GIS08 Development Of The Digestive System Flashcards
Revision: Gastrulation
Day 16-18
- Epiblast cells migrate through primitive streak (inwards and from caudal towards cephalic)
- Definitive endoderm cells (from epiblast) displace hypoblast
- Mesoderm spread between endoderm and ectoderm
Formation of primitive gut tube: Day 16
- Developing endoderm initially open to yolk sac
- Cardiac primodia formed cephalically
- Longitudinal (cephalic-caudal) folding at both ends bring endoderm inside and form gut tube
Formation of primitive gut tube: Day 18
Longitudinal folding creates:
- Anterior intestinal portal (future foregut)
- Posterior intestinal portal (future hindgut)
- Cardiac region brought to ventral side of gut tube
- ***Oropharyngeal membrane: future mouth
- ***Cloacal membrane: future anus
Formation of primitive gut tube: Day 22
- **Stomodeum: **Ectodermal depression at head end of embryo –> front part of mouth
- Gut associated organs: forms bud from endoderm
- Midgut opening to yolk sac progressively narrows
Formation of primitive gut tube: Day 30
- **Stomach bulge + **Dorsal pancreatic bud become visible
- Connection of midgut to yolk sac reduced to Yolk stalk
Invagination of lateral body wall
- folding of embryo to enclose endoderm layer –> forms gut tube
- Lateral folding –> embryo with a gut suspended by dorsal mesentery (***Splanchnic mesoderm)
- ***Somatic mesoderm –> lines body cavity –> muscles and CT
Subdivision of gut tube and and their derivatives
Foregut:
- esophagus
- stomach
- liver + gallbladder
- pancreas
- proximal duodenum (up to major duodenal papilla)
Midgut:
- lower duodenum
- small intestine
- large intestine (cecum, appendix, ascending colon)
- right half to 2/3 of transverse colon
Hindgut:
- 1/3 left transverse colon
- descending colon
- sigmoid colon
- rectum
- upper part of anal canal
- epithelium of urinary bladder and urethra
Development of foregut derivatives: Septum transversum
Portions of gut tube and derivatives are suspended from ventral + dorsal body wall by ventral + dorsal mesenteries (***mesodermal tissue)
- **Septum transversum (future diaphragm):
- formed by aggregation of ***mesenchyme tissue that develops within caudal part of ventral mesentery of foregut
- between
- -> Primitive thoracic cavity
- -> Abdominal cavities
- contributes to formation of **CT in liver and **central tendon of diaphragm
Esophagus formation
- ***Respiratory diverticulum (lung bud) at ventral wall of foregut (4 weeks)
- -> ***Tracheoesophageal septum partitions diverticulum from dorsal foregut
- -> esophagus grows in length to keep up with descent of heart and lungs
Foregut:
- -> ventral: trachea, lung buds
- -> dorsal: esophagus
Stomach formation
- Dorsal wall grows faster than ventral wall
- -> greater and lesser curvatures
- stomach rotates 90o clockwise (around longitudinal axis)
- -> ***left side face anteriorly
- -> **left vagus nerve –> innervates **anterior wall (vice versa)
Omental bursa / Lesser sac formation
Stomach attached to dorsal abdominal wall by ***dorsal mesogastrium (mesentery) (vice versa)
- -> rotation around longtudinal axis
- -> pull dorsal mesogastrium to the left (vice versa)
- -> formation of Omental bursa / Lesser sac (pouch of peritoneal cavity / potential space behind stomach)
5th week:
Spleen primordium appears as ***Mesodermal proliferation between 2 leaves of dorsal mesogastrium
Congenital malformations of stomach
Pyloric stenosis:
- one of most common abnormalities in newborn
- Circular (mainly) + Longitudinal musculature of stomach in pylorus enlarges (hypertrophy)
- -> extreme narrowing of pyloric lumen
- -> obstruct passage of food
- -> severe projectile vomiting
Duodenum formation
- forms at junction of foregut and midgut
- as stomach rotates, duodenum takes on form of ***C-shaped (anterior) loop and rotates to the right
- duodenum rotation + pancreas growth
- -> swings duodenum from midline position to the right and lie against dorsal wall
- **dorsal mesoduodenum (mesentery) fuses with back body wall leaving main portion of duodenum in retroperitoneal position (only anterior covered with mesentery) —> **secondarily extraperitoneal
Liver and gallbladder formation
Liver, Biliary system, Pancreas:
appear as outgrowth of **endodermal epithelium at **distal foregut (4th week)
- connection between liver bud and duodenum narrows
