Embryology Flashcards
How does the midgut remain connected to the yolk sac
Via the vitelline duct
What does the foregut form
Oesophagus
Stomach
Pancreas
Liver
Gall bladder
Duodenum - proximal to bile duct
What does the midgut form
Duodenum - distal to bile duct
Jejunum
Ileum
Caecum
Ascending colon
Proximal 2/3 of transverse colon
What does the hindgut form
Distal 1/3 of transverse colon
Descending and sigmoid colon
Rectum
Upper anal canal
Internal lining of bladder and urethra
What forms the stroma, muscle, CT and peritoneal components of the gut wall
The visceral mesoderm
What is the blood supply of the primitive gut tube
Foregut - celiac trunk
Midgut - superior mesenteric artery
Hindgut - inferior mesenteric artery
What are mesenteries
Double layers of peritoneum that enclose an organ and connect it to the body wall - neurovascular supply runs in mesentery
What are peritoneal ligaments
Double layers of peritoneum that pass from one organ to another or from an organ to the body wall
What does the dorsal mesentery form in the following regions:
Stomach
Duodenum
Colon
Stomach - forms greater omentum/dorsal mesogastrium
Duodenum - forms dorsal mesoduodenum
Colon - forms dorsal mesocolon
What does the ventral mesentery divide into and what causes this divide
Liver growth cause the ventral mesentery to divide into:
Lesser omentum
Falciform ligament - liver to ventral body wall
Describe what happens during the oesophagus development
In 4th week, respiratory diverticulum/lung bud appears in ventral wall of foregut
Tracheoesophageal septum then separates diverticulum from dorsal part of foregut to divide the foregut into respiratory primordium and oesophagus
Oesophagus develops rapidly as heart and lungs descend
Describe rotation of the stomach and how this affects nerve innervation
Stomach rotates 90o clockwise around its longitudinal axis -> left side faces anteriorly and right side faces posteriorly
Original posterior wall grows faster during rotation so have formation of greater and lesser curvatures
Causes left vagus nerve to innervate anterior wall and right vagus nerve to innervate posterior wall
How is the lesser sac formed
Rotation of stomach around longitudinal axis pulls dorsal mesogastrium left to create lesser sac behind the stomach
Rotation also pulls ventral mesogastrium right
Describe the formation of the spleen - where it develops and where it ends up
Spleen primordium appears as mesodermal proliferation between two leaves of dorsal mesogastrium
As rotation continues, spleen is pushed left and the dorsal mesogastrium between the spleen and dorsal midline fuses with peritoneum of posterior abdominal wall
Spleen is conencted to body wall by lienorenal ligament and to the stomach by gastrolienal ligament
How is the greater omentum formed
Stomach rotation around anteroposterior axis causes dorsal mesogatrium to bulge down and continue to grow downwards to form double-layered sac extending over the transverse colon and small intestinal loops
Why is there a bare area of the liver on its superior surface
Mesoderm on liver surface differentiates into visceral peritoneum expect on cranial surface where liver remains in contact with septum transversum - forms central tendon of the diaphram
What is the accessory pancreatic duct
A duct which remains if the proximal part of the dorsal pancreatic duct does not obliterate
Where do the main and accessory pancreatic ducts enter the duodenum
Main duct - major papilla
Accessory duct - minor papilla
What does the cephalic limb of the primary intestinal loop form
Distal duodenum
Jejunum
Part of the ileum
What does the caudal limb of the primary intesrinal loop form
Distal ileum
Caecum
Appendix
Ascending colon
Proximal 2/3 of transverse colon
Describe physiological herniation
During 6th week, primary intestinal loop enters the extraembyronic cavity in the unbilical cord due to the abdominal cavity becoming too small to contain the intestinal loops
Describe rotation of the midgut
Primary intestinal loop 90o counterclockwise around the axis formed by superior mesenteric artery - occurs three times
During each rotation, elongation continues
Describe how the herniated loops return into the abdominal cavity
During 10th week, herniated loops return to abdominal cavity
Proximal jejunum re-enters first -> lies on left side
Describe the development and movement of the cecal bud
Cecal bud forms in 6th week as a small dilation of caudal limb
It is the last part of the gut to re-enter the cavity
Initally lies below the right lobe of the liver before descending into the right iliac fossa to place the ascending colon and hepatic flexure on the right side of the abdominal cavity
Distal end of cecal bud forms appendix
What happens to the mesenteries of the ascending and descending colons after they find their final positions
The mesenteries press against the peritoneum of the posterior wall and then the layers fuse
This results in the ascending and descending colons becoming retroperitoneal
What happens to the transverse mesocolon during development
Transverse mesocolon fuses with posterior wall of greater omentum but retains its motility
Describe how the mesentery of the jejunoileal loops changes its attachment site
At the start, jejunoileal loop mesentery is continuous with mesentery of ascending colon
Once ascending colon mesentery fuses with posterior wall, mesentery of jejunoileal loop forms a new line of attachment to keep it mobile
What is the cloaca
Endoderm-lined cavity covered at its ventral boundary by surface ectoderm
Boundary between endoderm and ectoderm forms cloacal membrane
What is the anal canal derived from and what divides the anal canal
Upper 2/3 of anal canal is from endoderm of hindgut
Lower 1/3 of canal is from ectoderm
Anal canal is divided by the pectinate line
Describe the cranial part of the anal canal - neurovasculature, lymph and epithelium
Cranial part of anal canal:
Superior rectal artery from inferior mesenteric artery
Innervated by visceral nerves - S2,3,4 pelvic parasympathetic
Columnar Epithelium
Internal iliac nodes
Describe the caudal part of the anal canal - neurovasculature, lymph and epithelium
Caudal part of the anal canal:
Inferior rectal arteries from internal pudendal
Somatic nerves - pudendal nerve (S2,3,4)
Stratified squamous epithelium
Superficial inguinal nodes
