Short Notes On Git

Gastrointestinal Tract

Embryology of GIT In the first 3 Weeks of Intrauterine life the embryo forms the Trileminar germ disc which eventually undergoes a folding process This folding process takes place in 4th week and it is Cephalo-caudal with a concomitant lateral folding

These folding results in the formation of Gut tube by incorporation of part of Yolk Sac, lined by primitive epithelial tissue called endoderm The lamina propria, muscularis mucosa, submucosal, muscularis externa and serosa is derived from Mesoderm

The Primitive Gut tube is divided into three parts according to separate arterial supply

1. Foregut: (Coeliac trunk) Gives rise to Esophagus, Stomach, Duodenum 1st and 2nd parts, Liver, biliary tract and gallbladder, pancreas, pharyngeal pouches, Lungs and Thyroid 2. Midgut: (Superior Mesenteric Artery) Gives rise to Duodenum (2nd, 3rd and 4th parts), Jejunum, Ileum, Caecum, Appendix, Ascending colon and Transverse colon (proximal 2/3rd ) 3. Hindgut: (Inferior Mesenteric Artery) Gives rise to Transvers colon (Lateral 1/3rd ), Descending colon, Sigmoid colon, Rectum, Anal canal

Formation of Mesenteries: (5th Week) Ventral Mesentery: Extends from Terminal part of Esophagus up to the beginning of the duodenum. It differentiates from Septum Tranversum

Dorsal Mesentery: Differentiates dorsally between the posterior abdominal wall and primitive intestine. Its suspends the entire intestine in the abdominal cavity

Initially whole of the intestine is "Intraperitoneal", but with subsequent rotations some of its parts become "Secondarily retroperitonealized"

Formation of Liver Liver originates as a diverticulum from terminal Foregut endodermal lining. It then invaginate the Septum Tranversum. The terminal part develops into liver and gallbladder and the proximal part into biliary duct system The portion of ventral mesentery between liver and ventral body wall becomes Falciform ligament while the portion between liver and stomach becomes Lesser Omentum

Formation of Respiratory Bud The respiratory bud differentiates from the ventral part of the foregut This bud will form the whole respiratory system (Trachea, Primary, secondary and Tertiary bronchi) after separating completely from the foregut. Malformations may give rise to Tracheo-esophageal fistulae

Formation of Pancreas Pancreas originates as two buds (Endodermal lining of gut tube) Ventral bud: (Ventral mesentery) Together with the liver will form Uncinate process and caudal part of head Dorsal bud: (Dorsal mesentery) will form rest of body and tail of Pancreas Ventral bud eventually grows, rotates and fuses with the rest of the Pancreas

Rotation of the Gut Rotation occur around two axes

1. Around its own long Axis: (90°) During 11th week the Foregut (Stomach) rotates 90 degrees around its own long axes (Clock wise if viewed from above) so that the dorsal structures rotate to the left (Spleen and pancreas)and the ventral structures rotate to the right. After the pancreas and duodenum becomes secondarily retroperitonealized the lesser sac (Omental Bursa) is formed. The communication between lesser and Greater Sac (General peritoneal cavity) is Epiploic foramen (Foramen of Winslow)

2. Around Superior Mesenteric Artery (270°) The intestinal loop grows in the umbilical cord outside the abdominal cavity at the end of 5th week (Physiological herniation) Midgut rotates 270° Counterclockwise around superior Mesenteric artery as viewed from the front. This results Jejunum on the left and Ilium and caecum on the right Primitive intestine still connected to the Yolk sac via the Vitelline duct (Lack of regression of the duct ? Mackles diverticulum)

The hindgut opens in the Cloaca The posterior part of Cloaca will give rise to ano-rectal canal, while the anterior part will develop into uro-genital sinus (Urinary bladder) Improper division of these two parts might result in Uro-rectal Fistulas The allantois will develop into Urachus which will ultimately be obliterated and in the adult is represented by Median Umbilical ligament