Abdominal Organ Development

Question 1

Primordial gut

Answer 1

Blind-ended tube
1. Initially closed at cranial end by oropharyngeal membrane
2. Closed caudal end by cloacal membrane
3. Mesenteries
A. Portion of gut tube and derivatives suspended from dorsal and ventral body walls by mesentery
B. Dorsal mesentery -> dorsal mesogastrium, mesoduodenum, dorsal mesocolon, and mesentery proper
C. Ventral mesentery from septum transversum -> lesser omentum and falciform ligament

Question 2

Pharyngeal gut

Answer 2

Oropharyngeal membrane -> resp diverticulum (lung bud)

Question 3

Esophageal development

Answer 3

1. Week 4
   A. Respiratory diverticulum (lung bud) on ventral foregut
   B. Tracheoesophageal septum separates resp diverticulum from dorsal foregut
   C. Foregut -> respiratory primordium (ventral) and esophagus (dorsal)
   1. Incomplete division -> fistulas

Question 4

Stomach and spleen development

Answer 4

1. Week 4
   A. Bud on distal foregut (medial planes)
   B. Position change in 2 axes
   
   1. Rotates 90 clockwise (longitudinal axis)
      A. Left side -> ant right side -> post
      
      1. Left and right vagus nerves transition same
         B. Posterior wall grows faster -> greater curvature
         C. Dorsal mesogastrium (spleen) pulled left -> omental bursa
   2. Rotation around anteroposterior axis
      A. Cephalon and caudal ends from midline
      B. Pylorus -> right and upward
      C. Cardia -> left and outward
      D. Dorsal mesogastrium -> greater omentum
      C. Spleen
   3. From mesoderm
   4. Week 5: de novo in dorsal mesogastrium
   5. Fuses w/ posterior abdominal wall peritoneum
   6. Lienorenal ligament: attaches left kidney
   7. Gastrosplenic lig: attaches stomach
   8. Position explains route of splenic a.

Question 5

Duodenum development

Answer 5

1. Week 4:
   A. Dev from foregut and midgut
   B. C-shape from rapid growth and stomach rotation
   C. Dorsal mesogastrium fuses post body wall -> secondarily retroperitoneal
   
   1. Part connected pylorus stays intraperitoneal
      D. Most ventral mesogastrium disappears
2. Week 5 and 6
   A. Lumen temporarily obliterated by epithelial proliferation
   B. Recanalized from epithelial degeneration

Question 6

Liver, gallbladder, and, biliary development

Answer 6

1. Week 4
   A. Hepatic diverticulum: ventral outgrowth from distal foregut
   
   1. Liver
   2. Gallbladder
   3. Biliary duct
      B. Rapidly enlarges into ventral mesogastrium
      C. Connection to foregut -> bile duct
      D. Outgrowth off bile duct
   4. Bile duct
   5. Gallbladder primordium -> gallbladder
      E. Bile duct attaches ventral duodenum loop -> passes post duodenum
      F. Hepatic diverticulum. -> falciform lig and lesser omentum

Question 7

Pancreas development

Answer 7

1. Dev in mesentery layers from 2 buds
   A. Large dorsal bub in dorsal mesogastrium
   B. Small ventral bud in ventral mesogastrium
   C. Ventral part swings around to join dorsal when duodenum forms C-shape -> uncinate process
   D. Ducts anastomose
   
   1. Main = combo
   2. Accessory
   3. Secondarily retroperitoneal

Question 8

Midgut development

Answer 8

1. Week 5
   A. Suspended in dorsal mesentery
   B. Primary intestinal loop
   
   1. Cephalon limb - rapid growth
   2. Caudal limb
   3. Attached to yolk sac via vitelline duct
2. Week 6
   A. Liver growth -> dec space -> primary loop into remnant of extra embryonic coelom
   B. 90 counterclockwise rotation around SMA
   
   1. Cranial coils and lengthens
   2. Caudal lengthens
      C. Primordial cecum and appendix appear on caudal limb
   3. Appendix elongates
3. Week 10
   A. Loop -> abdominal cavity
   
   1. Small intestines first
   2. Additional 180 degree turn (270 total)
      B. Caudal returns to right side
   3. Dorsal mesentery twists around SMA
   4. Ascending and descending secondarily retroperitoneal

Question 9

Hindgut development

Answer 9

1. Cloaca (primitive anorectal canal): endoderm-lined distal chamber in contact w/ ectoderm and continues w/ allantois
2. Urorectal septum separate regions between allantois and hindgut
   A. Folds -> partition between rectum and urogenital sinus in cloaca
3. Week 7
   A. Cloacal membrane rupture (apoptosis) -> anal opening and ventral opening
   B. Proctodeum -> anal pit
   C. Sup 2/3 anal canal from endoderm of hindgut
   D. Inf 1/3 anal canal from ectoderm around anal pit