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124 Cards in this Set
- Front
- Back
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During what stage does formation of the GI tract begin?
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During gastrulation (week 3)
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What is the first step information of the GI tract?
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Formation of the three germ layers
- Endoderm - Mesoderm - Ectoderm |
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What does the endoderm form in the GI tract?
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Epithelial components of the gut
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What does the mesoderm form in the GI tract?
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Splanchnic Mesenchyme → Muscle, CT, and other layers of the wall of the gut
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What does the ectoderm form in the GI tract?
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Ectoderm-Derived Neural Crest → Enteric Nervous System
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What happens following gastrulation to the GI precursors?
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Small indentations develop first in the anterior and then in the posterior of the embryo
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What does the anterior indentation in the GI of the embryo form?
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Foregut diverticulum (anterior intestinal portal)
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What does the posterior indentation in the GI of the embryo form?
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Hindgut diverticulum (caudal intestinal portal)
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Does the foregut or hindgut diverticulum form first?
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Foregut diverticulum (anterior intestinal portal) forms before the hindgut diverticulum (posterior intestinal portal)
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What do the foregut and hindgut diverticulum form? How?
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They elongate to form to tubes that fuse into a single, straight tube consisting of the foregut, midgut, and hindgut
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What does the foregut give rise to?
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Epithelium of:
- Esophagus - Stomach - Proximal duodenum (Also the Thyroid, Lung, Liver, and Pancreas) |
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What does the midgut give rise to?
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Epithelium of the small intestine
- Distal duodenum - Jejunem - Ileum Contributes to the epithelium of the large intestine - Cecum - Appendix - Ascending colon - First 1/3-1/2 of the transverse colon |
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What does the hindgut give rise to?
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Contributes to the epithelium of the large intestine:
- Remaining 1/2-2/3 of the transverse colon - Descending colon - Sigmoid colon - Rectum - Superior part of anal canal |
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What is the general organization of the wall of the GI tract?
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Four tunics / layers:
- Mucosa (innermost) - Submucosa - Muscularis Externa - Adventitia or Serosa |
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What is the innermost layer of the GI tract? What is it composed of?
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Mucosa
- Epithelium - Lamina propria - Muscularis mucosa |
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What is the second layer of the GI tract? What is it composed of?
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Submucosa
- Loose collagenous and adipose supporting tissues - Large vessels and lymphatics - Innervated by submucosal / Meissner's plexus |
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What is the third layer of the GI tract? What is it composed of?
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Muscularis Externa
- Inner circular layer of smooth muscle - Outer longitudinal layer of smooth muscle - Innervated by myenteric / Auerbach's plexus |
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What is the outermost layer of the GI tract? What is it composed of?
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Adventitia / Serosa
- Outer layer of tissue that surrounds the entire tube |
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What are the plexuses that innervate the GI tract? Which layer do they innervate?
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- Meissner's / Submucosal Plexus - innervates submucosa
- Auerbach's / Myenteric Plexus - innervates the Muscularis Externa |
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What happens to food in the stomach?
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Mechanical and chemical digestion → forms chyme
- Strong churning action breaks down solid food - Chemical breakdown is accomplished by gastric juices secreted by mucosal epithelial glands |
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What is formed in the stomach?
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Chyme
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Where are the gastric juices released from in the stomach?
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Mucosal epithelial glands
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What does the stomach form from? How?
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- Forms from the foregut
- "Fusiform" or "spindle-shaped" dilation of foregut around week 4 |
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What is the stomach attached to?
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- Between the esophagus and intestine
- Attached to the body wall by dorsal and ventral mesenteries |
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What nerves are on the left and right sides of the stomach?
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L and R Vagus nerves
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Along what axis does the primordial stomach enlarge along?
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Dorsal-ventral axis
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What part of the stomach expands more quickly? More slowly? Implications?
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- Dorsal wall of stomach expands more quickly → Greater Curvature of Stomach
- Ventral wall of stomach expands more slowly → Lesser Curvature of Stomach |
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What happens after formation of the greater and lesser curvatures?
