Embryology L3 - P1 PDF

Embryology L 3 - P1 Third Week of Development PART 01 Third Week of Development {Trilaminar Germ Disc}  GASTRULATION: is the process of formation of mesoderm & endoderm. The most characteristic event occurring during the third week of gestation is gastrulation...

Embryology L 3 - P1 Third Week of Development PART 01 Third Week of Development {Trilaminar Germ Disc}  GASTRULATION: is the process of formation of mesoderm & endoderm. The most characteristic event occurring during the third week of gestation is gastrulation the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) Gastrulation begins with formation of the primitive streak on the surface of the epiblast Initially, the streak is vaguely defined, but in a 15- to 16-day embryo, it is clearly visible as a narrow groove with slightly bulging regions on either side. The cephalic end of the streak, the primitive node, 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 become flask-shaped, detach from the epiblast, and slip beneath it —> This inward movement is known as invagination Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8) —> which is synthesized by streak cells themselves —> it controls cell specification into the mesoderm  GASTRULATION: Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm, and others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm. Thus, the epiblast, through the process of gastrulation, is the source of all of the germ layers cells in these layers will give rise to all of the tissues and organs in the embrycells in these layers will give rise to all of the tissues and organs in the embryoo As more and more cells move between the epiblast and hypoblast layers, they begin to spread laterally and cranially , Gradually, they migrate beyond the margin of the disc and establish contact with the extraembryonic mesoderm covering the yolk sac and amnion , In the cephalic direction, they pass on each side of the prechordal plate  GASTRULATION: The prechordal plate itself forms between the tip of the notochord and the oropharyngeal membrane and is derived from some of the first cells that migrate through the node in the midline and move in a cephalic direction. Later, the prechordal plate will be important for induction of the forebrain. The oropharyngeal membrane at the cranial end of the disc consists of a small region of tightly adherent ectoderm and endoderm cells that represents the future opening of the oral cavity.  FORMATION OF THE NOTOCHORD Prenotochordal cells invaginating in the primitive node move forward cranially in the midline until they reach the prechordal plate These prenotochordal cells become intercalated in the hypoblast so that for a short time, the midline of the embryo consists of two cell layers that form the notochordal plate. As the hypoblast is replaced by endoderm cells moving in at the streak, cells ofthe notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord, which underlies the neural tube and is a signaling center for inducing the aXial skeleton. Because elongation of the notochord is a dynamic process, the cranial end forms first, and caudal regions are added as the primitive streak assumes a more caudal position. The notochord and prenotochordal cells extend cranially to the prechordal plate (an area just caudal to the oropharyngeal membrane) and caudally to the primitive pit. At the point where the pit forms an indentation in the epiblast, the neurenteric canal temporarily connects the amniotic and yolk sac cavities  FORMATION OF THE NOTOCHORD the cloacal membrane is formed at the caudal end of the embryonic disc. This membrane, which is similar in structure to the oropharyngeal end of membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm. When the cloacal membrane appears, the posterior wall of the yolk sac forms a small diverticulum that extends into the connecting stalk. This diverticulum, the allantoenteric diverticulum, or allantois, appears around the 16th day of development. Although in some lower vertebrates, the allantois serves as a reservoir for excretion products ofthe renal system, in humans, it remains rudimentary but may be involved in abnormalities of bladder development Summary THANK YOU！  Embryology L3 - P2 third week P2 PART 02 third week of development {trilaminar germ disc} FATE MAP ESTABLISHED DURING GASTRULATION Regions of the epiblast that migrate and ingress through the primitive streak have been mapped, and their ultimate fates have been determined. For example: cells that ingress through the cranial region of the node become pre- chordal plate and notochord, those migrating at the lateral edges of the node and from the cranial end of the streak become paraxial mesoderm, cells migrating through the midstreak region become intermediate mesoderm, those migrating through the more caudal part of the streak form lateral plate mesoderm, cells migrating through the caudalmost part of the streak contribute to extraembryonic mesoderm (the other source of this tissue is the primitive yolk sac [hypoblast]).  GROWTH OF THE EMBRYONIC DISC The embryonic disc, initially flat and almost round, gradually becomes elongated, with a broad cephalic and 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 the same size. Growth and elongation of the cephalic part of the disc are caused by a continuous migration of cells from the primitive streak region in a cephalic direction. Invagination of surface cells in the primitive streak and their subsequent migration forward and laterally continues until the end of the fourth week. At that stage, the primitive streak shows regressive changes, rapidly shrinks, and soon disappears.  GROWTH OF THE EMBRYONIC DISC That the primitive streak at the caudal end of the disc continues to supply new cells until the end of the fourth 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 third week, whereas in the caudal part, differentiation begins by the end of the fourth week. Thus, gastrulation, or formation of the germ layers, continues in caudal segments while cranial structures are differentiating, causing the embryo to develop cephalocaudally.  FURTHER DEVELOPMENT OF THE TROPHOBLAST By the beginning of the third week, the trophoblast is characterized by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer. During further development, mesodermal cells penetrate the core of primary villi and grow toward the decidua. The newly formed structure is known as a secondary villus. By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system. The villus is now known as a tertiary villus or a definitive placental villus.  FURTHER DEVELOPMENT OF THE TROPHOBLAST Capillaries in tertiary villi make contact with capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk. These vessels, in turn, establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo. Hence, when the heart begins to beat in the fourth week of development, the villous system is ready to supply the embryo with proper essential nutrients and oxygen. Meanwhile, cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium. Here, they establish contact with similar extensions of neighboring villous stems, forming a thin outer cytotrophoblast shell. This shell gradually surrounds the trophoblast entirely and attaches the chorionic sac firmly to the maternal endometrial tissue.  FURTHER DEVELOPMENT OF THE TROPHOBLAST Villi that extend from the chorionic plate to the decidua basalis (decidual plate: the part of the endometrium where the placenta will form) are called stem or anchoring villi. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur. The chorionic cavity, meanwhile, becomes larger, and by the 19th or the 20th day, the embryo is attached to its trophoblastic shell by a narrow connecting stalk. The connecting stalk later develops into the umbilical cord, which forms the connection between the placenta and embryo. Summary THANK YOU！