Development of GI System Flashcards
Primordial Gut forms during ___ week
4th
Primordial Gut closed cranially by the:
Oropharyngeal Membrane (layer of endoderm lined with ectoderm)
Primordial Gut closed caudally by the:
Cloacal Membrane (layer of endoderm lined with ectoderm)
____ of the primordial gut and surrounding _____ mesoderm form most of the gut, epithelium, and glands
- Endoderm
- Splanchnic
Epithelium of cranial and caudal ends of GI tract is ectodermally derived via the ____ and ___ ____
- Stomodeum
- Anal Pit (Proctodeum)
What signals regulate regional differentiation of primordial gut to form its parts?
- Hox
- ParaHox
- Shh
- BMP
- Wnt
Foregut: Derivatives
- Primordial pharynx
- Lower respiratory system
- Esophagus
- Stomach
- Liver
- Biliary Apparatus (Hepatic Ducts, Gallbladder, Bile Duct)
- Pancreas
- Duodenum proximal to the opening of the bile duct
Foregut: Blood Supply
Celiac Trunk
Midgut: Derivatives
- Duodenum distal to the opening of the bile duct
- Jejunum
- Ileum
- Cecum
- Appendix
- Ascending Colon
- Proximal 2/3 of Transverse Colon
Midgut: Blood Supply
Superior Mesenteric Artery
Hindgut: Derivatives
- Distal 1/3 of Transverse Colon
- Descending Colon
- Sigmoid Colon
- Rectum
- Superior part of Anal Canal (to pectinate line)
- Epithelium of urinary bladder & most of urethra
Hindgut: Blood Supply
Inferior Mesenteric Artery
Junction of Transverse Colon derived from Midgut vs. Hindgut is indicated by change in blood supply from branch of ___ to branch of ____
- SMA
- IMA
Esophageal Atresia
- Blockage of esophageal lumen
- Associated with Tracheoesophageal Fistula in 90% of cases
- Fetus is unable to swallow amniotic fluid –> fluid cannot pass to GI for disposal –> polyhydramnios
Two Main Causes of Esophageal Atresia:
(1) Deviation of Tracheoesophageal septum posteriorly
(2) Incomplete separation from Laryngotracheal tube
Esophageal Atresia: Neonatal Symptoms
- Excessive drooling
- Reject oral feeding
- Regurgitation / coughing
Esophageal Stenosis
- Narrowing of esophagus, usually in distal 1/3
- Results from incomplete recanalization or from failure of esophageal blood vessels to develop correctly
Dorsal Mesentery
- Future abdominal viscera
- Becomes Greater Omentum
- Important in anchoring gut tube to posterior abdominal wall during development
Ventral Mesentery
- Formed when septum transversum thins during 5th week
- Connects stomach and developing liver to ventral body wall
- Becomes lesser omentum
In early development (<5wks), gut tube is connect to ventral body wall by ___ ___
Septum Transversum
Intraperitoneal Organs
- Contain visceral serosa, parietal serosa and mesentery
- Means organ is inside the peritoneal cavity, suspended within it by a mesentery
Retroperitoneal Organs
- Develop within body wall; separated from coelom by serous membrane covering
- Organs formed outside the peritoneal cavity
- Do not have mesentery, visceral serosa or parietal serosa
Structure that anchors Retroperitoneal Organs to Posterior Abdominal Wall:
Adventitia
(dense, fibrous, irregular connective tissue)
Primary Retroperitoneal Organs
- Never had a mesentery
- Located posterior to the peritoneal cavity
Secondary Retroperitoneal Organs
- Organs initially had a mesentery but during development, the mesentery obliterated so organs then developed outside the peritoneal cavity
List of Secondary Retroperitoneal Organs:
- Ascending Colon
- Descending Colon
- Duodenum
- Pancreas
Development of Stomach
(1) Caudal part of distal foregut dilates in 4th week
(2) Left wall broadens ventrodorsally but right wall grows faster, becoming the greater curvature of the stomach
(3) Stomach rotates 90 degrees in clockwise direction (from cranial end) around its longitudinal axis
(4) Ventral border (lesser curvature) moves right and dorsal border (greater curvature) moves left
Rotation of Stomach: Original Left side becomes ___ surface while original right side becomes the ____ surface.