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- Stomach rotates 90 degrees clockwise around its longitudinal axis
- This places the long axis of the stomach almost transverse to the long axis of the body - Lesser curvature faces right side of body |
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What are the implications of the rotation of the stomach by 90 degrees?
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- Left vagus nerve supplies the anterior wall of the mature stomach
- Right vagus nerve supplies the posterior wall of the mature stomach - Produces a space behind the stomach referred to as the lesser sac or omental bursa - Pulls the stomach and duodenum upward |
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Which nerve supplies the anterior wall of the mature stomach? Why?
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Left Vagus Nerve (renamed anterior vagal trunk nerve) d/t the 90 degree rotation
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Which nerve supplies the posterior wall of the mature stomach? Why?
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Right Vagus Nerve (renamed posterior vagal trunk nerve) d/t the 90 degree rotation
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What is the greater omentum formed from?
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Dorsal mesentery extends from greater curvature to form the greater omentum
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What is the lesser omentum formed from?
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Ventral mesentery attaches to the developing liver and lesser curvature to form the lesser omentum
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What is the space formed posterior to the stomach?
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Lesser sac / omental bursa
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What is the space formed anterior and inferior to the stomach?
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Greater sac
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What kind of cells form the epithelium of the mature stomach? What do they arise from?
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- Simple columnar cells
- Arise from the foregut endoderm |
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What do the smooth muscle and connective tissue components of the stomach derive from?
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Splanchnic Mesenchyme
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What components form the duodenum?
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- Foregut and midgut endoderm
- Splanchnic mesenchyme - Neural crest |
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What kind of cells do the foregut and midgut endoderm form in the duodenum?
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- Simple columnar epithelium that covers the villi
- Crypts of Lieberkuhn |
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What does the splanchnic mesenchyme form in the duodenum?
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Smooth muscle and connective tissue components of duodenum
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What does the neural crest form in the duodenum?
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Neurons that innervate the gut (enteric nervous system)
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What is the location of the junction between the foregut and midgut endoderm?
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Distal to the bile duct
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What is the shape of the duodenum? What happens as it expands?
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Transforms from a straight tube to a "C" shaped tube
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What happens to the duodenum as the developing stomach rotates?
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The duodenum also rotates to the right
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What happens to the lumen of the esophagus and the duodenum during development?
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The epithelial cells derived from the endoderm proliferate to occlude the lumen of the gut tube
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What locations in the GI tract have occlusion of the lumen during development?
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- Esophagus
- Duodenum |
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How do you restore the lumen in the esophagus and duodenum?
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Re-canalization
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What happens to the midgut as it elongates?
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Forms a ventral U-shaped tube termed the midgut loop or primary intestinal loop
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What are the components of the midgut U-shaped loop?
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- Cranial Loop
- Caudal Loop |
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What connects the Cranial and Caudal Loops of the Midgut Loop?
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Omphaloenteric duct (aka Vitelline duct, Omphalomesenteric duct, or Yolk Stalk)
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What does the Cranial Loop of the Midgut Loop give rise to?
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Gives rise to the bulk of the small intestine (distal duodenum, jejunum, and most of the ileum)
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What does the Caudal Loop of the Midgut Loop give rise to?
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Distal ileum, cecum, appendix, and parts of the colon (ascending colon and part of the proximal transverse colon)
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What happens to the midgut loop as it develops into the organs? Why?
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The organs expand more quickly than the body cavity expands, so the midgut loop herniates through the umbilicus into the umbilical cord forming a Physiological Umbilical Hernia
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What happens after the Midgut Loop herniates into the umbilicus?
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The midgut loop rotates 90 degrees counterclockwise around the axis of the superior mesenteric artery (as viewed from the front of the embryo)
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What are the implications of the Midgut Loop rotating 90 degrees clockwise after herniating into the umbilicus?
