GI Embryology Flashcards
What does lateral embryonic folding in week 4 cause in the GI tract?
Creates a ventral body wall
Primitive gut becomes tubular
What does craniocaudal embryonic folding in week 4 do to the GI tract?
Creates cranial and caudal pockets from yolk sac endoderm beginning primitive gut development
Where is the opening in the gut tube at the beginning of its formation?
In the midgut - foregut and hindgut are blind ended diverticula
What is the stromatodeum?
Future mouth
What is the proctodeum?
Future anus
What is the internal lining of the gut derived from?
Endoderm (future epithelial linings)
What is the external lining of the gut derived from?
Slanchnic mesoderm (future musculature and visceral peritoneum)
How is the primitive gut tube held in place?
Suspended in intraembryonic coelom by a double layer of splanchnic mesoderm
What are the adult derivatives of the foregut?
Oesophagus
Stomach
Pancreas, liver and gall bladder
Duodenum (prox to entrance of bile duct)
Blood supply by celiac trunk
What are the adult derivatives of the midgut?
Duodenum (distal to entrace of bile duct)
Jejunum
Ileum
Cecum
Ascending colon
Prox 2/3 transverse colon
Blood supply by superior mesenteric artery
What are the adult derivatives of the hindgut?
Distal 1/3 transverse colon
Descending colon
Sigmoid colon
Rectum
Upper anal canal
Internal lining of bladder adn urethra
Blood supply by Inferior mesenteric artery
How does the arterial blood supply of the gut reflect embryonic development?
Each embryonic segment receives blood supply from a distinct branch of the abdominal aorta
What are the exceptions in the gut that have dual blood supply?
Dudodeum - proxial to bile duct by coeliac trunk and distal, by superior mesenteric artery
Pancreas - by both coeliac trunk and superior mesenteric artery
What are the body cavities formed from?
Intraembryonic coelom
Begins as one large cavity. Later subdivided by the future diaphragm into abdominal and thoracic cavities.
What is the peritoneal membrane and cavity?
Membrane lines the abdominal cavity and invests the viscera. During development it grows, changes shape and specialises.
Cavity is a potential space but under normal conditions it should contain nothing
What are mesenteries and what are their purpose?
Double layer of peritoneum suspending the gut tube from the abdominal wall. Allows a conduit for blood and nerve supply and mobility where needed.
How are mesenteries formed?
From a condensation of the splanchnic mesoderm surrounding the primitive gut in the intraembryonic coelom
Where are the mesenteries?
Dorsal mesentery suspends the entire gut from the dorsal body wall
Ventral mesentery only in the region of the foregut
What does the attachment of the dorsal and ventral mesenteries to the foregut mean?
Divides the cavity into left adn right sacs/ greater and lesser sacs respectively.
Where does teh lesser sac lie?
Behind the stomach
What are omenta?
Specialised regions of the peritoneum
Describe the formation of the greater omentum
Formed from the dorsal mesentery
First structure seen when the abdominal cavity is opened anteriorly
Describe the formation of teh lesser omentum.
Formed from the ventral mesentery
Free edge conducts the portal triad
How are the greater and lesser sacs and omenta formed?
Rotation of the stomach
How does the primitive stomach rotate?
2 diretions:
Around teh longitudinal axis adn around the anteroposterior axis
What is the result of stomach rotation?
Greater and lesser curvature come to lie first on the right and left side then cardia and pylorus move horizontally, pushing greater curvature inferiorly
How does rotation of the stomach and growth of the liver effect the right/lesser sac?
Moves it round to become posterior/dorsal to the left/greater sac
How does rotation of the stomach change the location of the vagus nerves?
Moves them to lie anterior and posterior instead of left and right
What is a peritoneal reflection?
A change in direction from parietal peritoneum to mesentery, from mesentery to visceral peritoneum etc…
What does “retroperitoneal” mean?
Were never in the peritoneal cavity and never had a mesentery
What does ‘secondarily retroperitoneal’ mean?
Began development invested by peritoneum, had a mesentery but, with successive growth and development, the mesentery is lost through fusion at posterior abdominal wall parietal peritoneum due to massive expansion of GI tract during development. Produces fusion of fascia
Eg duodenum (except duodenal cap) and pancreas
Where does the foregut extend from?
The lung bud to the liver bud
Describe the formation ofthe lung bud in relation to the foregut.
In the 4th week a respiratory diverticulum forms in teh ventral wall of teh foregut at the junction with the pharyngeal gu t- respitatory primordium (ventrally) and oesophagus (dorsally) separated by the tracheoesphageal septum
What are some abnormal positioning of the tracheosophageal septum?
Proximal blind-end oesophagus
Tracheoesophageal fistula
Why does the dorsal border create a greater curvature than the ventral?
Faster growth
What foregut-derived glands are formed in the ventral mesentery?
Liver
Biliary system
Part of pancreas (uncinate process and inferior head)
What foregut derived gland(s) are formed in the dorsal mesentery?