- Ventral
- Dorsal
Right wall of stomach becomes:
Greater Curvature
Left wall of stomach becomes:
Lesser Curvature
Hypertrophic Pyloric Stenosis
- Most common anomaly of the stomach
- Affects males more often than females
- Marked muscular thickening of the pylorus due to hypertrophy of circular and longitudinal muscles; obstructs passage of food
- Leads to stenosis of the pyloric canal and obstruction which makes stomach markedly distended
Hypertrophic Pyloric Stenosis: Symptoms in Infants
Infant expels the stomach’s contents with considerable force (projective vomiting)
Hypertrophic Pyloric Stenosis: Treatment
Requires pyloromyotomy (longitudinal incision through anterior wall of pyloric canal)
Duodenum develops from:
- Caudal foregut
- Cranial midgut
- Splanchnic mesenchyme
Development of Duodenum
(1) Early 4th week it develops from caudal foregut, cranial midgut and splanchnic mesenchyme
(2) Grows rapidly, forming a C-shaped loop that projects ventrally
(3) While stomach rotates, duodenal loop rotates right and is pressed against posterior wall of abdominal cavity (retroperitoneal position)
(4) Lumen is temporarily obliterated due to epithelial proliferation (5-6th weeks) – vacuolation occurs as epithelial cells degenerate and duodenum is recanalized
(5) Ventral mesentery disappears to free duodenum from anterior body wall
Duodenal Stenosis
- Partial occlusion of duodenal lumen from incomplete recanalization (defective vacuolation)
- Most involve horizontal (3rd) and/or ascending (4th) segments
- Causes vomiting that typically contains bile
Duodenal Atresia
- Complete occlusion of duodenum
- Located at junction of bile and pancreatic ducts or hepatopancreatic ampulla
- May be isolated but often associated with annular pancreas, anorectal anomalies and gut malrotation
Duodenal Atresia: Symptoms
- Vomiting (typically containing bile)
- Abdominal distention
- “Double Bubble” sign on US (looks like 2 bubbles)
- Polyhydramnios also occurs due to lack of intestinal absorption of swallowed amniotic fluid
Liver, Gallbladder, and Biliary Ducts arise from ____ ___
Hepatic Diverticulum – ventral outgrowth of foregut (4th week)
Liver & Biliary Apparatus Development: Hepatic Diverticulum
- Forms during the 4th week as a ventral outgrowth of foregut
- Divides into cranial and caudal parts as it grows within the ventral mesogastrium
Cranial part of hepatic diverticulum becomes the primordium of the ____
Liver
What determines developmental and functional segmentation of the liver?
Quantity (volume) of O2-rich blood from Umbilical Vein into Liv3r
Caudal part of Hepatic Diverticulum becomes the:
Gallbladder
Stalk of Hepatic Diverticulum becomes the:
Cystic Duct
Proliferating endodermal cells form ___ ___, which anastomose around endothelium-lined spaces, creating the ___ of ___ ___; Gives rise to the epithelial lining of the ___ part of the biliary apparatus
- Hepatic Cords
- Primordia of Hepatic Sinusoids
- Intrahepatic
Liver grows rapidly from ___-___ weeks
5th-10th
Hematopoiesis begins during ___ week
6th
Bile formation begins during ___ week
12th
Initially the Extrahepatic Biliary Apparatus is occluded with ___ cells, but will be canalized
Epithelial
Cholangiocytes of extrahepatic ducts are derived from:
Endoderm
Stalk of diverticulum connecting hepatic and cystic ducts becomes the:
Bile Duct
Anomalies of the Biliary Apparatus: Accessory Hepatic Ducts
Cystic duct opens into an accessory hepatic duct rather than into the common hepatic duct
Anomalies of the Biliary Apparatus: Extrahepatic Biliary Atresia
- Most serious anomaly; very rare
- Obliteration of bile ducts at or superior to porta hepatis
Extrahepatic Biliary Atresia: Symptoms
- Jaundice soon after birth
- Acholic stools
- Dark-colored urine
Anomalies of the Biliary Apparatus: Agenesis of the Gallbladder
- Very rare
- Usually associated with the absence of the cystic duct
Ventral Mesentery gives rise to:
- Lesser Omentum, including Hepatogastric and Hepatoduodenal Ligaments
- Falciform Ligament (extends from liver to ventral abdominal wall)
Ventral Mesentery is derived from the:
Mesogastrium
Ventral Mesentery forms the __ ___ of the liver
Visceral Peritoneum
Bare Area
Area of the liver that is not covered by peritoneum; area is in direct contact with the diaphragm
Pancreas develops between ___ layers from pancreatic buds of the ___ foregut
- Mesentery
- Caudal
Dorsal Pancreatic Bud
- Appears first, crainal to the ventral bud
- Larger of the two buds
Ventral Pancreatic Bud
- Develops near entry of bile duct into the duodenum and grows between the layers of the ventral mesentery
