Third Week of Embryonic Development PDF

Embryology Trilaminar Germ Disc (Third Week of Development) By Dr. Sally Mohsen Assistant Professor of Anatomy and Embryology  objectives 1. Outline the formation of intra-embryonic mesoderm and intraembryonic coelom. 2. Describe the process of gastrulation an...

Embryology Trilaminar Germ Disc (Third Week of Development) By Dr. Sally Mohsen Assistant Professor of Anatomy and Embryology  objectives 1. Outline the formation of intra-embryonic mesoderm and intraembryonic coelom. 2. Describe the process of gastrulation and fate map established during this process. 3. Outline the formation of primitive streak and primitive node. 4. Outline formation of notochord. 5. Describe the development of trophoblast. www.vision.edu.sa  Gastrulation GASTRULATION: It is the most characteristic event occurring during the third week of gestation. The process establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins with formation of the primitive streak on the surface of the epiblast. The primitive streak is a narrow groove with slightly bulging regions on either side. It is vaguely defined, but in a 15- to 16-day embryo, it is clearly visible. The primitive node is the cephalic end of the streak, consists of a slightly elevated area surrounding the small primitive pit. The primitive pit is a depression in the center of primitive node. www.vision.edu.sa  Cells of the epiblast migrate toward the primitive streak. They become flask-shaped, detach from the epiblast, and slip beneath it. This inward movement is known as invagination. 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. Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8), which is synthesized by streak cells themselves. As more and more cells move between the epiblast and hypoblast layers, they begin to spread laterally and cranially. 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. In the cranial part of the embryonic disc, the cells of the hypoblast become tall columnar forming a thickening called the prochordal plate which is firmly attached to the overlying epiblastic disc (oral membrane) on 14th day. With the appearance of prochordal plate, the pole of the embryo is determined & also the central axis (right & left halves). This becomes the cranial end & other tail end. www.vision.edu.sa www.vision.edu.sa  Notochord The notochord is a midline structure develops from upper end of primitive streak to lower end of prochordal plate. Notochordal Process: cells multiply in the midline between ectoderm and endoderm, reaching lower end of prochordal plate. (17th to 18th day) Notochordal canal: primitive pit extends into the notochordal process, converting it into a canal. Floor of the notochordal canal break down, notochordal canal communicates with yolk sac and also with amniotic cavity through primitive pit. www.vision.edu.sa  At the point where the pit forms an indentation in the epiblast, the neurenteric canal temporarily connects the amniotic and yolk sac cavities. Intra-embryonic mesoderm spreads everywhere in the disc except in prochordal plate, where ectoderm & endoderm are adherent without mesoderm inbetween & is very thin. This will later forms buccopharyngeal membrane (oral cavity) 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 (future urogenital region). 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. www.vision.edu.sa  Prenotochordal cells invaginating in the primitive node move forward cranially in the midline until they reach the prechordal plate. 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. The notochordal plate is a two cell layers in the midline of the embryo where prenotochordal cells become intercalated in the hypoblast. 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 Notochord acts as a forerunner in the development of vertebral column & induces the formation of neural tube. Notochord cells persists in first decade of life as nucleus pulposus of inter-vertebral disc. www.vision.edu.sa www.vision.edu.sa 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 by a continuous migration of cells from the primitive streak in the cephalic region. 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. 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. www.vision.edu.sa 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 as follows: Cells that ingress through the cranial region of the node become prechordal plate and notochord Cells 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. Cells migrating through the more caudal part of the streak form lateral plate mesoderm. Cells migrating through the caudal most part of the streak contribute to extraembryonic mesoderm (the other source of this tissue is the primitive yolk sac [hypoblast]). www.vision.edu.sa 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. 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 proper with essential nutrients and oxygen. www.vision.edu.sa  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. 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 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. www.vision.edu.sa www.vision.edu.sa  References 1. Langman’s Medical Embryology, 12th edition. 2. Essentials of Human Embryology, by A.K. Datta, 5 th edition 3. Before we are born, by More & Persuad 4. Internet websites (for videos) www.vision.edu.sa www.vision.edu.sa