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- Positions the cranial limb on the right
- Positions the caudal limb on the left |
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What happens to the cranial limb of the Midgut Loop after it has rotated 90 degrees counterclockwise?
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Cranial limb undergoes looping to form the primordial jejunum and ileum
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What happens to the caudal limb of the Midgut Loop after it has rotated 90 degrees counterclockwise?
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Caudal limb develops the cecal bud, which ultimately forms the cecum
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What happens at 10 weeks, once the body cavity has grown sufficiently?
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- The midgut loop retracts into the body cavity (out of the umbilicus)
- Cranial limb retracts before the caudal limb - Midgut undergoes another 180 degree rotation counterclockwise around the axis of the SMA |
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Where do the cranial and caudal limbs of the Midgut loop go once the body cavity expands sufficiently?
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- Cranial loop returns into the left side of the body cavity
- Caudal loop returns to the right side of the body cavity - Transverse colon rests in front of the duodenum - Initially, cecum and short ascending colon rest under the liver |
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What happens to the cecum and short ascending colon after it returns into the body cavity?
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- Initially the cecum and short ascending colon rest underneath the liver
- As the ascending colon grows and elongates, the cecum descends, placing the cecum and appendix in the RLQ of the body |
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What happens to the vitelline duct during the fetal period?
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It regresses and disappears
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What marks the division between the midgut-derived and hindgut-derived ascending colon?
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Transition of the blood supply from the superior mesenteric artery (to the midgut) to the inferior mesenteric artery (to the hindgut)
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What is the term for the expanded terminal region of the hindgut?
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Cloaca
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What partitions the cloaca?
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Urorectal septum - mesenchyme grows and expands to separate the cloaca
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How does the urorectal septum form? Function?
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- Septum develops at an angle between the allantois and hindgut
- As it grows toward the cloacal membrane, it extends fork-like projections that result in infolding of the lateral walls of the cloaca - As the infolds grow toward each other and fuse, leading to a partition of the cloaca |
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What forms after the urorectal septum partitions the cloaca?
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- Dorsal side produces the rectum and part of the anal canal
- Ventral side produces the urogenital sinus |
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What does the urorectal septum fuse with?
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Fuses with the cloacal membrane to divide it into a dorsal anal membrane and a ventral urogenital membrane
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What does the urorectal septum partition the cloacal sphincter into?
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External anal sphincter (posterior part) and multiple muscles (superficial transverse perineal, bulbospongiosus, and ischiocavernosus) (anterior part)
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What muscles are separated from the cloacal sphincter by the urorectal septum? How are they related?
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- Superficial transverse perineal
- Bulbospongiosus - Ischiovavernosus - They are all innervated by the pudendal nerve |
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What forms the anal canal?
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- Superior - from hindgut
- Inferior - from proctodeum (ectodermally-derived) |
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What do the liver, gallbladder, and biliary duct system develop from?
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Outgrowth of the ventral foregut endoderm
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What happens in the first phase of the liver bud (hepatic diverticulum) formation?
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- The foregut endoderm, composed of polarized columnar epithelial cells, protrudes into the surrounding septum transversum mesenchyme
- Apical surface faces the gut lumen and basal surfaces contact laminin-rich basement membrane |
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What is the septum transversum mesenchyme derived from?
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Splanchnic mesoderm between heart and midgut
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What happens in the second phase of the liver bud (hepatic diverticulum) formation, after the foregut endoderm protrudes into the septum transversum mesenchyme?
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Simple columnar epithelium transforms into a pseudostratified epithelium encased in basement membrane
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What happens in the third phase of the liver bud (hepatic diverticulum) formation, after the epithelium transforms to a pseudostratified epithelium encased by BM?
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- BM is degraded, and bipotential hepatoblasts delaminate and migrate into the septum transversum mesenchyme
- This forms cords of hepatic cells within the mesenchyme - Hepatoblasts have potential to differentiate into hepatocytes, the epithelial cells of the liver parenchyma, or into cholangiocytes, the epithelial cells of the biliary system |
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What are the epithelial cells of the liver parenchyma?