- Carried dorsally as the duodenum rotates – will lie posterior to and fuse with dorsal bud
Ventral Bud forms ___ ___ and part of the ___ of the ___
- Uncinate Process
- Head of the Pancreas
Developing pancreas is pulled _____ as stomach, duodenum and ventral mesentery rotate
Retroperitoneal
Pancreatic Duct forms from:
(1) Duct of the Ventral Bud
(2) Distal part of the Duct of Dorsal Bud
____ part of the Duct of the Dorsal Bud can persist as an ___ ___ ___ that opens into ____ ____ ___
- Proximal
- Accessory Pancreatic Duct
- Minor Duodenal Papilla
Ectopic Pancreatic Tissue
- Pancreatic tissue that is located separately from pancreas
- Can be located in stomach mucosa, proximal duodenum, jejunum, pyloric antrum, and ileal (Meckel) diverticulum
Annular Pancreas
- Rare but can cause duodenal obstruction
- Ring-like annular part of the pancreas surrounds descending or 2nd part of the duodenum
- Females affected more than males
Annular Pancreas: Symptoms
Infants present with symptoms of complete or partial bowel obstruction
Annular Pancreas: Cause
Growth of bifid ventral pancreatic bud around the duodenum and parts of the bifid ventral bud then fuse with the dorsal bud, forming a pancreatic ring
Herniation of Midgut Loop
(1) Midgut elongates as it grows, and forms the U-shaped midgut loop
(2) Midgut Loop projects into the extraembyronic coelom via umbilical cord (6th week)
Midgut Loop communicates with the yolk sac through the ___ ___ until the 10th week – Physiological Umbilical Herniation
Omphaloenteric Duct
Cranial and Caudal limbs of Midgut Loop are suspended by:
Dorsal Mesogastrium
____ ____ is attached to the apex of the Midgut Loop where the two limbs join
Omphaloenteric Duct
Cranial Limb of Midgut Loop grows rapidly and forms the:
Small Intestinal Loops
Caudal Limb of Midgut Loop undergoes very little change and forms the:
Cecal Swelling (Diverticulum) – the primordium of the cecum and appendix
Rotation of Midgut Loop
- Results from differential growth
- Loop rotates 90 degrees counterclockwise around axis of the Superior Mesenteric Artery
- Brings cranial limb (small intestine) to the right and caudal limb (large intestine) to the left
- Cranial limb elongates to form intestinal loops (primordia of jejunum and ileum)
Retraction of Intestinal Loops:
- Intestine return to the abdomen –> reduction of the midgut hernia (10th week)
- Small intestine (cranial limb) returns first and occupies central abdomen
- Large intestine returns and undergoes a further 180 degrees counterclockwise rotation – moves descending and sigmoid colon to the right side of the abdomen
- Ascending colon becomes recognizable with the elongation of the posterior abdominal wall
Stomach and Duodenal rotation leads pancreas to fall to the ___
Right
As intestine assume their final positions, mesentery of Ascending Colon fused with posterior abdominal wall and becomes:
Retroperitoneal
Jejunum and Ileum mesentery is remodeled and extends from:
Duodenojejunal-to-the-ileocecal junction
Descending colon becomes ____ as its mesentery fuses with ___ ___ ___ ___
- Retroperitoneal
- Left Posterior Abdominal Wall
What initially holds the gut tube in place?
Primordial Dorsal and Ventral Mesogastriums
Stomach rotation and growth of Dorsal Mesogastrium create the:
Lesser Sac of the Peritoneal Cavity
(Alcove that is dorsal to the stomach)
Greater Omentum
- Formed from a suspended fold of mesogastrium that forms when the Lesser Sac enlarges
- Hangs from dorsal body wall and greater curvature of the stomach
Principal passageway between the Greater and Lesser Sacs:
Omental (Epiploic) Foraman (of Winslow)
Congenital Omphalocele
- Persistent herniation of abdominal contents (covered by peritoneum and amnion) into the proximal umbilical cord
- Very rare; half of cases are associated with chromosomal abnormalities
- Occurs when do not get re-entrance of gut tube/intestinal loop back into abdominal cavity by the 10th or 11th week
- Surgical repair is required
Congenital Omphalocele: Cause
Results from impaired growth of mesodermal (muscle) and ectodermal (skin) components of abdominal wall
Infants with very large Omphaloceles can also suffer from:
Pulmonary and Thoracic Hypoplasia
Gastroschisis
- Malformation of the anterior abdominal wall results from failure of the lateral body folds to fuse completely when the anterior abdominal wall forms during 4th week
- Permits extrusion of the abdominal viscera without involving the umbilical cord – viscera protrude into the amniotic cavity where it is exposed to amniotic fluid
Most intestinal anomalies result from:
Incomplete rotation and/or fixation of intestines
Non-Rotation of the Midgut
