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Digestive system


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Digestive system

  3. 3. Formation of primitive gut • Digestive system is mainly derived from primitive or primordial gut. • The primordial gut is endodermal in origin and develops from the dorsal part of the yolk sac. • It (primordial gut) forms during the fourth week of intrauterine life. • It develops as a result of longitudinal and transverse folding of the embryo.
  4. 4. Longitudinal folding • It is mainly due to rapid growth of central nervous system. • Ends of the trilaminar germ disc folds ventrally and produce head and tail folds.
  5. 5. • Neural tube • Prochordal plate • Cloacal plate • Connecting stalk and allantois • Developing heart tube & pericardial cavity. • Septum transversum
  6. 6. • Later (day 22) the developing forebrain grows cranially beyond the buccopharyngeal membrane and overhangs the developing heart. • Concomitantly, the septum transversum, primordial heart, pericardial cavity, and buccopharyngeal or oropharyngeal membrane move onto the ventral surface of the embryo. • A small foregut and hindgut has developed. • Buccopharyngeal membrane is vertical (ectoderm is anterior, endoderm is posterior).
  7. 7. • Pericardial cavity is now inferior to heart tube. • Septum transversum is posterior to heart tube and pericardial cavity. • Cloacal membrane is vertical. • Connecting stalk and allantois has gone inferior to hindgut but they are directed posteriorly.
  8. 8. • Small portions of yolk sac have been engulfed and incorporated anteriorly and posteriorly. This is the beginning of foregut and hind gut formation.
  9. 9. • Day 25. Now the buccopharyngeal/oropharyngeal membrane and cloacal membrane have completed 180 degree turns (ectoderm is inferior and endoderm is superior). Note that the rotation is more in tail fold than head fold. • Foregut and hindgut have elongated. • The connecting stalk and allantois are now directed (project) inferiorly/ventrally. • The constriction of yolk sac is now obvious.
  10. 10. • Relatively advanced stage of cephalo-caudal folding. Now foregut and hindgut are clearly visible. Yolk sac is reduced in size. Midgut is still widely communicating with yolk sac.
  11. 11. • Day 28. Oropharyngeal and cloacal membranes have rotated more and now facing each other. • The yolk sac has further constricted. Midgut is now clear but still connected to yolk sac through narrow canal called vitelline duct. • Connecting stalk along with allantois has gone nearer to vitelline duct and associated splanchnic mesoderm.
  12. 12. • Advanced stage of cephalo-caudal folding. foregut, midgut and hindgut are clearly visible. Yolk sac is reduced in size. Midgut is still communicating with yolk sac through a narrow duct called vitelline duct.
  13. 13. • During longitudinal folding, part of the endoderm of the yolk sac is incorporated into the embryo as the foregut and hindgut. • The foregut lies between the brain and heart. Oropharyngeal membrane separates the foregut from the stomodeum. • The terminal part of the hindgut soon dilates slightly to form the cloaca (primordium of the urinary bladder and rectum).
  14. 14. • Lateral view of developing 26 day old embryo showing different parts of gut.
  15. 15. • After folding, the septum transversum lies caudal to the heart where it subsequently develops into the central tendon of the diaphragm. • Before folding, the primitive streak lies cranial to the cloacal membrane; after folding, it lies caudal to it. • After folding the connecting stalk and the vitelline duct join each other and finally form umbilical cord.
  16. 16. • The folding also affects the arrangement of the embryonic coelom (primordium of body cavities). • Before folding, the coelom is a flattened, horseshoe-shaped single continuous cavity.
  17. 17. • Trilaminar germ disc by the end of third week
  18. 18. • After folding, the coelom has become divided into thoracic and abdominal cavity. Thoracic cavity or pericardial coelom lies ventral to the heart and cranial to the septum transversum. Abdominal (peritoneal) cavity lies caudal to septum transversum. • Thoracic (pericardial) and abdominal (peritoneal) cavities communicate each other through pleuropericardial canals. The canals are posterior to septum transversum and lateral to primitive developing vertebral column.
