1.2.3 GI Development Flashcards
Gut tube begins to appear in the ____________________ – formed from the endoderm lining of the yolk sac which is enveloped by the developing coelom (and partially incorporated into the gut tube)
o This is a result of the cephalo-caudal and lateral body folding of the embryo
- The gut tube is a blind tube, closed at both ends by the ________________ membrane and ________ membranes
- The gut tube is divided into the foregut, midgut and hindgut in a cranial to caudal position – based on the arterial supply
o The midgut remains connected to the yolk sac by the narrow vitelline duct
- The _____________ of the gut tube gives rise to the mucosal epithelium, mucosal and submucosal glands of the gastrointestinal tract
o The ____________________ gives rise to the muscle layer surrounding the gut tube
4th week of development;
buccopharyngeal; cloacal;
endoderm;
splanchnopleuric mesoderm
Development of the peritoneum – serous membrane lining the cavity of the abdomen and covering the abdominal organs
- Parietal peritoneum lines the body wall – derived from the _________________
- Visceral peritoneum covers the abdominal and some of the pelvic viscera – derived from the ________________
o These layers are continuous with each other as the
dorsal mesentery
o The cavity between both layers is known as the
peritoneal cavity (derived from the _________________)
- The peritoneum is covered by a single layer of _____________ cells which secretes fluid that lubricates between the two layers
- As the peritoneum envelopes the abdominal organs, it folds over itself to form ligaments, mesenteries and omenta
o Mesenteries – folds connecting an organ to the abdominal wall
o Apart from suspending portions of the gut tube and derivatives from the dorsal and ventral body wall, the mesenteries also provide a pathway for blood vessels, nerves and lymphatics to the organs - Dorsal mesentery suspends the caudal foregut (terminal oesophagus), midgut and hindgut (cloaca) to the posterior abdominal wall
- Ventral mesentery (exists only in the foregut) – derived from the _______________ (block of mesoderm connecting the liver to the central tendon of the diaphragm)
o As the liver grows into the septum transversum, it divides it into the _________________ and the ventral mesogastrium (lesser omentum)
somatopleuric mesoderm;
splanchnopleuric mesoderm;
intraembryonic coelom;
mesothelial;
septum transversum;
falciform ligament
Development of oesophagus and trachea
- During the 4th week of development, the foregut divides into a respiratory primordium and oesophagus separated by a ___________________
o The oesophagus lengthens during development as the heart and lungs descend
- By the end of the 4th week, the respiratory diverticulum and the oesophagus are separated by the tracheoesophageal septum
o Failure of development of this septum results in a _________________ (abnormal connection between the trachea and oesophagus)
§ New-borns present with copious salivation associated with choking, coughing, vomiting, cyanosis (with the onset of feeding)
tracheoesophageal septum;
tracheoesophageal fistula;
Stomach – begins as the fusiform dilation at the 4th week of development
- Enlargement of the mesentery and adjacent organs result in
o Enlargement of the ____________ of the stomach more than the ventral border
o Rotation of the stomach 90° _______________ around the longitudinal axis and around the anteriorposterior axis
- The stomach assumes its final position with its long axis in line with the long axis of the body
- Lesser omentum – the ventral mesentery of the stomach connecting the stomach to the liver
o As the stomach rotates, so does the lesser omentum, resulting in the formation of a sac – lesser sac/omental bursa ____________ to the stomach - Greater omentum – formed by the dorsal mesentery of the stomach
o The dorsal mesogastrium bulges downwards to form a double-layered sac like an apron over the transverse colon and small intestines
o These layers fuse and eventually forms a single sheet hanging from the greater curvature of the stomach
o The posterior layer fuses with the _________________
dorsal border;
clockwise;
posterior;
mesentery of the transverse colon
Spleen – develops during the 5th week from the __________________ between the layers of the dorsal mesogastrium
- The final position of the spleen is contributed to by the rotation of the stomach and its related mesenteries
- It is connected to the greater curvature of the stomach via the gastrosplenic (gastrolienal) ligament and the left kidney via the splenorenal (lienorenal) ligament
- Functions of the spleen
o Haematopoietic centre in the foetus
o Potential for extra-medullary haematopoiesis in adults
o Part of reticuloendothelial system in adults
mesenchymal cells
original ventral border of stomach: movement, new structure?
right; lesser curvature
original dorsal border of stomach: movement, new structure?
left; greater curvature
original left side of stomach: movement?
ventral surface (left vagus nerve supplies the anterior wall)
original right side of stomach: movement?
dorsal surface; right vagus nerve supplies posterior wall
original cranial end of stomach: movement, new structure?
left, inferiorly; cardic end
original caudal end of stomach: movement, new structure?
