Lo 4

Learning Objective 4 Outline the embryological development of the GIT Primordial Gut • This is present by the 4th week, but is non functional • It is closed off o Oropharyngeal membrane closes it off at the cranial end o Cloacal membrane at its caudal end • Epithelium and glands are almost entirely derived from endoderm • Cranial epithelium o Derived from the ectoderm of the stomodeum (Primordial mouth) o Derived from the ectoderm of the proctodeum (Anal pit) • Other layers of the digestive tract including muscular tissue and connective tissue derived from splanchnic mesenchyme • Foregut: Pharynx – Duodenum (Including Liver and gall bladders). • Midgut: Small Intestine (most of duoenum) – 2/3 Transverse Colon • Hindgut: Final 1/3 Transverse Colon – Anus Definitive Gut • 5-6th week o Epithelium proliferates and obliterates the lumen o This is then vacuolated, through apoptosis, and recanalised o This is done y the 8th week Mesentery • Dorsal mesentery is preserved along the length of the gut o Blood vessels, Lymphatics and nerves are found here • Ventral Mesentery mostly disappears, which allows for lengthening and folding of the gut o It is preserved in the foregut and the cloacal regions o It gives rise to 2 structures: Lesser omentum (Fold of peritoneum) Falciform ligament

FOREGUT Oesophagus • Partioning of trachea from the oesophagus (See respiratory embryology) • The oesophagus is initially short but elongates rapidly • Undergoes obliteration and recanalisation of its lumen • The upper 1/3 is comprised of striated muscle in its muscularis externa, allows for volutary control of swallowing • Lower 2/3 has smooth muscle • Both types of muscle are supplied by the Vagus nerve Stomach • Middle of 4th week dilation in foregut indicates primordial stomach • Following 2 weeks: The Dorsal border grows faster then ventral border o This results in the greater curvature Stomach rotation As it enlarges it undergoes a 90o rotation which has a number of affects: o Lesser curvature moves to the right o Greater curvature to the left o Original left side becomes the ventral surface o Original right side becomes the dorsal surface o Cranial region moves left and slightly inferiorly o Caudal region moves right and superiorly Mesenteries of the Stomach o Dorsal mesogastrium suspends it from the dorsal wall of the abdominal cavity o Ventral mesogastrium attaches it to the duodenum and liver and ventral abdominal wall Omental Bursa • Several small cavities develop in the dorsal mesogastrium o These coalesce to form a single cavity the Omental Bursa • The omental bursa lies between the stomach and the posterior abdominal wall o As the stomach rotates it pulls the dorsal mesogastrium to the left, enlarging the bursa

•

•

•

•

The developing diaphragm cuts the bursa in two o The top half becomes the infracardiac bursa o The bottom part persists as the superior recess of the omental bursa The Omental Bursa expands with the enlarging stomach and develops the Inferior recess of the Omental Bursa o This is found between the layers of the dorsal mesogastrium and is called the Greater Omentum The greater omentum overhangs over the intestines o The two layers of this membrane fuse around he intestines o As this occurs the inferior recess of the omental bursa disapppears The omental foramen is the opening through which the Omental Bursa communicates with the peritoneal cavity

Duodenum • There are two part to the developind duodenum o Cranial part develops from the forgut o Caudal part from the midgut o These join distal to the bile duct • Undergoes obliteration and recanalisation of its lumen Pancreas • It is derived from endoderm • It develops from 2 buds: Dorsal Pancreatic Bud Ventral Pancreatic Bud • The pancreas is mostly derived from the Dorsal Bud o It forms the body and tail of the pancreas o It also forms the distal portion of the main pancreatic duct o Part of the dorsal duct often persists as an accessory pancreatic duct • The ventral bud develops near the bile duct o The duodenum rotates to the right and forms a ‘C’ shape o The carries the ventral bud and bile duct dorsally o The 2 buds now lie next to each other and later fuse o It forms the uncinate process, head of the pancreas, as well as the proximal part of the main pancreatic duct

