BILAMINAR

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BILAMINAR TRILAMINAR EMBRYONIC DISCS
Dr. Ahmed Fathalla Ibrahim
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SECOND WEEK

• Completion of implantation of the blastocyst
• Production of a bilaminar embryonic disc
• Formation of extraembryonic structures amniotic
  cavity amnion, yolk sac, chorionic sac
  connecting stalk
• Appearance of primary chorionic villi
• Appearance of the prechordal plate

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THIRD WEEK

• Appearance of the primitive streak
• Development of the notochord
• Differentiation of the 3 germ layers (formation
  of a trilaminar germ disc)
• Beginning of formation of neural tube
• Completion of formation of chorionic villi

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BILAMINAR DISC

• A flat, almost circular bilaminar plate of cells
  composed of 2 layers
• Epiblast a layer of high columnar cells
• Hypoblast a layer of small cuboidal cells

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AMNIOTIC CAVITY AMNION

• Amniotic cavity a space appears dorsal to the
  epiblast (primordium of amniotic cavity)
• Amniogenic cells (amnioblasts) separates from
  epiblast forms the amnion which encloses the
  amniotic cavity
• N.B.
• The amnion forms the roof of amniotic cavity
• The epiblast forms the floor of amniotic cavity
  is continuous peripherally with amnion

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YOLK SAC

• The hypoblast becomes continuous peripherally
  with the exocoelomic membrane to enclose the
  exocoelomic cavity
• PRIMARY YOLK SAC has
• A roof formed by hypoblast
• A floor formed by exocoelomic membrane
• A cavity formed by exocoelomic cavity

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YOLK SAC

• SECONDARY YOLK SAC
• Smaller than primary yolk sac
• Formed by migration of endodermal cells from
  hypoblast to enclose a portion of primary yolk
  sac
• The rest of primary yolk sac is pinched off

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YOLK SAC

• SECONDARY YOLK SAC has
• A roof formed by hypoblast
• A floor formed by endodermal cells migrating
  from hypoblast
• A cavity formed by a portion of the cavity of
  primary yolk sac

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EXTRAEMBRYONIC MESODERM

• Formed by
• Endodermal cells of yolk sac
• later on by primitive streak
• Surrounds amnion yolk sac

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CHORION

• It is formed of
• Trophoblast
• Extraembryonic mesoderm

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EXTRAEMBRYONIC COELOM

• Isolated spaces filled with fluid appear within
  the extraembryonic mesoderm
• The spaces fuse to form the coelom
• The coelom divides the meosderm into 2 layers
• Extraembryonic somatic mesoderm between
  trophoblast coelom
• Extraembryonic splanchnic mesoderm between
  coelom yolk sac

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CONNECTING STALK

• It is a part of extraembryonic mesoderm that does
  not contain coelom
• It attaches the amnion yolk sac to the chorion
• It indicates the future site of umbilical cord

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CHORIONIC SAC

• It contains embryo amniotic sac yolk sac
  suspended by the connectiong stalk
• It is enclosed by the chorion

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PRIMARY CHORIONIC VILLI
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SECONDARY CHORIONIC VILLI
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TERTIARY CHORIONIC VILLI
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CHORIONIC VILLI

• DEVELOPMENT During implantation, the trophoblast
  thickens at region of contact with endometrium
  differentiates into
• Outer syncytiotrophoblast a layer containing
  multiple nuclei in a common cytoplasm with no
  cell boundaries (secretes HCG)
• Inner cytotrophoblast a layer of mononucleated
  cells

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STAGES OF CHORIONIC VILLI

• PRIMARY At the end of 2nd week, finger-like
  processes formed of outer syncytiotrophoblast
  inner cytotrophoblast appear
• SECONDARY Early in 3rd week, extraembryonic
  mesoderm extends inside the villi
• TERTIARY During 3rd week, arterioles, venules
  capillaries develop in the mesenchyme of villi
  join umbilical vessels
• By the end of 3rd week, embryonic blood begins to
  flow slowly through capillaries in chorionic
  villi

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PRECHORDAL PLATE
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PRECHORDAL PLATE

• In a localized area in the median plane, the
  cuboidal cells of the hypoblast become columnar
  form a thickening prechordal plate
• The prechordal plate indicates the future cranial
  region future site of mouth
• It is an important organizer of the head region

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PRIMITIVE STREAK
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PRIMITIVE STREAK NODE
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BILAMINAR TRILAMINAR DISCS
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PRIMITIVE STREAK

• It appears as a linear opacity in the epiblast
  of the caudal part of the embryo, in the median
  plane
• It results from proliferation of epiblast
• Primitive node the enlarged cranial end of
  primitive streak
• Primitive Pit a depression in primitive node
• Primitive groove a narrow groove in primitive
  streak that is continuous with the primitive pit

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PRIMITIVE STREAK

• IMPORTANCE
• It forms embryonic mesoderm
• It replaces cells of hypoblast from the roof of
  yolk sac forms embryonic endoderm
• The primitive node forms the notochordal process
• It shares in the formation of extraembryonic
  mesoderm

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