right, superiorly;
pyloric end
Duodenum – represents the transition from foregut to midgut, with the junction distal to the origin of the liver bud
- As the duodenum develops and rotates, it swings to the __________side of the abdomen from the midline to form a C-shaped loop and lies against the dorsal body
- Most of the mesoduodenum fuses with the peritoneum resulting in the duodenum becoming secondarily retroperitoneal except for a small area around the pylorus and a portion of the 1st part of the duodenum (_________________)
right ;
duodenal cap
Liver – begins to develop in the 4th week as an outgrowth of the endodermal epithelium at the distal foregut
- Grows into the ________________ and divides it
o Falciform ligament – part connecting the liver to the anterior abdominal wall
o Lesser omentum – part connecting the liver to the stomach
- Hepatocytes derive from the endoderm while ______________ (haematopoietic cells, Kupffer cells and connective tissue) derive from the mesoderm
the distal foregut
- The parenchyma of the liver ____________ within the tissue of the septum transversum and the plexus of the vitelline vessels (to and from the yolk sac) – accounts for the eventual architecture in the adult liver
septum transversum;
sinusoids;
intercalates
Bile duct
- The entrance of the bile duct into the duodenum gradually shifts from the initial position to a more posterior one– thus, bile duct in an adult passes posterior to the ____________________
duodenum and the head of the pancreas
Pancreas
- Begins as 2 endodermal outgrowths (fuse together later) between the layers of mesentery during the 4th week of development
o The ventral and dorsal pancreatic buds develop into the ventral and dorsal mesenteries respectively – caudal to the developing liver
- Dorsal pancreatic bud – forms the majority of the pancreas, carrying with it the _____________
o ____________ part of its pancreatic duct joins the ventral pancreatic duct to form the main pancreatic duct (of Wirsung) – drains into major papilla with bile duct
o Proximal part of dorsal pancreatic duct often persists as _________________, which drains into minor duodenal papilla that lies slightly more proximal - Ventral pancreatic bud – forms the _______________________, carrying with it the ventral pancreatic duct
o Moves dorsally during rotation of the stomach/duodenum to lie ___________ dorsal bud during rotation of the stomach and the duodenum
o Ventral pancreatic duct joins the distal part of the dorsal pancreatic duct to form the main pancreatic duct - In the______ month of development, the parenchyma of the pancreas develops to form the Islets of Langerhans (endocrine pancreas – insulin, glucagon, somatostatin)
o Insulin secretion begins from the 5th month onwards
dorsal pancreatic duct;
Distal;
accessory pancreatic duct (Duct of Santorini);
inferior part of the head and the uncinate process;
below and behind
3rd
Rotation of duodenum and ventral pancreatic duct
- Growth and rotation of the duodenum brings the ventral pancreatic bud towards the dorsal bud, where they subsequently fuse
- Annular pancreas – rare but potentially serious developmental anomaly
o Occasionally, the right and left sides of the ventral bud may migrate in __________ directions – results in a ring of pancreatic tissue around the duodenum (annular pancreas)
o This malformation constricts the duodenum and may even cause complete obstruction
opposite
- During the 4th week of intrauterine life, the embryo undergoes a head fold, a tail fold and 2 lateral folds
o As a result of folding, the _________ is incorporated in the embryo to form the primitive gut tube - The primitive gut tube is made up of endoderm and is surrounded by _______________
o Subdivided into foregut, midgut and hindgut – the midgut is connected to the yolk sac by the ______________.
