development of the gi tract

development of the gut tube

1. endoderm folds and elevates. splachnic mesoderm comes from the LPM

2. endoderm fold around ventrally. endoderm fuses ventrally

what is the primitive gut tube divided into

• foregut

• mid gut

• hind gut

which membrane is in the oral and anal region

oral- buccopharyngeal

anal- cloacal

layers of the gut- what do endoderm and splanphic mesoderm form

endoderm- epithelial cell adjacent to lumen

splanchnic mesoderm- connective tissue, smooth muscle and mesothelial layers (mesentery and peritoneum)

failure to perforate membranes

• persistent buccopharyngeal membrane in human- feeding is impossible, can perforate to treat

• persistent cloacal membrane in cow, must be perforated

foregut

• forms oesophagus, stomach, liver, gallbladder, bile ducts, pancreas, proximal duodenum

• must rotate and twist to elaborate into these structures

rotation of the foregut

what does the greater and lesser omentum contain

what can developmental defects of the omentum causse

• greater omentum contains gastrophrenic, gastrocolic, gastrosplenic ligaments

• lesser omentum contains hepatophrenic and hepatooesophageal ligaments

• developmental defects of the omentum can cause internal hernia

development of the spleen

• what type of cells does it form from and what is it also known as

• what is it in foetal stages

• forms from mesodermal cells in the dorsal mesentery- also called mesogastrium

• is a major haemopoetic tissue in foetal stages

what does the midgut consist of

• distal half of duodenum

• jejunum

• ileum

• cecum

• ascending colon

• proximal half of transverse colon

the equine caecum

• hindgut fermenter

• continues to grow in embryo

• in adults roughly 1m in length with a 30L capacity

species differences in ascending colon

• horse

• ruminant and pig

• horse- ascending colon continues to grow forming loop connected by remnants of dorsal mesentery

• ruminant and pig- ascending colon continues to grow forming spiral

rotation of the stomach

• what does it begin as

• what attaches it to the body walls

• what lies on the ventral and dorsal surfaces

• what happens first and what does it do the the other structures

• at the end what is the ventral and dorsal mesentery attachment

• begins as a spindle shaped tube

• ventral and dorsal mesenteries attach tube to body walls

• branches of the left and right vagus nerves lie on ventral and dorsal surfaces

• 90 degree clockwise rotation along the longitudinal axis then occurs

• this alters the course of the vagus nerve branches: right now innervates the ventral surface and left lies on the dorsal aspect

• as the stomach rotates along the ventral dorsal axis, the caudal end is displaced towards the right and cephalic end to the left

• ventral and dorsal mesenteries also displaced to right and left respectively

• lesser curvature is the ventral mesentery attachment

• greater curvature is the dorsal mesentery attachment

what does the ventral and dorsal mesentery give rise to

• ventral to falciform ligament which secures the liver ventrally

• also the lesser omentum which connects the liver and stomach and proximal duodenum

• dorsal mesentery gives rise to the greater omentum which is an apron like fold of mesentery that attaches to the greater curvature of the stomach and drapes over the small intestine

final arrangement of the biliary and pancreatic ducts step one

• what arise as outgrowths of the foregut

• what extend ventrally and dorsally

• what do liver buds outgrow into

• what happens to the proximal duodenum

1. abdominal accessory digestive organs arise as outgrowths of foregut prior to stomach and duodenal rotation

2. dorsal pancreatic duct extends dorsally

3. ventral pancreatic duct, gallbladder and liver buds extend ventrally

4. liver buds develoop as an outgrowth of the foregut into the septum transversum

5. proximal duodenum rotates clockwise

part 2

• what happens to the dorsal and ventral pancreatic buds and ducts

• then where does it go and exttnd

• what does the ventral pancreatic bud form

• where does the main and accessory pancreatic duct drain into and how

• what does the pancreatic duct join to and why

• how does the common bile duct form

• what direction does the common bile duct wrap around the duodenum

• what does it join with

• dorsal and ventral pancreatic buds and their ducts fuse

• the now formed complete pancreas nestles in the c curve of the duodenum and extends toward the left side of the body

• the uncinate process is the portion derived from the ventral pancreatic bud

• main pancreatic duct empties into the duodenum via major papilla

• accessory drains via minor

• the bile and pancreatic ducts join to drain bile and pancreatic juices at the major papilla

• liver drains bile into hepatic duct

• hepatic and cystic ducts merge to form the common bile duct

• due to the rotation of foregut and displacement of papillae the common bile duct wraps posteriorly around the duodenum

• it joins with the main pancreatic duct to drain bile at the major papilla to the duodenum

what are the four steps in midgut development

1. herniation and counterclockwise rotation: counterclockwise rotation about the axis of the superior mesenteric artery. midgut comprises the primary intestinal loop which elongates so rapidly that it temporarily outgrows the abdominal cavity, herniating into the umbilical cord

2. elongation: the caecum is now on the right hand side, as it has undergone a 270 degree rotation now (in pigs this is 450) . the rotated primary loop elongates rapidly creating the jejunoileal loops

3. retraction and counterclockwise rotation: loops retract from the umbilical cord into the abdominal cavity, and the intestines rotate 180 degrees counterclockwise as they continue to elongate. caecum now in upper right.

4. final positions: large intestine frames the small intestine. caecum now in the lower right

3 steps of hindgut development

• what closes off the cloaca

• what forms between the allantois and the hindgut

• what grows towards the cloacal membrane and what does it separate

• what happens to the cloacal membrane

• what separates the urogenital and anal membranes

1. cloaca: initially the cloaca is the common end of the hindgut and urogenital tract. the cloacal membrane (endoderm and ectoderm) close off the cloaca. the urorectal septum forms between the allantois and hindgut

2. urorectal septum: urorectal septum grows towards the cloacal membrane. it now separates the primitive urogenital sinus which appears as an anterior swelling. cloacal membrane ruptures before the urorectal septum reaches

3. perineal body: represents the tup of the urorectal septum, separates the urogenital and anal membranes. now completely divides the urinary and digestive tracts so that the urinary bladder lies anteriorly and anorectal canal remains posteirorly