Lec5 Third Week Of Development Trilaminar Germ Disc PDF

College of Medicine Dr. Firas Al-Masoody Second stage Medical Embryology Lecture 5 Third Week of Development Trilaminar Germ Disc Learning objectives: 1- Describe developmental changes occur in w...

College of Medicine Dr. Firas Al-Masoody Second stage Medical Embryology Lecture 5 Third Week of Development Trilaminar Germ Disc Learning objectives: 1- Describe developmental changes occur in week 3. 2- Define gastrulation, primitive node, notochord. 3- Describe the fate map. 4- Illustrate the developmental changes in trophoblast at week 3. Gastrulation The most characteristic event occurring during the third week of gestation is Gastrulation, the process that establishes all the three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation converts the bilaminar embryonic disc into trilaminar embryonic disc. 1- Primitive streak: Gastrulation begins with formation of the primitive streak on the surface of the epiblast near the caudal end of the bilaminar embryonic disc. It becomes visible in a 15- to 16- day embryo as a narrow groove (Primitive groove) surrounded by slightly bulging regions on either side. 2- Primitive node: the cephalic (cranial) end of the streak is expanded and known as the primitive node. It contains a circular depression called the Primitive pit. 3- Invagination: cells of the epiblast migrate toward the primitive streak, then they detach and slip beneath it. This inward movement of epiblast cells is known as invagination. Invagination occur at the region of the streak and node. Once the cells have invaginated: a) Some displace hypoblast, creating the embryonic endoderm. b) Others will lie between epiblast and newly formed endoderm, to form the mesoderm. c) Cells that do not migrate and remain in the epiblast form the ectoderm. Thus, through the process of gastrulation, the epiblast is the source of all the germ layers, and cells in these layers will give rise to all of the tissues and organs in the embryo. 1 Formation of the primitive streak also defines for the first time all major body axes. These consist of the cranial-caudal (or head-tail) axis , the dorsal-ventral (or back-belly) axis , and the left-right axis. 4. Prechordal plate: it is derived from the first cells that migrate through the primitive node in the midline, and move in a cephalic direction. Prechordal plate lies between the tip of notochord and oropharyngeal membrane and it is important for the induction of forebrain. 5. Oropharyngeal membrane: lies at the cranial end of embryonic disc, consists of tightly adherent ectoderm and endoderm with no mesoderm in between. It represents the future opening of oral cavity. 6. Neuroenteric canal: lies at the primitive pit, and temporarily connects the amniotic and yolk sac cavities. 7. Cloacal membrane: Lie at the caudal end of embryonic disc. It is similar in structure to the buccopharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm. 8. Allantoenteric diverticulum, or Allantois: a small diverticulum that extends into the connecting stalk, from the posterior wall of the yolk sac. It appears around the 16th day of development. 9. Formation of the Notochord Prenotochrodal cells invaginating in the primitive node move forward cranially in the midline until they reach the prechordal plate. These cells become intercalated with 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 of the notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord. 2 Fate Map Established During Gastrulation Cells of the epiblast that migrate and ingress through the primitive streak have been mapped and their ultimate fates determined. For example, cells that ingress through the cranial region of the node become prechordal 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; and cells migrating through the caudal-most part of the streak contribute to extraembryonic mesoderm. 3 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 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. 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, continues in caudal segments while cranial structures are differentiating, causing the embryo to develop cephalo-caudally. Clinical Correlate The beginning of the third week is a highly sensitive stage for teratogenic insult. A. High doses of alcohol at this stage kill cells in the anterior midline of the germ disc, producing a deficiency of the midline in craniofacial structures and resulting in holoprosencephaly. B. Gastrulation itself may be disrupted by genetic abnormalities and toxic insults. In caudal dysgenesis (sirenomelia), insufficient mesoderm is formed in the caudalmost region of the embryo. Affected individuals have hypoplasia and fusion of the lower limbs, vertebral abnormalities, renal agenesis, imperforate anus, and anomalies of the genital organs. C. Tumors Associated with Gastrulation: Sometimes, remnants of primitive streak persist in the sacrococcygeal region. These clusters of pluripotent cells proliferate and form tumors, known as sacrococcygeal teratomas , that commonly contain tissues derived from all three germ layers. 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 definitive placental villus. 4 Capillaries in the tertiary villi establish contact with capillaries in the mesoderm of chorionic plate and connecting stalk. These vessels in turn, establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo. Meanwhile, cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium. 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 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. 5