1.2.3 GI Development

Question 1

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

Answer 1

4th week of development;

buccopharyngeal; cloacal;

endoderm;

splanchnopleuric mesoderm

Question 2

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)

Answer 2

somatopleuric mesoderm;

splanchnopleuric mesoderm;

intraembryonic coelom;

mesothelial;

septum transversum;

falciform ligament

Question 3

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)

Answer 3

tracheoesophageal septum;

tracheoesophageal fistula;

Question 4

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 _________________

Answer 4

dorsal border;

clockwise;

posterior;

mesentery of the transverse colon

Question 5

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

Answer 5

mesenchymal cells

Question 6

original ventral border of stomach: movement, new structure?

Answer 6

right; lesser curvature

Question 7

original dorsal border of stomach: movement, new structure?

Answer 7

left; greater curvature

Question 8

original left side of stomach: movement?

Answer 8

ventral surface (left vagus nerve supplies the anterior wall)

Question 9

original right side of stomach: movement?

Answer 9

dorsal surface; right vagus nerve supplies posterior wall

Question 10

original cranial end of stomach: movement, new structure?

Answer 10

left, inferiorly; cardic end

Question 11

original caudal end of stomach: movement, new structure?

Answer 11

right, superiorly;

pyloric end

Question 12

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 (_________________)

Answer 12

right ;

duodenal cap

Question 13

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

Answer 13

septum transversum;

sinusoids;

intercalates

Question 14

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 ____________________

Answer 14

duodenum and the head of the pancreas

Question 15

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

Answer 15

dorsal pancreatic duct;

Distal;

accessory pancreatic duct (Duct of Santorini);

inferior part of the head and the uncinate process;

below and behind

3rd

Question 16

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

Answer 16

opposite

Question 17

• 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

Answer 17

yolk sac;

splanchnic mesoderm;

vitelline duct;

broad mesodermal attachment;

mesentery proper;

dorsal mesentery;

Question 18

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

Answer 18

embryonic;

proximal two-thirds of the transverse colon;

Question 19

Rotations of the midgut – during the 6th to 11th weeks of development to place the intestines in the correct abdominal position with associated mesenteries

1. 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
2. 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

Answer 19

superior mesenteric artery;

right; left;

retraction/reduction;

right upper quadrant;

right iliac fossa

Question 20

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

Answer 20

10th or 11th week;

parietal peritoneum

Question 21

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

Answer 21

inches;

ileocecal junction ;

appendicitis

Question 22

______________: 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

Answer 22

Omphalocele;

umbilical ring;

physiological herniation

Question 23

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

Answer 23

urorectal septum;

urinary bladder and most of the urethra;

median umbilical ligament

Question 24

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 _______________

Answer 24

adrenal gland;

metanephric duct;

extra renal arteries

Question 25

Malfunctions related to the ascent of kidneys
- Malformations may occur due to errors in the ascent of the kidney
- Pelvic kidney – kidney completely fails to ascend unilaterally or bilaterally
- Horseshoe kidney – inferior poles of both kidneys fuse
o The kidneys become caught under the ___________________ and are thus unable to reach their normal sites

Answer 25

inferior mesenteric artery

Question 26

Embryological basis for aberrant renal arteries
- Normally, each kidney is supplied by a single renal artery arising from the abdominal aorta
- In 30% of the cases, the kidneys are supplied by more than one artery – aberrant (accessory) renal arteries
o Failure of one or more of the transient inferior renal arteries to regress
o Polar renal artery – accessory renal artery piercing the inferior pole of the kidney
§ Sometimes compresses the ureter, causing the
accumulation of urine (____________________)

Answer 26

hydronephrosis