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Hepatocytes
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What are the epithelial cells of the biliary system?
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Cholangiocytes
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What do the hepatoblasts have the potential to differentiate into during formation of the liver bud?
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- Hepatocytes (epithelial cells of the liver parenchyma)
- Cholangiocytes (epithelial cells of the biliary system) |
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What growth factors are secreted from the heart and the septum transversum? Function?
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- FGFs and BMPs
- Essential for proper specification and outgrowth of the primordial liver bud |
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What surrounds the pseudostratified liver bud? Function?
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- Necklace of endothelial cells
- Required for delamination and expansion of the liver |
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What is the fetal liver an important site for?
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Hematopoiesis - remains the key site of hematopoiesis prior to the onset of BM hematopoiesis
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Why is the liver characteristically bright red?
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Hematopoiesis
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What are the sinusoids of the liver? What are they derived from?
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- Blood vessels residing at the basal surface of polarized hepatocytes
- Derived from vessels resident in the septum transversum mesenchyme via angiogenesis |
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What does the gall bladder originate from?
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Small caudal region of the liver bud
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What does the cystic duct originate from?
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Stalk of the liver bud
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What does the stalk connecting the hepatic and cystic ducts to the duodenum become?
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Bile duct
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What does the bile duct connect to?
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- Initially attaches to the ventral aspect of the duodenal loop
- As the duodenum grows and undergoes rotation, the bile duct is carried to the dorsal aspect of the duodenum |
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What does the pancreas originate from?
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Two buds emanating form the dorsal and ventral foregut endoderm
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What does the dorsal and ventral endoderm give rise to?
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Exocrine and endocrine epithelial cells of the pancreatic parenchyma
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How does the rotation of the duodenum to form a "C" shape affect the developing pancreas buds?
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Ventral pancreatic bud is carried dorsally to lie posterior to the dorsal pancreatic bud, the buds eventually fuse
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Which pancreatic bud emerges first? What does it give rise to?
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- Dorsal pancreatic bud
- Gives rise to the majority of the mature pancreas |
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What signals are important for the development of the dorsal pancreatic bud?
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- Signals received from the notochord, which resides above the endoderm fated to form the pancreas
- Notochord secretes FGF2 and Activin to inhibit SHH, promoting pancreatic development instead of intestinal development |
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What happens to the notochord during development that impacts the formation of the dorsal pancreatic bud?
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- Notochord is displaced by the fusing dorsal aorta
- Endodermal cells of the dorsal pancreatic bud now receive signals from the aorta to promote bud expansion and endocrine cell differentiation |
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What induces exocrine cell differentiation of the dorsal pancreatic bud?
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Caused by mesenchyme comes between the aorta and dorsal pancreas
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How does SHH affect the development of the pancreas?
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- Dorsal pancreatic bud: SHH is inhibited by FGF2 and Activin from the notochord, which promotes pancreatic development instead of intestinal development
- Ventral pancreatic bud: SHH is not involved |
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What signals are important for the development of the ventral pancreatic bud?
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- It is automatically fated to become pancreatic, but can become fated to become hepatic by instructive factors like FGFs and BMPs
- SHH is not involved in the signaling |
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What is the affect of FGF and BMP on the ventral pancreatic bud? Where do these signals come from?
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- FGF and BMP are secreted from the closely associated cardiac and septum transversum mesenchyme tissues
- They cause the pancreatic fated tissues to a adopt a hepatic fate |
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What does the ventral pancreatic bud require for its development?
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- Splanchnic mesoderm
- Signals from the vasculature, namely the vitelline veins, for expansion and differentiation of the endocrine and exocrine cell lineages |
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What are the possible defects in duodenal development?
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- Duodenal atresia - failure to recanalize, complete occlusion
- Duodenal stenosis - failure to completely recanalize, partial occlusion |
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What are the implications of blockage of the duodenum (duodenal atresia and stenosis)?