MIDGUT Physiological Umbilical Hernia • This takes place between the 6-10th week • All the midgut is supplied by the superior mesenteric artery • Midgut loop is formed when the midgut elongates • It projects into the imbilical cord and forms the ‘Physiological Umbilical Hernia’ • This forms as there is not enough room in the abdomen for the rapidly growing midgut Rotation of the Midgut • Around the 6th week while in the umbilical cord the midgut loop rotates 90o counterclockwise • This takes place around the superior mesenteric artery • During the 10th week the intestines return to the abdomen o When returning it undergoes a further 180o rotation counterclockwise o The mesentery and blood vessels undergo the same rotation • Complications: The midgut could untwist Could have excessive rotation causing a Volvulus

•

• •

HINDGUT All the hindgut is supplied by inferior mesenteric artery o The junction between midgut and hindgut is indicated by the change in blood supply to the transverse colon(superior to inferior mesenteric artery) The descending colon becomes retroperitoneal The partitioning of the cloaca has been covered in the Urinary System embryology

ABMNORMALITIES Tracheoesophageal Fistula (TEF) • Occurs in about 1:3000 to 1:4500 (Mainly males) • It is an abnormal passage between the trachea and oesophagus • This allows saliva and gastric secretions to be aspirated into the lung • It results because of an incomplete division of the cranial part of the foregut into respiratory and oesophageal parts in the 4th week • Surgical procedure involves removing the affected portion Congenital Hypertrophic Pyloric Stenosis • Males 1:150 Females 1:750 • It involves a thickening of the pylorus, resulting in stenosis and obstruction to the food pathway • Stomach becomes distended • Classical sign is projectile vomiting • Surgical relief involves releasing the muscle (Not necessary to cut through into mucosa, just into the muscle layers) Meckel’s Diverticulum • Also known as Ileal Diverticulum • Very common anomaly of the digestive system • Occurs in 2-4% of infants (Male:Female = 5:1) • Often occurs in conjunction with other anomalies • Clinical significance: Can become inflames, mimicking appendicitis Gastric mucosa often produces acid causing ulceration and bleeding • An Ileal diverticulum is a remnant of a proximal portion of the yolk sac o Where the vitteline duct attached to the yolk sac • Appears as a fingerlike pouch (3-6cm) roughly 2 feet from the ileocecal valve • It contains all the layers of the ileum and can contain patches of gastric and pancreatic tissue Congenital Omphalocele

• • • • •

Herniation of the intestines 1:5000 Herniation of liver and intestines 1:10000 It is as a result of failure of the intestines to return to the abdomen cavity (At about the 10th week) The covering of the sac is epithelium of the umbilical cord The abdominal cavity is relatively small as there is no impetus for it to grow

Congenital Megacolon (Hirschsprung disease) • Male:Female 4:1 • Part of the colon is dilated • There absence of autonomic ganglion cells in the portion of the colon distal to the dilation • This is as a result of failure of peristalsis in the distal portion, which prevents movement of intestinal contents o The proximal portion dilates as a result of the obstruction Imperforate Anus and Anorectal Anomalies • 1:5000 (Males more common) • Most develop as a result of abnormal development of the urorectal septum • This results in incomplete separation of the cloaca • Lesions are classified as low or high if the rectum ends superior or inferior to the puborectalis muscle Anal Agenesis (with or without fistula) 1. Anal canale may end blindly 2. May be an ectopic anus 3. Or it may be an anoperineal fistula • The abnormal canal my eneter in the vagina in females, or urethra in males • Most anorectal anomalies are associated with a fistula (A fistula results from incomplete separation of the cloaca Membranous Atresia of the Anus • A thin layer of tissue separates the anal canal from the exterior • This is as a result of the failure of the anal membrane to perforate (Around the 8th week) Anorectal Agenesis (with or without fistula) • This is the most common type of anorectal anomaly (2/3) • The rectum ends superior to the puborectalis muscle

• • •

This is as a result of a failure The rectum ends blindly but there is usually a fistula The fistula develops because there is an incomplete separation of the cloaca by the urorectal septum o To the bladder (rectovesicular fistula) o In males to the urethra (rectourethral fistula) o In females to the vagina (rectovaginal fistula)