes BMP4 to
dorsalize mesoderm into notochord and paraxial mesoderm in caudal
regions of the embryo.

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FGF8, secreted by the node and
primitive streak, establishes
expression of Nodal, a member of
the TGF-b superfamily, and the nodal
protein then accumulates on the left
side near the node.
Later, as the neural plate starts to
form, FGF8 induces expression of
Nodal and LEFTY-2 in the lateral
plate mesoderm, whereas LEFTY-1
is expressed on the left side of the
ventral aspect of the neural tube.

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These signals are dependent upon
serotonin (5HT). Products from the
Brachyury (T) gene, expressed in the
notochord, also participate in induction of
these three genes.
In turn, expression of Nodal and
LEFTY-2 regulates expression of the
transcription factor PITX
2, which, through further downstream
effectors, establishes left-sidedness.
SHH, expressed in the notochord, may
serve as a midline barrier and also
represses expression of left-sided genes
on the right. Expression of the
transcription factor Snail may regulate
downstream genes important for
establishing right-sidedness.

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Cells migrating more posteriorly
through the node and most cranial
aspect of the streak will form
paraxial mesoderm (pm;
somitomeres and somites);
Fate map established during
gastrulation
Cells migrating at the most
cranial part of the node will
form the notochord (n);
Cells migrating through the next portion
of the streak will form intermediate
mesoderm (im; urogenital system);
Cells migrating through the more caudal
part of the streak will form lateral plate
mesoderm (lpm; body wall);
Cells migrating through the most
caudal part will contribute to
extraembryonic mesoderm (eem;
chorion).

19. Molecular Regulation of
Genital Duct
Development
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20. SOX
9
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22. Estrogr
n
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24. Molecular Regulation of
Pharyngeal Arches
Development
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25. Crest cells originating from Rhombomeres migrate to specific
arches
Patterning of pharyngeal
arches
This is regulated
by
1st Arch 2nd to 6th Arch
By
OTX 2 gene
MidbrainandForebrainExpressed in
regions and migrate with crest cells to
1st arch.
By
HOX genes
inHindbrainExpressed in
specific overlapping
pattern
There may be
an interaction
with HOX
genes in 1st
arch
patterning
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26. HOX
genes
Crest cells alone can’t maintain HOX genes
expression but require interaction with
mesoderm of the pharyngeal arches
HOX expression is
regulated by
SHH
Upstream regulator
expressed in the
arches
Regulate HOX expression in concentration
RAREdependent manner via
Retinoi
c Acid
: ( Retinoic Acid Response Elements)RARE
Binding sites for retinoic acid in promoter regions of HOX genes4/28/2013

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Patterns of HOX gene expression in the
hindbrain. HOX genes are expressed in overlapping
patterns
ending at specifi c rhombomere boundaries. Genes
at
the 3′ end of a cluster have the most anterior
boundaries,
and paralogous genes have identical expression
domains.
These genes confer positional value along the
anteriorposterior
axis of the hindbrain, determine the identity of
the rhombomeres, and specify their derivatives.

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30. Molecular Regulation of
Spinal Cord&Brain
Development
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32. Forebrain
&
Midbrain
Inhibition of BMP 4
Chordin
Noggin
Follistati
n
B
y
Expression of ( OTX 1,2 , EMX1, EMX2) genes in specific
& overlapping pattern
Differentiation of Forebrain & Midbrain
Regions
Once Boundaries of Forebrain & Midbrain Regions are
established
2 additional Organizing Centers appear
ANR
( at the junction of cranial border of
neural plate & non- neural
ectoderm
Isthmus
( at the junction Midbrain &
Hindbrain)
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33. AN
R
FGF
8
BF
1
Regulate
Development of
Telencephalon
Induce expression of
Ishmu
s
FGF
8
Induce expression of
WNT 1
EN
1
EN
2
Regulate
Development of
Tectum &
Cerebellum
Regulate
Development of
Cerebellum only
Interact with
EN1 & EN2
to regulate
development
of the region
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34. Isthmus
FGF
8
EN
1
EN
2
WNT 1
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35. Hindbrain
WNT 3a & FGF
Induce differentiation of
Hindbrain region into 8
segments called
Rhombomeres
Express HOX
genes
Expressed in overlapping pattern
Determine identity of these Rhombomeres & specify their
derivatives
Retinoic Acid Organizes Craniocaudal Axis
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Patterns of HOX gene expression in the
Hindbrain:
HOX genes are expressed in overlapping patterns
ending at specifi c rhombomere boundaries. Genes at
the 3′ end of a cluster have the most anterior boundaries,
and paralogous genes have identical expression domains.
These genes confer positional value along the
anteriorposterior
axis of the hindbrain, determine the identity of
the rhombomeres, and specify their derivatives.

