12. git[1]

ALIMENTARY SYSTEM II GASTROINTESTINAL TRACT

Profile view of a human embryo estimated at twenty or twenty-one days old. (Dorsal aorta labeled at center left.)

Dorsal aorta Each primitive aorta receives anteriorly a vein—the vitelline vein —from the yolk-sac , and is prolonged backward on the lateral aspect of the notochord under the name of the dorsal aorta . The dorsal aortæ give branches to the yolk-sac, and are continued backward through the body-stalk as the umbilical arteries to the villi of the chorion . The two dorsal aortae combine to become the descending aorta in later development

TOPICS Highlights. Introduction . Derivation of individual parts of alimentary tract. /foregut, midgut , hindgut/ Rotation of the gut. Fixation of the gut. Timetable of some events described in this lecture.

HIGHLIGHTS ENDODERM At first it is in the form of a flat sheet , Converted into a tube by formation of head, tail and lateral folds of embryonic disc. This tube is the gut . THE GUT consists of foregut, midgut and hindgut . The midgut is at first in wide communication with the yolk sac. Later it becomes tubular. Part of midgut forms a loop that is divisible into prearterial and postarterial segments.

Cloaca It is the most caudal part of the hind gut. It is partitioned to form the primitive rectum (dorsal) and the primitive urogenital sinus . The oesophagus is derived from the foregut. The stomach is derived from the foregut. DUODENUM The superior part and the upper part of the descending part is derived from the foregut , The rest of the duodenum develops from the midgut loop . HIGHLIGHTS (continue)

The jejunum and ileum are derived from the prearterial segment of the midgut. The postarterial segment of the midgut loop gives off a caecal bud. The caecum and the appendix are formed by enlargement of the caecal bud. The ascending colon develops from the postarterial segment of the midgut loop. After ascending colon formation the gut undergoes rotation. As a result of rotation; the caecum and ascending colon come to lie on the right side; The jejunum and ileum lie mainly in the left-half of the abdominal cavity. HIGHLIGHTS (continue)

INTRODUCTION Epithelial lining of the various parts of the gastrointestinal tract is endodermal origin. In the mouth and anal canal , some of the epithelium is derived from ectoderm (stomatodaeum, proctodaeum). Head and tail folds Part of the Yolk sac is enclosed within the embryo to form the primitive gut . Gut is in free communication with the yolk sac. Foregut cranial to communication, Hind gut ------? Midgut - -------?? Cranially Buccopharyngeal membrane separates the foregut from the stomatodaeum. Caudally Cloacal membrane separate the hindgut from the proctodaeum. Later both membrane disappear and gut opens to the exterior at 2 ends.

While the gut is being formed, the circulatory system of the embryo undergoes considerable development. A midline artery, the dorsal aorta , is established and comes to lie just dorsal to the gut. It gives off a series of branches to the gut. Vitelline arteries connect midgut with the yolk sac. Most of the ventral arteries disappear, only three of them remain; Coeliac, superior and inferior mesenteric arteries . SMA, IMA Wide communication between midgut and yolk sac is gradually narrowed down, The midgut assumes the form of a loop. The superior mesenteric artery runs in the mesentery of this loop to its apex. The loop has Prearterial (proximal) and postarterial (distal) segments. INTRODUCTION (continue)

After a number of weeks, the midgut loop comes to lie outside the abdominal cavity of the embryo. It passes through the umbilical opening into a part of the extra-embryonic coelom (that persists in relation to the most proximal part of the umbilical cord) . The loop is subsequently withdrawn into the abdominal cavity. Allantoic diverticulum opens into the ventral aspect of the hindgut. The part of the hindgut caudal to the attachment is called the cloaca. The cloaca shows subdivision into a broad ventral part and narrow dorsal part. Urogenital septum separate the two parts. INTRODUCTION (continue)

The ventral subdivision is called the primitive urogenital sinus and gives origin to some parts of the urogenital system. The dorsal subdivision is called the primitive rectum. It forms the rectum and part of the anal canal. The urogenital septum grows towards the cloacal membrane and fuses with it. The cloacal membrane is divided into ventral urogenital membrane (related to the urogenital sinus). and dorsal anal membrane (related to the rectum). Mesoderm around the anal membrane becomes heaped up with the result that the anal membrane comes to lie at the bottom of a pit called the anal pit, or proctodaeum. The anal pit contributes to the formation of the anal canal. INTRODUCTION (continue)

DERIVTIVES OF THE FOREGUT Part of the floor of the mouth, including the tongue. Pharynx. Various derivatives of the pharyngeal pouches, and the thyroid. Oesophagus. Stomach. Duodenum: (1 st part + 1 st ½ of 2 nd part up to the major duodenal papilla) Liver and extra-hepatic biliary system. Pancreas. Respiratory system.