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- Vomiting of the stomach contents as well as bile
- Often associated with other congenital anomalies - Polyhydramnios occurs w/ duodenual atresia because the blockage prevents proper intestinal absorption of the swallowed amniotic fluid |
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What is Polyhydramnios? When does it occur?
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Occurs w/ duodenual atresia because the blockage prevents proper intestinal absorption of the swallowed amniotic fluid
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What is the most serious anomaly of extrahepatic biliary system development?
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Extrahepatic Biliary Atresia
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What is the most common cause of Extrahepatic Biliary Atresia?
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Obliteration of bile ducts (85% of cases)
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What are the symptoms of Extrahepatic Biliary Atresia? Prognosis?
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- Jaundice occurs soon after birth
- Stools are acholic (clay colored) - If unable to repair ducts surgically, biliary atresia will be fatal without a liver transplant |
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What causes Gastroschisis?
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- Defect lateral to the median plane of the anterior abdominal wall
- Poorly understood etiology, thought to be multifactorial (vascular events and/or environmental factors may contribute by impacting abdominal wall development) |
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What happens if there is a defect lateral to the median plane of the anterior abdominal wall?
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Gastroschisis
- Abdominal viscera are extruded through the wall (does not involve the umbilical cord) - Protruding viscera are bathed in amniotic fluid, which causes Serositis |
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Where does Gastroschisis occur?
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Most commonly on the right side lateral to the umbilicus
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What is Gastroschisis categorized as?
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- Not a hernia because it is not covered in a sac
- It is an evisceration |
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What is the term for herniation of the abdominal contents into the proximal umbilicus?
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Omphalocele
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What happens in Omphalocele?
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- Herniation of the abdominal contents (small and large intestine, liver, stomach, and gonads) into the proximal umbilicus
- Failure of the intestine to return to the abdomen during development |
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How does Gastroschisis compare to Omphalocele?
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Gastroschisis
- Evisceration (not a hernia) - Not associated with other congenital anomalies Omphalocele - Hernia into proximal umbilicus - Commonly associated with other congenital anomalies (including cardiac and urogenital defects) |
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What commonly causes congenital anomalies of the small intestine?
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Defects in gut rotation including non-rotation or mal-rotation, also reverse rotation (rare)
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What are the impacts of defects in gut rotation on the intestines?
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- Leads to misplacement of the intestines within the body cavities
- Improperly positioned and fixed intestines can lead to twisting of the midgut = Volvulus |
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What is the term for twisting of the midgut d/t a fixed intestine?
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Volvulus
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What are the implications of Volvulus?
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Intestinal:
- Obstruction - Infarction - Gangrene |
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What direction does the gut normally rotate during development? What direction can it turn rarely, called "reverse rotation"?
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- Normal: counterclockwise
- Reverse: clockwise |
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What is the term for an out-pocketing of the ileum?
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Meckel Diverticulum
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What is one of the most common anomalies of the GI tract?
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Meckel Diverticulum
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What is a Meckel Diverticulum a remnant of? What is it?
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- Remnant of the Omphaloenteric duct
- It is an outpocketing of the Ileum |
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What are the symptoms of Meckel Diverticulum?
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- Can become inflamed
- Mimics appendicitis - Can lead to ulceration and bleeding because of secretion of gastric acid and gastric and pancreatic enzymes |
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What is the anatomical organization of a Meckel Diverticulum?
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Wall of the diverticulum contains all layers of the ileum and may also contain gastric and pancreatic tissues
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What can cause ulceration and bleeding in Meckel Diverticulum?
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Secretion of gastric acid and gastric and pancreatic enzymes
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What is the disease that presents as a "mega-colon" (enlarged colon)?
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Hirschsprung disease
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What causes Hirschsprung disease?
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Aganglionosis - enlarged, dilated region contains normal ganglion cells but the affected tissue lacks ganglion cells and fails to relax, thus preventing movement of bowel contents
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