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BILAMINAR & TRILAMINAR EMBRYONIC DISCS

BILAMINAR & TRILAMINAR EMBRYONIC DISCS. Dr. Ahmed Fathalla Ibrahim. 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

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BILAMINAR & TRILAMINAR EMBRYONIC DISCS

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  1. BILAMINAR & TRILAMINAR EMBRYONIC DISCS Dr. Ahmed Fathalla Ibrahim

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. YOLK SAC SECONDARY YOLK SAC: • Smallerthan 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

  8. 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

  9. EXTRAEMBRYONIC MESODERM • Formed by: • Endodermal cells of yolk sac • later on by primitive streak • Surrounds amnion & yolk sac

  10. CHORION • It is formed of: • Trophoblast • Extraembryonic mesoderm

  11. 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

  12. 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

  13. CHORIONIC SAC • It containsembryo + amniotic sac + yolk sacsuspended by the connectiong stalk • It is enclosed by the chorion

  14. PRIMARY CHORIONIC VILLI

  15. SECONDARY CHORIONIC VILLI

  16. TERTIARY CHORIONIC VILLI

  17. 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

  18. 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

  19. PRECHORDAL PLATE

  20. 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

  21. PRIMITIVE STREAK

  22. PRIMITIVE STREAK & NODE

  23. BILAMINAR & TRILAMINAR DISCS

  24. 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

  25. 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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