Duodenum : (2 nd ½ of the 2 nd part distal to the major duodenal papilla; horizontal and ascending part). Jejunum. Ileum. Caecum and appendix. Ascending colon. Right 2/3 rd of the transverse colon. DERIVTIVES OF THE MIDGUT

Left 1/3 rd of the transverse colon. Descending and pelvic colon. Rectum . Upper part of the anal canal. Parts of the urogenital system derived from the primitive urogenital sinus. DERIVTIVES OF THE HINDGUT

Note At this stage, The endoderm of the foregut, midgut and hindgut gives rise only to the epithelial lining of the intestinal tract. The smooth muscle, connective tissue and peritoneum are derived from splanchnopleuric mesoderm .

DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT OESOPHGUS The oesophagus is developed from the foregut, Between the pharynx and the stomach. At first, it is short , but elongates with the; Formation of the neck, Descent of the diaphragm, Enlargement of the pleural cavities. The musculature is derived from mesenchyme surrounding the foregut. Around the upper 2/3 rd the mesenchyme forms striated muscle. Around lower 1/3 rd the mesenchyme forms smooth muscle (as over the rest of the gut).

At first, it is seen as a fusiform dilatation of the foregut, just distal to the oesophagus. Dorsal mesogastrium attaches the dorsal border of the stomach to the posterior abdominal wall. Ventral mesogastrium attaches the ventral border of the stomach to the septum transversum . The liver and the diaphragm are formed in the substance of the septum transversum. The ventral mesogastrium now passes from the stomach to the liver and from the liver to the diaphragm and anterior abdominal wall. Lesser omentum = ventral mesogastrium between liver and stomach. Coronary ligament = ventral mesogastrium between liver and diaphragm. Falciform ligament = ventral mesogastrium between liver and anterior abdominal wall. DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT STOMACH

The dorsal mesogastrium is divided by the development of the spleen into; Gastrosplenic ligament = between stomach and spleen. Lienorenal ligament = between spleen and posterior abdominal wall. The stomach undergoes differential growth resulting in considerable alteration in its shape and orientation: The original left surface becomes anterior surface. The original right surface becomes the posterior surface. The original ventral border comes to face upward and to the left = lesser curvature . The original dorsal border comes to face downwards and to the left = greater curvature . DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT STOMACH (continue)

The superior (1 st ) part + the upper half of the descending (2 nd ) part of the duodenum are derived from the foregut . The rest of the duodenum develops from the most proximal part of the midgut. Mesoduodenum is a mesentery attaches the duodenum to the posterior abdominal wall. Mesoduodenum then fuses with the peritoneum of the posterior abdominal wall, with the result that most of the duodenum becomes retroperitoneal. Mesoduodenum persists in relation to a small part of the duodenum adjacent to the pylorus. (duodenal cap/ radiograph). Branches of Coeliac artery supply the proximal part of the duodenum. Branches of Superior mesenteric artery supply the distal part of the duodenum. DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT DUODENUM

The jejunum and most of the ileum are derived from the prearterial segment of the midgut loop. The terminal portion of the ileum is derived from the postarterial segment proximal to the caecal bud. DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT JEJUNUM AND ILEUM

CAECUM AND APPENDIX Caecal bud is derived from the postarterial segment of the midgut. The caecum and the appendix are formed by the enlargement of this bud. The proximal part of the caecal bud grows rapi dly to form the caecum. The distal part of the caecal bud remains narrow and forms the appendix. The appendix arises from the apex of the caecum . The lateral (right) wall of the caecum grows much more rapidly than the medial (left) wall, The point of attachment of the appendix with caecum comes to lie on the medial side. DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT

Ascending colon develops from the postarterial segment of the midgut loop distal to the caecal bud. Transverse colon; The right 2/3 rd develop from the postarterial segment of the midgut loop. The left 1/3 rd arises from the hindgut. The right 2/3 rd are supplied by the superior mesenteric artery. The left 1/3 rd is supplied by the inferior mesenteric artery. Descending colon develops from the hindgut. The rectum is derived from the primitive rectum (dorsal subdivision of the cloaca). Anal canal is formed partly from the endoderm of the primitive rectum, And partly from the ectoderm of the anal pit (proctodaeum). Pectinate line = the line of junction of the endodermal and ectodermal parts of the anal canal is represented by the anal valves. DERIVATION OF INDIVIDUAL PARTS OF THE ALIMENTARY TRACT

ROTATION OF THE GUT After its formation , the midgut loop lies outside the abdominal cavity of the embryo (in a part of the extra-embryonic coelom that persists near the umbilicus) . The loop has a prearterial (proximal) segment and postarterial (distal) segment. Initially , the loop lies in the sagittal plane , its proximal segment being cranial and ventral to the distal segment. The midgut loop now undergoes rotation. This rotation plays a very important part in establishing the definitive relationships of various part of the intestine.

