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INDI-555
Anatomy and Pathophysiology
Trilaminar Germ Disc
Carlos A C Baptista, MD., PhD. MPH
Department of Neurosciences
Two Layers
1
Implantation
Cell and Tissue Lineage
2
Gastrulation
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Gastrulation is the process through
which the bilaminar germ disc
(composed of two layers: epiblast
and hypoblast) becomes a
trilaminar germ disc, which is
composed of three germ layers:
Ectoderm
Mesoderm
Endoderm
Primitive Streak (15 day old)
3
Hypoblast-Epiblast
Primitive Streak
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During the initial phase of gastrulation, a groove appears
in the midline axis of the caudal portion of the bilaminar
germ disc.
On both sides of the groove, epiblast cells proliferate.
At the cranial end of the groove, cells migrate inward
forming a pit (primitive pit).
The proliferation of epiblast cells around the pit creates a
dense concentration of cells called a node, the primitive
node.
Collectively, the groove, pit and node create an area
called the primitive streak. The primitive streak gives
bilateral symmetry and a midline axis to the developing
embryo.
4
Primitive Streak
Non-Migrating Epiblast Cells

Non-migrating epiblast cells become
the embryonic ectoderm.
5
Migrating Epiblast Cells
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
Some epiblast cells around the primitive
streak are induced to loose their
connections with one another, and to
migrate through the primitive streak.
The migrating epiblasts are destined to:


replace the hypoblast cells and become
embryonic endoderm, and
create a third germ layer – the mesodermal
(intraembryonic) layer that becomes
sandwiched between the epiblasts and
endodermal cells of the hypoblast.
Endoderm and Mesoderm
6
Paths of Migration
Form Prechordal
Plate and Notochord
Primitive Node
Primitive Groove
Form the Mesoderm
Exceptions to Mesodermal Layer

Some migrating epiblast cells
become mesodermal cells which
form a continuous layer between
the ectodermal and endodermal
layers, except in two regions:


Buccopharyngeal area (site of future
mouth)
Cloacal area (site of distal openings
of the digestive and urogenital
tracts)
7
Fate Map of the Epiblast
8
Organization of Embryonic Mesoderm


Cells of the mesoderm layer become
organized into regionally distinct cell
masses along the midline axis of the
embryo.
The distinct masses of mesoderm are:




Axial mesoderm
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
9
Differentiation of the Mesoderm
10
Axial mesoderm

Some epiblast cells, which migrate
through the primitive streak, form
an axial midline mass that gives
rise to the prechordal plate and
the notochordal process.
Notochordal and Prechordal Plate
11
Axial mesoderm
The notochord process:
 It is a hollow tube of mesodermal cells as it
forms from the nodal region of the primitive
streak.
 Over embryonic days 16-22, the notochord
process fuses with the underlying midline
endoderm to form the notochordal plate.
 The notochordal plate infolds and detaches
from the endoderm, and then moves back
into the mesoderm space, forming the
notochord. Some cells of endoderm origin
become incorporated in the notochord.
Notochordal Transformation
12
Axial mesoderm


The axial mesodermal structures (the
prechordal plate + the cranial portion of
the notochoral plate, secrete inducing
substances that cause the overlying
ectoderm to differentiate into neural
ectoderm and form the neural plate.
A distinct population of cells located in
the lateral margins of the neural plate,
the neural crest cells, detach from the
neural plate and migrate to specific
regions.
Neural Plate



During the third week, the neural
plate begins to differentiate into
the brain and spinal cord.
The cranial portion of the neural
plate undergoes differentiation into
the forebrain, midbrain and
hindbrain.
The caudal portion of the neural
plate becomes the spinal cord
13
Lateral Plate Mesoderm
Lateral Plate Mesoderm
14
Paraxial Mesoderm

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Cells migrating through the primitive streak
form a sheet-like mass of mesoderm on either
side of the notochord during the third and fourth
weeks.
The bilateral masses of mesoderm, which are
nearest the notochord, the paraxial mesoderm,
become condensed into cube-like masses that
are segmentally arranged.
These masses are called Somitomeres.
Cells of the paraxial mesoderm give rise to
cells of the axial skeleton, skeletal musculature,
and contribute to dermal portion of the skin.
Somitomeres Development
15
Paraxial Mesoderm
16
Somites
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Axial skeleton
Vertebral column
Occipital bone
Muscles of the Neck (voluntary)
Muscles of body wall
Muscles of the limbs
Part of the dermis of neck and trunk
Part of the dermis of the abdomen
Intermediate Mesoderm


Distinct condensations of
mesodermal cells immediately
lateral to the paraxial mesoderm.
The cells of the intermediate
mesoderm differentiate into cells
of the urinary system and
contribute cells to the reproductive
system.
17
Intermediate Mesoderm
Lateral Plate Mesoderm


Formed by cells lateral to the intermediate
mesoderm
Organized into two layers:




somatopleuric mesoderm, that is nearest the
overlying ectoderm
splanchnopleuric mesoderm, which is nearest
the underlying endoderm.
The somatopleuric mesoderm contributes to the
dermis of the skin in the limb buds and body
wall.
The splanchnopleuric layer of mesoderm forms
the walls of the developing internal organs.
18
Lateral Plate Mesoderm
Lateral Plate Mesoderm
19
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