  19. 19. • Diagram of a five week old embryo showing: • Septum transversum • Pericardioperitoneal canals
  20. 20. Transverse folding • The flat germinal disc not only folds cephalo-caudally but also folds in horizontal or transverse direction. • The right and left lateral edges of trilaminar germ disc not only grow laterally but also move ventrally and finally medially to meet in the midline.
  21. 21. • As the lateral edges of embryonic disc grow they form right and left lateral folds. • Transverse folding is produced by the rapidly growing somites and neural tube. • The primordium of the ventrolateral wall grows laterally, ventrally and medially towards the median plane, rolling the edges of the embryonic disc and forming a roughly cylindrical embryo.
  22. 22. • By the middle of 3rd week (day 17) a flat trilaminar germinal disc has formed.
  23. 23. • The development of paraxial mesoderm and formation of somites is mainly responsible for transverse folding of embryo. • As the paraxial mesoderm starts developing the ectoderm is raised bilaterally and neural groove forms between these bilateral ridges. • Paraxial mesoderm later forms somites. The formation of somites and neural tube further raises surface ectoderm dorsally.
  24. 24. • By the day-19 paraxial mesoderm has formed. • In the lateral plate of mesoderm small spaces have started developing.
  25. 25. • By the day-20 somites have started forming.
  26. 26. • Day-21. intra-embryonic coelom is well developed now. The yolk sac is decreasing in size. The embryo is assuming a globular form.
  27. 27. • Day-22. Somites have further enlarged. Neural tube has formed. Intra-embryonic coelom is wide. Embryo is more globular in shape.
  28. 28. • Day 23. • Yolk sac has further reduced in size. • Dorsal aortae have joined to form one aorta. • Intra-embryonic coelom is wider. • Embryo is more globular in shape.
  29. 29. • Day-25. • amnion is almost surrounding the developing embryo. • yolk sac is constricted. after lateral folding the connection is reduced to a yolk stalk. • Intra-embryonic coelom and extra-embryonic coeloms still communicating.
  30. 30. • Day-28. • right & left edges of amnion, surface ectoderm and parietal mesoderm have joined each other. • Intra-embryonic coelom is now within body
  31. 31. • As a result of cephalo-caudal (longitudinal) and lateral (transverse) folding the embryo assumes globular form. • The area of attachment of amnion to the ventral surface of the embryo is reduced to a relatively narrow umbilical region. • Connecting stalk and vitelline duct join and form umbilical cord. • The amnion surrounds the umbilical cord from all sides.
  32. 32. • Lateral view of developing 26 day old embryo showing different parts of gut. The allantois and vitelline duct have come closer.
  33. 33. • As a result of longitudinal and transverse folding, a portion of endoderm-lined yolk sac is incorporated into the developing embryo to form primitive or primordial gut.
  34. 34. • The endoderm of the primordial gut gives rise to the epithelium and glands of digestive system. • Only a small portion of epithelium at the cranial and caudal extremities of the tract is derived from ectoderm of stomodeum (primordial mouth) and proctodeum (anal pit), respectively. • All other layers i.e., lamina propria, muscularis mucosa, submucosa, mauscularis externa and fibrosa/serosa of the wall of the digestive tract are derived from the splanchnic mesenchyme surrounding the primordial gut.
  35. 35. • For descriptive purposes the primordial gut is divided into four parts: 1.Pharyngeal gut 2.Foregut 3.Midgut 4.Hindgut.
  36. 36. • Pharyngeal gut extends from oropharyngeal membrane to the respiratory diverticulum. • Primordial pharynx and its derivatives (oral cavity, pharynx, tongue, tonsils, salivary glands, and respiratory system) develop from pharyngeal gut.
  37. 37. • I believe that Pakistan has come into being to stay forever and is not like a dune of sand which appears and vanishes with strong winds. • Although the economy is confronted with difficult times yet it is a temporary phase and the country has the strength to get through it sooner or later. • Hence we should not lose hope to make country a greater Pakistan in the days to come.