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Carlos A C Baptista, MD., PhD. MPH
Department of Neurosciences
Trilaminar Germ Disc
INDI-555
Anatomy and Pathophysiology
Two Layers
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Implantation
Cell and Tissue Lineage
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Gastrulation
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)
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Hypoblast-Epiblast
Primitive Streak
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.
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Primitive Streak
Non-Migrating Epiblast Cells
Non-migrating epiblast cells become
the embryonic ectoderm.
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Migrating Epiblast Cells
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
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Paths of Migration
Primitive Node
Form Prechordal
Plate and Notochord
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)
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Fate Map of the Epiblast
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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
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Differentiation of the Mesoderm
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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
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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
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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
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Lateral Plate Mesoderm
Lateral Plate Mesoderm
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Paraxial Mesoderm
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
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Paraxial Mesoderm
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Somites
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.
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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.
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