3rd Week Embryonic Development PDF

Lecture (46-47) Third week of development By Dr. Sally Sayed Anwer Department of human anatomy and embryology Assiut university Learning objectives After this lecture, student should be able to: Know Gastrulation (formation of embryonic mesoderm and endo...

Lecture (46-47) Third week of development By Dr. Sally Sayed Anwer Department of human anatomy and embryology Assiut university Learning objectives After this lecture, student should be able to: Know Gastrulation (formation of embryonic mesoderm and endoderm). Know formation of the Notochord. Describe the further development of the trophoblast. Content of the lecture Formation of the trilaminar germ disc. Formation of the Notochord. Further development of the trophoblast and development of a villus. Gastrulation The most characteristic event occurring during the third week of gestation is gastrulation. -Definition of gastrulation: It is the process that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Evidence of gastrulation is the appearance of primitive streak.  -Steps of gastrulation 1- The Primitive streak appears as vaguely defined groove on the on the surface of the epiblast. 2- Later on, 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. 3- 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. 4-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. 5-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, and cells in these layers will give rise to all of the tissues and organs in the embryo. 6-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. 7- In the cephalic direction, they pass on each side of the prechordal plate. 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. 8-Later, the prechordal plate will be important for induction of the forebrain. 9-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 notochord 1-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 2-cells of the 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 serves as the basis for the axial skeleton. 3- The elongation of the notochord is a dynamic process, the cranial end forms first, and caudal regions are added after that. 4-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. 5-At the point where the pit forms an indentation in the epiblast, the neurenteric canal temporarily connects the amniotic and yolk sac cavities 6-The cloacal membrane is formed at the caudal end of the embryonic disc. This membrane, which is similar in structure to the oropharyngeal membrane, consists of tightly adherent ectoderm and endoderm cells with no intervening mesoderm. 7-The allantoenteric diverticulum, or allantois, appears as extension of posterior wall of yolk sac into the connecting stalk. This structure is rudimentary in human. Fate of notochord: The notochord disappears with the exception of a part that will form the followings: 1- Apical ligament of the skull. 2- Part of bones of skull base. 3- Nucleus pulposus of intervertebral disc.  Body of sphenoid 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 the same size. growth and elongation of the cephalic part of the disc that is caused by a continuous migration of cells from the primitive streak region in a cephalic direction.  The primitive streak shows regressive changes at the end of fourth week , rapidly shrinks, and disappears. In the cephalic part of embryonic disc, germ layers begin their differentiation by the middle of the third week while the differentiation of the caudal part begins by the end of fourth week.  Teratogenesis Associated With Gastrulation Caudal dysgenesis (sirenomelia): insufficient mesoderm is formed in the caudal most region of the embryo. Because this mesoderm contributes to formation of the lower limbs, urogenital system (intermediate mesoderm), and lumbosacral vertebrae, abnormalities in these structures occur. Affected individuals exhibit a variable range of defects, including hypoplasia and fusion of the lower limbs, vertebral abnormalities, renal agenesis, imperforate anus, and anomalies of the genital organs. Tumors Associated With Gastrulation Sacrococcygeal teratoma: tumors result from remnants the primitive streak that persist in the sacrococcygeal region commonly contain tissues derived from all germ layers.  Further development of trophoblast: 1-At the beginning of third week, the trophoblast contains primary villi which are composed of cytotrophoblastic core surrounded by syncytiotrophoblastic shell. 2- During further development, mesodermal cells derived from extraembryonic mesoderm penetrate into the core of the primary villi. The newly formed structure is called secondary chorionic villus. 3-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. 4-Capillaries in tertiary villi make contact with capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk. 4-These vessels, in turn, establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo. When the heart begins to beat in the fourth week of development, the villous system is ready to supply the embryo proper with essential nutrients and oxygen. 5- Cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium. 6-The cytotrophoblastic cells establish contact with 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. 7-Villi that extend from the chorionic plate to the decidua basalis 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. syncytiotrophoblast cytotrophoblast The chorionic cavity, meanwhile, becomes larger, and by the 19th or 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 placenta and embryo.