Embryology of GI tract Flashcards
How is gut tube formed
Ventral body wall closure and embryo folding
Endoderm is incorporated more and more into the body of the embryo
With what does the midgut communicate with
Midgut communicates with the yolk sac via the vitelline duct and will eventually become long and narrow
What forms the inner lining of the GI tract and associate organs
Endoderm
What parts of the mesoderm form the rest of the GI tract
Somites
Intermediate mesoderm
Lateral plate mesoderm
What does the lateral plate mesoderm form in terms of GI system
Parietal mesoderm - parietal serosa of the peritoneum
Visceral mesoderm - visceral serosa of the peritoneum
Layers of the GI tract wall except inner epithelial lining
What does the intermediate mesoderm form
Kidneys and gonads
What does the para-axial mesoderm form
Somites, which go on to form dorsal dermatomes, myotomes, sclerotomes
What is the cranial end of gut tube closed by
oropharyngeal membrane
What is the oropharyngeal membrane
Separates the primitive oral cavity (formed from ectoderm) from the pharyngeal part of the foregut (formed from endoderm)
Endoderm and ectoderm are in direct contact with each other
By 4th week, membrane ruptures, allowing communication between oral cavity and gut tube
What is the cloacal membrane
Separates the upper part of anal canal (hindgut), derived from endoderm, from the lower anal canal formed from ectoderm (called proctodeum)
The endoderm and ectoderm here are in direct contact with each other
Breaks down in 7th week to create opening of the anus
Why is allantois important
With folding it gets incorporated into the body of the embryo and will form the urogenital sinus
Distal part remains in the connecting stalk
By week 5 - yolk sac, connecting sac and allantois are restricted to the umbilical region
Explain what the pharyngeal pouches form
Epithelial lining of tympanic cavity and auditory tube (pharyngeal pouch 1)
Part of tonsils (pharyngeal pouch 2)
Parathyroid glands (pharyngeal pouches 3 & 4)
Part of thymus gland (pharyngeal pouch 3)
What is the mesentery derived from and were do the extend from in an embryo
Derived from mesoderm
Dorsal mesentery - extends from terminal part of the oesophagus down to the cloacal region of the hindgut
Ventral mesentery - exists from the terminal oesophagus, stomach and upper part of duodenum only
How does oesophagus form
Respiratory diverticulum on the ventral wall of foregut
Tracheoesophageal septum partitions this region of foregut into trachea ventrally and oesophagus dorsally
Rapidly lengthens with descent of heart and lungs
Muscular coat of oesophagus is derived from visceral mesoderm
What is oesophageal atresia
Oesophagus ends as a blind ended sac
What is tracheoesophageal fistula
Distal part is connected to the trachea
How does the stomach form
Dilation of foregut
Oesophageal lengthening is important for positioning the stomach in the abdomen
What is diaphragmatic hernias
Failure of the oesophagus to lengthen can cause diaphragmatic hernias
Explain the longitudinal rotation of the stomach
90 degrees clockwise
During this rotation, posterior wall grows faster forming the greater and lesser curvatures
Explain the anteroposterior rotation of stomach
the cranial and caudal ends of the stomach were originally in midline
rotates so that the caudal/pyloric part moves right and upwards, and the cephalic or cardiac portion moves left and slightly downward
How is the lesser omentum derived
Liver cords grow into the septum transversum to form the liver
This causes the ventral mesentery to become the falciform ligament and lesser omentum
How does greater omentum form
Dorsal mesentery bulges inferiorly from greater curvature of the stomach
Grows downwards, forming a double-layered sac extending over the transverse colon and small intestine like an apron
How is the duodenum formed
Distal end of foregut and proximal part of midgut = duodenum
Stomach rotation with rapid growth of the pancreas causes the duodenum to swing from its midline position to the right side and take on a C-shape
How does liver form
Liver primordium appears as an outgrowth of the endoderm epithelium at the distal end of the foregut
= hepatic diverticulum or liver bud
Rapidly proliferating cells that penetrate the septum transversum
How does bile duct form
the connection between the hepatic diverticulum and the foregut (specifically the duodenum) narrows to become the bile duct
Why does bile duct travel to the posterior part of duodenum
Due to the rotation and movement of the duodenum, the bile duct entrance shifts from its initial anterior position to a posterior one, hence why the bile duct travels behind the proximal part of the duodenum
What are some gall bladder abnormalities
Can get duplication of the gallbladder
Can get accessory pancreatic ducts
Can get abnormalities that involve ducts not forming a lumen
How is the pancreas formed
Formed by 2 buds originating from the endoderm of the duodenum
Dorsal pancreatic bud in dorsal mesentery
Ventral pancreatic bud is close to bile duct
As the duodenum rotates right and becomes C-shaped, the ventral bud moves dorsally
Ventral bud becomes to lie immediately below and behind the dorsal bud
Ventral bud = uncinate process
Dorsal bud = head, neck, body, tail
Where does main pancreatic duct form from
Distal part of the dorsal pancreatic duct
Entire ventral pancreatic duct merge to form the main pancreatic duct
(=major duodenal papilla)
What does the proximal part of dorsal pancreatic duct form?
Accessory pancreatic duct
=minor duodenal papilla
