Dr Ahmed Fathalla Ibrahim SECOND WEEK Completion of

Dr. Ahmed Fathalla Ibrahim

SECOND WEEK • Completion of implantation of the blastocyst • Production of a bilaminar embryonic disc • Formation of extraembryonic structures: 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) 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: 1. Epiblast a layer of high columnar cells 2. Hypoblast: a layer of small cuboidal cells

AMNIOTIC CAVITY & AMNION • Amniotic cavity: 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: 1. A roof: formed by hypoblast 2. A floor: formed by exocoelomic membrane 3. A cavity: 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: 1. A roof: formed by hypoblast 2. A floor: formed by endodermal cells migrating from hypoblast 3. A cavity: cavity formed by a portion of the cavity of primary yolk sac

EXTRAEMBRYONIC MESODERM • Formed by: 1. Endodermal cells of yolk sac 2. later on by primitive streak • Surrounds amnion & yolk sac

CHORION • It is formed of: 1. Trophoblast 2. 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: 1. Extraembryonic somatic mesoderm: between trophoblast & coelom 2. 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: 1. Outer syncytiotrophoblast: a layer containing multiple nuclei in a common cytoplasm with no cell boundaries (secretes HCG) 2. Inner cytotrophoblast: a layer of mononucleated cells

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

PRECHORDAL PLATE

PRECHORDAL PLATE • In a localized area in the median plane, 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: 1. It forms embryonic mesoderm 2. It replaces cells of hypoblast from the roof of yolk sac & forms embryonic endoderm 3. The primitive node forms the notochordal process 4. It shares in the formation of extraembryonic mesoderm