o The primitive gut tube gives rise to epithelium and glands of the gastrointestinal tracts
o The surrounding layer of mesenchyme gives rise to the muscular wall of the gastrointestinal tract - Initially, the midgut is in broad contact with the mesenchyme of the posterior abdominal wall
- By the 5th week, the midgut is withdrawn in the ventral direction, resulting in a ___________________ thinning out as a suspending sheet of mesoderm – dorsal mesentery of the midgut
o The midgut is suspended from the posterior abdominal wall by this broad sheet of mesenchyme
o The dorsal mesentery becomes the ______________ in later development - The dorsal aorta gives a branch (superior mesenteric artery) to the midgut – the superior mesenteric artery passes through the ____________ to supply the midgut
- The midgut is still in contact with the yolk sac through the vitelline duct but both the yolk sac and vitelline duct are getting narrower
- The foregut has both dorsal and ventral mesenteries whereas the midgut and hindgut only have a dorsal mesentery
yolk sac;
splanchnic mesoderm;
vitelline duct;
broad mesodermal attachment;
mesentery proper;
dorsal mesentery;
Rapid growth of midgut – physiological umbilical herniation
- During the 5th and 6th weeks of intrauterine life, the midgut grows rapidly and forms a U-shape loop
- At this point in time, the liver is also expanding quickly – limited space in the abdominal cavity to accommodate midgut loops as the liver is also expanding at the same time
o Hence, the midgut loops project into the ______ end of the umbilical cord during the 6th week of development – physiological umbilical herniation
- The midgut gut is divided into cranial (cephalic) limb and caudal limb
o Cranial limb of midgut gives rise to distal part of the
duodenum (below the major duodenal papilla),
jejunum and part of the ileum
o Caudal limb of midgut gives rise to – lower portion of
the ileum, cecum, appendix, ascending colon and
the ___________________________
o All of these derivatives of the midgut are supplied by the superior mesenteric artery
embryonic;
proximal two-thirds of the transverse colon;
Rotations of the midgut – during the 6th to 11th weeks of development to place the intestines in the correct abdominal position with associated mesenteries
- Umbilical cord (herniation) – rotates 90° counter clockwise around the axis of the _________
- The cranial limb moves to the ____ whereas the caudal limb moves to the _______
- During this rotation, the cranial limb of the midgut grows rapidly and gives rise to intestinal loops (jejunum and ileum)
- There are no major changes happening to the caudal limb of the midgut at this point in time - Retraction of intestinal loops – by the 10th week of development, growth of the liver and kidney slows down and abdominal cavity of the embryo grows in size
- The combination of these events allow the intestines to return into the abdomen
- This is known as the _________________ of the intestinal loops
- The cranial limb (future small intestines) returns first to the abdominal cavity and occupies the centre part of the abdomen – as the small intestines return, they pass posterior to the superior mesenteric artery
- During the reduction of the intestines, the midgut undergoes a 90° rotation twice counter clockwise around the superior mesenteric artery
o The cecum is located in the _________________ of the abdomen just under the liver but in the later life descends into the _____________, placing the ascending colon and hepatic flexure on the right side of the abdominal cavity
Hence, the midgut rotates a total of 270° counter clockwise around the superior mesenteric artery – 90° during physiological herniation and 180° during the retraction of intestinal loops
superior mesenteric artery;
right; left;
retraction/reduction;
right upper quadrant;
right iliac fossa
The intestines completely return to the abdominal cavity by the 11th week – usually, the vitelline duct also disappears by the __________________
- The mesentery of the ascending colon and most of the duodenum fuses with the ____________________ of the posterior abdominal wall and disappears
o Hence, the ascending colon and most of the duodenum (except the 1st part) becomes secondarily retroperitoneal
o The rest of the midgut derivatives (jejunum and ileum) retain their mesentery and are classified as intraperitoneal
10th or 11th week;
parietal peritoneum
Meckel’s diverticulum – due to an incomplete regression of the vitelline duct, forming an outpouching of the ileum
- ‘Rule of twos’ – has a 2% incidence rate, is 2 ________ long and is 2 feet from the _________________(sizes are given are adult and not foetal dimensions)
- It may be free (75%) or attached to the umbilicus
(25%)
- The inflammation of Meckel’s diverticulum can mimic
___________ or peptic ulcer disease – the ileum should be inspected thoroughly during an abdominal surgery e.g. appendectomy
inches;
ileocecal junction ;
appendicitis
______________: involves the herniation of the abdominal viscera through an enlarged _________
- Due to the failure of the midgut loops to return to the abdominal cavity after the ______________ – intestines are located outside the abdominal cavity and are covered by amnion
- This condition is also associated with a defect in the development of the anterior abdominal wall
Omphalocele;
umbilical ring;
physiological herniation
The lower end of the hindgut is dilated – cloaca
- The hindgut is partitioned into ventral (urogenital sinus) and dorsal (anorectal canal) parts by the ____________
o Ventral part gives rise to ________________
o Dorsal part gives rise to distal one-third of the transverse colon, descending colon, sigmoid colon, rectum and upper part of the anal canal
o The derivatives of the hindgut are supplied by the inferior mesenteric artery
- Allantois – diverticulum from the hindgut, no major function in humans
o Most of allantois obliterates and becomes a fibrous cord known as _______________ (urachus) connecting the urinary bladder to the umbilicus
urorectal septum;
urinary bladder and most of the urethra;
median umbilical ligament
Kidneys are formed in the pelvic region, ascending to the lumbar region just inferior to the __________ between the 6th and 9th week of development
o Relocation of the kidney – due to growth of the body in the lumbar and sacral region and diminished body curvature
- As the kidney ascends, the _______________ elongates to become the ureter
- The kidney receives new segmental arteries from the aorta and loses those vessels below (sites of former renal arteries)
o Sometimes, lower vessels fail to disappear and form _______________
adrenal gland;
metanephric duct;
extra renal arteries