- Occurs when intestines do not rotate completely as they re-enter the abdomen
- Caudal limb of midgut returns first
- SI lies on right side of abdomen and LI is on left side
- Usually the 270 degree counterclockwise rotation is no completed
- Cecum and appendix located inferiorly to the pylorus (Subhepatic Cecum and Appendix)
Intestinal Atresia
Formation of Peritoneal (Ladds) bands and intestinal vovulus (twisting) due to improper fixation of duodenum and proximal colon to abdominal wall
Midgut Volvulus can obstruct _____ which can lead to ____ and _____
- SMA
- Infarction
- Gangrene
Infants with Intestinal Malrotation/Volvulus present with:
Bilious Emesis
Ileal (Meckel) Diverticulum
- Outpouching of part of the ileum (remnant of proximal part of the Omphaloenteric Duct)
- Typically blind-ending, finger-like pouch that may be connected to the umbilicus by fibrous cord
- Occurs when vitelline duct does not obliterate during the 6th week,; vitelline duct becomes a ligament that connects the midgut to the anterior abdominal wall; forms out-pouching of SI
Ileal (Meckel) Diverticulum: Composition
- Contains all GI tunics
- May contain gastric/pancreatic tissue that could become inflammed
Ileal Diverticulum predisposes individuals to:
Intestinal obstruction and volvulus – intestine may wrap around cord or form omphaloenteric fistula
Cloaca
Expanded terminal part of hindgut where hindgut and allantois empty
Cloaca is lined with:
Endoderm
At the Cloacal Membrane:
Endoderm meets surface ectoderm of anal pit
Cloaca is divided into dorsal and ventral parts by the:
Urorectal Septum (between allantois and hingut)
The Urorectal Septunm mdivides the Cloaca into:
Rectum, cranial part of anal canal, and urogenital sinus
Does the Urorectal septum fuse with the cloacal membrane?
NO
Cloacal Membrane ruptures by ____ and the anorectal lumen is temporarily closed by an ___ ___ ___
- Apoptosis
- Epithelial Anal Plug
Recanalization of the Anorectal Canal occurs by ____ and forms the __ ___
- Apoptosis
- Anal Pit (Proctodeum)
Superior 2/3 of the adult anal canal are from:
Hindgut
Inferior 1/3 of anal canal are from:
Anal Pit
Pectinate Line
Indicates epithelium junction derived from Ectoderm (anal pit) and endoderm (hindgut)
Other layers of the wall of the anal canal are derived from:
Splanchnic Mesenchyme
Anal Transition Zone
Zone where there is a transition from simple columnar to stratified squamous epithelium
3 Anomalies of the Low Anorectal Region:
(1) Imperforate Anus
(2) Anal Stenosis
(3) Membranous Atresia
3 Anomalies of the High Anorectal Region:
(1) Anorectal Agenesis
(2) Anorectal Agenesis with a Fistula
(3) Rectal Atresia
Imperforate Anus
- Anomaly of low anorectal region
- Rare; more common in males
- Anal canal may end blindly or ectopically, or anoperineal fistula that opens into the perineum, vagina, or penile/spongy urethra
- > 90% of low anorectal defects are associated with a fistula connecting the rectum and urethra
Anal Stenosis
- Anomaly of low anorectal region
- Anus is in typical position but the anus and anal canal are narrow
- Likely caused by a slight deviation of the urorectal septum as it grows caudally
Membranous Atresia
- Anomaly of low anorectal region
- Anus is in typical position but a thin layer of tissue separates the anal canal from the exterior
- Remnant of epithelial anal plug is thin enough to bulge on straining – appears blue from the meconium superior to it
- Failure of epithelial plug to perforate at the end of the 8th week (Failure of the anal plug to completely re-canalize out)
Anorectal Agenesis
- Anomaly of high anorectal region
- Rectum ends superior to the puborectalis muscle
- Typically a rectovesicle fistula (bladder)
- Rectourethral (males) or rectovaginal/rectovesibular fistula (females)
What is the most common type of Anorectal birth defect?
Anorectal Agenesis
Anorectal Agenesis with Fistula
- Anomaly of high anorectal region
- Results from incomplete separation of cloaca from the urogenital sinus by the urorectal septum
- In newborn males with this condition they may have meconium in the urine
- In newborn females with this condition they might have meconium in the vestibule of the vagina
Rectal Atresia
- Anomaly of high anorectal region
- Anal canal and rectum are present but separated
- Maybe connected by a fibrous cord, the remnant of an atretic portion of the rectum
- Results from recanalization of the colon or possibly defective blood supply (doesn’t let hindgut grow properly)
Pancreatic Islets form as isolated clumps of cells that bud from:
Endodermal Tubules
Epithelial lining of the Intrahepatic and Extrahepatic Biliary Ducts is derived from:
Endoderm
