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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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Lateral Plate Mesoderm

Lateral Plate Mesoderm

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