37. Spinal
Cord
Induced byWNT 3a &
FGF
Neural plate in spinal cord region
expresses the following
Transcription Factors
(PAX3, PAX7, MSX1, MS
X2)
This expression is
controlled by
SHH (secreted by prochordal
plate)
Inhibit their
expression
Ventralize the Neural plate
Motor Neurons formation
BMP4,7(secreted by Non-neural
Ectoderm)
Upregulate PAX3,7
Dorsalize the Neural plate
Sensory Neurons
formation
needed for
Neural
Crest cells
formation
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38. Molecular Regulation of
LIMB
Development
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39. Limb outgrowth is initiated by FGF10 secreted by
lateral plate mesoderm in the limb-forming regions
Once outgrowth is initiated, the AER is induced by
BMPs and restricted in its location by the gene
Radical fringe expressed in dorsal ectoderm. In
turn, this expression induces that of SER2 in cells
destined to form the AER.
After the ridge is established, it
expresses FGF4 and FGF8 to maintain
the progress zone, the rapidly
proliferating mesenchyme cells adjacent
to the ridge.
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40. Anteroposterior patterning of the limb is controlled
by cells in the ZPA at the posterior border.
These cells produce retinoic acid (vitamin A), which
initiates expression of SHH, regulating patterning.
The dorsoventral limb axis is directed by
WNT7a, which is expressed in the dorsal
ectoderm.
This gene induces expression of the
transcription factor LMX1 in the dorsal
mesenchyme, specifying these cells as
dorsal.
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Longitudinal section through the
limb bud of a chick
embryo, showing a core of
mesenchyme cover by a layer of
ectoderm that thickens at the distal
border of the limb to form the AER.
In humans, this occurs during the
fifth week of development.
External view of a chick limb at
high magnification showing the
ectoderm and the specialized
region at the tip of the limb called
the AER

42. Bone type and shape are regulated by HOX genes, whose
expression is determined by the combinatorial expression of
SHH, FGFs, and WNT7a.
HOXA and HOXD clusters are the primary determinants of bone
morphology.
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43. Molecular Regulation of
EYE
Development
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44.  The eyes begin to develop as a pair of outpocketings that will
become the optic vesicles on each side of the forebrain at the end
of the fourth week of development .
 The optic vesicles contact the surface ectoderm and induce lens
formation.
 When the optic vesicle begins to invaginate to form the pigment
and neural layers of the retina, the lens placode invaginates to form
the lens vesicle.
 Through a groove at the inferior aspect of the optic vesicle, the
choroid fissure, the hyaloid artery (later the central artery of the
retina) enters the eye .
 Nerve fibers of the eye also occupy this groove to reach the optic
areas of the brain.
 The cornea is formed by:
(a) a layer of surface ectoderm,
(b) the stroma, which is continuous with the sclera, and 4/28/2013

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47. The master gene for
Eye Development
PAX
6
Expressed in the single eye field at the neural
plate stage.
SHH
Separate the
eye field into
two optic
primordia.
upregulates
PAX2 expression in
the optic stalks
while
downregulating
PAX6.
Restricting
(PAX6)expressio
n to the optic cup
and lens
Epithelial–mesenchymal interactions between prospective lens
ectoderm, optic vesicle, and surrounding mesenchyme then
regulate lens and optic cup differentiation.
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48. Molecular Regulation of
TOOTH
Development
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49.  Teeth develop from epithelial–mesenchymal interactions
between oral epithelium and neural crest–derived
mesenchyme.
 Enamel is made by ameloblasts.
 It lies on a thick layer of dentin produced by
odontoblasts,
a neural crest derivative.
 Cementum is formed by cementoblasts, another
mesenchymal derivative found in the root of the tooth.
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50. Enamel Knot
(Circumscribed
Region of dental
epith. at tips oftooth
buds
The organizer for the
TOOTH
Development is
Express
FGF 4 BMP 4
Regulate
outgrowth of the
cap.
Regulate timing of
apoptosis in Knot
cells 4/28/2013

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 http://www.imeg.kumamoto-u.ac.jp/en/index.html

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