Pancreas - superior head, neck, body and tail
Describe the deveolpment of the liver
Earliest associated GI tract gland
Develops from a hepatic bud withhin the ventral mesentery
Occupies a large proportion of the abdomen during development
Surrounded by the falciform ligament anteriorly and the lesser omentum posteriorly. Bare area superiorly
Describe the formation of the duodenum.
In 5th and 6th week lumen is obliterated, then recanalised by the end of the embryonic period
Rotation of the stomach pushes it to the right, then against the posterior abdominal wall
Why does the primary intestine loop?
Elongates enormously and runs out of space
Describe the primary intestinal loop.
SMA axis
connected to the yolk sac by the vitilline duct
Has cranial and caudal limbs
How does physiological herniation occur in development?
During 6th week, growth of the primary intestinal loop is very rapid - elongation
Liver is also growing rapidly - abdominal cavity is too small to accommodate both
Intestines herniate into the umbilical cord
What structures are seen in the umilical cord at week 6 of development?
Umbilical vein
Umbilical artery (x2)
Allantois
Intetine
Amnion covering cord
How many times does the midgut rotate?
3 x 90 degrees around superior mesenteric artery
90 during herniation
90 to twisting intestine around itself
90 so cranial limb returns to abdominal cavity first, moving to the left side
After rotation, where does the cecal bud lie?
Just under the liver - must descend
What are the derivitives of the cranial limb of the midgut?
Distal duodenum
Jejunum
Proximal ileum
What are the derivitives of teh caudal limb of the midgut?
Distal ileum
Cecum
Appendix
Ascending colon
Proximal 2/3 transverse colon
What are the 2 type od malrotation of teh midgut?
Incomplete - midgut lopp makes only 90 degree rotation - left sided colon
Reversed - midgut loop makes a 90 degree rotation clockwise - transverse colon passes posterior to the duodenum
What are the major complications of the midgut defects?
Volvulus - strangulation/ischaemia
What abnormailities can result from a persistant yolk sac?
Vitelline cyst - vitelline duct forms fibrous strands
Vitelline fistula - Direct communication betwen the umbilicus and intestinal tract
Meckel’s diverticulum - AKA ileal diverticulum, most common GI anomaly
What is recanalisation?
In some gut structures, cell growth becomes so rapid that the lume is partially or completely obliterated - oesophagus, bile duct, small intestine
Recanalisation occurs to restore the lumen
What happens if recanalisation is wholly or partially unsuccessful?
Atresia - lumen obliterated
Stenosis - lumen narrowed
Most occur in duodenum
Vascular accidents may also contribute to these conditions (common contribution in the lower duodenum)
What is pyloric stensosis and what is the cause?
Hypertrophy of the circular muscle in the region of the pyloric sphincter - NOT a recanalisation failure
Common abnormality of the stomach in infants - causes characteristic projectile vomiting
What is gastrochisis?
Failure of closure of the abdominal wall during folding of the embryo
Leaves gut tube and derivatives outside the body cavity
What is Omphalocoele/exomphalos and how does it differ from a hernia?
Persistance of physiological herniation
Differs from umilical hernia because hernias have covering of skin and subcut. tissue
ie. Hernia, gut had complete physiological herniation sequence.
What deos teh hindgut give rise to?
Distal 1/3 transverse colon
Descending colon
Rectum
Superior part of anal canal
Epithelium of urinary bladder
What divides the anal canal histologically?
Pectinate line - indicates differences in arterial supply, venous and lymphatic drainage and innervation
Above (endoderm origination) - IMA, S2-4 parasympathetic, columnar epithelium, lymph drainage = internal iliac nodes. Only sensitive to stretch sensation
Below (ectoderm origination) - pudenal A, S2-4 pudenal nerve, stratified spitheliu, lymph drainage = superficial inguinal nodes. Sensitive to temperature, touch and pain sensation
How is the anal canal formed?
At 6 weeks, the hindgut ends in the cloaca
Separated from teh outside by the cloacal membrane
The cloaca is subject to anteroposterior subdivision
A wedge of mesoderm grows down into the cloaca dividing it into the urogenital sins anteriorly and the anorectal canal posteriorly.
What makes up the cloacal membrane?
Endoderm adn ectoderm
What hindgut abnormalities can occur?
Imperforate anus - failure of anal membrane to rupture
Anal / anorectal agenesis
Hindgut fistulae
What mid and hindgut derivatives retain mesenteries?
Jejunum
ileum
appendix
transverse colon
sigmoid colon
What sturctures of the mid/hindgut have fused mesenteries?
Duodenum
Ascending colon
Descending colon
Rectum (no peritoneal covering in distal 1/3)
What deos teh dorsal mesentery become?
Greater omentum
Gastrolienal ligament - stomach to spleen
Lienorenal ligament - spleen to kidney
Mesocolon
Mesentery proper - Jejunal and ileal loops