- -> forming bile duct
- -> give rise to gallbladder and cystic duct
Formation of liver:
Liver bud grows and penetrates ventrally from duodenum into septum transversum (plate between pericardial cavity and yolk stalk)
–> invade entire **septum transversum
–> portion of septum transversum / mesentery between **liver and foregut / stomach
–> becomes membranous
–> ***Lesser omentum
- -> portion of septum transversum / mesentery between ***liver and ventral abdominal wall
- -> becomes membranous
- -> ***Falciform ligament
Pancreas formation
Outgrowth of dorsal + ventral endodermal lining of duodenum
–> Dorsal + Ventral pancreatic anlage
Rotation of duodenum
- -> ventral anlage moves dorsally
- -> lie below and behind dorsal anlage
- **fusion of ducts from dorsal, ventral anlagen
- -> formation of ***Main pancreatic duct
- -> enter duodenum together with bile duct at site of ***major duodenal papilla
original duct of dorsal anlage
- -> ***accessory pancreatic duct
- -> exit at ***minor papilla
dorsal anlage –> body and tail of pancreas
ventral anlage –> head and uncinate process
Pancreatic divisum
- most common congenital anomaly of pancreas
- failure of fusion of dorsal and ventral pancreatic anlages
- -> drainage of fluid from **main pancreatic duct via **minor papilla
Rotation of midgut (6-8th week)
- rapid elongation of midgut and mesentery –> formation of ventral U-shaped midgut loop
- ***Superior mesenteric artery forms axis of rotation (dorsoventral axis 穿向前) and herniates into umbilical cord (physiological umbilical herniation)
- U-shape mid gut loop rotate ***90o anticlockwise
- -> ***Small intestine loop (jejnum, ileum) become on the right
- -> ***Cecal diverticulum become on the left (primordium of cecum and appendix)
- Yolk stalk / Vitelline duct attached to apex of midgut loop where two limbs join (small intestine loop, cecal diverticulum)
Rotation of midgut (10th week)
Abdominal cavity larger: midgut return into abdominal cavity (sequential manner)
- small intestine loop first
- then cecal diverticulum
Midgut loop further rotate ***180o anticlockwise
- -> small intestine loop now on the left (lower left)
- -> cecal diverticulum now on right (upper right)
- Displacement of cecum and appendix caudally (向下)
- -> places them in right lower quadrant
- -> ascending colon on right
Both ascending colon and descending colon back mesenteries fuse with back wall –> Retroperitoneal
Gut rotation defects
Rotate 90o anticlockwise **without further 180o rotation
–> **left and right reverse
Rotate 90o **clockwise (in 1st rotation) (with subsequent 180o rotation)
–> Duodenum **in front of transverse colon
–> may result in twisting of bowel (volvulus) and compromise blood supply
Omphalocele
- Failure of herniated gut to return to abdominal cavity
- herniated gut covered by amnion
- associated with other malformations e.g. heart / neural tube defects / chromosomal abnormalities
Remnants of vitelline duct
- Vitelline duct regresses between 5-8th week and later obliterates into ***fibrous cord and degenerate completely
- -> ***Umbilicus
Not degenerate completely:
- ***Meckel’s diverticulum (outpouch at ileum)
- Vitelline cyst
- Vitelline fistula (remain patent)
Cecum and appendix development
midgut: cecal diverticulum
- -> cecal bud
- -> cecum + appendix (posterior to cecum)
Development of hindgut
Partitioning of cloaca (by ***Urorectal septum —> Mesoderm)
- separate region between Allantois and Hindgut
- -> anterior: Urogenital sinus
- -> posterior: Anorectal canal
- Hindgut enter posterior region of cloaca –> future anorectal canal
- Allantois enter anterior region –> future urogenital sinus
Primitive urogenital sinus –> bladder, pelvic urethra, definitive urogenital sinus
Anal canal
- upper 2/3 of anal canal from endoderm of hindgut
Invagination of ectoderm in region of Proctodeum
–> lower 1/3 of anal canal + anal pit
- degeneration of anal membrane (cloacal membrane)
- -> establish continuity between upper and lower anal canals
- -> leftovers: ***Pectinate line
Congenital hindgut malformations
- **Imperforate anus: failure of cloacal membrane to break down
- -> rectum cannot connect with anus
Imperforate anus with rectal atresia
- -> urorectal septum cannot separate allantois and hindgut
- -> ***urorectal fistula (rectum connect with urethra)
