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 Appearance of primary chorionic villi Appearance of the prechordal plate

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

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

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

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

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

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

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

CHORION It is formed of: Trophoblast Extraembryonic mesoderm

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

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

CHORIONIC SAC It contains embryo + amniotic sac + yolk sac suspended by the connectiong stalk It is enclosed by the chorion

PRIMARY CHORIONIC VILLI

SECONDARY CHORIONIC VILLI

TERTIARY CHORIONIC VILLI

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

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

PRECHORDAL PLATE

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

PRIMITIVE STREAK

PRIMITIVE STREAK & NODE

BILAMINAR & TRILAMINAR DISCS

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

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