The loop undergoes Anticlockwise rotation by 90°, (the prearterial segment lies on the right, and the postarterial segment lies on the left). The prearterial segment undergoes great increase in length to form the coils of the jejunum and ileum. (loops still out side the abdominal cavity, to the right of the distal limb). The coils of the jejunum and ileum (proximal segment) return to the abdominal cavity. As they do so , the midgut loop undergoes further anticlockwise rotation . Jejunum and ileum pass behind the SMA into the left half of the abdominal cavity. Duodenum comes to lie behind the artery. The jejunum and ileum occupy the posterior and left part of the abdominal cavity. ROTATION OF THE GUT STEPS OF ROTATION (viewed from ventral side)

Finally , the postarterial segment of the midgut loop returns to the abdominal cavity. It also rotates in an anticlockwise direction. With the result the transverse colon lies anterior to the SMA , and the caecum comes to lie on the right side. At this stage the caecum lies below the liver , and an ascending colon cannot be demarcated . Gradually , the caecum descends to the right iliac fossa, and the ascending, transverse and descending parts of the colon become distinct. ROTATION OF THE GUT STEPS OF ROTATION (viewed from ventral side)

FIXATION OF THE GUT At first all parts of the small and large intestines have a mesentery by which they are suspended from the posterior abdominal wall. After the completion of rotation of the gut , the duodenum , the ascending colon , the descending colon and the rectum become retroperitoneal (by fusion of their mesenteries with the posterior abdominal wall). The original mesentery persists as; The mesentery of small intestine , The transverse mesocolon, The pelvic mesocolon.

ANOMALIES OF THE GUT CONGENITAL OBSTRUCTION. ABNORMAL COMMUNICATION OR FISTULA. DUPLICATION. DIVERTICULA . ERRORS OF ROTATION. ERRORS OF FIXATION . SITUS INVERSUS.

COGENITAL OBSTRUCTION (SKIP) This may be due to a variety of causes. Atresia (continuity of the lumen is interfered by, a segment of the gut may be missing, replaced by fibrous tissue, or by a septum block the lumen) . Stenosis (abnormal narrowing) . Non-development of nerve plexuses in the wall of a part of the intestinal tract. (megacolon or Hirschsprung’s disease) Abnormal thickening of muscular wall . (congenital pyloric stenosis) External pressure by abnormal band or abnormal blood vessels . (bands seen in relation to the duodenum or compressed by annular pancreas) Imperforate anus . (caused by stenosis or atresia of the lower part of the rectum or anal canal).

ABNORMAL COMMUNICATION OR FISTULAE (SKIP) Fistula is an abnormal communication with other cavities or with the surface of the body. Fistulae are most frequently seen in relation to the oesophagus and the rectum and usually associated with atresia of the normal passage. Tracheo-oesophageal fistula . Incomplete septation of the cloaca . The rectum may communicate with the ; Urinary bladder. Urethra. Vagina. Or open onto the perineum at an abnormal site. These conditions are associated with imperforate anus.

DUPLICATION Varying length of the intestinal tract may be duplicated. The duplication may form only a small cyst, Or may be considerable length . It may or may not communicate with the rest of the intestine.

DIVERTICULA Diverticula may arise from any part of the gut. Diverticula are most common in and near the duodenum . (pylorus, fundus of stomach) Meckel’s diverticulum ( diverticulum ilei ); Persistence of vitello-intestinal duct. It is of surgical importance. May contain pancreatic tissue. May contain gastric mucosa. May give rise to fecal fistula, umbilical sinus, cyst (enterocystoma or vitelline cyst), fibrous band or growths.

ERRORS OF ROTATION Non-rotation of the midgut loop . (small intestine lies towards the right side of the abdominal cavity, and the large intestine towards the left). Reversed rotation . (the transverse colon crosses behind the SMA and the duodenum crosses in front of it). Non-return of umbilical hernia ; Omphalocoele or exomphalos (herniated parts are covered only by omentum). Congenital umbilical hernia (muscle layer and skin are absent in the region of the umbilicus, creating a defect).

ERRORS OF FIXATION Volvulus; where parts of intestine, that are normally retroperitoneal, may have mesentery. Adhesion; where parts of intestine, which normally, have a mesentery, may be fixed by abnormal peritoneal attachment. Sub-hepatic caecum, or may descend only to the lumbar region. Alternatively, it may descend into the pelvis.

SITUS INVERSUS All the abdominal and thoracic viscera are laterally transposed. All parts normally on the right side are seen on the left side, and vice versa. For example, the appendix and duodenum lie on the left side and the stomach on the right side.

TIMETABLE OF SOME EVENTS DESCRIBED ABOVE Age Developmental events 16 days Allontoic diverticulum starts appearing 3 weeks Gut begins to acquire tubular form because of head and tail foldings. At the end of 3 rd week the buccopharyngeal membrane ruptures. 4 weeks The fusiform shape of the stomach becomes visible. 5 weeks Stomach rotates and dilates. Intestinal loop begins to form. Caecal bud can be identified. 6 weeks Intestinal loop is well formed. Urorectal septum starts dividing the cloaca. Allantois and appendix become clearly visible. Stomach complete its rotation. 7 weeks Septation of cloaca into rectum and urogenital sinus is completed. Intestinal loop herniates out of the abdominal cavity. 8 weeks Intestinal loop rotates counterclockwise. 9 weeks Anal membrane breaks down. 3 months Head and tail foldings are completed. Herniated coils of intestine return to the abdominal cavity.

12. git[1]

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