Week 3 Development PDF

Third week of development I 1) (Trilaminar germ disc) 2 1) Department of Anatomy and Embryology Faculty Of Medicine Arab American University - AAUP Spring semester 2024 sig! Va 0 -...

Third week of development I 1) (Trilaminar germ disc) 2 1) Department of Anatomy and Embryology Faculty Of Medicine Arab American University - AAUP Spring semester 2024 sig! Va 0 - S Gastrulation where occur fastrulation ?  The most characteristic event occurring during the 3rd Els week of gestation. J - - ~  Begins - with formation of the primitive streak - - & on the surface of the epiblast. - ( S it is clearly visible as a  In a 15-16 day embryo, - ( [ ↑⑮ To o Pos ,ja narrow groove with slightly bulging regions pinative steak - on either side. - 5)jejs 3  Gastrulation: the process that establishes &- - - - all three germ layers (ectoderm, mesoderm, - - - & endoderm). - ⑮(  i Gastrulation - > CP - --- I Gig permitive I. a Epibla m 5 ,91,53 > F - Giriu S treak - > & 1,Xs Endoderm gr a -. > - blasts 1999. hypo - mesoderm - Gastrulation 0 - - ↳ E - - - - - C - - Gastrulation do  The cephalic end of the- - streak, the primitive node, 00 consists of a slightly elevated area surrounding the small primitive pit. -  Cells of the epiblast migrate toward the primitive - - streak.  Upon arrival in the region of the streak, they T - ( become flask-shaped, detach from epiblast, & slip - beneath it. - - voi) jjs L  This inward movement is known as invaginationC. = Gastrulation  Once the cells have invaginated, some displace the - - - hypoblast, creating the embryonic endoderm& others come to lie between the epiblast & newly created endoderm to form mesoderm. -  Cells remaining in the epiblast then form - ectoderm. -  Thus, the epiblast is the source of all of the - germ - layers, & cells in these layers will give rise to all of the tissues & organs in the embryo. -  - Gastrulation Gradually, cells migrate - - - beyond the margin of the - - disc & establish contact - - - with the extraembryonic - - ( S m mesoderm covering the -- - - - yolk -- sac & amnion. In the cephalic direction, - The prechordal plate itself forms between the tip of the notochord & the buccopharyngeal membrane & is they pass on each side of I - derived from some of the first cells that migrate through 1 the prechordal plate. the node in a cephalic direction. Gastrulation O ( C  Later, - - the prechordal plate will be - - important for induction of the -- jo ; a forebrain. yes  The buccopharyngeal membrane at O the cranial end of the disc consists - of a small region of tightly - adherent ectoderm & endoderm cells that represents the future jig - opening of the oral cavity. Formation of the Notochord we , =5 - Prenotochordal cells invaginating in the - primitive pit move forward cephalad until - - they reach the prechordal plate. As the- hypoblast is replaced by endoderm 0 cells moving in at the streak, cells of the -- E - - - notochordal plate proliferate & detach from ~ - the endoderm. j , & form a - They then- solid cord of cells, the Sis so definitive notochord, which & - underlies the = E 7 neural tube & serves as the basis for the - - &axial skeleton.C - Formation of the Notochord Tel  Because elongation of the notochord is a - - # dynamic process, the cranial end forms first & - -goin caudal regions are added as the primitive - gi'disS S caudal position. streak assumes more - &  The notochord - & prenotochordal - cells extend -- cranially to the prechordal plate & caudally to - - the primitive pit.  At the point where the pit forms an indentation in the epiblast, theC - neurenteric canal& Im temporarily connects the amniotic & yolk sac cavities. - - Formation of the Notochord  The cloacal membrane is formed at the- - caudal end -S of the embryonic disc. -  This membrane consists of tightly adherent = - ectoderm & endoderm cells with no mesoderm.  When the cloacal membrane appears, the posterior & O - wall of the yolk sac forms a- small diverticulum that G - extends into the connecting stalk. -  This diverticulum, the C - allantoenteric diverticulum, G or allantois, appears around the 16th day of - - development. Growth of the Embryonic Disc - E  The embryonic disc, initially flat & almost round, gradually becomes elongated, with a broad cephalic & a narrow caudal end. -  Expansion of the embryonic disc occurs mainly in the cephalic region; the region of the - primitive streak remains more or less ⑮n - - the same - size. - - -  Growth & elongation of the cephalic part of the disc are caused by a continuous migration of cells from the primitive streak in - - a cephalic direction.  Invagination of surface cells in the primitive streak & their subsequent migration forward & laterally continues until the end of - the 4th week. W - G & / 6 - -I ( - E S Growth of the Embryonic Disc  At that stage, the primitive - - streak shows regressive changes, rapidly shrinks & soon disappears. -  That the primitive streak at the caudal end of the disc continues to supply new cells - until the - > -- end of the 4th week has an important bearing on development of the embryo.  In the cephalic part, germ layers begin their specific differentiation by the middle of the 3rd week, whereas in the caudal part, differentiation begins by the end of the 4th - week.  Thus gastrulation continues in &caudal segmentsC while cranial structures are ( differentiating, causing the embryo to develop cephalocaudally. - S Teratogenesis Associated With Gastrulation  The beginning of the 3rd week of development, when gastrulation is initiated, is a &highly sensitive stage for teratogenic insult.(  At this time, fate maps can be made for various organ systems, these cell populations - - may be damaged by teratogens, as high doses of alcohol. - - ( &  This stage is reached 2 wks after fertilization it is approximately 4 wks from the last menses. jollig -  Therefore, the woman may not recognize she is pregnant & she may not take Skel jets precautions. -  Gastrulation itself may be disrupted by genetic abnormalities && toxic insults. Further Development of the Trophoblast By the beginning of the 3rd week, the trophoblast is characterized byG - primary villi that consist - of a cytotrophoblastic core covered by a syncytial layer. During further development,T mesodermal cells penetrate the core of W primary villi & grow Ctoward the decidua. T -jijs The newly formed structure is known as a secondary villus. - om 00 Further Development of the Trophoblast  By the end of the- 3rd week, mesodermal cells in the core of the villus begin to differentiate into blood - - cells & small blood vessels, forming the - villous capillary system.  The villus is now known as a tertiary villus or definitive placental villus.  Capillaries in tertiary villi make contact with capillaries developing in mesoderm of the chorionic plate & in the connecting stalk. - placenta & the embryo. (  These vessels, in turn, establish Icontact with the intraembryonic circulatory system, connecting the - - > - ⑧ O O Further Development of the Trophoblast -/GN : , o When the heart begins to beat in the 4th week of - - development, the villous system is ready to supply the - -- - embryo proper with essential - nutrients & oxygen o Villi that extend from the chorionic plate to the- decidua - basalis are called stem or anchoring villi. - I S - o Those that branch from the sides of stem villi are free &(terminal)& villi, through which exchange of nutrients & other factors will occur. 12) - I - W o The chorionic cavity, becomes larger & by the 19th or 20th - day, the embryo is attached to its11 trophoblastic shell by a 21 narrow connecting stalk which later develops into the - &umbilical cord.&