T Germ Disc - Wiki Germ Disc 2013...  which the bilaminar germ disc (composed of two layers: epiblast and hypoblast) becomes a trilaminar germ disc, which is composed of three germ

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    Carlos A C Baptista, MD., PhD. MPH

    Department of Neurosciences

    Trilaminar Germ Disc


    Anatomy and Pathophysiology

    Two Layers

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    Cell and Tissue Lineage

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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:




    Primitive Streak (15 day old)


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    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


    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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    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


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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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